ESICM LIVES 2026: Haemodynamic effects of prone positioning in ARDS Prone positioning is usually discussed as a strategy to improve oxygenation. At LIVES 2026, Dr Jennifer Hastings highlighted a further physiological dimension: turning a patient prone can profoundly change right ventricular afterload and systemic venous return. In ARDS, the right ventricle faces several simultaneous threats. Hypoxia, hypercapnia, inflammatory vasoconstriction and microvascular thrombosis increase pulmonary vascular resistance. Atelectasis narrows extra-alveolar vessels, while overdistension compresses alveolar capillaries. The consequence may be RV dilatation, septal displacement, reduced LV filling and circulatory failure. Proning redistributes lung inflation and perfusion. By recruiting dependent regions while limiting overdistension, it may reduce pulmonary vascular resistance and unload the RV. Improved gas exchange can further attenuate hypoxic and hypercapnic pulmonary vasoconstriction. RV function may therefore improve even when the change in the PaO₂/FiO₂ ratio is modest. Moreover, oxygenation indices can be influenced by low mixed venous oxygen saturation or intracardiac shunting and cannot substitute for circulatory assessment. But the effect on cardiac output is more complex. Prone positioning can increase intra-abdominal pressure and mean systemic filling pressure, potentially increasing the driving pressure for venous return. Yet resistance to venous return may rise as well. In a physiological study of 22 patients, cardiac index increased by at least 15% in only seven, all of whom were preload responsive. Increased filling pressure does not automatically mean increased flow. This distinction matters at the bedside. A patient with RV pressure overload may benefit from proning through afterload reduction, whereas a patient without preload reserve may show little increase in cardiac output. Abdominal compression, intravascular volume and RV function all modify the response. The PROSEVA trial demonstrated a major survival benefit of early prolonged proning in severe ARDS, with 28-day mortality of 16.0% versus 32.8% in the supine group. Whether cardiovascular unloading contributes to this benefit remains an important physiological question, not a proven mechanism of mortality reduction. Prone positioning is therefore not merely a way to increase PaO₂. Its effects should be assessed through respiratory mechanics, echocardiography, cardiac output and tissue perfusion alongside gas exchange. #ESICM #LIVES2026 #ARDS #PronePositioning #RightVentricle #Hemodynamics #CriticalCare