Step response analyses of the cardiovascular system and their application to the measurement of systemic and pulmonary vein compliances.

A theoretical analysis of the step response in the closed cardiovascular system induced by a sudden shift of the right cardiac output curve predicted that if the relations of the right heart output (COr) and the total systemic capillary flow (CFs) to the systemic venous pressure (Psv) are linear, then the time course ofPsv change will become monexponential with a time constant T given by T=Csv/(Gr + Gs), whereCsv is the systemic vein compliance andGr andGs are the conductances of the transientCOr - Psv andCFs - Psv relationships. A similar prediction was obtained for the time constant T of the pulmonary vein pressure (Ppv) response to the step change in the left cardiac output (COl) curve, pulmonary vein compliance (Cpv) and the conductances of theCol curve, and the pulmonary capillary flow (CFp) curve againstPpv. The actualPsv orPpv changes following sudden alteration of theCOr orCOl curve by inflation and deflation of the balloon in the right or left atrium revealed monoexponential time courses. Semilogarithmic plots of the transient vein pressure changes yielded correlation coefficients of −0.995 ± 0.006 (means ± SD) in 11 curves forPsv and 0.977 ± 0.017 in 16 curves forPpv (P<0.01). The assumed linearity of dynamicCOr andCOl curves was confirmed by beat by beatCOr - Psv andCOl - Ppv relationships during the step responses, except for the first few beats immediately after the balloon maneuver. The linearity of the dynamicCFs curve was examined by measuringCFs with a double-step balloon maneuver so as to cause rapid equilibrium betweenCOr andCFs at varied moments of the transient process. The correlation coefficient betweenCFs andPsv thus obtained was 0.98 ± 0.04 (P<0.01). A similar linearity of the dynamicCFp -Ppv relationship was suggested from their steady-state curves. The values ofCsv calculated from the experimental data were 1.70 ± 0.12 ml/mmHg/kg body wt in 11 curves and those ofCpv were 0.13 ± 0.03 ml/mmHg/kg in 15 curves. These results are mostly consistent with those previously reported.