Problems associated with the determination of pulmonary vascular resistance.

The presence of critical pressures in the pulmonary circulation complicates the traditional use of pulmonary vascular resistance (PVR). The recruitable nature of the pulmonary circulation violates a basic assumption of the PVR formula, that is, that the involved vessels are rigid-walled. Flow through collapsible blood vessels is subject to the influence of critical opening pressures in addition to inflow and outflow pressures. As a result, PVR has a variable relationship to the Poiseuille resistance, approximating it better when zone 3 conditions predominate. In addition to being flow-dependent, PVR cannot easily distinguish among vasodilation, recruitment, and rheologic changes. PVR may be viewed as an index of steady-state power dissipation by the circulation, describing the relationship between power dissipation and flow, but it will still underestimate power dissipation by as much as 50%, since it cannot express oscillatory and kinetic power components. Laboratory data regarding the pulmonary circulation are predicted and explained by positing the existence of critical pressures in the pulmonary circulation and allow estimation of Poiseuille resistance. Unfortunately, clinical application of this approach is difficult owing to the necessity of generating pressure-flow plots under very stringent conditions. The clinical use of both pressure-flow and PVR-flow plots is impaired by shifting to different curves during hemodynamic manipulation. PVR must be interpreted in light of its considerable limitations.