ISSN NUMBER: 1938-7172
Issue 6.6 VOLUME 6 | NUMBER 6

Editor:
Michael A. Fiedler, PhD, CRNA

Contributing Editors:
Penelope S Benedik, PhD, CRNA, RRT
Mary A Golinski, PhD, CRNA
Gerard Hogan Jr., DNSc, CRNA
Alfred E Lupien, PhD, CRNA, FAAN
Lisa Osborne, PhD, CRNA
Dennis Spence, PhD, CRNA
Cassy Taylor, DNP, DMP, CRNA
Steven R Wooden, DNP, CRNA

Assistant Editor
Jessica Floyd, BS

A Publication of Lifelong Learning, LLC © Copyright 2012

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

EQUIPMENT & TECHNOLOGY
Relationship between bispectral index values and volatile anesthetic concentrations during the maintenance phase of anesthesia in the B-Unaware trial
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GENERAL
Predictors and clinical outcomes from failed Laryngeal Mask Airway Unique: a study of 15,795 patients
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The effects of isoflurane and desflurane on cognitive function in humans
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ORTHOPEDICS
Timing of acute myocardial infarction in patients undergoing total hip or knee replacement
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Comparative effectiveness of regional versus general anesthesia for hip fracture surgery in adults
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PEDIATRIC ANESTHESIA
The effect of obesity on the ED95 of propofol for loss of consciousness in children and adolescents
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Equipment & Technology
Relationship between bispectral index values and volatile anesthetic concentrations during the maintenance phase of anesthesia in the B-Unaware trial

Anesthesiology 2011;115:1209-1218

Whitlock E, Villafranca A, Lin N, Palanca BJ, Jacobsohn E, Finkel KJ, Zhang L, Brunside BA, Kaiser HA, Evers AS, Avidan MS


Abstract

Purpose The purpose of this study was to determine the correlation between Bispectral Index Values (BIS values) and End Tidal Anesthetic Concentration (ETAC). The study also examined the effects of four patient characteristics on this correlation:

  • ASA status (3 or less vs. 4)
  • men vs. women
  • age < 60 vs. age over 60
  • those alive 1 year postoperatively vs. those who had died

 

Background Inadequate anesthetic depth can result in intraoperative awareness. Greater attention was directed at awareness as manufacturers began releasing monitors purported to measure depth of anesthesia. Most such depth of anesthesia monitors use processed electroencephalograph (EEG) data. Many, but not all, anesthetics cause known changes in the EEG. Investigators have suggested the following criteria as being required, though not completely sufficient, for any depth of anesthesia monitor to be useful in guiding titration of general anesthesia during anesthetic maintenance:

  • a high correlation between the depth of anesthesia displayed on the monitor and anesthetic concentration in the brain
  • a predictable depth of anesthesia value at which emergence from anesthesia reliably occurres in the majority of patients
  • the same association between depth of anesthesia and the depth of anesthesia monitor value when anesthesia is deepened and when it is lightened (no hysteresis)

 

 

Methodology This was a randomized, prospective study of 1,941 adult patients undergoing surgery with isoflurane, desflurane, or sevoflurane anesthesia. The BIS group used a protocol for titrating maintenance of anesthesia based upon BIS values (BIS Quatro, software version XP). In the BIS group, anesthesia providers could see the BIS values. The BIS manufacturer had no role in the study nor did they support the study financially or in kind. The End Tidal Anesthetic Concentration (ETAC) group used a protocol for titrating maintenance of anesthesia based upon End Tidal agent concentration and clinical signs. In the ETAC group, anesthesia providers could not see the BIS display or any part of the monitor, but, BIS data was recorded by investigators. End Tidal Anesthetic Concentrations were converted into MAC equivalents adjusted for age. Age Adjusted MAC equivalents were used in order to standardize the description of depth of anesthesia across patients of different ages and different inhalation agents.

 

Result Data from over 800 patients were excluded from the study because of incomplete data collection, thus the analysis included data from 1,100 patients. A total of 930 hours of stable, maintenance anesthesia data points were included.

 

The mode (most common) BIS value was in the low 40’s irrespective of age adjusted MAC values across a range of 0.42 to 1.51 MAC. This clustering of BIS values in the low 40’s irrespective of actual MAC delivered to the patient was seen not only in the BIS group, where BIS values could be seen and targeted; but also in the End Tidal Anesthetic Concentration group, where BIS values were not visible to the anesthesia provider. For every 0.1 increase in age adjusted MAC, the BIS value decreased by approximately 1.5 units. Thus, the BIS would go down by only 9 units when the agent concentration was doubled from 0.6 age adjusted MAC to 1.2 age adjusted MAC. At equivalent MAC values, patients 60 or younger tended to have lower BIS values than patients older than 60 years. Lower BIS values at the same MAC were also seen in:

  • women vs. men
  • ASA IV patients vs. ASA I thru III patients

In a subset of patients, a linear regression was performed to determine the correlation between BIS values and age adjusted MAC. The correlation coefficient was -0.16; a week correlation. And, in about 25% of patients the slope of the BIS vs. age adjusted MAC relationship was nearly zero, indicating almost no variation in BIS values as age adjusted MAC (depth of anesthesia) was increased or decreased. Conversely, a few patients did show a good correlation between BIS values and age adjusted MAC values.

