prone ventilation improves RV hemodynamics

For patients with severe ARDS, prone ventilation confers several salutary effects, including a more homogeneous distribution of ventilation, improved oxygenation, higher respiratory system compliance, reduction in ventilator-induced lung injury, and right ventricular (RV) unloading. One or more of these may underlie the mortality benefit of prone ventilation for those with severe ARDS.

RV dysfunction is present in roughly 25% of patients with severe ARDS. Common echocardiographic findings include an increase in the RV-to-left ventricular (LV) end-diastolic area ratio, tricuspid regurgitation, and an abnormal eccentricity index reflecting septal deviation toward the LV (the eccentricity index is the ratio of the anteroposterior and septolateral LV dimensions, reflecting how circular the LV appears in the short axis). When RV dysfunction is present, the beneficial hemodynamic impact of prone ventilation is revealed as a reduction in RV size compared with the LV, less tricuspid regurgitation, and a tendency to normalize the position of the interventricular septum as reflected in the LV eccentricity index. For example, in one trial of ARDS patients with acute RV dysfunction, prone ventilation lowered the RV:LV end-diastolic area ratio from 0.91 to 0.61, while the LV eccentricity index fell from 1.5 to 1.1. Improved RV function may restore cardiac output and hemodynamic stability.

Prone ventilation tends to reduce compliance of the chest wall but to raise compliance of the lung. The net result is typically no change or a modest increase in overall compliance of the respiratory system. Prone ventilation typically raises central venous and pulmonary artery occlusion pressures as the heart is lowered relative to the large splanchnic reservoir of blood. Although one can imagine that reducing RV afterload could lower right atrial pressure, this is not seen, even in those with acute cor pulmonale. Oxygenation usually improves with prone ventilation, often dramatically, as signaled by a rise in P:F ratio. This, however, is not likely the basis for the improvement in survival.12345678

Footnotes

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