ISSN NUMBER: 1938-7172
Issue 2.5 VOLUME 2 | NUMBER 5

Editor:
Michael A. Fiedler, PhD, CRNA

Contributing Editors:
Mary A. Golinski, PhD, CRNA
Alfred E. Lupien, PhD, CRNA

Guest Editors:
Penelope S. Benedik, PhD, CRNA, RRT
Terri M. Cahoon, MSN, CRNA
Steven R. Wooden, MS, CRNA

Assistant Editor
Jessica Floyd, BS

A Publication of Lifelong Learning, LLC © Copyright 2008

New health information becomes available constantly. While we strive to provide accurate information, factual and typographical errors may occur. The authors, editors, publisher, and Lifelong Learning, LLC is/are not responsible for any errors or omissions in the information presented. We endeavor to provide accurate information helpful in your clinical practice. Remember, though, that there is a lot of information out there and we are only presenting some of it here. Also, the comments of contributors represent their personal views, colored by their knowledge, understanding, experience, and judgment which may differ from yours. Their comments are written without knowing details of the clinical situation in which you may apply the information. In the end, your clinical decisions should be based upon your best judgment for each specific patient situation. We do not accept responsibility for clinical decisions or outcomes.

Table of Contents

AIRWAY
  THE PENTAX-AWS RIGID INDIRECT VIDEO LARYNGOSCOPE: CLINICAL ASSESSMENT OF PERFORMANCE IN 320 CASES
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CARDIOVASCULAR
CIGARETTE SMOKING AND THE HAEMODYNAMIC RESPONSE TO TRACHEAL INTUBATION
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GENERAL
CARDIAC ARRESTS AND DEATHS ASSOCIATED WITH MALIGNANT HYPERTHERMIA IN NORTH AMERICA FROM 1987 TO 2006
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  MONITORING OF NEUROMUSCULAR BLOCKADE AT THE P6 ACUPUNCTURE POINT REDUCES THE INCIDENCE OF POSTOPERATIVE NAUSEA AND VOMITING
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REGIONAL ANESTHESIA
  THE INFLUENCE OF SPINAL NEEDLE ORIENTATION DURING ADMINISTRATION OF SUBARACHNOID BLOCKS ON DISCHARGE CRITERIA IN SAME-DAY SURGICAL PATIENTS
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  COMPARISON OF TWO DIFFERENT METHODS OF LIMB EXSANGUINATION (ESMARCH BANDAGE AND LIMB ELEVTION) IN INTRAVENOUS REGIONAL ANESTHESIA
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PEDIATRIC ANESTHESIA
  THE IMPACT OF HEAD POSITION ON THE CUFF AND TUBE TIP POSITION OF PREFORMED ORAL TRACHEAL TUBES IN YOUNG CHILDREN
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QUALITY IMPROVEMENT
  PUBLICATIONS IN ANESTHESIA JOURNALS: QUALITY AND CLINICAL RELEVANCE
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PHARMACOLOGY
  THE ONSET TIME OF ATRACURIUM IS PROLONGED IN PATIENTS WITH SICKLE CELL DISEASE
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RESPIRATION & VENTILATION
PRESSURE-CONTROLLED VENTILATION IMPROVES OXYGENATION DURING LAPAROSCOPIC OBESITY SURGERY COMPARED WITH VOLUME-CONTROLLED VENTILATION
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Airway

Suzuki A, Toyama Y, Katsumi N, Kunisawa T, Sasaki R, Hirota K, Henderson JJ, Iwasaki H


 

The Pentax-AWS rigid indirect video laryngoscope: clinical assessment of performance in 320 cases

Anaesthesia 2008;63:641-647

Suzuki A, Toyama Y, Katsumi N, Kunisawa T, Sasaki R, Hirota K, Henderson JJ, Iwasaki H

 

Abstract

Purpose            The purpose of this study was to compare endotracheal intubation with the Pentax-AWS and a conventional Macintosh laryngoscope.

Background            The Pentax-AWS is a rigid, fiberoptic, indirect laryngoscope with a disposable blade (“Pblade”) and a built in guide for the endotracheal tube (ETT). A stylet in the ETT is unnecessary when the Pentax-AWS is used. Normal use for laryngoscopy does not require lateral displacement of the tongue, in contrast to traditional direct laryngoscopy. The built in color LCD video monitor includes an aiming point where the ETT guide points the tip of the ETT when it is being advanced. Several other devices are available to aid indirect laryngoscopy or blind intubation but an improved view of the glottis does not always result in successful intubation.

Methodology            This prospective, observational study included adult ASA physical status I, II, and III patients scheduled for elective surgery. Patients with unstable teeth or a mouth opening less than 1.8 cm were excluded (the Pentax-AWS Pblade is 1.8 cm thick). A Mallampati classification (without phonation) was assigned and thyromental distance measured on all study patients prior to data collection. Intubations were performed by individuals with at least two years experience who were already proficient with both the Pentax-AWS and the Macintosh laryngoscope.

Subjects were placed  in the supine position with the head on a pillow. They were preoxygenated for three minutes. General anesthesia was induced with 1 µg/kg fentanyl and 1.5 to 2.0 mg/kg propofol. Vecuronium was administered after the ability to ventilate by mask was confirmed. Once complete paralysis was verified with a peripheral nerve stimulator, laryngoscopy was performed with a Macintosh laryngoscope. The laryngoscopist was permitted to move the subject’s head and neck but external laryngeal manipulation was not allowed. Once the Cormack and Lehane (CL) classification (glottic view) was established and the Percentage of Glottic Opening (POGO) visible was measured the Macintosh laryngoscope was withdrawn. Next, after mask ventilation, laryngoscopy was performed with the Pentax-AWS. The CL class and POGO were recorded according to the view on the Pentax-AWS LCD video screen and the ETT was advanced into the trachea. Again, head and neck movement was allowed, but external laryngeal manipulation was not.

Result            There were 320 patients in the study. The Mallampati class was 3 or 4 in 55 patients (17.2%). Thyromental distance was < 7 cm in 63 patients (19.7%).

On average, the Cormack and Lehane view of the glottis was better with the Pentax-AWS than with the Macintosh laryngoscope (P<0.001). The entire glottis was visible 99.1% of the time when the Pentax-AWS was used and 54.7% of the time when the Macintosh blade was used. In 3 patients (0.94%) only part of the glottis was visible when the Pentax-AWS was used. This compares to 68 patients (21.3%) in whom only part of the glottis was visible when the Macintosh blade was used and 75 patients (23.8%) in whom none of the glottis was visible during Macintosh laryngoscopy. The Percentage of Glottic Opening (POGO) visible was greater when the Pentax-AWS was used as well (P<0.0001). When the glottic opening could not be seen with the Macintosh blade (77 patients, 24%), on average 88% of the glottis was visible using the Pentax-AWS.

The overall success rate for intubation with the Pentax-AWS was 96% on the first attempt and 100% in two attempts. (Intubation was not performed during Macintosh laryngoscopy.)

Conclusion            The Pentax-AWS produced a better view of the glottis than did a Macintosh blade.

 

Comment

The development of a number of small, easily cleaned videolaryngoscopes with their own built in LCD monitors is likely to reduce the difficulty of a lot of “difficult” intubations. Each of the currently available devices generally allows a good view of the glottis, at least in patients with normal airways. But, as this study points out, a better view and getting the tube in the right place quickly, easily, and without airway trauma are two different things.