 

Conclusion The three criteria identified by the investigators as being necessary for a depth of anesthesia monitor to be clinically useful were not found in this study [see background section for list]. The correlation between BIS values and stable age adjusted MAC in most patients was week; and in about 25% of patients it was non-existent. Furthermore, the BIS reading was influenced by patient age, gender, and ASA physical status and not depth of anesthesia alone. The BIS was most often insensitive to clinically significant changes in End Tidal Anesthetic Concentration.

 

Comment

This study is one in a long list of studies that reveal the futility of using current depth of anesthesia monitors. There is lots of scientific evidence that the BIS does not reliably monitor depth of anesthesia and no credible evidence I’m aware of that suggests it does. An often cited study that looks on the surface as though BIS use might help anesthetists avoid recall in those at greatest risk for postoperative recall has such a wide variability in it’s results that it is simply not credible.

 

This well executed study provides solid information in three general categories.

First, the BIS doesn’t correlate well with actual anesthetic depth. When the inhalation agent is turned up, and the patient’s anesthetic deepened, the BIS should go down. If there was a perfect relationship between an increase in the End Tidal Anesthetic Concentration / Depth of Anesthesia and the BIS value the correlation would be -1.0. Don’t let the minus sign fool you. This would be a perfect correlation. The minus sign simply means that as one goes up (agent concentration or MAC) the other goes down (BIS). But the correlation wasn’t anywhere near -1.0. It was -0.16. The investigators generously called this correlation “weak.” I’d call it almost nonexistent.

Second, in about 25% of patients the correlation wasn’t even this good. It was even more nonexistent than a correlation of -0.16. Over a wide range of MAC values the BIS simply didn’t change.

Third, the BIS value was partly determined by patient age, gender, and ASA physical status. While it would be inconvenient, there could be a physiologic basis for a difference due to gender. To work well, manufacturers would have to define any gender influence and put a switch on the monitor for us to specify whether the patient being monitored was male or female. But age was already adjusted in the MAC values used; it had already been compensated for age. I can’t think of a conceptual basis by which ASA physical status should influence BIS values. But these are not the only factors that have nothing to do with depth of anesthesia that have been shown in the scientific literature to influence BIS values. Other documented factors include:

  • muscle relaxants
  • patient position
  • drug used to induce / maintain general anesthesia

 

In fairness, it should be noted that in a very few patients there was a good correlation between anesthetic depth and BIS values. From the manufacturer’s perspective this is encouraging. It may mean that they are close to right in some way so it works as intended once in a while. But it may also mean that random chance sometimes results in readings that only look good. From a clinician’s perspective, however, this just doesn’t fly. Who among us would accept a BP monitor, pulse oximeter, or ETCO2 monitor that got it right once in a while and the rest of the time gave us results that were totally unrelated to the real BP, pulse ox, or ETCO2? No one I know.

 

I occasionally speak to anesthetists that regularly use a consciousness monitor and are convinced that they are accurate and clinically helpful. I respect my colleagues’ viewpoint. There may be a narrow range of patient circumstances and anesthetic techniques under which consciousness monitors work much better than their overall level of accuracy. But, my colleagues may also have been deceived by marketing, expectation bias, or coincidence. This is why basing our practice on evidence, such as this research, is so important. To be useful, a monitor has to work for everyone, all the time. I can be fooled. The systematic, controlled, unbiased approach used in well done studies such as this one are much harder to fool than my observations are. We would be wise to pay attention to it.

Michael A. Fiedler, PhD, CRNA


Anesthesia awareness and the Bispectral Index. Anesthesia Abstracts April 2008 (Vol. 2 Num. 3) [N Eng J Med 2008;358:1097-1108]

Does patient position influence the reading of the bispectral index monitor. Anesthesia Abstracts November 2009 (Vol. 3 Num. 11) [Anesth Analg 2009;109:1843-1846]

Prevention of intraoperative awareness in a high-risk surgical population. Anesthesia Abstracts October 2011 (Vol. 5 Num. 10) [N Engl J Med 2011;365:591-600]


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012




General
Predictors and clinical outcomes from failed Laryngeal Mask Airway Unique: a study of 15,795 patients

Anesthesiology 2012;116:1217-26

Ramachandran SK, Mathis MR, Tremper KK, Shanks AM, Kheterpal S


Abstract

Purpose The purpose of this study was to identify clinical predictors and outcomes of failed Laryngeal Mask Airway Unique (uLMA) placement.

