Vigorous spontaneous effort can potentially worsen lung injury. This study hypothesized that the prone position would diminish a maldistribution of lung stress and inflation after diaphragmatic contraction and reduce spontaneous effort, resulting in less lung injury.
A severe acute respiratory distress syndrome model was established by depleting surfactant and injurious mechanical ventilation in 6 male pigs (“mechanism” protocol) and 12 male rabbits (“lung injury” protocol). In the mechanism protocol, regional inspiratory negative pleural pressure swing (intrabronchial balloon manometry) and the corresponding lung inflation (electrical impedance tomography) were measured with a combination of position (supine or prone) and positive end-expiratory pressure (high or low) matching the intensity of spontaneous effort. In the lung injury protocol, the intensities of spontaneous effort (esophageal manometry) and regional lung injury were compared in the supine position versus prone position.
The mechanism protocol (pigs) found that in the prone position, there was no ventral-to-dorsal gradient in negative pleural pressure swing after diaphragmatic contraction, irrespective of the positive end-expiratory pressure level (–10.3 ± 3.3 cm H2O vs. –11.7 ± 2.4 cm H2O at low positive end-expiratory pressure, P = 0.115; –10.4 ± 3.4 cm H2O vs. –10.8 ± 2.3 cm H2O at high positive end-expiratory pressure, P = 0.715), achieving homogeneous inflation. In the supine position, however, spontaneous effort during low positive end-expiratory pressure had the largest ventral-to-dorsal gradient in negative pleural pressure swing (–9.8 ± 2.9 cm H2O vs. –18.1 ± 4.0 cm H2O, P < 0.001), causing dorsal overdistension. Higher positive end-expiratory pressure in the supine position reduced a ventral-to-dorsal gradient in negative pleural pressure swing, but it remained (–9.9 ± 2.8 cm H2O vs. –13.3 ± 2.3 cm H2O, P < 0.001). The lung injury protocol (rabbits) found that in the prone position, spontaneous effort was milder and lung injury was less without regional difference (lung myeloperoxidase activity in ventral vs. dorsal lung, 74.0 ± 30.9 μm · min–1 · mg–1 protein vs. 61.0 ± 23.0 μm · min–1 · mg–1 protein, P = 0.951). In the supine position, stronger spontaneous effort increased dorsal lung injury (lung myeloperoxidase activity in ventral vs. dorsal lung, 67.5 ± 38.1 μm · min–1 · mg–1 protein vs. 167.7 ± 65.5 μm · min–1 · mg–1 protein, P = 0.003).
Prone position, independent of positive end-expiratory pressure levels, diminishes a maldistribution of lung stress and inflation imposed by spontaneous effort and mitigates spontaneous effort, resulting in less effort-dependent lung injury.
- Prone positioning during mechanical ventilation for patients with acute lung injury has been shown to increase oxygenation and possibly improve outcome
- It is now widely used for patients with COVID-19 failing routine ventilation protocols
- Its use during spontaneous ventilation has increased as result of the pandemic, yet detailed data on its ventilatory effects have not been well established
- The authors utilized porcine and rabbit models of lung injury to evaluate pulmonary mechanics, distribution of ventilation, and biochemical and histologic effects on lung injury with varying positive end-expiratory pressure levels
- Independent of positive end-expiratory pressure levels, prone positioning reduced maldistribution of lung stress and reduced effort-dependent evidence of lung injury