Pulmonary arterial compliance in dogs and pigs: the three-element windkessel model revisited

In six dogs and six weight-matched miniature pigs at baseline and after pulmonary embolization, pulmonary arterial compliance was determined using the pulse pressure method (C-PPM), the three-element windkessel model (CWK-3), and the ratio of stroke volume to pulse pressure (SV/PP). C-PPM was lower in pigs than in dogs at baseline (0.72 +/- 0.23 vs. 1.14 +/- 0.29 ml/mmHg, P < 0.05) and after embolism (0.37 +/- 0.14 vs. 0.54 +/- 0.16 ml/mmHg, P = 0.07) at; matched flow, but not at matched flow and pressure. C-PPM showed the expected inverse relation with pressure and a direct relation with flow. CWK-3 was closely correlated with C-PPM, except for all dogs at baseline where CWK-3 was up to 100% higher than C-PPM. Excluding these data, regression analysis yielded CWK-3 = -0.01 +/- 1.30.C-PPM (r(2) = 0.97). CWK-3 was found to be unreliable when input impedance first harmonic modulus was close to characteristic impedance, i.e., when reflections were small. SV/PP correlated well with C-PPM (SV/PP = -0.10 + 1.76.C-PPM, r(2) = 0.89). We conclude that 1) C-PPM is a consistent estimate of pulmonary arterial compliance in pigs and dogs, 2) CWK-3 and SV/PP overestimate compliance, and 3) CWK-3 is unreliable when wave reflections are small.