 

Background Laryngeal mask airways have been used in clinical practice since 1981. They have revolutionized how anesthesia providers manage the airway. These supraglottic airway devices are safe and have a low failure rate. The incidence of life-threatening complications with supraglottic airway devices has been estimated to be 1 in 46,174 cases. The incidence of adverse airway events, such as airway obstruction and laryngospasm, with the Classic LMA are reported to be 0.15% - 7%, with placement failure rates ranging from 0.19% to 4.7%. The uLMA is a single-use LMA which is used in many institutions in lieu of the Classic LMA. The failure rates for the uLMA are lower than the Classic LMA, 0 - 2.5%. Unfortunately, little is known about what risk factors might predict which patients may be at risk for failed uLMA placement.

 

Methodology This was a retrospective, observational study conducted at the University of Michigan. The investigators collected data from their anesthesia information management system computer database to identify adult patients that underwent general anesthesia with the use of a uLMA between January 2006 and October 2009. Exclusion criteria included children, instances in which laryngoscopy was performed prior to uLMA placement, and when the uLMA was removed because of a change in the surgical procedure.

 

Multiple patient, anesthetic, and surgical characteristics were examined to identify predictors of failed uLMA placement. A failed uLMA was defined as any acute airway event occurring between induction and completion of the surgery which required removal and rescue with an endotracheal tube. Airway events included the inability to ventilate because of a leak or airway obstruction, need to administer succinylcholine to facilitate uLMA reinsertion attempts, and adverse airway events. Adverse airway events were defined as desaturation ≤85%, hypercapnia with ETCO2 >50 mm Hg, and increased peak inspiratory pressures ≥25 cm H2O on two consecutive readings 1 minute apart.

 

The primary outcome of the study was to identify predictors of uLMA failure. The secondary outcomes were the incidence of difficult mask ventilation in patients with failed placement and frequency of unplanned hospital admissions. Statistical analysis and sample size were appropriate.

 

Result A total of 15,795 adult patients had a uLMA placed at this facility between 2006 and 2009. Of these cases, 170 patients (1.1%) were identified as having failed uLMA placement requiring subsequent intubation. Failures occurred prior to surgical incision (61%), during table rotation (16%), and during maintenance of anesthesia (23%).

 

In 48% of the cases of uLMA failure, the device placed was outside the recommended size for patient body weight. When uLMA failure occurred, there was no documented attempt to reinsert the uLMA in 61% of cases, 1 attempt at reinsertion in 25% of cases, and 2 or more attempts in 14% of cases. Of the 27 cases of uLMA failure during surgical table rotation, 9 occurred during head and neck surgery.

 

Inadequate ventilation secondary to a leak occurred in 42% of cases. Significant adverse respiratory events occurred in 62% of uLMA failures (n = 106). Twenty-two percent (22%) of failed uLMA cases experienced severe desaturation of ≤85%. Gastric contents in the uLMA were seen in three of the 170 uLMA failures. Eight patients required succinylcholine to break a laryngospasm, 4.7% of failures.

 

Logistic regression identified three independent clinical predictors of uLMA failure and the odds ratio of failure when the risk factor was present. They included:

  • surgical table rotation (OR = 5.0, P <0.0001)
  • male gender (OR = 1.7, P = 0.002)
  • poor dentition (OR = 1.6, P = 0.049

In the 1,089 patients in whom mask ventilation was attempted, the incidence of difficult mask ventilation was 3 times higher in those with failed uLMA placement as compared to those without failed placement (P < 0.05).

 

Most uLMA cases occurred in the ambulatory setting and they included 131 of the 170 recorded uLMA failures. In 14% of these uLMA failure cases, an unplanned hospital admission was required (reason for admission not specified). Only 2 out of the 170 patients who had uLMA placement failure required an unplanned intensive care unit admission for persistent hypoxemia (1 inpatient and 1 outpatient).

 

Conclusion The incidence of failed uLMA placement was 1 in 93 cases (1.1%). Difficult mask ventilation occurred in 6% of patients with failed uLMA placement. Laryngeal Mask Airway Unique failure was 5 times more likely to occur if the surgical table was rotated. Other predictors of failure included, male gender and poor dentition. Unplanned hospital admission occurred in 14% of patients with uLMA failure; however the reasons for the admissions were not identified.

 

Comment

This is one of the largest studies ever published that examined predictors of failed uLMA placement. To me, the results are not surprising. The investigators found that the largest portion of the airway events (40%) were due to inability to ventilate related secondary to leaks. This is not surprising since almost half the uLMAs placed were outside the size recommendations for the patients’ body weight. This finding highlights the importance of placing the appropriate sized LMA.

