Time-domain representation of ventricular-arterial coupling as a windkessel and wave system

The differences in shape between central aortic pressure (P_Ao) and flow waveforms have never been explained satisfactorily in that the assumed explanation (substantial reflected waves during diastole) remains controversial. As an alternative to the widely accepted frequency-domain model of arterial hemodynamics, we propose a functional, time-domain, arterial model that combines a blood conducting system and a reservoir (i.e., Frank's hydraulic integrator, the windkessel). In 15 anesthetized dogs, we measured P_Ao, flows, and dimensions and calculated windkessel pressure (P_wk) and volume (V_wk). We found that P_Wk is proportional to thoracic aortic volume and that the volume of the thoracic aorta comprises 45.1 ± 2.0% (mean ± SE) of the total Vwk. When we subtracted Pwk from P_Ao, we found that the difference (excess pressure) was proportional to aortic flow, thus resolving the differences between P_Ao and flow waveforms and implying that reflected waves were minimal. We suggest that P_Ao is the instantaneous summation of a time-varying reservoir pressure (i.e., P_wk) and the effects of (primarily) forward-traveling waves in this animal model.