The Relationship Between Pulmonary System Impedance and Right Ventricular Function in Normal Sheep

Right ventricular (RV) afterload is a key determinant of RV function and is increased in many cardiopulmonary pathologies. Pulmonary circulation input impedance has been used to quantify afterload previously but due to its complexity has not been widely applied. This study examines the effect of a subset of the impedance spectrum, the zeroth and first harmonic impedance moduli (Z 0, Z 1), on RV performance in large animals. An artificial circuit with adjustable resistance and compliance (C) was implanted into the pulmonary circulation of five sheep. Resistance was varied to increase Z 0 in increments of 2 mmHg/(L/min) until Z 0 was 8 mmHg/(L/min) above baseline. At each Z 0, C was adjusted between 0, 0.5 and 2 mL/mmHg or 0, 1, and 5 mL/mmHg. Fourier transforms of the pulmonary artery pressure and flow in each situation were used to calculate the pulmonary impedance. Results show that the percent change in cardiac output (%ΔCO) is linearly related to the change in Z 0 (ΔZ 0). Increases in Z 1 (ΔZ 1) decreased %ΔCO but to a much smaller degree, with the effect of ΔZ 1 increasing with ΔZ 0. Regression of these results produce the equation: %ΔCO = (−0.0829ΔZ 1 − 3.65)ΔZ 0 − 9.02 (R 2 = 0.69). Blood flow and pressure moduli are small at harmonics higher than the first and are unlikely to affect RV function. Therefore, during acute, high afterload states, Z 0 is the primary determinant of CO, while the effect of Z 1 is minor.