Lung compression by both the heart and the diaphragm, especially in obese patients and when the abdomen is left unsupported, can be reduced by prone positioning, improving ventilation and oxygenation as well. The ability to attenuate mechanical lung injury may be more beneficial clinically. CT imaging modeling data demonstrated that the asymmetry of lung shape leads to a greater induced pleural pressure gravity gradient when supine is compared to prone positioning. Multiple physiological studies support the theory that placing a person in the prone position promotes more homogeneous aeration of the lung in ARDS. The outcome is an improvement in ventilation and oxygenation. As a result, there is reduced alveolar distension limiting ventilator-associated lung injury, and allowing for opening of alveoli that had collapsed during supine ventilation. Prone positioning reduces the difference between the dorsal and ventral transpulmonary pressure, making ventilation more homogeneous, and leading to a decrease in ventral alveolar overinflation and dorsal alveolar collapse. Changes in the distribution of extravascular lung fluid and secretions may also play a role. The improvement of oxygenation during prone ventilation is multifactorial, but occurs mainly by reducing lung compression and improving lung perfusion. Despite evidence in favor of prone ventilation, adoption of this strategy has been slow in the United States compared with Europe, likely due to the perceived operational barriers to performing it. Subsequent observations of improvement in oxygenation with simple patient rotation dominated the next several decades of research. Prone positioning was then established as a rescue strategy for severe hypoxemia. Subsequent studies suggested that prone positioning improves oxygenation in most patients (70–80%) with ARDS. Clinical case series supported this concept, documenting significant improvement in oxygenation with prone positioning. To reduce atelectasis in injured lungs, Bryan proposed prone positioning, theorizing that it would reduce pleural pressure gradients and restore aeration to dorsal lung segments. It may be used for the treatment of acute respiratory distress syndrome (ARDS) as a strategy to improve oxygenation and was first proposed in the 1970s as a method to improve gas exchange in ARDS. Prone ventilation refers to the delivery of mechanical ventilation with the patient lying in the prone position. As such, this article aims to review the physiological principles and effects of the prone ventilation, positioning, as well as its contraindications and complications. In view of different roles, we surgeons had to take during the COVID-19 pandemic, it is of importance to learn how to implement this therapeutic measure, especially in a surgical critical care unit setting. Although proning is indicated in patients with severe ARDS who are not responding to other ventilator modalities, this technique has moved away from a salvage therapy for refractory hypoxemia to an upfront lung-protective strategy intended to improve survival in severe ARDS, especially due to the current COVID-19 pandemic. CT imaging modeling data demonstrated that the asymmetry of lung shape leads to a greater induced pleural pressure gravity gradient when supine as compared to prone positioning.
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