BACKGROUND HVAD left ventricular assist device flow waveforms provides graphical real-time information linking device performance with invasive hemodynamics. Previous studies have demonstrated a good correlation between the slopes of the ventricular filling phase slope (VFPS) and directly measured pulmonary capillary wedge pressure (PCWP). We aimed to validate the utility of VFPS to estimate PCWP and predict clinical outcomes. METHODS In this prospective blinded study, screenshots from the HVAD monitor and simultaneous invasive hemodynamic measurements were obtained. Each screenshot was digitized and the VFPS was calculated by 2 independent reviewers who were blinded to the hemodynamic results. The equation PCWP = 7.053 +1.365 × (VFPS) was derived from a previously published dataset and the estimated PCWP was correlated to the actually measured PCWP. RESULTS One hundred thirty-one sets of simultaneous measurements (VFPS and PCWP) were obtained from 27 HVAD patients (mean age 55 years, 47% male). A previously proposed cutoff of VFPS ≥5.8 L/min/s predicted PCWP ≥ 18 mmHg with 91.5% sensitivity and 95.2% specificity with the area under curve of 0.987. The estimated PCWP significantly correlated with measured PCWP (R2 = 0.65, P < .001) and showed acceptable agreement with measured PCWP. Patients with VFPS ≥ 5.8 L/min/s experienced significantly higher heart failure readmission rates than those without (0.24 vs 0.05 events/y, P < .001). CONCLUSIONS VFPS of the HVAD flow waveform is a novel noninvasive parameter that can estimate PCWP.