Pulse Contour Cardiac Output Monitoring

In an attempt to provide continuous measurement of cardiac output, a number of techniques have focused on analysis of the arterial pulse pressure waveform. These methods, generally termed pulse contour cardiac output, determine cardiac output from computerized analysis of the pressure waveform recorded at various sites, including the aorta,^[785] radial artery,^[786] and even a finger in a noninvasive technique.^{[787] [788]} All these methods determine stroke volume and cardiac output from analysis of the arterial pressure pulse.^[789] Although early studies comparing these techniques showed fairly poor agreement with other cardiac output measurements, more recent clinical trials in cardiac surgical patients have shown that the pulse contour methods provide an acceptable level of accuracy.^{[790] [791] [792] [793] [794] [795]} Two pulse contour methods are clinically available that differ in regard to the site of arterial pressure monitoring and the initial calibration technique. One method uses transpulmonary thermodilution calibration and requires femoral arterial catheterization, ^[796] which may limit its applicability in some centers. The other uses lithium dilution calibration and arterial pulse wave analysis from radial or brachial measurement sites.^[797]

These pulse contour methods are based on solid physical principles, offer the potential for continuous beat-to-beat monitoring, and are calibrated with sound methods. Both systems use transpulmonary measurement of an indicator (either cold or lithium), thereby eliminating the confounding effects of the respiratory cycle encountered during standard PAC thermodilution cardiac output measurements. However, these techniques involve substantial computations and are based on physiologic models that may not always apply clinically. A number of issues remain with the pulse contour technique, including the nonlinearity of aortic compliance, the relationship between peripheral and aortic pressure, and the expected clinical problems encountered with the recording of arterial pressure waveforms, including resonance and damping. ^[789]

The clinical role of all these alternative methods for monitoring cardiac output remains in evolution. Advocates of flow monitoring argue that early preemptive therapy guided by these devices is more likely to improve patient outcome than delayed therapy is in critically ill patients who already have evidence of end-organ failure. In the acute care setting, noninvasive cardiac output monitoring methods that are easily applied during surgery or on initial arrival at the emergency department or intensive care unit may have a unique role in providing a bridge to more comprehensive, invasive monitoring with a PAC.