The isobaric pulmonary arterial compliance in pulmonary hypertension

Pulmonary hypertension (PH) is associated with stiffening of pulmonary arteries which increases right ventricular pulsatile loading. High pulmonary artery wedge pressure (PAWP) in postcapillary PH (Pc-PH) further decreases PA compliance (PAC) at a given pulmonary vascular resistance (PVR) compared to precapillary PH, thus responsible for a higher total arterial load. In all other vascular beds, arterial compliance is considered as mainly determined by the distending pressure, due to non-linear stress-strain behaviour of arteries. We tested the applicability, advantages and drawbacks of two comparison methods of PAC depending on the level of mean PA pressure mPAP (isobaric PAC) or PVR. Right heart catheterisation data including PAC (stroke volume/pulse pressure) were obtained in 112Pc-PH (of whom 61 had combined postcapillary and precapillary PH) and 719 idiopathic pulmonary arterial hypertension (iPAH). PAC could be compared over the same mPAP range (25–66 mmHg) in 792/831 patients (95.3%) and over the same PVR range (3–10.7 WU) in only 520/831 patients (62.6%). The main assumption underlying comparisons at a given PVR was not verified as the PVR×PAC product (RC-time) was not constant but on the contrary more variable than mPAP. In the 788/831 (94.8%) patients studied over the same PAC range (0.62–6.5 mL·mmHg−1), PVR and thus total arterial load tended to be higher in iPAH. Our study favours comparing PAC at fixed mPAP level (isobaric PAC) rather than at fixed PVR. A reappraisal of the effects of PAWP on the pulsatile and total arterial load put on the right heart is needed, and this point deserves further studies.