 

The strongest predictor of failed uLMA placement was surgical table rotation, and in 9 of these cases the procedure was on the head and neck. I suspect that the uLMA either got dislodged or became obstructed when the table was turned and/or the patient’s head was rotated. The patient most likely experienced some sort of airway event (i.e., inability to ventilate, desaturation) that necessitated conversion to an endotracheal tube. This finding highlights the importance of being vigilant when deciding to place an uLMA in a patient when the bed is going to be rotated.

 

When the bed is turned we lose access to the airway, so our ability to rapidly respond if there is an airway event may be delayed. If you choose to place an uLMA when the table is rotated, I would make sure the uLMA is appropriately sized, and that it is secured in place so it will not get dislodged. Reevaluate placement after rotation of the bed or when the patient’s head is turned. If the patient is a man and has poor dentition or dentures that were removed, I would be extra vigilant for the possibility of a uLMA failure, or consider placing an endotracheal tube rather than an uLMA.

 

A trick I have learned in placing LMAs in patients with no teeth is to use two soft bite blocks or rolled 4x4s placed on either side of the LMA between the molars. I then wrap the tape around the LMA and the two 4x4s and secure the tape to the maxilla.

Dennis Spence, PhD, CRNA


The views expressed in this article are those of the author and do not reflect official policy or position of the Department of the Navy, the Department of Defense, the Uniformed Services University of the Health Sciences, or the United States Government.


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012





The effects of isoflurane and desflurane on cognitive function in humans

Anesth Analg 2012;114:410-5

Zhang B, Tian M, Zhen Y, Yue Y, Sherman J, Zheng H, Li S, Tanzi RE, Marcantonio ER, Zhongcong X


Abstract

Purpose The purpose of this study was to determine the effects of isoflurane and desflurane on postoperative cognitive function in elderly patients at one week after surgery.

 

Background Postoperative cognitive dysfunction (POCD) is associated with cognitive impairment and increased morbidity and mortality after major surgery. Isoflurane has been reported to induce neurotoxicity which is associated with POCD, and impairment in learning and memory. The exact cause of the neurotoxicity is not well understood, but some research suggests isoflurane is associated with increased caspase activation, apoptosis, neuroinflammation, and impairment in learning and function. Desflurane, on the other hand, has not been reported to cause similar neurotoxicity. It was hypothesized that isoflurane metabolism may generate greater amounts of trifluoroacetic acid in the blood compared to desflurane, and this acid may induce cytotoxicity and possibly POCD.

 

Methodology This was a prospective, randomized, controlled pilot study conducted at the Capital Medical University in Beijing, P.R., China. A total of 45 ASA I or II patients, aged 64 to 73, undergoing elective lower extremity or lower abdominal surgery were randomized to one of three groups:

  1. spinal anesthesia alone
  2. spinal anesthesia plus desflurane
  3. spinal anesthesia plus isoflurane.

Patients were excluded if they had a Mini Mental Status Examination score below 24 (out of 30 points), history of alcoholism or drug dependence, psychiatric or neurological disease, severe visual or auditory disorders, or terminal status.

 

All patients had a battery of 11 cognitive tests administered preoperatively and at 1 week postoperatively. These tests were highly sensitive to different types of cognitive impairment. After baseline data was collected, patients in all three groups had spinal anesthesia induced with 2 mL of 1% tetracaine. No sedatives or opioids were administered during the surgery. Patients in the isoflurane or desflurane groups had general anesthesia induced with propofol and a laryngeal mask airway was placed. Inhaled anesthetics were titrated to keep the Bispectral Index between 50 and 60.

 

The investigators calculated a change score for each of the 11 tests. POCD was defined when 4 or more of the change scores were negative and the absolute value of each of these change scores was larger than 1 standard deviation of the baseline score of the same cognitive test, indicating lower cognitive function. Similar criteria were used to establish POCD in previous research studies. The investigators compared the incidence of POCD and the mean number of tests with cognitive function decline between the three groups. Statistical analysis was appropriate. This was a pilot study so no power analysis was performed.

 

Result There were no significant differences in demographics, BIS values in the isoflurane and desflurane group, surgical procedures, blood loss, or surgical duration between the three groups. Over half the patients were men (53%) with an average age of 69 ± 2 years. There were no differences in the preoperative Mini Mental Status exam between groups.

 

There was no significant difference in the mean number of 11 tests administered that showed a cognitive function decline between groups (P=0.77, Figure 1). However, when POCD was defined as “impairment in 4 or more” tests, there were more subjects in the isoflurane group that showed postoperative impairment on 4 or more tests (27%) then either the desflurane group (0%) or spinal anesthesia alone group (0%; P = 0.028). A study including about 30 times as many subjects as this pilot study would be needed to unequivocally demonstrate an increased rate of cognitive decline in any one group.