Previous studies evaluating these devices have included only patients with fairly normal airways. The kind of patients we really don’t need the device for in the first place. From a research perspective that is a reasonable place to start, but from a clinical perspective it isn’t as helpful as we’d like. This is the first study evaluating the Pentax-AWS I’ve read that included a noteworthy number of patients who might be difficult to intubate in a clinical setting. It is encouraging to note that all patients were successfully intubated with the Pentax-AWS and that the view of the glottis was better with the Pentax-AWS than with a Macintosh blade. Of course, what we’d all really like to see is a study where all the patients are predicted difficult intubations. In the meantime, this study begins to shed some light on the utility of one of these devices in patients who are at least slightly more challenging to intubate.

 

Michael Fiedler, PhD, CRNA

 

 

If you have clinical experience with the Pentax-AWS, McGrath videolaryngoscope, or one of the new GlideScope models please share your experiences in the “Blog with the Editors” section of the Anesthesia Abstracts web site.

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Cardiovascular

Cuvas O, Er A, Ikeda O, Dikmen B, Basar H


 

Cigarette smoking and the haemodynamic response to tracheal intubation

Anaesthesia 2008;63:463-466

Cuvas O, Er A, Ikeda O, Dikmen B, Basar H

 

Abstract

Purpose            The purpose of this study was to describe the effects of smoking on heart rate and blood pressure following induction of general anesthesia and endotracheal intubation in men and women.

Background            Cigarette smoking increases the risk for cardiovascular disease and sudden death. Cigarette smoke contains nicotine and carbon monoxide (CO). CO may reduce oxygen delivery to the heart. Nicotine has cardiovascular effects mediated by the sympathetic nervous system due both to increased release and duration of catecholamine effects. The half life of nicotine has been reported to be between 30 minutes and 2.5 hours. Sympathetically induced tachycardia and hypertension following induction and intubation may result in myocardial ischemia in at risk patients. The Rate Pressure Product (RPP = systolic BP x HR) has been correlated with increased myocardial oxygen requirement and ischemia in individuals with ischemic heart disease.

Methodology            This prospective study included ASA physical status I patients between 20 years and 49 years of age scheduled for elective surgery. Both smokers (≥ 10 cigarettes per day for ≥ 10 years) and non-smokers were recruited. Those with hypertension,  a body mass index (BMI) > 30 kg/m2, anticipated difficult intubation, cardiovascular disease, and those taking maintenance medications were excluded. Patients with a sustained systolic blood pressure (BP) > 160 torr or diastolic BP > 90 torr before anesthetic induction were also excluded. All patients were premedicated with diazepam 10 mg PO 60 minutes before induction.

Patients were divided into four groups by gender and smoking status (male smoker, male non-smoker, female smoker, female non-smoker). After baseline vital signs were recorded general anesthesia was induced with pentothal 4-5 mg/kg and fentanyl 1.5 µg/kg. Vecuronium 0.1 mg/kg was administered prior to intubation. Anesthesia was maintained with sevoflurane 2% and 50% nitrous oxide with EtCO2 maintained between approximately 34 and 38 torr. All patients were intubated using a Macintosh 3 laryngoscope blade. A 7.5 mm endotracheal tube was placed in women and an 8.5 mm ETT was placed in men. Vital signs were recorded for three minutes before intubation and five minutes after intubation by an observer unaware of the patient’s smoking status.

Result            Duration of intubation was similar in all groups. The number of cigarettes smoked each day and the number of hours since the last cigarette was similar between smoking groups.

Heart rate and Rate Pressure Product increased in all groups after intubation. Male smokers experienced the greatest increases. Mean heart rate (HR) increased 30% in male smokers (P<0.05 compared to female non-smokers). RPP increased 40% in male smokers (P<0.05 compared to male and female non-smokers). Following intubation, changes in HR and RPP were most similar between male non-smokers and female smokers. Female non-smokers had the smallest increases in HR and RPP following intubation.

Conclusion            Smokers, especially male smokers, are at increased risk of increases in HR and RPP following intubation. Increased HR and RPP may increase the risk of myocardial ischemia in at risk patients.

 

Comment

We are all aware that induction and intubation is a critical time during which heart rate and blood pressure may increase. In patients at risk for myocardial ischemia we are especially careful to prevent increases in HR and BP during intubation. Of course, we stand a better chance of attenuating increases in HR and BP when we can identify patients who are more likely to respond to the sympathetic stimulation of intubation. This simple study with appropriate statistical analysis shows that not only are HR and BP changes following intubation different between smokers and non-smokers, they are also different between men and women. When I look at the graphs of HR and RPP before, during, and after intubation I see three different patterns. The male smokers are up there by themselves with the largest increases in HR and RPP. The male non-smokers and female smokers have amazingly similar post intubation changes in vital signs. They could almost be the same group. Then comes the female non-smokers in a class all by themselves. Their average HR and RPP went up less than 10% after intubation. This study has improved my awareness of the risk of increased HR and BP following intubation in smoking males. I can put this awareness to work for my patients by taking extra steps to prevent increases in HR and BP in patients at risk for myocardial ischemia. (This is the essence of quality improvement.)

It is interesting to note that none of the smokers included in this study were “hard core” smokers. The average number of cigarettes smoked per day was about 14. The maximum number of cigarettes smoked per day by any smoker in the study was only 20, one pack per day. It would be interesting to know if the effects of chronic smoking on changes in HR and RPP following intubation were greater in people who smoked three or four packs per day. While perhaps not a linear relationship, I suspect that big time smokers are prone to greater increases in HR and BP following intubation.

Just as an aside, I used to think that if my intubation technique was good enough I could minimize the hemodynamic effects of intubation. But, as it turns out, the greatest stimulus to increased HR and BP is not laryngoscopy and placement of the endotracheal tube but inflating the cuff of the ETT. (This seems a bit counterintuitive but the data supporting it are quite convincing.) So unless I can inflate the cuff “better” than everyone else, this study will probably help me identify patients who will benefit from extra measures to prevent hypertension and tachycardia during intubation.

 

Michael Fiedler, PhD, CRNA

 

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




General

Larach M, Brandom B, Allen G, Gronert G, Lehman E


 

 

Cardiac arrests and deaths associated with malignant hyperthermia in north america from 1987 to 2006

Anesthesiology 2008;108:603-611

Larach M, Brandom B, Allen G, Gronert G, Lehman E

 

 

Abstract

Purpose            The purpose of this study was to analyze 19 years of data from the North American Malignant Hyperthermia (MH) Registry, and to ascertain from the analysis if associations existed between severe MH adverse outcomes and a plethora of variables (demographic, geographic, anesthetic, etc). It was assumed that if strong associations were discovered, at risk individuals could be identified and MH crisis could be avoided in those individuals.

Background            Malignant Hyperthermia is an inherited syndrome of sustained skeletal muscle hypermetabolism. The sustained skeletal muscle hypermetabolism is caused by altered or mutant calcium homeostasis. MH susceptibility is known to be inherited as an autosomal dominant trait. It is associated with mutations in two genes: the ryanodine receptor type 1 and the dihydropyridine receptor. In humans, Malignant Hyperthermia is triggered by exposure to volatile inhalation agents and/or depolarizing muscle relaxants and only in those with the myopathy. The true incidence of MH in North America is unknown because of a lack of reporting to the registry. Dantrolene was approved as a treatment for MH in 1980. The mortality rate of MH prior to dantrolene was estimated to be 64%. This study attempted to test the hypotheses that severe outcomes of either cardiac arrest or death were associated with a distinguishing clinical history and presenting signs, as well as varying responses to dantrolene, depending upon dose and timing of administration.

