Total arterial compliance estimated from the time constant of aortic pressure decay: the influence of downstream pressure

The influence of downstream pressure (Po) on total arterial compliance (C) was studied in 16 adults (14 M, 2F; aged 44±16 yrs) undergoing left heart high fidelity pressure catheterization. A two‐element RC model of the circulation was assumed, where R is the vascular resistance. The time constant (Tau = RC) of the monoexponential fit of diastolic aortic pressure decay was calculated by using either the standard, semilogarithmic method with zero pressure asymptote, or two methods with variable pressure asymptote, namely the best‐fit method and the derivative method. Cardiac output was measured by the thermodilution technique. Diastolic aortic pressure was 82±10 mmHg. The best‐fit method and the derivative method led to similar value of Po (71±12 vs 75±12 mmHg), Tau, R and C. As compared with the semilogarithmic method, the best‐fit method led to 74% lower Tau (353±106 vs 1372±106 msec; P<0.05). When substituting Po for central venous pressure, the R was 72% less (0.42±0.17 vs 1.51±0.61 mmHg.sec.mL −1 ; P<0.05). This resulted in C that was 18% less (0.83±0.38 vs 1.02±0.40 mL.mmHg −1 ; P<0.05). The high Po value documented here was consistent with recent experimental and clinical findings suggesting that a Starling resistor mechanism could be present in vivo at the arteriolar precapillary level of the various systemic vascular beds. This may significantly impact on the clinical estimation of arterial load. Source of research support: University.