The effect of wedge pressure on the pulmonary compliance to resistance relationship

Background The resistance-compliance (RC) time derived from the pulmonary arterial pressure waveform, is an integrated measure of global right ventricular (RV) afterload that combines the pulmonary arterial compliance (PAC) and pulmonary vascular resistance (PVR) as the pulsatile and resistive components of RV load. Traditionally considered constant, RC time has shown variability in some cohorts. Controversy exists as to: i) whether this relationship is physiological, or mathematical artefact from interdependence between the PAC and PVR equations, and ii) whether the RC relationship persists at lower PVR, such as cohorts with post-capillary pulmonary hypertension (pcPH). Purpose i) Analyse the relationship between PVR and PAC using two calculation methods; ii) use mathematical modelling to evaluate the influence of stroke volume (SV) and pulse pressure, as well as the interaction between PAC and PVR; and iii) examine the PVR-PAC relationship across the hemodynamic spectrum of PH to determine whether the inverse hyperbolic relationship holds at lower PVR values (<180dynes.s.cm-5), and in pcPH. Methods Pulmonary pressure waveform data was collected during right heart catheterisation. PAC was calculated using the empirical (PACempirical) and area under the curve (PACAUC) methods. PVR was calculated using the standard equation. Mathematical modelling was used to characterise the relationship between PVR and PAC. Additional modelling was performed to explore for a mathematical interaction between SV, the inverse pulse pressure, post-capillary wedge pressure (PCWP), PVR, and RC time. Results Mean PACempirical was 2.63±1.74 mL/mmHg (n=1017) and PACAUC 2.70±1.89 mL/mmHg (n=222). Using both PAC methods, a strong and independent inverse hyperbolic relationship was observed between PVR and PAC, independent of equation-derived interactions ( = 1.48; 95% C.I. 1.39 – 1.57, z = 31.6, p < 0.001). The effect was modified by PCWP (Figure 1), which reduces the pulsatile rather than resistive component of RV load, and in the isolated pcPH cohort, the PVR-PAC relationship was significantly dependent on PCWP. Conclusion These findings confirm a significant effect of PCWP on the pulsatile loading of the RV, in particular in isolated post-capillary PH. This has important implications for the measurement of global RV load and response to left ventricular unloading by medical therapy and devices.Figure 1