A rapid increase in left atrial pressure reduces pulmonary arterial compliance in rats: Insights from pulmonary arterial impedance analysis.

Pulmonary arterial (PA) impedance is a comprehensive parameter that describes the dynamic properties of pulmonary circulation. PA impedance is estimated using the transfer function that relates PA flow to pressure waveforms. However, both PA flow and pressure may be affected by left atrial (LA) pressure, especially when LA pressure is elevated. This study aimed to evaluate the impact of increased LA pressure on the estimation of PA impedance and its associated parameters. We used eight male Sprague-Dawley rats to measure PA impedance using (1) one-input, one-output (I1O1) analysis that does not consider LA pressure in the calculation, and (2) two-input, one-output (I2O1) analysis that takes into account the potential effect of LA pressure on PA flow and pressure. Under normal physiological conditions, both I1O1 and I2O1 analyses provided accurate estimates of PA impedance with high coherence function. However, when LA pressure was increased by transverse aortic ligation with blood transfusion, I1O1 analysis showed reduced coherence in the low frequency range, whereas I2O1 analysis maintained reasonable estimation of PA impedance. When parameterizing PA impedance estimated by I2O1 analysis, rapid increase in LA pressure was associated with decreased pulmonary arterial compliance (Cp), without affecting peripheral pulmonary resistance (Rp), characteristic impedance of the proximal arteries (Zc) or transmission time from the main trunk to the reflection sites. In conclusion, increase in LA pressure significantly decreases Cp, while Rp and Zc remain unaltered. When estimating PA impedance under conditions of elevated LA pressure, the effect of LA pressure should not be neglected.