 

 

Figure 1. Comparison of Number of Tests with Cognitive Decline

Figure 1

 

Conclusion This pilot study suggested that isoflurane may be associated with a higher incidence of postoperative cognitive decline than desflurane. A larger study will be needed to establish a convincing difference in cognitive function between agents.

 

Comment

Elderly patients are at increased risk for POCD. These preliminary results suggest that isoflurane anesthesia may increase the incidence of POCD in elderly patients. What I found most interesting was that, by one measure, patients in the isoflurane group had some evidence of POCD within a week after surgery, while patients who received desflurane or spinal anesthesia alone did not. By another measure, the total number of subjects in each group that had a declines on postoperative cognitive tests, there was no discernible difference between groups. It should be noted that diagnostic criteria for POCD are not yet agreed upon.

 

It must be pointed out that this was a pilot study, so the results should be considered preliminary until larger studies are done. Additionally, these results may not apply to younger patients. However, given the fact that elderly patients recover faster and get out of the hospital sooner with desflurane (and sevoflurane) anesthesia when compared to isoflurane, anesthesia providers might want to consider avoiding the use of isoflurane in elderly patients based upon recovery time alone.

Dennis Spence, PhD, CRNA


The views expressed in this article are those of the author and do not reflect official policy or position of the Department of the Navy, the Department of Defense, the Uniformed Services University of the Health Sciences, or the United States Government.


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012




Orthopedics
Timing of acute myocardial infarction in patients undergoing total hip or knee replacement

Arch Intern Med (July 23, 2012 Online First)

Lalmohamed A, Vestergaard P, Klop C, Grove EL, de Boer A, Leufkens HGM, van Staa TP, de Vries F total knee, total hip, replacement, MI, myocardial infarction, risk


Abstract

Purpose The purpose of this study was to determine when and how often Acute Myocardial Infarction (MI) occurred after total hip or total knee replacement compared to individuals who had not undergone joint replacement. A secondary purpose was to identify risk factors or risk modifiers for MI after total joint replacement.

 

Background Total hip and total knee replacements are common surgeries performed an estimated 1.8 million times worldwide. These procedures themselves may result in embolization of bone marrow with subsequent morbidity and mortality. Ninety day Acute Myocardial Infarction mortality rates following total joint replacements have been reported to be as high as 1.8% and as low as 0.1%. Most studies place the 90 day postoperative mortality rate between 0.3% and 0.8%. The investigators suggested the differences in reported MI mortality rates may be due to differences in diagnostic criteria. In any case, most postoperative MIs occur within the first postoperative week. Nevertheless, little is known about specific risk factors for MI after total joint replacement.

 

Methodology This retrospective database review examined the Danish national registries. Records of all adult patients who underwent primary total hip or total knee replacement during a 10 year period were examined. Each total joint patient was “matched” for age and gender [quotes added by editor, see discussion in comment section] with three control patients in the same registry who did not undergo total joint replacement. Individuals who had an MI within the six weeks prior to admission for surgery were excluded from the study. Historically, 99% of total hip and total knee replacement patients in Denmark received thromboprophylaxis perioperatively, almost always with low molecular weight heparins. Hazard ratios (HR) were calculated to compare the rate of MI in total joint vs. control individuals.

 

Result The analysis included 66,524 total hip replacement patients, 28,703 total knee replacement patients, and over 286,000 control individuals. Total joint patients were more likely than controls to be taking NSAIDS and 2 percentage points more likely to have a diagnosis of ischemic heart disease. The adjusted Hazard Ratio for MI was 25 for total hip patients and 31 for total knee patients. [Editor’s note: the hazard ratio is basically the number of times greater the incidence of MI was in total joint replacement patients compared to individuals who did not have joint replacement.] The Hazard Ratio for MI returned to baseline six weeks postoperatively for total hip patients. The Hazard Ratio for MI returned to baseline two weeks postoperatively for total knee patients. The absolute incidences of MI during the six weeks postoperatively were 0.5% in total hip patients and 0.2% in total knee patients. Patients who had an MI in the 6 months before their surgery were 4 times more likely to have a new MI within 6 weeks postoperatively. 

 

Conclusion The risk of an MI within 2 weeks after a total hip or total knee replacement was determined by this retrospective study to be 25 times and 31 times higher than individuals who did not have a total joint replacement. This increase in risk did not return to baseline until 6 weeks following a total hip replacement and 2 weeks following a total knee replacement. Since patients who had an MI within 6 months before their total joint surgery were 4 times more likely to have a new MI within 6 weeks postoperatively, the investigators suggested that total joint replacements be contraindicated within 12 months of an MI.

