Relationship between pulmonary artery wedge pressure and left atrial pressure: pathophysiologic implications

Left atrial pressure (LAP) is clinically estimated by the occluded pulmonary artery (PA) pressure (PA wedge pressure, PAWP), which should equalize with downstream LAP. Early studies reported the potential for PAWP to exceed LAP, but the frequency and clinical significance of this finding is unclear.

This study aimed to investigate the hemodynamic characteristics, physiologic determinants, and prognostic impact of patients with PAWP exceeding LAP.

Patients who underwent right heart catheterization (RHC) for diagnosis or treatment with PAWP and simultaneous measurement of LAP were identified. Patients were divided into two groups based on differences in PAWP and LAP. Pulmonary capillary pressure (Pc) was measured using the PA occlusion technique. Pulmonary vascular resistance (PVR) was then partitioned into upstream PVRa ([mean PA pressure (PAP)-Pc]/cardiac output [CO]) and downstream PVRv ([Pc-PAWP]/CO), with each expressed as a percentage of PVR.

675 patients underwent RHC with transseptal puncture allowing measurement of both PAWP and LAP simultaneously. A strong correlation and agreement between PAWP and LAP were demonstrated (r=0.92, P<0.001; bias +0.5 mmHg), but 310 patients (46%) displayed PAWP exceeding LAP by at least 1 mmHg (mean difference 3±2 mmHg). Patients with PAWP exceeding LAP displayed higher mean PAP (Figure 1A) and PVR calculated using LAP as the downstream pressure (PVR-LA: 2.10 [1.13-3.53] vs 3.38 [2.19-5.66] WU, P<0.001). PAWP was significantly higher in patients with PAWP exceeding LAP, although there was no significant difference in LAP between the groups (Figure 1B-C). PA compliance, which does not rely on either PAWP or LAP for calculation, was significantly lower in patients with PAWP exceeding LAP (2.7 [1.8-4.0] vs 2.1 [1.5-3.0] mL/mmHg, P<0.001, Figure 1D). The difference between PAWP and LAP was significantly correlated with mean PAP (r=0.32, P<0.001, Figure 1E). In 63 patients where Pc could be measured, those with PAWP exceeding LAP showed a higher Pc and greater pulmonary venous (vs arterial) distribution of PVR (Figure 1F-H). Over 5.4 (IQR 1.7-10.2) years, the composite outcome of all-cause death or heart failure hospitalization occurred in 326 patients (48%). Patients with PAWP exceeding LAP had a higher risk of the composite outcome than those without (log-rank P<0.001, Figure 2). Patients with PAWP exceeding LAP had 37% greater risk of composite outcome than those without (hazard ratio 1.37, 95% CI 1.09-1.73; P=0.008) after adjusting for age, sex, hemoglobin, moderate or greater mitral regurgitation, and tricuspid regurgitation, LAP, and mean PAP.

While PAWP is a robust surrogate for LAP, the former commonly exceeds the latter, and patients with PAWP exceeding LAP have more severe pulmonary vascular disease, greater capillary hypertension and venous involvement, and a higher risk for adverse outcomes.