Abstract Body (Do not enter title and authors here): Background: Pulmonary vascular resistance (PVR) and arterial compliance (PAC) represent the two major hemodynamic determinants of PA load, and their interdependence (the PVR-PAC relationship), critical to our understanding of RV-PA coupling. Previously, it has been reported that the two are physiologically linked through an inverse hyperbolic relationship, such that any rise in PVR is followed by an inverse reduction in PAC, and vice versa. Recent modelling has questioned the 'physiological correctedness' of this relationship as both share stroke volume (SV) in their determination. Accordingly, we sought to re-explore the PVR-PAC relationship, comparing PAC_clinical with a new, modified area under the curve method, termed PAC_pcwp.

Methods: PAC_clinical was calculated as the ratio of SV to PA_PP. For PAC_pcwp, the time constant was estimated as: τ = (A-dt*PCWP)/(P₁-P₂), where A is the integral over the time period from t₁ to t₂ of the pressure wave, dt is the time difference between P₁ and P₂, PCWP is the mean post capillary wedge pressure, P₁ is the arterial pressure at t₁ and P₂ the arterial pressure at t₂. Once τ was determined, PAC_pcwp was measured as the τ/PVR ratio.

Results: Hemodynamic data from 79-patients with dyspnoea who underwent sequential right heart catheterisation/cardiac magnetic resonance for investigation of PH were analysed (24 pre-capillary PH, 25 post-capillary, 17 combined and 13 controls). PAC_pcwp values were significantly higher than PAC_clinical (5.5±4.0 vs 3.4±1.6 mL/mmHg; P<0.01), with a non-signifcant increase in τ amongst subjects with PH (Figure 1a). Whereas PVR demonstrated a strong inverse hyperbolic correlation with PAC_clinical (R²-0.77, P<0.01), it did not with PAC_pcwp (R²-0.09, P=0.59; Figure 1b) where there was significant random scatter at lower PVR values. Compared with data in those with a PCWP <15mmHg, the relationship was predictably shift downward in an almost parallel manner for PVR-PAC_clinical but not so PVR-PAC_pcwp (Figure 1c&d).

Conclusions: Although these observations may seem trivial at first, they bring into question the invariability of (i) a constant R-C time product and (ii) a hyperbolic resistance-compliance relationship, signalling the potential for greater fluctuations in PAC for any given change in PVR. Reconsideration of PAC determination using the PAC_pcwp method in those with a low PVR and/or PA_M (i.e., at the steep end of the PVR-PAC curve) may be the key to unlocking the prognostic utility of this index.