Methodology            Adverse metabolic and musculoskeletal reports received by the Registry from January 1, 1987 through December 2006, were examined by experts. Those reports ranked by the MH clinical grading scale as “very likely” or “almost certain” to be an MH event were included in the analysis. Reports that identified the patient as having received a triggering agent were also included. Reviewers studied entire case reports as well as anecdotal summaries by the providers involved in the cases. Cases were excluded if the experts could easily determine that the complications identified were related to the surgical procedure (clearly not MH) and/or a preexisting disorder other than MH.

The comprehensive set of variables that were analyzed included demographics (Canada or U.S., event year, age, gender, race, and body build), family history, previous medical history, previous anesthetic history, procedure type and urgency, adverse anesthetic response, location within the hospital, and MH signs - including the order in which signs appeared. Also analyzed were time intervals from induction of anesthesia to identification of the first sign of an adverse reaction, induction to development of maximum end-tidal partial pressure of carbon dioxide, anesthetic induction to the development of maximum temperature, anesthetic induction to discontinuation of volatile anesthetic agents, and anesthetic induction to recrudescence time. Data was also gathered on the time between the first MH sign and the discontinuation of volatile agents, time between first sign and first dantrolene dose, and time between first sign and recrudescence. Maximum temperatures and all pertinent venous and arterial laboratory values were determined, including coagulation studies. Lastly, the initial and total dantrolene dose, the need for CPR, MH complication type, recrudescence and survival were analyzed.

The strongest limitation of this study was related to the probable lack of reporting when MH events resulted in a poor outcome.

Result            A total of 597 reports were received by the registry; 291 met the inclusion criteria. Of the 291 subjects, 4 had CPR and survived the MH event, and 4 were given CPR but expired. A total of 8 patients (2.7%) suffered severe outcomes. There were significant differences between the cardiac arrest/death group and the non-cardiac arrest/non-death group. Factors which were associated with a statistically significant difference between the cardiac arrest resulting in death group and the no arrest no death group are shown in the following list. (Numbers compare cardiac arrest and death with no arrest and no death groups.)

§     Induction to maximum ET Pco2 interval, minutes 216 versus 87 (P=0.0025)

§     Maximum end tidal Pco2 (mmHg) 80 versus 67 (P=0.016)

§     Maximum temperature-Celsius 42.5 versus 38.9

§     Arterial pH 6.96 versus 7.22

§     Arterial Pco2, mmHg 94 versus 55          

§     Serum potassium 7.75 versus 4.90

§     Prothrombin time (seconds) 17 versus 13

§     Total dose of dantrolene (mg/kg) 10.1 versus 5.0

 

Further analysis revealed that the four deaths occurred in four different years, all patients resided in different states, and the median age was 22 years. For each of these death cases surgery was elective and the cardiac risk was either low or intermediate. Compared to the other incidences, cardiac arrest resulting in death patients were 18.7 times more likely to have a muscular body build; two were considered elite athletes. Those who arrested or died were 49.7 times more likely to develop coagulopathies. The significant differences between the fatal group and the non-fatal group, in terms of categorical variables were as follows:

  • Muscular build n = 4 (fatal) vs. n = 282 (non-fatal) (P=0.0128)
  • CPR required n = 4 (fatal) vs. n = 287  (non-fatal) (P=0.0001)
  • Coagulopathy n = 4 (fatal) vs. n = 182  (non-fatal) (P<0.0001)

 

Conclusion            It is important to note that these types of studies may be limited in scope due to inconsistencies of case determination and incomplete reporting by providers. If under reporting or biased reporting occurred, this could lead to under- or- overestimation of the severity of MH events. The authors of this study do not know whether cardiac arrests and deaths are proportionately underreported to the Registry. It is known however, that not all deaths were reported because the ASA Closed Claims Project team provided the authors with some facts regarding additional MH deaths. In terms of the reporting from Canada, there were no known Canadian deaths but disproportionately few MH cases were reported from Canada. This lead the authors to suspect underreporting.

The statistical analysis of this study was limited by the small number of cardiac arrests and death cases. The data could not be analyzed with more rigorous statistical procedures due to the small sample size. Patients unfortunately continue to die during an MH crisis, even when everything is done “right.” It is imperative for us to learn from these situations so we can prevent them from happening again. As providers, we should make sure when events do occur, we obtain blood specimens appropriate for molecular genetic analysis. For those patients who do survive, we should be encouraging them to undergo biopsy contracture studies. All providers should report adverse anesthetic events to the Registry.

 

Comment

With the 24 hour / 7 day a week expert resources available to us as providers from the Malignant Hyperthermia Association of the United States, it is incomprehensible to think that a lack of reporting occurs when our patients experience an MH event. Never should we feel “alone” or “isolated” during a suspected or real event, as the MHAUS is there to guide us through the most challenging of situations. There should never be self-blame or feelings of not knowing what to do with such available experts. It is our responsibility to be vigilant, and identify changes in the patient’s hemodynamics that may be suspect. Then we call and obtain guidance! There exists absolutely no reason for non-reporting. While the understanding of the syndrome itself has certainly increased, we unfortunately continue to have deaths. This isn’t something that we as professional providers accept. However, by reporting all information related to an MH event, and by following standards of preparedness and care delivery, we are allowing future research to be conducted that may help discover treatments that will prevent fatal outcomes. Additionally, and extremely helpful, the authors wrote up the individual case summaries for the four fatal malignant hyperthermia events in great detail at the conclusion of the original article. If this information had not been reported, it could not have been summarized, and we would not learn from it. It is extremely valuable and worth obtaining the original article to read!

 

Mary A. Golinski, PhD, CRNA

 

 

Malignant Hyperthermia Hotline            800-644-9737 (800 MH HYPER)

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008





Arnberger M, Stadelmann K, Alischer P, Ponert R, Melber A, Greif R, Unibe MME


 

Monitoring of neuromuscular blockade at the P6 acupuncture point reduces the incidence of postoperative nausea and vomiting

Anesthesiology 2007;107:903-908

Arnberger M, Stadelmann K, Alischer P, Ponert R, Melber A, Greif R, Unibe MME

 

Abstract

Purpose            The purpose of this study was to test the hypothesis that constant stimulation of the median nerve in the area of the P6 acupuncture point with a peripheral nerve stimulator would reduce Postoperative Nausea and Vomiting (PONV).

Background            Antiemetic drugs do not eliminate PONV. The incidence of PONV is between 20% and 70%. Some, but not all, reports of acupuncture and related therapies demonstrate a reduction in PONV. The P6 (Neiguan) acupuncture point is located 2 times the width of the patient’s thumb at the interphalangeal joint proximal to the distal skin crease of the wrist along the ulnar border of the flexor carpi radialis. Transcutaneous electrical stimulation of the P6 acupuncture point has been shown to reduce nausea to a greater extent than vomiting.

Monitoring neuromuscular blockade using a peripheral nerve stimulator is a common practice that, similar to electroacupuncture, also stimulates peripheral nerves.

