The purpose of applying the reservoir-wave model to the pulmonary circulation is to validate the resulting reservoir and wave parameters against known physiologic responses. The reservoir model represents a modified windkessel and describes the lumped resistance to flow, compliance of vessels and the downstream pressure at which modeled outflow ceases. Applied to arteries and veins in tandem, a complete description of the pulmonary circulation is possible. The reservoir model determines the reservoir pressure, which is subtracted from measured pressure. The difference is called excess pressure and it is used with wave intensity analysis to quantify the effects of incident waves and their reflections. The changes to reservoir function and wave patterns may be helpful in providing extra diagnostic information regarding pulmonary hypertension and providing targets for treatment of this disease.
Measurements of pressure and flow were made in the main pulmonary artery and pulmonary vein in 15 anesthetised, open-chest dogs. Six different conditions were created by the addition of blood volume and the application of positive end-expiratory pressure (PEEP). At each condition, hypoxic ventilation was used to create vasoconstriction and then, inhaled nitric oxide was used to create vasodilation. Ventilation changes are the focus of experimentation because hypoxia represents a pulmonary hypertension disease state and nitric oxide represents a common treatment. Both arterial and venous reservoir resistance increased with hypoxia and decreased with nitric oxide but only arterial reservoir resistance was fully reversed with nitric oxide. With excess pressure, arterial wave patterns revealed the presence of a prominent negative reflection occurring from the junction of arteries branching from the left or right pulmonary artery. Venous wave patterns showed that the left atrium and ventricle are responsible for the majority of measured pressure fluctuations but a wave transmitted from the pulmonary arteries is measurable during the filling of the left atrium. Overall, results are in general agreement with hypothesized physiological changes, which serves to validate the application of the reservoir-wave model and allows both the vascular characteristics and wave propagation to be quantified in ways that other models can not.