The technique of estimating the instantaneous aortic blood velocity in man from the pressure gradient

Abstract

mhe time relationships between pres1 sure and blood velocity in the asdending aorta can be used to define the mechanical function of the heart. To the extent that the mechanical function is altered early in disease, evaluation of these pressure-velocity relationships has important clinical as well as biophysical implications. It is possible to estimate both the aortic pressure and the instantaneous blood velocity by the use of a catheter method.’ This method requires accurate measurement of the instantaneous lateral pressures at two relatively close points along the axis of the aorta. The difference between these pressures is an approximate measure of the instantaneous pressure gradient from which the blood velocity may be computed by means of simple analog computer methods. The computation of the aortic blood velocity from the spatial pressure gradient requires extremely accurate measurement of the pressure and meticulous attention to a number of experimental details. A number of problems are raised by these requirements in the application of this technique to studies in human subjects. It is the purpose of this report to present certain approaches to many of these problems and to make available the details necessary to apply this method successfully in the clinical physiology laboratory. Examples are also given which illustrate the pressure-velocity relationships as measured in patients with nonvalvular myocardial disease and after the administration of certain pharmacologic agents. The discussion of the application of the computed pressure gradient technique may be divided into five general areas: catheter, gauges, electrical circuits and amplifiers, calibration, and evaluation of the total system response. Catheter. The catheter, as diagrammatically illustrated in Fig. 1, is of a special double-lumen design,? with two lateral pressure taps at the end of each lumen. The entry to each lumen is separated by a distance of 5 cm. The No. 69; French catheter will pass through a short beveled modification of a No. 12 gauge Robb needle. The stopcocks connecting the catheter to the gauges must be made to fine tolerance.1