 

Comment

I’m not sure whether this study is the glass half empty or the glass half full. The only thing I’ll do up front is encourage all of us not to give the findings of this study more weight than they have earned.

 

Databases like the Danish national registry that this study used have the potential to redefine “retrospective” research. Retrospective used to mean, in part, that we didn’t know what information we’d have available from patient to patient or if what we had would be accurate. We’re now starting to see large databases systematically collecting highly accurate and complete health information. In my judgment, when appropriate research methods are used, studies using these databases fall somewhere between traditional retrospective studies and prospective studies in terms of their level of evidence.

 

Even if we assume the Danish national registry is a robust database, however, it is difficult to be confident in the results of this study. The investigators noted that, “differences in reported MI mortality rates may [have been] due to differences in diagnostic criteria.” While this is a true statement, they may also have been due to any number of other factors not considered in the methodology of this study. For example, anesthetic technique (e.g. regional vs. general), perioperative care, and preexisting risk factors surely all played a part in the incidences of postoperative MI. But the study didn’t even consider the anesthetic technique and didn’t control for multiple potentially confounding variables. Another shortfall was the so called “matching” of total joint replacement patients with patients who did not have joint replacement surgery. Matching is a research technique in which each individual in the experimental group is compared to a control individual who is identical over a large number of characteristics that could influence the study. The best matching is when the subjects are compared to themselves or to an identical twin. In this study, subjects were “matched” on only two criteria; age and gender. This leaves out many factors that might influence the outcome of the study. Preexisting cardiovascular disease and medication history quickly come to mind. I cannot consider this a legitimately matched study. Being a very large study, over 380,000 individuals in total, simply cannot make up for these shortcomings.

 

So, why did I include this study? Is it good for anything? The magnitude of the increase in risk of postoperative MI in this study is hard to dismiss. Despite the methodological flaws, a 25 fold or greater increase in the risk of MI in total hip and knee replacement patients over the incidence of MI in the general population of the same age and gender is probably real to some degree. The increase may be partially due to the host of factors that weren’t controlled for, but it is improbable that all confounding factors stacked up on the side of making the incidence of MI greater in the total joint group.

 

So, if we accept this study as evidence highly suggestive of a significant increase in the incidence of MI in total joint replacement patients, what should be our response? Total hip and knee replacement patients should probably be thoroughly evaluated for cardiovascular risk factors. Following that, I suggest we consider a lower threshold for intervention in these areas:

  • cardioprotective techniques, such as beta blockade
  • even more vigilant monitoring & 2 lead ST monitoring including V5
  • a lower threshold for invasive monitoring
  • a lower threshold for treating perioperative hemodynamic deviations

We’re probably doing pain rounds on these patients for a few days postoperatively. We might use these visits to remind other personnel about the duration of increased risk of MI. And, finally, this study does seem to reinforce an adage we’ve had in anesthesia for decades; patients who have had an MI in the 6 months prior to surgery are at much higher risk for a new MI postoperatively. If the joint replacement is elective it seems reasonable to wait at least that 6 months to avoid the 4 fold increase in risk of MI from this factor alone.

 

In the background section, the investigators observe that, “little is known about specific risk factors for MI after total joint replacement.” Unfortunately, this study doesn’t change that fact. We need better evidence identifying the risk factors for MI following total joint replacement. When we have it, we can get much more specific about preventing them. Until then, we need to be aware and keep everything as tight as we can. If the risk of postoperative MI in total joint patients is anywhere near as elevated as this study reports, it demands everything we can do to prevent it.

Michael A. Fiedler, PhD, CRNA


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012





Comparative effectiveness of regional versus general anesthesia for hip fracture surgery in adults

Anesthesiology 2012;117:72-92

Neuman MD, Silber JH, Elkassabany NM, Ludwig JM, Fleisher LA


Abstract

Purpose The purpose of this study was to compare mortality rates as well as pulmonary and cardiac complication rates in patients who received regional anesthesia vs. general anesthesia for hip fracture surgery.

 

Background As the population ages worldwide it is anticipated that the number of elderly patients who experience hip fractures will increase. These elderly patients experience high rates of morbidity, mortality, and disability after hip fracture repair. Approximately 5% die during their hospitalization and 10% die within 30 days due to pulmonary or cardiovascular complications. Unfortunately, there are few interventions available to reduce hip fracture surgery mortality.

 

The type of anesthesia may influence the outcomes. Some previous investigations have found benefit with regional (i.e., spinal, epidural, or peripheral nerve blockade) versus general anesthesia for hip fracture repair. However, other studies have not. Additionally, it is not known if there are differences in outcomes based on the type of hip fracture; for example, femoral neck vs. intertrochanteric fractures. The authors of this study hypothesized that use of regional anesthesia for hip fracture repair would reduce the probability of inpatient mortality, and major pulmonary and cardiovascular complications when compared to general anesthesia. The authors also hypothesized that these outcomes would vary by type of fracture.