Methodology            This prospective, double-blind, randomized study included ASA physical status I, II, and III women between the ages of 18 years and 80 years scheduled for elective gynecological or abdominal laparoscopic surgery lasting more than one hour. Exclusion criteria included body mass index > 35 kg/m2, vestibular disease, preoperative vomiting, and preoperative antiemetic therapy.

In the control group, electrodes for the peripheral nerve stimulator were placed over the ulnar nerve at the wrist. The negative electrode was placed 1 cm proximal to the wrist crease. The positive electrode was placed 3 cm proximal to the distal electrode. In the P6 stimulation group the negative electrode was placed 2 cm distal to the P6 point between the tendons of the flexor carpi radialis and the palmaris longus. The positive electrode was placed 1 cm proximal to the P6 point. In both groups, the electrodes were placed on the dominant extremity (dominant by handedness). After induction of anesthesia, but before administration of a muscle relaxant, both groups received a supramaximal stimulation to define maximum twitch height. Following that, both groups received constant stimulation at 1 twitch per second (50 mA, 0.2 ms duration) throughout the anesthetic.

At the end of the case residual neuromuscular block was antagonized with 0.4 mg glycopyrrolate and 2.5 mg neostigmine and a return of maximum twitch height was verified. Rescue treatment for PONV included ondansetron 4 mg. All patients received IVPCA.

Patients were assessed for nausea and vomiting from arrival in recovery until 24 hours postoperatively. Neither the patient nor the individual assessing PONV knew which group the patient was in.

Result            The study included 220 subjects. Demographic factors were similar between the control and P6 stimulation groups. Overall, significantly less PONV was experienced in the P6 stimulation group than in the control group (P=0.022). The overall rate of PONV experienced over 24 hours was 45% in the P6 stimulation group and 61% in the control group. This overall reduction in PONV was due primarily to a reduction in the incidence of nausea (40% vs. 56%, P=0.022). The overall incidences of vomiting were similar between groups (23% vs. 28%, P=0.44).

The reduction in the 24 hour PONV rate seen in the P6 stimulation group was due almost completely to a reduction in the incidence of PONV in the first six hours postoperatively, 35% vs. 56% (P =0.003). And, again, reductions in nausea played the dominant role. The incidence of PONV from 6-24 hours postoperatively was no different between groups.

Conclusion            Stimulation of the P6 acupuncture point on the dominant forearm with a peripheral nerve stimulator reduced the incidence of PONV during the first six hours postoperatively.

 

Comment

There are many more receptor types involved in mediating nausea and vomiting than we have drugs to block. It seems unlikely that we will “solve” the problem of PONV with pharmacology any time soon. Given that, I wish I had a better idea whether or not some variation of acupuncture was effective for treating PONV and, if so, how it worked. There are a number of studies that use electrostimulation or pressure (“acupressure”) at the P6 point on the inner aspect of the forearm to prevent PONV. Some have shown a reduction in the incidence of PONV and others have not. Overall I’m not yet impressed. This study caught my attention because it uses a technique to prevent PONV that I’d be using anyway, peripheral nerve stimulation to assess neuromuscular block. The only difference is that the electrodes are placed to stimulate the median nerve rather than the ulnar nerve. Given that we don’t see complications of assessing neuromuscular block with a PNS, there seems to be little to loose trying this technique. If it helps reduce PONV, fine. If not, we still monitored muscle paralysis as we normally would. The only catch is to remember that if it does help reduce PONV, the effect only lasts for a few hours after we remove the nerve stimulator. I’m going to give this approach a try.

Perhaps the most interesting, and effective, study using an electrical current to prevent PONV didn’t involve the P6 acupuncture point at all.1 In that study the investigators applied a TENS unit to the neck of patients undergoing laparoscopic cholecystectomy. Compared to control patients, TENS patients had a 66% reduction in the incidence of nausea and dizziness, a 71% reduction in the incidence of vomiting, used 79% less antiemetic medication, and used 38% less pain medication (all statistically significant). If you are looking for a non-pharmacologic method of reducing PONV to add to your multi-modal approach I strongly recommend that you get a copy of the article cited following this comment and read the details of using a TENS unit for PONV.

 

Michael Fiedler, PhD, CRNA

 

1.            Cekmen N, Salman B, Keles Z et al. Transcutaneous electrical nerve stimulation in the prevention of postoperative nausea and vomiting after elective laparoscopic cholecystectomy. J Clin Anesth 2007;19:49-52.

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Regional Anesthesia

Brewer D, Baxter S, Marsh D, Saraceno D, Barber KR, Lebeck LL


 

 

the influence of spinal needle orientation during administration of subarachnoid blocks on discharge criteria in same-day surgical patients

AANA J 2008;76:203-207

Brewer D, Baxter S, Marsh D, Saraceno D, Barber KR, Lebeck LL

 

 

Abstract

Purpose            The purpose of this study was to evaluate spinal needle orientation to determine if it effected the time to discharge in a same day surgery setting.

Background            Reducing the length of patient stay is an important factor in reducing costs in a same day surgery setting. Spinal anesthesia is underused because it can delay the meeting of discharge criteria.

Methodology            Patients were recruited into the study that were from age 18-65, between 5 feet 2 inches and 6 feet 2 inches tall, and had a physical status classification of I or II. Those excluded were pregnant, allergic to lidocaine, had chronic back problems, or abnormal spinal anatomy.

Two study groups were established and subjects were randomly assigned to a group. Group A had the spinal injected with the needle orifice facing cephalad, and group B had the needle orifice facing laterally. Each patient received at least 500 mL of isotonic crystalloid solution prior to the procedure, pre-spinal sedation of 0 to 5 mg of midazolam and / or 0 to 150 µg of fentanyl, and the spinal was placed in the sitting position. A 25 gauge Whitacre spinal needle was used in all participants and oriented in the selected position prior to the administration of hyperbaric lidocaine sufficient to obtain a block between T-6 and T-12. All participants were immediately placed in the supine position once the block was placed. Those participants who had narcotics or anything except hyperbaric lidocaine in the spinal anesthesia were considered in violation of the protocol.

Sensory block was evaluated with an alcohol wipe and motor block was determined using the Bromage scale. The Aldrete assessment tool was used to determine discharge readiness. A score of 18, with a score of 2 in both ambulation and voiding, was required. Time to discharge was recorded from injection of lidocaine until the patient met discharge criteria.

Result            There were 83 patients enrolled in the study. Twenty were excluded because of missing data or protocol violations. Group A (cephalad) had 35 subjects while group B (lateral) had 28. There were no statistically significant demographic differences in the groups except for participant weight. Group A was heavier but received very little difference in lidocaine dose (mean dose of 65.3 mg for group A and 66.4 mg for group B). The doses of propofol and fentanyl sedation were not significantly different between the two groups. Comparing the two groups, there was not a significant difference from time of block until discharge criteria was met. Both groups were discharged between and average of 195 and 200 minutes after the block with time in the post care unit averaging between 140 and 150 minutes. Return to motor function in both groups averaged between 95 and 100 minutes while first void in both groups averaged between 195 and 200 minutes.

Conclusion            A number of studies have evaluated different techniques for spinal anesthesia to determine if one technique reduced discharge time more than another. Study results have been inconsistent. This study indicated that there was no significant difference between the two groups when comparing needle orientation during local anesthetic injection to time of discharge.

