Pulmonary vascular capacitance (PVC) is reduced in pulmonary arterial hypertension (PAH). In normal lung, PVC is largely a function of vascular compliance. In PAH, increased pulmonary vascular resistance (PVR) arises from the arterioles. PVR and PVC share pressure and volume variables. The dependency between the two qualities of the vascular bed is unclear in a state of intense vasoconstriction.
We compared PVC and PVR before and during nitric oxide (NO) inhalation during right-sided heart catheterization in eight NO-responsive patients with PAH. NO only directly affects tone in parenchymal vessels.
During NO inhalation, pulmonary arterial systolic pressure decreased, 80 ± 20 SD to 48 ± 20 mm Hg, and stroke volume increased, 62 ± 19 mL to 86 ± 24 mL (P < .01). PVR dropped from 10 ± 4.4 Wood units to 4.7 ± 2.2 Wood units (P < .012), and PVC increased from 1.4 ± 1.1 mL/mm Hg to 3.2 ± 1.8 mL/mm Hg (P < .018). The magnitude of PVR drop was 57% ± 6% and the decrease in 1/PVC was 54% ± 14% (P = not significant).
In vasoresponsive PAH, PVC is a function of the pressure response of the vasoconstricted arterioles to stroke volume. Immediately upon vasodilation, the capacitance increases markedly. The compliance vessels are, thus, the same as the resistance vessels. The immediate reduction in pulmonary arterial pressure during NO inhalation suggests that large vessel remodeling is not a major contributor to systolic pressure in these patients.