 

Methodology This was a retrospective study examining adult patients >50 years old or older who underwent hip fracture repair at hospitals in New York State between 2007 and 2008. The investigators used the New York State Inpatient Database to identify records of patients having hip fracture surgery. The investigators recorded data on anesthesia type (general or regional), patient and hospital characteristics, as well as pulmonary and cardiovascular complication rates from the database.

 

The primary outcome was in-hospital mortality; secondary outcomes included major pulmonary and cardiovascular complications. Complications included: pneumonia, empyema, aspiration, respiratory failure, acute myocardial infarction, congestive heart failure, and cardiac arrest. The investigators used logistic regression techniques to compare their primary and secondary outcomes while controlling for potential patient and hospital confounding variables that might influence the outcomes. A P value < 0.05 was considered significant.

 

Result A total of 18,158 patients were included in the sample. Of these patients, 29% (5,254) received regional anesthesia and 71% (12,904) received general anesthesia. An unknown number of patients may also have received both general and regional anesthesia. The average age of patients was 83.5 years with a majority of patients being Caucasian (83%) and female (74%). Patients in the regional anesthesia group had a slightly lower rate of pathologic fractures (1.9% vs. 2.7%). In the regional group, 24% had a history of chronic obstructive pulmonary disease compared to 20% in the general group (P < 0.0001). Likewise, 21% in the regional group had a history of dementia compared to 19% in the general group (P = 0.005).

 

Logistic regression found that patients who received regional anesthesia for hip fracture surgery had a 29% lower adjusted odds of mortality compared to those who received general anesthesia (P = 0.014). Pulmonary complications were 24% lower in the regional anesthesia group compared to the general anesthesia group (P <0.0001). No difference was found in the incidence of cardiovascular complications between the two groups. Of those patients who had intertrochanteric fracture repair, regional anesthesia was associated with a 42% reduction in mortality (P = 0.013), and 37% reduction in any pulmonary complication compared to patients who received general anesthesia as the primary anesthetic (P = 0.001). No differences in mortality or cardiac and pulmonary complications were found between the groups in patients who had femoral neck fracture repair.

 

Conclusion Regional anesthesia was associated with a lower risk of mortality and pulmonary complications in patients who had hip fracture repair, especially in those with intertrochanteric fractures.

 

Comment

Patients who present with hip fractures tend to be elderly females with multiple comorbidities. Additionally, these patients can be in a significant amount of pain, and this may make it difficult to position them for the procedure while they are still awake. These factors combined make it challenging to administer anesthesia to these patients. Furthermore, anesthesia providers may feel pressured to administer general anesthesia rather than regional anesthesia (i.e., spinal, epidural, or peripheral nerve block) because of the concern that a regional technique may delay the start of the surgery.

 

It is important to point out this was a retrospective study, so the strength of the evidence is weaker. However, I believe this was a well designed study that used advanced statistical analysis to control for many confounding variables. Therefore, I think the results of this study do provide good evidence supporting improved outcomes with regional anesthesia in elderly patients who require hip fracture repair for intertrochanteric fractures. Also, it is impossible to determine which specific type of regional technique was used (i.e., spinal, epidural, or peripheral nerve block). I suspect the most common technique was a neuraxial technique such as spinal anesthesia. Furthermore, an unknown number of patients in the general anesthesia group may have received a regional technique as well, such as a peripheral nerve block for postoperative pain. According to the authors, this might bias the results and thus underestimate the true effect regional anesthesia had on morbidity and mortality after hip fracture repair.

 

The greatest benefit found was on pulmonary complications and overall mortality. Overall mortality was most likely decreased secondary to reduced pulmonary complications. Given the known effects of general anesthesia on postoperative lung function, especially in elderly patients with COPD, it seems prudent to consider administering a regional anesthetic for intertrochanteric hip fracture repair.

 

Anesthesia providers should share these results with their orthopedic and anesthesia colleagues and talk about how regional anesthesia may reduce morbidity and mortality in hip fracture patients. I think careful titration of opioids and sedatives to assist in positioning for the regional technique and procedure would make the procedure more tolerable for the patient. It would also be important to know if there are any contraindications to placing a regional anesthetic (i.e., aortic stenosis, coagulopathy), and how long the surgeon may take to do the repair, as these factors may be the rate limiting step.

Dennis Spence, PhD, CRNA


The views expressed in this article are those of the author and do not reflect official policy or position of the Department of the Navy, the Department of Defense, the Uniformed Services University of the Health Sciences, or the United States Government.


