Continuous Thermodilution Cardiac Output Monitoring

Newer technologies applied to PAC monitoring allow nearly continuous CCO monitoring with the use of either warm^{[664] [665] [666] [667] [668] [669] [670] [671]} or cold^[672] thermal indicators. The warm thermal technique is the one now widely accepted in clinical practice. In brief, this method involves the release of small quantities of heat from a 10-cm thermal filament incorporated into the right ventricular portion of a PAC approximately 15 to 25 cm from the catheter tip.^{[667] [673]} Commercially available CCO monitors use proprietary stochastic identification algorithms to analyze the thermal signal measured by the thermistor at the tip of the catheter. The heating filament is cycled on and off in a pseudorandom binary sequence, and cardiac output is derived from cross-correlation of the measured pulmonary artery temperature and the known sequence of heating filament activation. A basic assumption inherent in this technique is that the cardiac output remains constant during the interval analyzed for measurement.^[674]

Typically, the displayed value for cardiac output is updated every 30 to 60 seconds and represents the average value for the cardiac output measured over the previous 3 to 6 minutes.^[675] In a laboratory investigation examining how CCO measurements respond to unstable hemodynamic conditions such as hemorrhage and fluid resuscitation, Siegel and colleagues showed that CCO changes were markedly slower than those detected by ultrasonic flow probe, blood pressure, or mixed venous oxygen saturation.^[674] In view of these inherent time delays, current warm thermal PAC CCO methods should not be considered continuous, real-time monitors, but rather techniques that provide continual, frequently updated cardiac output values.

In general, CCO methods appear to have good agreement with standard bolus thermodilution cardiac output measurements or electromagnetic flow probe techniques. ^{[668] [669] [676]} In a small, multicenter study, Mihm and associates found that CCO provided a clinically reliable measurement in a group of 47 intensive care unit patients.^[665] The device performed well in patients with a wide range of cardiac outputs (1.6 to 10.6 L/min) and core temperatures (33.2°C to 39.8°C), and the method showed no deterioration in performance over the 72-hour monitoring period. In general, PAC-based CCO monitoring appears to be more reproducible or precise than standard bolus thermodilution techniques, although this may simply be a function of the fact that the continuous methods display a time-weighted, average cardiac output value as opposed to a single instantaneous measurement.^{[664] [665] [677] [678]}

Warm thermal CCO PACs have been fairly rapidly and widely accepted into clinical practice for a number of practical reasons. Although these catheters are more expensive than standard PACs, bolus injections for measurement of cardiac output are obviated, thereby potentially reducing nursing efforts and the risk of fluid overload or infection. However, like cold bolus thermodilution techniques, warm

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Because the CCO PAC measures a cardiac output value that is an average derived over the previous several minutes, the beat-to-beat variations in stroke volume that occur during a single respiratory cycle are all equally represented in the derived average value for cardiac output. In contrast, bolus thermodilution measurements will show quite different cardiac output values, depending on the phase of respiration during which they are recorded (see earlier discussion). Thus, the CCO-averaging algorithm obviates this physiologic effect that confounds bolus thermodilution cardiac output techniques. As a result, cardiac output measured by the CCO method may provide a more accurate measurement of average cardiac output for patients receiving positive-pressure mechanical ventilation. Although electronic smoothing or filtering is standard for all hemodynamic values displayed on the bedside monitor, such as heart rate, blood pressure, and so forth, the delay inherent in warm thermal CCO monitoring is much longer—minutes rather than seconds. In effect, the CCO technique involves a fundamental tradeoff between rapid response time and overall accuracy of measurement. The standards for these performance characteristics have not been defined but must balance the response time against the stability of the displayed value and its immunity from thermal noise.^[678]

In summary, the CCO PAC monitoring technique is more reproducible and precise than bolus thermodilution cardiac output measurement, but the time delays inherent in the CCO technique require the clinician to rely on other monitored variables to detect acute circulatory changes. Because an external system for cold fluid injection is not required, the CCO technique requires less nursing time and may result in fewer measurement errors, less risk of fluid overload, and less risk of infection. The CCO computer and catheter require a significant amount of time to warm up and may work poorly in an environment with a great deal of thermal noise or rapid hemodynamic changes, such as the cardiac operating room.