Reliability of stroke volume to pulse pressure ratio for estimating and detecting changes in arterial compliance.

Total arterial compliance is an important haemodynamic variable which is difficult to measure in vivo. Using a Windkessel model, it can be determined from the ratio of the diastolic-decay time constant (tau) of the arterial system and the peripheral resistance. Using this technique, paired estimates of arterial compliance were determined in control and in mechanically produced low-compliance steady states. Since determination of compliance based on the three-element Windkessel model is tedious the same data were retrospectively analysed to evaluate the reliability of the stroke volume to pulse pressure ratio (SV:PP) for estimating and predicting changes in compliance. The linear correlation coefficient for 148 paired values of compliance determined by the two methods was r = 0.85, P less than 0.001. Both methods indicated similar values (mean +/- s.e.m.) for compliance in the control steady state: 0.35 +/- 0.01 ml/mmHg (using tau/R) versus 0.36 +/- 0.01 ml/mmHg (using SV:PP), P was NS, and both methods detected a significant decrease in compliance, P less than 0.001, in the low-compliance steady state: 0.18 +/- 0.01 ml/mmHg (using tau/R) and 0.21 +/- 0.01 ml/mmHg (using SV:PP). Thus, SV:PP compared with (tau/R) was a good index of compliance and changes in compliance and may prove to be a useful index for estimating compliance clinically.