Effect of compliance on a time-domain estimate of the characteristic impedance of the pulmonary artery during acute pulmonary hypertension.

In 11 pigs under general endotracheal anaesthesia, the time-domain method of determining the pulmonary arterial input impedance was compared with the frequency-domain equivalent under normal conditions as well as acute pulmonary hypertension induced by glass microspheres. The time-domain methods of determining the pulmonary arterial compliance C and pulmonary vascular bed resistance Rp compared favourably with the frequency-domain equivalents (r = 0.774, n = 60, p < 0.001 and r = 0.906, n = 60, p < 0.001, respectively), even at mean pulmonary artery pressures (MPAP) of 35 mmHg and above. A consistent and ever-increasing difference between characteristic impedance Zo(omega), estimated by averaging input impedance modulus values over a selected frequency range, and its time domain equivalent Ro with increasing MPAP was shown to be the cause of the poor fit between the measured and remodelled pulmonary blood flow. By analysing a time-domain estimate of the pulmonary characteristic impedance Ro(C, Rp, t), it was demonstrated that the characteristic impedance was dependent on C. Ro is therefore not an accurate representation of the characteristic impedance, especially under conditions of acute pulmonary hypertension. Ro(C, Rp, t) should therefore be calculated instead.