Reliability of continuous pulse contour cardiac output measurement during hemodynamic instability

J Clin Monit Comput 2007;21:237-242

Johansson A, Chew M

 

 

Abstract

Purpose            The purpose of this study was to evaluate the reliability of pulse contour cardiac output (PCCO) (PiCCO catheter, Pulsion Medican Systems, Germany) measurements in an animal model of hemodynamic instability.

Background            Cardiac output is a useful monitor in anesthesia and critical care but is difficult to measure safely and accurately. It is especially useful when intravascular volume is changing and when vasopressors are being administered. Clinically, cardiac output is most commonly measured by thermodilution with a pulmonary artery (PA) catheter. The use of PA catheters is limited, however, by associated risks and the general lack of evidence that the information from PA catheters contribute to improved patient outcomes.

Pulse Contour Cardiac Output (PCCO) monitoring generates a cardiac output value by analysis of an arterial pressure waveform. PCCO must be periodically calibrated against a transpulmonary thermodilution cardiac output (TpCO) measurement. TpCO requires a central venous line and a central arterial line (often femoral or axillary).

Studies evaluating the accuracy of PCCO measurements have yielded contradictory results. Few studies have examined the accuracy of PCCO in hemodynamically unstable patients.

Methodology            This prospective, randomized study was conducted in 15 pigs weighing between 20 and 25 kg. The pigs were anesthetized, intubated, and mechanically ventilated. IV fluid was infused at 10 ml/kg/h. Nine pigs received an infusion of bacterial endotoxin to simulate sepsis. Six pigs served as controls. All pigs received a central venous and central arterial line for TpCO monitoring. The PCCO monitor was connected to the central arterial line.

At time zero (T0), after the pigs were prepared, the PCCO was calibrated to the TpCO. Each hour thereafter for six hours (T1 through T6) PCCO measurements were taken before and after the PCCO was calibrated to the TpCO. In the experimental pigs, the endotoxin infusion was begun at T1. Heart rate (HR), mean arterial pressure (MAP), and central venous pressure (CVP) measurements were also recorded at all data collection points.

Result            Pigs given endotoxin became hypotensive and their TpCO changed by 46% over time. Their PCCO values before calibration were statistically and/or clinically significantly different from the TpCO values at multiple time points during the study. PCCO values immediately after calibration were reflective of TpCO values.

Conclusion            In hemodynamically unstable animals, PCCO was unacceptably different from TpCO values; their agreement differing by 56%. The hemodynamic changes that accompany hemodynamic instability resulted in grossly inaccurate PCCO values.

 

Comment

We regularly treat patients undergoing significant changes in stroke volume, cardiac output, and systemic vascular resistance without knowing which values have changed or how much they have changed. We often make good guesses about what has changed based upon our knowledge of physiology, pharmacology, indirect indicators, and experience. I don’t think any of us would be willing to give up blood pressure monitoring during an anesthetic. I suspect there will come a time when we will feel the same way about cardiac output monitoring. Of course, we don’t use cardiac output monitoring very much right now because it is highly invasive (and thus adds risk), expensive, and fairly labor intensive to set up and use. If and when cardiac output monitoring becomes as low risk, inexpensive, and accurate as oscillometric blood pressure monitoring I think we’ll feel differently about it.

A number of alternatives to Swan-Ganz thermodilution cardiac output monitoring have been under development for decades with little success. This one analyzes the area under the curve of an arterial line waveform to generate a cardiac output. Since it must be calibrated to a transpulmonary cardiac output value (a technique closely related to the pulmonary artery catheter cardiac output measurements we are familiar with) PCCO isn’t a replacement for the more invasive techniques. It simply adds information by providing hands off, continuous, real time cardiac output monitoring.

My first thought when reading this article was that the investigators had chosen the wrong “gold standard” against which to compare the PCCO. While thermodilution cardiac output monitoring has long been the best we had clinically, it isn’t all that accurate; about ?19%. (Every wonder why you shot multiple COs and averaged them?) Given how far off the PCCO values in this study were from the thermodilution values, however, the inaccuracy of the thermodilution measurements is unlikely to be an issue.

This study is important because we are most anxious to know what the cardiac output is when a patient is unstable. What the study shows is that when hemodynamics are unstable, pulse contour cardiac output monitoring is currently a non-starter.

 

Michael Fiedler, PhD, CRNA