Rationale: Blood flow rate affects mixed venous oxygenation (SvO₂) during venovenous extracorporeal membrane oxygenation (ECMO), with possible effects on the pulmonary circulation and the right heart function.

Objectives: We aimed at describing the physiologic effects of different levels of SvO₂ obtained by changing ECMO blood flow, in patients with severe ARDS receiving ECMO and controlled mechanical ventilation.

Methods: Low (SvO₂ target 70-75%), intermediate (SvO₂ target 75-80%) and high (SvO₂ target > 80%) ECMO blood flows were applied for 30 minutes in random order in 20 patients. Mechanical ventilation settings were left unchanged. The hemodynamic and pulmonary effects were assessed with pulmonary artery catheter and electrical impedance tomography (EIT). Measurements and Main Results: Cardiac output decreased from low to intermediate and to high blood flow/SvO₂ (9.2 [6.2-10.9] vs 8.3 [5.9-9.8] vs 7.9 [6.5-9.1] L/min, p = 0.014), as well as mean pulmonary artery pressure (34 ± 6 vs 31 ± 6 vs 30 ± 5 mmHg, p < 0.001), and right ventricle stroke work index (14.2 ± 4.4 vs 12.2 ± 3.6 vs 11.4 ± 3.2 g*m/beat/m², p = 0.002). Cardiac output was inversely correlated with mixed venous and arterial PO₂ values (R² = 0.257, p = 0.031 and R² = 0.324, p = 0.05). Pulmonary artery pressure was correlated with decreasing mixed venous PO₂ (R² = 0.29, p <0.001) and with increasing cardiac output (R² = 0.378 p < 0.007). Measures of ventilation/perfusion mismatch did not differ between the three steps.

Conclusions: In severe ARDS patients, increased ECMO blood flow rate resulting in higher SvO₂ decreases pulmonary artery pressure, cardiac output, and right heart workload.