 

Comment

I thought this was a reasonably well developed, simple study that looked at something most clinical practitioners find important. That is, can we find ways to reduce the time to discharge when using regional anesthesia. Regional anesthesia can be safe and effective in any setting, but in an outpatient and rural surgery setting time to discharge becomes an important factor. Discharge time is not only important for cost considerations, but also because manpower resources are limited and the longer a patient stays in the facility the more difficult it becomes for the staff to manage the environment safely.

It was interesting to note how quickly motor function returned in both groups, which was half the time to discharge. It appears from this data that first void was the limiting factor in patient discharge. I certainly have found that urinary retention is a concern with spinal anesthesia and I personally think this tends to be the limiting factor in discharging patients after spinal anesthesia.

I practice as a solo practitioner in a rural setting, and I prefer to use regional anesthesia when it is as safe as or safer than a general anesthetic because patients are generally easier to manage postoperatively and will allow me to start the next case more quickly. However, if the time to discharge is significantly greater than a general anesthetic, the limited manpower resources will quickly overwhelm the recover room and potentially create problems for a smooth continuation of the surgery schedule. I am always looking for regional anesthesia methods that will safely reduce the time to discharge, but orientation of the spinal needle does not appear to be one of those methods. I think if we could find a short acting spinal anesthetic that did not produce urinary retention, we would be on the right track.

 

Steven R. Wooden, MS, CRNA

 

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008





Farbood A, Shahbazi S


 

 

Comparison of two different methods of limb exsanguination (Esmarch bandage and limb elevtion) in intravenous regional anesthesia

J Clin Anesth 2008;20:196-199

Farbood A, Shahbazi S

 

 

Abstract

Purpose            The purpose of this study was to compare limb elevation and Esmarch bandage compression for exsanguination of the arm prior to instituting an intravenous regional anesthetic (IVRA or Bier block).

Background            Bier block is an effective anesthetic for many types of forearm and hand surgery. Exsanguination of the arm is commonly achieved by wrapping it tightly with an Esmarch bandage but this can be uncomfortable. Some information indicates that elevating the arm for a period of time may be as effective as wrapping the arm with an Esmarch bandage. Wrapping the arm in an Esmarch bandage has, however, been shown to exsanguinate the arm to a greater degree than elevation.

Methodology            This randomized, double-blind study included ASA physical status I and II patients aged 19 years to 44 years old undergoing soft-tissue procedures of the hand, forearm, or elbow. In all patients a distal vein was cannulated for local anesthetic injection. Esmarch patients had their arm wrapped with an Esmarch bandage while Elevation patients had their arm elevated 90° for five minutes. Next, a double cuffed tourniquet was inflated to 150 torr above systolic blood pressure and 5 mg/kg of 0.5% lidocaine was injected slowly through the distal IV. Following local anesthetic injection, pain was assessed with a 0-5 verbal rating scale at 2, 4, 6, and 8 minutes and then every 10 minutes throughout the surgical procedure. Complaints of pain after the 4 minute assessment were treated with midazolam and fentanyl. If complaints of pain continued general anesthesia was induced.

Result            A total of 70 patients were studied (35 in each group). There were no differences in pain at any time period. Four doses of rescue medication were administered to patients in each group. No patient complained of pain during the surgical procedure. No signs of local anesthetic toxicity were identified during the surgical procedure or upon deflation of the tourniquet.

Conclusion            Elevating the arm 90° for five minutes was as effective as wrapping the arm with an Esmarch bandage prior to performing a Bier block.

 

Comment

I have always concentrated on wrapping the arm tightly with an Esmarch bandage, elevating the arm only for as long as it took to wrap it and inflate the tourniquet. I’ve never elevated the arm for a period of time prior to wrapping it (as some of my colleagues do) nor have I ever done a Bier block without wrapping the arm with an Esmarch. The data in this study clearly show that elevating the arm for five minutes is sufficient without wrapping the arm with an Esmarch. I say “clearly” because while a verbal rating scale is a fairly imprecise measure, these investigators did use a proper statistical analysis and the results weren’t even close to showing a significant difference between groups. (If the results had been “nearly” statistically significant I might think that a more precise measure of pain could have revealed a difference, but that was not the case.)

I do wonder if a cultural element might have affected the pain reported in this study. Patients were from Iran. I don’t know anything about Iranian medical care but I know much of the country is quite poor and I imagine that access to care might be somewhat limited. It is not hard to think that the availability of any degree of anesthesia might be so appreciated that patients are unlikely to complain of pain unless it is quite severe. If that is the case, it would be difficult to “see” a difference in pain scores in a study such as this.

Years ago I was taught the following keys to a good Bier block:

  • IV as distal in the hand as possible
  • Double tourniquet
  • Exsanguination with a tightly wrapped Esmarch bandage
  • Tourniquet to 300 torr irrespective of the patient’s blood pressure (if their BP is high, it should be treated before the block rather than increasing the pressure in the tourniquet)
  • Inject the lidocaine slowly, over at least two minutes by the clock

They have served me so well that I am strongly reluctant to depart from them. I will, however, file the information in this study away for a time when the patient cannot tolerate having their arm wrapped with an Esmarch and they prefer not to have a general anesthetic.

 

Michael Fiedler, PhD, CRNA

 

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Pediatric Anesthesia

Jordi Ritz EM, Von Ungern-Sternberg BS, Keller K, Frei FJ, Erb TO


 

The impact of head position on the cuff and tube tip position of preformed oral tracheal tubes in young children

Anaesthesia 2008;63:604-609

Jordi Ritz EM, Von Ungern-Sternberg BS, Keller K, Frei FJ, Erb TO

 

Abstract

Purpose            The purpose of this study was to describe the position of the tip and cuff of an ETT with a preformed bend during neck extension and flexion in children between the ages of 1 year and 8 years.

Background            Oral endotracheal tubes (ETTs) with a preformed bend are often used in ENT and oral surgery. These ETTs have the advantage of exiting the mouth and curving down over the chin without kinking. A disadvantage is that the length between the preformed bend and the tip of the ETT may not facilitate proper placement of the ETT in some patients. Cuffed ETTs are being used in children more frequently than in the past. Proper ETT cuff positioning below the cricoid cartilage is important in children to avoid airway complications.

The length of the trachea varies with head and neck position. Neck extension elongates the trachea while flexion shortens the trachea. During neck extension or flexion the tip of an ETT secured to the face is withdrawn or advanced respectively. Extubation or endobronchial intubation may result. To take advantage of the bend in a preformed ETT, insertion depth is dictated by tube size (internal diameter in mm). This increases the chance that the tip of the ETT will be malpositioned. Using an insertion depth that does not result in the preformed bend lying at the lower incisors increases the likelihood of the tube becoming kinked.

Methodology            This prospective, descriptive study included healthy children between 13 months and 8 years old who were undergoing elective surgery with general anesthesia. Neuromuscular block was not used. All children were mechanically ventilated. All children were intubated with an oral ETT with a preformed bend (Microcuff PET, Microcuff GmbH, Weinheim, Germany, now Kimberly Clarke, Zaventem, Belgium). Tube size was selected based upon normal procedure. Sizes 3.5 mm to 5 mm were used. After direct laryngoscopy and intubation, the bend in the tube was positioned even with the lower incisors and the tube secured. The child’s head was placed in a neutral position resulting in a 110° angle between the horizontal surface of the operating room table and an imaginary line drawn between the external auditory meatus and the superior orbital margin. Endotracheal tube measurements were taken with the head in the neutral position (110°), and during neck flexion (80°) and extension (130°).

