CHANGES IN FREQUENCY AND IN COMPLIANCE AFFECT THE PULSATILITY INDEX OF THE DOPPLER FLOW VELOCITY WAVE-FORM AT CONSTANT FLOW IN AN IN-VITRO PULSATILE FLOW MODEL

Objective: We have investigated the effect on the pulsatility index (P I) of the Doppler flow velocity waveform (FVW) of changes in frequency and in compliance, while maintaining mean flow rate and pressure in a n in vitro pulsatile flow model. Methods: Changes in frequency simulat ed changes in heart rate and were modeled by changing the rate of a ro tary pump that imposed regular pulsations in flow. Changes in complian ce were modeled by increasing the inflation pressure around a Starling -type tube or a Windkessel incorporated in the system. Results: An inc rease in frequency produced an increase in the maximum rate of change of the pressure oscillation (DELTAP(max)) relative to that of the flow oscillation (DELTAQ(max)) and an increase in the phase lead of the pr essure wave relative to that of the flow wave. It also produced a fall in PI even though mean flow and pressure remained constant. Decreasin g compliance also led to an increase in DELTAP(max) relative to DELTAQ (max), but in contrast to changes in frequency, it produced a fall in the phase lead of pressure relative to the phase lead of flow. As comp liance decreased, a critical point was reached beyond which PI increas ed. Conclusions: These results indicate that changes in frequency and in compliance affect the shape of the FVW. This can produce changes in Doppler indices such as PI even at constant flow and pressure. These studies highlight the problems of using indices such as PI to examine the fetal circulation, particularly under circumstances where changes in heart rate and the distribution of blood to vascular beds of differ ing compliance occur, e.g., in hypoxemia.