Approximately 35 years ago, the use of the prone position was proposed to improve arterial oxygenation in patients with acute respiratory failure (ARF).1–3 The prone position, however, may have variable effects on gas exchange. Moreover, it has been suggested that, independent of gas exchange, the prone position may decrease the harm of mechanical ventilation, improving the outcome of patients with ARF. This chapter discusses the mechanisms affecting the changes in gas exchange consequent to the prone position in patients with ARF, and the effects of prone positioning on outcome of critically ill patients.
The effects of prone positioning on gas exchange may result from a combination of the following mechanisms: (a) changes in regional lung inflation, (b) redistribution of ventilation, and (c) redistribution of perfusion. These three mechanisms apply to both the normal and diseased lung. In the diseased lung, however, these mechanisms are also affected by the underlying pathology. Moreover, because the underlying pathology is an evolving process, it is likely that the effects of positioning on arterial oxygenation will vary with time.
Most of the studies dealing with regional lung inflation in normal subjects were performed with radioactive xenon.4,5 We used computed tomography (CT) to quantify regional lung inflation.6,7 The CT scan provides a computer-reconstructed image that is composed of several hundred elementary units (voxels). Each voxel is characterized by a given level of absorption of X-rays, which mainly reflects the density of the material being studied. The density is usually expressed in CT numbers or Hounsfield units (H).8 A density equal to 0 H characterizes a voxel composed of water, while a voxel with a density of −1000 H is composed of gas. A voxel with a CT number equal to −500 H has a composition of 50% gas and 50% tissue. By analysis of the CT numbers, we quantitatively describe the regional lung inflation of a single CT section, at the level of the lung base, that is representative of the entire lung.6 The CT section is divided into ten levels along the vertical axis, each level including approximately 300 to 400 voxels (Fig. 49-1). The gas-to-tissue ratio, which is our index of regional lung inflation, was computed from the average CT number at each lung level.
Regional analysis of the CT scan image. The vertical distance from ventral to dorsal surface (height) is divided into ten equal intervals. Ten lung levels are then obtained. Each level is composed of 300 to 400 elementary units (voxels), each characterized by a given CT number expressed in Hounsfield units (H). (Used, with permission, from Pelosi et al.16)
Regional Inflation in the Supine Position