Result            Measurements were taken in 128 children. The length of the trachea increased with age and body length (P<0.0001). Neck extension lengthened the trachea (P<0.0001) and neck flexion shortened the trachea (P<0.0001). The mean distance between the carina and the tip of the ETT with the head in neutral position varied with the internal diameter of the ETT.

The ETT tip advanced an average of 0.5 – 1.0 cm further into the trachea during neck flexion depending upon the size of the ETT. In two children, neck flexion and the ensuing ETT advancement resulted in endobronchial intubation. The ETT tip was pulled back an average of 0.5 – 1.2 cm during neck extension depending upon the size of the ETT. The average total distance the tip of the ETT moved in the trachea between flexion and extension was 1.0 cm in younger children and 2.1 cm in older children.

The ETT was not advanced far enough into the trachea in 22 subjects (17.2%) resulting in the cuff being positioned near the cricoid cartilage and increasing the risk of airway complications. In 5 children inadequate cuff advancement was observed during flexion, in 7 with the head in neutral position, and in 10 during extension.

Conclusion            Practical limitations on the insertion depth of pediatric ETTs with a preformed bend increases the risk that the tube will be improperly positioned in the trachea. Making these ETTs longer and the cuff length shorter might result in more optimal ETT placement in the trachea.

 

Comment

This well-executed study provides the evidence to support the adage that the tip of the endotracheal tube (ETT) follows the direction of the nose when the head position is altered from the neutral to extension or flexed positions. The researchers utilized a clinically-applicable angle of extension that is commonly seen in pediatric otorhinolaryngological surgery. They did recognize that a weakness of their results was the omission of the mouth gag utililized in those same cases. This device has been shown to force the ETT tip more caudally. The standardization of patient positioning was performed with great precision for accuracy and to allow for replication.

The evidence demonstrated the change of cuff position as well as displacement of the tip of the ETT. Because of the pediatric airway anatomy with the cricoid ring as the narrowest point even up to the age of five years, it is important that clinicians think about the pressure-related injury to the mucosa, not simply the risk of extubation. Based on these findings, the authors suggested the restructuring of the preformed ETT to be longer from tip to preformed bend and employ a shorter cuff. They recognized the increased risk of endobronchial misplacement, but also reported that, in each case of endobronchial movement, the malposition was easily detected with auscultation, a commonly employed assessment technique. The malpositioned cuff placing pressure on the mucosa at the cricoid ring required the extraordinary measure of fiberoptic bronchoscopy. It is impressive that the intubation depth mark was located above the cords in seven patients in the neutral position and in 5 patients in the flexed position that moved the ETT more caudad.

The researchers improved on a previous study that measured depth changes during laryngoscopy. The measurements and findings in this study are more clinically applicable because the action of the laryngoscope blade on the soft tissues was removed as a variable. I appreciated this well-thought out and executed study that has excellent applicability to clinical practice.

 

Terri M. Cahoon, MSN, CRNA

 

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Quality Improvement

Lauritsen J, Moller AM


 

Publications in anesthesia journals: quality and clinical relevance

Anesth Analg 2004;99:1486-91

Lauritsen J, Moller AM

 

Abstract

Purpose            The purpose of this study was to evaluate the number and quality of clinically oriented articles in prominent anesthesia journals.

Background            As evidence based practice becomes a focus it becomes more important to have access to relevant clinically based anesthesia journal articles. It would be assumed that respected anesthesia journals would contain numerous clinically based articles and that those articles be of high quality. This study reviewed the number of clinically based articles in five anesthesia journals and the quality of the methodology used in each article. It is important to ask how focused these journals were on clinical questions and whether or not they used valid statistical analysis methods to answer those questions.

Methodology            Five respected and widely read anesthesia journals were selected for review. They were Anesthesiology, Anesthesia & Analgesia, British Journal of Anesthesia, Anesthesia, and Acta Anaesthesiologica Scandinavica. Articles reviewed were published between January and June of 2000. All articles were read twice and classified by the reviewers. Articles were classified as primary studies (original research) or secondary studies (summarization of primary studies). Randomized clinical trials were classified as primary studies, while systematic reviews, narrative reviews, editorials, correspondence, and other articles were classified as secondary studies. Study outcomes were classified as clinically relevant, surrogate (laboratory), practice (impact on provider but not on patient), and apparatus. Clinical relevance was calculated as the proportion of articles using a primary or secondary clinical methodology and a clinical endpoint.

Result            There were 1379 articles classified and reviewed. There was a large variation in the number of primary studies found between the five journals. Primary studies varied between a low of 12.2% and a high of 35.3% with the median being 20.4%. Of the primary study articles found, laboratory research and animal studies comprised 31.2%, randomized clinical trials 20.4%, other designs and other types 28.3%.

In the randomized clinical trials, 54% measured clinically relevant outcomes while 33% measured surrogate outcomes. Overall, only 18.6% of all articles were determined to be clinically relevant.

Conclusion            Of all the articles reviewed, 81.4% were found to be not clinically relevant because of the suboptimal methodology used, or they investigated nonclinical or surrogate outcomes. The research showed that the majority of pages from these five journals included mostly narrative reviews and editorials rather than systematic reviews and meta-analyses. Primary clinical research requires specialized training in research methods and statistical analysis, is time consuming, and is often costly which may partially explain why clinically relevant research was infrequently found in these journals.

Surrogate (laboratory) outcome measurements have resulted in inaccurate conclusions, do not always measure outcomes of real clinical interest, and have a risk of serious fallacies. Surrogate studies are often used because they are faster, less expensive, and more efficient than clinical studies. It is important for clinicians to understand that they should not rely solely on surrogate studies to base their clinical decisions. Randomized clinical trials, systematic reviews, or meta-analyses using rigorous methods and appropriate statistical analysis are more valid articles upon which to base clinical practice.

It is often thought that the “Journal Impact Factor” is an indicator of the quality of research. Journal Impact Factor is a calculation of how often a research article is cited in other articles. For a number of reasons, including publication timing and indexing, the Journal Impact Factor as a determination of article quality is questionable, at best.

This study had a number of weaknesses. Various definitions of how articles can be classified were found leading to difficulty in standardizing classifications. As with most peer review articles, it is impossible to be absolutely objective. Biases are introduced in such articles that can not be easily controlled.

It is desirable to reduce the number of surrogate articles and increase the number of clinical trials. Because this study looked at a very small period of time and a limited number of journals, it is impossible to determine if the outcome would be different when comparing other journals and time periods.

 

Comment

While this study is from a number of years ago, the importance of its message can not be understated. Too often we assume that all articles published are relevant and accurate. We also assume that all research with a clinical focus has a clinically relevant outcome. This is just simply not true and a good clinician should be very careful about assuming any of this to be true.

I enjoyed the discussion this article provided about types of research, clinical relevance of research types, why certain types of studies are often undertaken, and other comments about misconceptions concerning the relevance of research articles and journals.

It is certainly possible that the finding that 18.6% of reviewed articles were determined to be clinically relevant could be optimistically high. This study did not evaluate the quality of statistical analysis, but simply that a statistical analysis was accomplished. There are many studies that include a statistical analysis but the methods used may not be appropriate for the data, creating misleading results. Other such studies have noted the need for better study quality in addition to more clinically relevant research.

