Pressure Pulse Contour-derived Stroke Volume and Cardiac Output in the Morbidly Obese Patient

Abstract

The pressure pulse contour method for measuring stroke volume (SV) and cardiac output (CO) has come of age. Various methods have been proposed, but at this time no single technique has shown clear superiority over the others. This commentary and review discusses the various methods, and particularly the pressure recording analytical method (PRAM). Dissection of the method shows that vascular wall abnormalities, which are not unique to the morbidly obese state, represent one more biophysical perturbation causing inaccuracy in stroke volume and cardiac output determination. As PRAM is an uncalibrated method, its accuracy depends on certain assumptions that may not fully explain the multitude of combinations and permutations that define pulsatile blood flow; specifically, the area under the pressure curve during systole and the morphologic characteristics of the waveform throughout the cardiac cycle. As a result of incomplete theory, referenced specifically to the morbidly obese individual, PRAM does not faithfully mimic established reference standards of flow; it systematically underestimates stroke volume and cardiac output. Field equations, that is, equations that are applicable over the full gamut of hemodynamic conditions and vascular pathology, are analytically derived truisms. They require input variables that satisfy the natural state of affairs. To realize this state of absolute biophysical bliss, these variables should ideally be measured. Unfortunately, because of the constraints of practicality, shortcuts to the absolute truth are obligatorily required. As a result, pressure pulse contour methods have evolved that employ curve analysis and neural networking techniques, providing uncalibrated facsimiles of SV and CO.