Right Ventricular Function in Relation to Pulmonary Arterial Impedance

Summary

The mechanical function of the right ventricle (RV) and the input impedance of the pulmonary arterial (PA) tree define in combination the RV-PA hydraulic power production and dissipation. Mutual RV-PA adaptation may be discussed in the light of the efficiency of the hydraulic energy transfer. In pump-perfused rabbit lungs, characteristic PA impedance and pulsatile (= wasted oscillatory) power were minimal when PA pressure was normal, and this seemed to be associated with a slight degree of vasoconstriction. Stimulation of sympathetic nerves to intact cat lungs resulted in increased PA impedance; however, this effect diminished if RV flow was increased. The flow and pressure of an isolated supported and working cat heart RV preparation showed characteristic dependence of the PA load impedance. The produced hydraulic power exhibited a distinct maximum, indicating the existence of an optimum load impedance. Analysis of RV flow and pressure over broad ranges of load impedance indicated that the relation: pressure = E(t, volume) · (volume—V_d) + Pc (where V_d and Pc are constants, t is time, and E(t, volume) is time- and volume dependent cavity volume elastance), described RV pressure according to its volume (and flow). The above function may also be expressed by a three-dimensional surface with coordinates of pressure, volume and time, i.e., Pressure = F(Volume, t). Such RV descriptions were coupled to PA impedance in computer computations, which gave flows and pressures close to those observed. Also, analysis depending on such computations, together with the assumption of maximum efficiency in terms of external and produced RV hydraulic power, indicated that optimum efficiency is obtained when the PA impedance is close to normal.

Analysis of the RV-PA system indicated, roughly, that the end-systolic pressure-volume relation E_max, heart rate (HR), and PA peripheral resistance (Rp) should be related as E_max/HR = Rp for optimum power output. In the cat, RV E_max was 1.2 × 10⁴ dyne. Cm^-5. Rp may be around 4 × 10³ dyne, s cm^-5 and HR 3 s^-1, which would balance the above relation and assure that power was around optimum. Although the evidence is only indirect, this and the other observations point to mutual RV-PA adaptation to achieve efficient power transfer from heart to vessel bed.