LAUSR

RATIONALE The inspiratory rise in transpulmonary pressure during mechanical ventilation increases right ventricular (RV) afterload. One mechanism is that when alveolar pressure (Palv) exceeds left atrial pressure, West zone 1 or 2 (non-zone 3) conditions develop and Palv becomes the downstream pressure opposing RV ejection. The tidal volume (VT) at which this impact on the RV becomes hemodynamically evident is not well established. OBJECTIVES To determine the magnitude of RV afterload and prevalence of significant non-zone 3 conditions during inspiration across the range of VT currently prescribed in clinical practice. METHODS In post-operative passively ventilated cardiac surgery patients, we measured right atrial, RV, pulmonary artery, pulmonary artery occlusion (Ppao), plateau (Pplat), and esophageal (Peso) pressures during short periods of controlled ventilation with VT increments ranging between 2-12 ml/kg PBW. The inspiratory increase in RV afterload was evaluated hemodynamically and echocardiographically. The prevalence of non-zone 3 conditions was determined using 2 definitions based on changes in Peso, Ppao and Pplat. RESULTS Fifty-one patients were studied. There was a linear relationship between VT, driving pressure and transpulmonary pressure and the inspiratory increase in the RV isovolumetric contraction pressure. Echocardiographically, increasing VT was associated with a greater inspiratory increase in markers of afterload and a decrease in stroke volume. Non-zone 3 conditions were present in >50% of subjects at a VT ≥ 6 ml/kg PBW. CONCLUSIONS In the range of VT currently prescribed, RV afterload increases with increasing VT. A mechanical ventilation strategy that limits VT and driving pressure is cardio-protective.