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012




Pediatric Anesthesia
The effect of obesity on the ED95 of propofol for loss of consciousness in children and adolescents

Anesth Analg 2012;115:147-53

Olutoye OA, Yu X, Govindan K, Tjia I, East DL, Spearman R, Garcia PJ, Coulter-Nava C, Needham J, Abrams S, Kozinetz CA, Andorpoulos DB, Wachta MF


Abstract

Purpose The purpose of this study was to determine whether or not BMI effected propofol requirements for children and adolescents. This was an attempt to determine the appropriate induction dose for obese children and adolescents.

 

Background With obesity rates among children and adolescents growing at an epidemic pace, the authors noted that there were few clinical studies that directly look at the ED95 of propofol in relation to BMI. The Food and Drug Administration recommends dosing propofol by body weight, not taking into consideration ideal weight. Since propofol is lipophilic, there are concerns with many providers that a significantly increased amount of adipose tissue affects uptake, distribution, and redistribution. There are very few studies that have determined the right “formula” for the induction dose in obese patients, and even fewer for obese children. The authors give excellent suggestions for the administration of propofol in this population.

 

Methodology This study was a Biased Coin Design. The technique, introduced in the 1970’s, uses previously collected data to drive decisions for subsequent subjects, in this case, the induction dose of propofol. There were 40 subjects in each of two groups. Subjects were not randomized – they were placed in groups based on BMI. Those >95th percentile for age were in the obese group, those between the 25th and 84th percentile were in the non-obese group. An independent observer blinded to the propofol dose documented loss of lash reflex, level of sedation (using the University of Michigan Sedation Score UMSS) and Bispectral Index values at 20, 60, and 120 seconds. Both the obese and non-obese groups were studied simultaneously. After propofol was administered, if the desired result was not achieved, the dose for the next patient was adjusted. This technique was continued until a range and target point were obtained.

 

Result There were no differences demographically between the two groups. The study determined that the ED95 of propofol was actually higher for the non-obese group than the obese group, 3.2 mg/kg vs. 2 mg/kg respectively. There was no correlation between BIS score and loss of lash reflex in either group. As a side note, the obese group had significantly higher starting systolic blood pressures than the non-obese group. It is unknown whether or not this affected the distribution time for the drug.

 

Conclusion This study showed that some accommodation was required in propofol dosing for obese children and adolescents. The mg/kg dose was significantly lower in the obese group. Using a straight-forward mg/kg dose regardless of weight may not be the best approach. The authors suggested that when caring for any child or adolescent with a BMI >95th percentile that a dose of 2 mg/kg be used.

 

Comment

This is a very interesting study. As a practicing CRNA who does a significant amount of IV sedation, I am always interested in articles that attempt to quantify propofol dose based on body weight. The authors suggested that the dose relationship between Total Body Weight and clearance are non-linear. I’ve heard it suggested to use ideal body weight, actual body weight, and body weight in kg x 0.75 to determine the propofol dose. There are few studies that actually attempt to determine the effect of obesity on the required dose of propofol. In one study the authors stated that “Total propofol dose (mg/kg) required for syringe drop and time to loss of consciousness were similar between control subjects and morbidly obese subjects receiving propofol infusions based on Lean Body Weight.” “Syringe drop” is a commonly used technique to determine loss of consciousness – placing a 20 - 60 mL syringe in the patients hand and asking them to hold it during induction. The time to drop is recorded when they drop the syringe. They suggested that lean body weight be used as the standard for propofol infusion. The current study did not find this to be the case.

 

One of the downfalls of the study is the design; it didn’t allow for a logistic regression which could have determined whether or not age or gender affected the ED95 of propofol. Another issue was that propofol studies often have different end-points to determine effectiveness. There is no standard for determining when useful loss of consciousness occurs. Is the determination syringe drop, loss of lash reflex, apnea, bispectral index?

 

Suggesting that providers use a child’s BMI calculator and maintain age/weight gender specific growth charts is unrealistic. The Centers for Disease Control and Prevention offer an on-line child/adolescent BMI calculator. This tool requires current height and weight, birthdate, and date of measurement. It reports the percentile, which is of use according to the authors. Dose can be determined based on their percentile. Having a formula based on research would be more helpful.

Gerard Hogan Jr., DNSc, CRNA


 

Ingrande J, Broadsky JB, Lemmens HJM. Lean body weight scalar for the anesthetic induction dose of propofol in morbidly obese subjects. Anesth Analg. 2011;113:57-62.

 

Centers for Disease Control and Prevention. BMI percentile calculator for child and teen. Retirieved July 12, 2012 from http://apps.nccd.cdc.gov/dnpabmi/calculator.aspx


© Copyright 2012 Anesthesia Abstracts · Volume 6 Number 6, June 30, 2012