As I continue to write for Anesthesia Abstracts I am finding myself more frequently exposed to both good clinical research and trash. As this study shows, there is a lot of trash out there. Anesthesia Abstracts is unique in its ability to present research purported to be clinically relevant in a concise manner, and then have the contributor critically evaluate the article’s claims of clinical importance and research validity. With the knowledge that less than 20% of the articles contained in those “high impact” journals are clinically relevant, it is important to read carefully and evaluate completely. For the busy clinical practitioner, I think that Anesthesia Abstracts is very helpful in that respect.

 

Steven R. Wooden, MS, CRNA

 

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Pharmacology

Dulvadestin P, Gilton A, Hernigou P, Marty J


 

The onset time of atracurium is prolonged in patients with sickle cell disease

Anesth Analg 2008;107:113-116

Dulvadestin P, Gilton A, Hernigou P, Marty J

 

Abstract

Purpose            The purpose of this study was to compare the onset time of an intubating dose of atracurium in patients with and without sickle cell disease.

Background            Patients with sickle cell disease are more likely to undergo a surgical procedure during their lifetime than those without sickle cell disease. Sickle cell patients may respond differently to a variety of drugs used during anesthesia.

In the general population, the onset of an intubating dose of atracurium has been widely reported to be 3 minutes or less. The onset time of muscle relaxants is largely controlled by two factors: muscle blood flow and the initial plasma concentration of the drug. The initial plasma concentration of atracurium may be lower in sickle cell disease patients due to an increase in plasma volume caused by chronic anemia.

Methodology            This study included 13 patients with sickle cell disease and 17 ASA physical status class I or II patients without sickle cell disease. In all patients, general anesthesia was induced with propofol 2-3 mg/kg and sufentanil 1.5 µg/kg. Anesthesia was maintained with 0.5% to 2% sevoflurane and 50% nitrous oxide.

Neuromuscular blockade was monitored with Train-of-four stimulation every 15 seconds. A force displacement transducer (PCB Piezotronics, Orsay, France) was placed at the wrist to quantify the magnitude of the twitches. After a stabilization period, atracurium 0.5 mg/kg was administered intravenously (time zero). Intubation was performed three minutes later without regard to the level of neuromuscular block. The times to various levels of neuromuscular blockade, and recovery of blockade, were monitored.

Result            There were some differences between the healthy control and sickle cell disease groups. While only 23.5% of healthy control patients were African American, 100% of the sickle cell patients were African American (P<0.001) Sickle cell patients also had a lower body mass index and a lower hemoglobin (P<0.05 and 0.01 respectively).

The onset of atracurium was defined as the time from administration of the drug to a 10% reduction in twitch height. Onset was longer in sickle cell patients (P<0.01). Time to onset (average ± SD (range)) was 165 ±36 (110-230) seconds in healthy control patients and 323 ±124 (165-490) seconds in sickle cell patients. Recovery from neuromuscular block began slightly sooner in healthy control patients (44 minutes vs. 53 minutes, P<0.05) but the total time to complete recovery was no different between groups.

Intubating conditions 3 minutes after atracurium administration were excellent or good in 94% of healthy control patients and 61.5% of sickle cell patients. Intubating conditions were unacceptable in 1 (6%) healthy control patient and 5 (38.5%) sickle cell patients.

Conclusion            The onset time of an intubating dose of atracurium was longer in patients with sickle cell disease than in healthy patients. The time to recovery from atracurium block was no different between the two groups. Neuromuscular blockade monitoring may be advisable in patients with sickle cell disease to guide the timing of laryngoscopy and intubation.

 

Comment

This simple study probably has something important to teach us about paralysis and intubation in sickle cell patients. I say, “probably,” because I have to assume a couple of things the investigators were silent about in their report to make that statement, but the assumption seems a fairly good bet. Also, they didn’t say anything about why their groups had different numbers of patients (17 healthy patients and 13 sickle cell patients) or why they had so many Caucasian patients in the healthy group. Other than that, their methods and statistical analysis appear trustworthy.

The differences in atracurium onset time between these two groups was quite remarkable. First, the average onset time in healthy patients was 2.75 minutes, which is consistent with what we expect in the general population. By comparison, the sickle cell group needed an average of 5.38 minutes for the same level of block; almost twice as long. The standard deviation (an indication of the variability of each group) was about half a minute in the healthy group and two minutes in the sickle cell group. But perhaps most striking was the range of times needed for the onset of neuromuscular block. (And remember, this is not the time to 100% block but the time block is starting to set up well.) The longest onset time for any patient in the healthy group was 3.8 minutes, which was 30% shorter than the average time of 5.38 minutes in the sickle cell group. The longest onset time in the sickle cell group was over 8 minutes! (I’m visualizing the surgeon as I wait 8 minutes to intubate.)

I anesthetize patients with sickle cell disease only occasionally so I don’t have a “gut” feeling for how these results compare with my clinical practice in general or with the use of other muscle relaxants. (I almost never use atracurium and only occasionally use cis-atracurium.) But if these results are generalizable to all sickle cell patients and multiple muscle relaxants they indicate the need for either a lot more patience when intubating or a different plan to make it possible to intubate faster. Given that the incidence of complications during intubation is lower when there is good muscle relaxation, I would have to agree with the investigators recommendation to use a peripheral nerve stimulator to judge the onset of paralysis in sickle cell patients.

 

Michael Fiedler, PhD, CRNA

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008




Respiration & Ventilation

Cadi P, Guenoun T, Journois D, Chevallier J-M, Kiehl J-L, Safran D


 

Pressure-controlled ventilation improves oxygenation during laparoscopic obesity surgery compared with volume-controlled ventilation

Br J Anaesth 2008;100:709-716

Cadi P, Guenoun T, Journois D, Chevallier J-M, Kiehl J-L, Safran D

 

Abstract

Purpose            Morbidly obese patients undergoing a pneumoperitoneum may require high airway pressures to achieve adequate oxygenation. This study hypothesized that plateau pressure, arterial oxygenation, and carbon dioxide elimination would be improved with the use of pressure-controlled ventilation (PCV) as opposed to volume-controlled ventilation (VCV) during operation.

Background            Morbidly obese patients generally suffer from altered respiratory mechanics in the form of reduced chest wall and lung compliance. Chest wall compliance is reduced by the weight of excess adipose tissue on the rib cage. Both subcutaneous and abdominal fat push the diaphragm cephalad and restrict diaphragmatic motion. Lung compliance may be limited by a shift in the pressure-volume curve such that tidal breathing occurs at lower lung volumes. Add to these limitations a pneumoperitoneum and it takes substantially higher inflation pressure to achieve an adequate ventilation.

Mechanical ventilation during routine anesthesia is normally volume-limited, meaning that when a preset volume is reached, volume delivery stops and exhalation begins. Volume delivery will only be truncated if and when a preset pressure limit is reached. In pressure-limited ventilation, tidal volume is determined by a operator-set pressure limit, gas flow, and the length of inspiration. In this case, gas flows in the lungs along a pressure gradient; as the lungs fill, the gradient decreases and eventually the pressure limit is reached and flow (and volume) delivery stops. The preset pressure is maintained for the duration of the inspiratory time.

