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Pulse contour analysis: should good‐looking curves be trusted?

RELIABLE assessments of haemodynamic variables are important in the monitoring, diagnosis and treatment of patients during major surgery or in critically ill patients. For more than 30 years, pulmonary artery catheterization, with monitoring of cardiac filling pressures (CVP, PCWP) and intermittent or continuous determination of thermodilution cardiac output, has been the mainstay in invasive haemodynamic assessment of patients in intensive care units. More recently, methods based on analysis of the contour of the arterial blood pressure curve have gained wide acceptance as a means of continuously monitoring cardiac output. The major problem, when attempting to determine stroke volume/ cardiac output from arterial blood pressures, is the non-linearity of the pressure/volume relationship of the arterial system. The compliance of the arteries decreases as blood pressure increases in an unpredictable but, supposedly, also in a subject-specific way. To circumvent the problem, most algorithms that calculate stroke volume from the pulse contour have to be calibrated with the result of a precise method of measuring cardiac output. The most extensively investigated and used pulse contour monitor, in adults, is the PiCCO monitor (Pulsion Medical Systems, Munich, Germany). Advocates of this device emphasize (i) that it is possible, for extended periods, to continuously monitor cardiac output without recalibrating the monitor and (ii) the low invasiveness of the method compared with pulmonary artery catheterization. The first contention I shall address later in the editorial but the second may as well be dealt with immediately. The PiCCO system requires a central venous access plus the insertion of a 5F/4F thermistor-tipped arterial catheter, via a femoral or axillary artery, into the aorta. Therefore, if there is a difference in invasiveness it is minimal and I doubt that it translates into a difference in complications. It is, however, an important point that it is frequently faster and technically less demanding to start monitoring with the PiCCO system than with a pulmonary artery catheter. Consequently, the speed and ease with which haemodynamic status can be assessed increases the likelihood that a critically ill patient receives immediate, appropriate treatment. And this is not unimportant. In a recent paper, Rivers et al. have clearly shown the importance of starting suitable therapy as soon as the critically ill patient reaches the hospital (1). And, on the other hand, two newly published papers plainly demonstrate the futility of starting extensive invasive haemodynamic monitoring and therapy when multiple organ failure has already been established (2,3).