Treatment of ARDS With Prone Positioning

Abstract

Prone positioning was first proposed in the 1970s as a method to improve gas exchange in ARDS. Subsequent observations of dramatic improvement in oxygenation with simple patient rotation motivated the next several decades of research. This work elucidated the physiological mechanisms underlying changes in gas exchange and respiratory mechanics with prone ventilation. However, translating physiological improvements into a clinical benefit has proved challenging; several contemporary trials showed no major clinical benefits with prone positioning. By optimizing patient selection and treatment protocols, the recent Proning Severe ARDS Patients (PROSEVA) trial demonstrated a significant mortality benefit with prone ventilation. This trial, and subsequent meta-analyses, support the role of prone positioning as an effective therapy to reduce mortality in severe ARDS, particularly when applied early with other lung-protective strategies. This review discusses the physiological principles, clinical evidence, and practical application of prone ventilation in ARDS.

Keywords: ARDS; critical care; hypoxemia; lung injury; ventilation.

Figure 1

Column I shows an isolated lung (cone) and alveolar units (circles) removed from the chest wall. This illustrates how the unhindered lung contains more alveolar units in the dorsal regions than in the ventral regions and how a gravitational pleural pressure gradient leads to compression of dependent segments. When the patient is in a prone position, this results in a smaller fraction of compressed alveolar units than when the patient is supine. Column II illustrates the effects of compressing the native conical shape of the lungs into the rigid chest wall. While the patient is supine, the compressive effects of gravity are magnified by the chest wall, further compressing the dorsal segments while expanding the ventral segments. Conversely, when the patient is prone, the chest wall effects oppose gravimetric effects, leading to more homogeneous aeration. Column III displays experimental data supporting this model. The curves describe how pulmonary aeration (gas to tissue ratio on CT) varies as one moves along the lung's vertical axis in human patients with ARDS. Note the marked asymmetry in aeration (and thus ventilation) along the ventral/dorsal axis when supine and a much more uniform gas to tissue ratio when prone. The white arrows signify recruitment of dependent regions, and the black arrows signify reduced regional hyperinflation in well-aerated lung.

Figure 2

In a sheep model, pulmonary perfusion along a lung-height axis is displayed for both the supine (left side of graph) and prone (right side) positions. The total length of each of the eight bars along the x-axis represents the relative perfusion of each of the eight stacked coronal planes. The red shading within each bar represents the fraction of perfusion that is shunt perfusion, whereas the blue coloring is the nonshunt perfusion. The number adjacent to the bar is the precise value of the shunt fraction for that plane. For example, in the supine position, the most dorsal (dependent) plane receives approximately 13% of total perfusion (x-intercept) and of that perfusion, 91% of it is shunt perfusion. Note that prone positioning does not significantly change the distribution of perfusion but it does markedly reduce the total shunt fraction.