Relationship Between Instantaneous Aortic Flow and the Pressure Gradient

Abstract

The relationship between the pressure gradient and the instantaneous blood flow in the descending thoracic aorta was studied in six dogs. Instantaneous flow was calculated from the pressure gradient using the simple analog approach of Fry and the more elaborate calculations of Womersley. The flows computed by either of these techniques were essentially indistinguishable from one another and compared favorably with the "true" flow. The "true" flow at the point of pressure gradient measurement was determined indirectly from the flow signals of two electromagnetic flowmetering systems, one placed upstream and the other downstream from the point of pressure gradient measurement. The probes were placed sufficient distances from the site at which pressure gradients were measured so that the pulsations of the vessel at that site were unaffected by application and removal of the probes. Thus, the vessel was presumed to be in its normal pulsating state. It is concluded, first, that the simpler approach of Fry is the method of choice in large vessels such as the aorta because the results do not differ appreciably from those using the Womersley approach. Second, the favorable comparison of either of the pressure gradient techniques with the "true" flow is strong evidence for the validity of this method and places the theoretical considerations of Womersley and others on a firmer experimental footing than previously.