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

Departments of 1 Medicine and Physiology/Biophysics and 2 Civil Engineering, University of Calgary, Calgary, Alberta, Canada T2N 4N1; 3 Physiological Flow Studies Group, Department of Bioengineering, Imperial College of Science, Technology, and Medicine, London SW7 2AZ, United Kingdom; and 4 National University of Ireland, National Centre for Biomedical Engineering Science, Galway, Ireland 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 V Wk . When we subtracted P Wk 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. aortic pressure; aortic flow; compliance; left ventricular ejection; waves