Pressure Waveform Analysis
Monitoring cardiac output is of special interest for detecting early hemodynamic impairment and for guiding its treatment. Among the techniques that are available to monitor cardiac output, pressure waveform analysis estimates cardiac output from the shape of the arterial pressure curve. It is based on the general principle that the amplitude of the systolic part of the arterial curve is proportional to cardiac output and arterial compliance. Such an estimation of cardiac output has the advantage of being continuous and in real time. With “calibrated” devices, the initial estimation of cardiac output by pressure waveform analysis is calibrated by measurements of cardiac output made by transpulmonary thermal or lithium dilution. Later, at each time transpulmonary dilution is performed, the estimation by pressure waveform analysis, which may drift over time, is calibrated again. By contrast, uncalibrated devices do not use any independent measurement of cardiac output. Unlike calibrated devices, they can be plugged to any arterial catheter. Nevertheless, uncalibrated devices are not reliable in cases of significant short-term changes in arterial resistance, as for instance in patients undergoing liver surgery or those with vasodilatory shock receiving vasopressors. Perioperative hemodynamic monitoring is recommended for high-risk surgical patients since it reduces the number of complications in these patients. The pressure waveform analysis monitoring, especially with uncalibrated devices, is suitable for this purpose. In the intensive care setting, hemodynamic monitoring is recommended for patients with acute circulatory failure, who do not respond to initial therapy. Since these patients often experience large changes in arterial resistance, either spontaneously or due to vasoactive drugs, calibrated devices are more suitable in this context. Not only are they more reliable than uncalibrated devices but also they provide a comprehensive hemodynamic assessment through measurements of a variety of transpulmonary thermodilution-related variables. In this review, we summarize the characteristics of the monitoring devices using the pressure waveform analysis and discuss the appropriate use of different devices in the perioperative and intensive care unit settings.