Tidal volume delivery during mechanical ventilation partially depends upon gas flow waveforms.1 Ventilator waveforms may be classified as exponential-decay, rectangular, ramp (ascending or descending), or sinusoidal. Volume ventilators used in anesthesia are generally designed to deliver a rectangular waveform, while pressure control ventilation uses a decelerating flow pattern. Decelerating flow patterns have been shown to generate lower peak airway pressures and improve gas distribution. This forms the theoretical basis for this study: the use of PCV will improve oxygenation and ventilation parameters whilst doing so at lower plateau pressures and therefore with a reduction in barotrauma.

Methodology            This was a prospective, randomized, unblinded study on 36 obese subjects undergoing laparoscopic gastric banding under general endotracheal anesthesia. Inclusion criteria were BMI > 35 kg/m2, 18 years or older, no major pulmonary disease (no volume or flow parameters < 70% predicted), and preoperative PaCO2 < 45 torr.

All subjects underwent preoperative pulmonary function testing with blood gases, cardiac evaluation, and polysomnography if sleep apnea was suspected. Subjects were randomized to either PCV or VCV. Anesthesia care was standardized to achieve appropriate levels of anesthesia and neuromuscular relaxation using propofol, sufentanil, and atracurium. A ventilation protocol was designed based on each subject’s ideal body weight with respiratory frequency (RR) set at 14 bpm, I:E 1:2, FIO2 0.6, and PEEP 5 cm H2O. Subjects in the VCV group had VT set at 8 mL/kg, while PCV subjects had plateau pressure (Ppl) set such that VT = 8 mL/kg. A specific protocol was followed to keep PaCO2 between 33 and 34.9 torr by altering RR in the VCV group and Ppl in the PCV group. Respiratory and cardiovascular data were collected after 45 minutes of CO2 pneumoperitoneum and when minute ventilation and end-tidal CO2 were at steady state for 10 minutes.

Result            There were no differences between groups for patient characteristics, incidence of co-morbidities, or results of preoperative testing, therefore reported results were not due to group variability. There were no differences in ventilatory parameters (VT, VT/kg, minute ventilation, bpm, mean airway pressure, dynamic compliance, resistance, or dead space) between VCV and PCV groups. Subjects in the PCV group had higher peak inspiratory flow rate (p < 0.01), higher proportion of VT delivered during the first half of the inspiratory time (p < 0.01), and lower end-tidal CO2 to PaCO2 gradient (5 v. 7 torr, p = 0.01). There were no differences between groups in intra- or post-operative systolic, diastolic, mean arterial pressure, or heart rate.

Statistically significant differences were found between groups for intraoperative, but not postoperative, arterial blood gases. Subjects in the PCV group had higher pH (7.4 v. 7.38, p = 0.041), higher PaO2 (168 v. 119 torr, p = 0.011), lower PaCO2 (39 v. 40.5 torr, p = 0.014), higher arterial oxygen saturation (99 v. 98%, p = 0.01) and PaO2/FIO2 (281 v. 199, p = 0.011).

Conclusion            PCV statistically improved oxygenation and ventilation in morbidly obese subjects undergoing laparoscopic banding, although plateau pressure and mean airway pressures were similar in both the PCV and VCV groups. The authors hypothesize that the higher inspiratory flow rate coupled with a decelerating flow pattern that continues during the inspiratory plateau time served to improve ventilation/perfusion relationships in the lung.

 

Comment

Anesthesia providers are often advised that pressure control ventilation is beneficial in the morbidly obese primarily, because it can ventilate these patients at lower airway pressures. Volume ventilation, even at large tidal volumes of 1000 to 1200 mL, effectively hyperventilates the morbidly obese patient, but fails to improve oxygenation.2 It is an appropriate goal to reduce the mechanical stress on the lung by using the lowest peak and plateau pressure possible, particularly in patients who may require postoperative or long term ventilation. This study has demonstrated that using PCV with decelerating flow patterns in obese patients can make a clinically significant improvement in oxygenation (intraoperative PaO2 and the ratio PaO2/FIO2). It also illustrates that a statistically significant result may not have any clinical significance; in this case, the statistically significant change in PaCO2  (40.5 to 38 mm Hg) and pH (7.38 to 7.40) during PCV was not a clinically significant change.

A problem with this study is that the Evita 2 ventilator is designed for intensive care use and is not routinely used in an operating room; recall that these subjects received TIVA so that the use of a proper anesthesia machine was not necessary. The Evita 2 can provide the high flow rates that may be necessary in the PCV mode to achieve adequate tidal volumes in patients with low pulmonary compliance; the typical anesthesia machine—especially older models—cannot replicate this. I would suspect that many institutions do not provide the newest models of anesthesia ventilators with a PCV mode, therefore an anesthesia provider may not be able to reproduce this technique in their own OR. The authors note, and it is very important to remember, that the use of pressure control inherently means that volume delivery is compromised when compliance decreases or resistance increases. This is common when neuromuscular paralysis begins to abate or under light anesthesia; therefore close attention to exhaled tidal volume and minute ventilation is required. This is not a problem as long as you utilize the requisite exhaled VT and/or minute volume alarms and they are set within reasonably narrow limits.

What is the explanation for lack of improvement in oxygenation in the post-operative period? It is well known that over 90% of patients develop anesthesia-induced atelectasis within minutes of induction. In the morbidly obese under anesthesia, up to 10% of lung volume may be lost to atelectasis. Without active preventive or rescue interventions, this population will not recover full lung inflation for over 24 hours. In caring for this population, I apply an alveolar recruitment maneuver (ARM) immediately after induction and use from 8 to 10 cm H2O PEEP during the course of the anesthetic.3 An ARM is a manual inflation to 40 cm H2O held for 20 seconds; Rothen has elegantly demonstrated that most of the re-expansion actually occurs within the first 7 to 8 seconds of the maneuver. 4 Once the ARM is complete, the use of 10 cm H2O PEEP serves to maintain inflation while lower levels of PEEP or ZEEP permit alveolar collapse to recur. These actions serve to treat anesthesia-induced atelectasis and then maintain alveolar re-inflation, improving V/Q relationships and oxygenation. The authors performed neither of these now routine interventions, which may explain the similar blood gas results in the postoperative period.

That said, in the absence of these maneuvers, PCV may well be more advantageous than VCV in particular because of the apparently improved distribution of ventilation and oxygenation. A quick review of your ventilator’s operating manual on setting the parameters and alarms is all that is required.

 

Penelope S. Benedik PhD, CRNA, RRT

 

1. Kacmarek RM & Chipman D. Basic principles of ventilator machinery. In Tobin MJ (Ed.) Principles and practice of mechanical ventilation (2nd Ed.) New York, McGraw-Hill; 2006: 53-96.

2. Sprung J, Whalley DG, Falcone T, Wilks W, Navratil JE, Bourke DL. The effects of tidal volume and respiratory rate on oxygenation and respiratory mechanics during laparoscopy in morbidly obese patients. Anesth Analg 2003;97:268-274.

3. Coussa M, Proeitti S, Schnyder P, Frascarolo P, Suter M, Spahn DR, Magnusson L. Prevention of atelectasis formation during the induction of general anesthesia in morbidly obese patients. Anesth Analg 2004; 98:1491-1495.

4. Rothen HU, Neumann P, Berglund JE, Valtysson J, Magnusson A, Hedensteirna G . Dynamics of re-expansion of atelectasis during general anesthesia. Br J of Anaesth, 1999;82(4):551-6.

 

© Copyright 2008 Anesthesia Abstracts · Volume 2 Number 5, June 30, 2008