• Prone positioning is used as an adjunct short-term supportive therapy in an attempt to recruit alveoli to improve gas exchange in a critically ill patient with severely compromised lungs.

• The last major outcomes to be examined in the meta-analyses were the presence of significant complications when the prone position was compared with the supine position. Two of the three analyses reported on complications. One analysis showed a statistically significant higher risk for the development of pressure ulcers in the prone position,²⁸ and Abroung and group¹ showed no significant difference in major airway complications in the prone position. All three meta-analyses concluded that an adequately sized study optimizing the duration of proning and ventilation strategy is warranted to be able to draw definitive conclusions. A phase III trial looking at these issues has been completed but the results have not been published to date. To enhance an understanding of how prone positioning may affect gas exchange, understanding the factors that influence the distribution of ventilation and perfusion within the lung is important.

• Distribution of ventilation: Regional pleural pressures and local lung compliance jointly determine the volume of air distributed regionally throughout the lungs. Three major factors—gravity and weight of the lung, compliance, and heterogeneously diseased lungs—influence regional distribution. In an upright individual, the pleural pressure next to the diaphragm is less negative than at the pleural apices. The weight of the lung and the effect of gravity on the lung and its supporting structures in the upright position create this difference in regional pleural pressures. This relationship results in a higher functional residual capacity (FRC) in the nondependent zone or the apices, redirecting ventilation to the dependent zone.8,15,35 When body position changes, changes occur in regional pleural pressures, compliance, and volume distribution. In the supine position, distribution becomes more uniform from apex to base. The ventilation of dependent lung units exceeds that of nondependent lung units, however, and a reduction in FRC is seen.^8,35 The two factors that contribute to the reduction in FRC seen in moving from the upright to the supine position include: 1, the pressure of the abdominal contents on the diaphragm⁸; and 2, the position of the heart and the relationship of the supporting structures to the lung and its influence on pleural pressure gradients.^17,20

• Distribution of perfusion: Similar to ventilation, regional distribution of perfusion is influenced by three factors: cardiac output, pulmonary vascular resistance, and gravity or body position.

• In an upright individual, blood flow decreases as it moves from base to apex with virtually no flow at the apex. This decrease is caused by the influence of gravity on pulmonary vascular pressures within the lung (Fig. 18-1).

• In supine and lateral positions, apical region blood flow changes. No real change is seen in basilar units, but a greater dependent versus nondependent blood flow occurs. In the prone position, a marked reduction occurs, however, in the gravitational perfusion gradient, which suggests no gravity-dependent benefit to flow in the prone position.²¹

• On the basis of the current available data as outlined here, changes in oxygenation seem to be related to differences in the regional inflation/ventilation of the lung while prone and are not related to a redistribution of blood flow.6,17,23

• Suggested criteria for use of the prone position include:

• Contraindications and precautions to manual pronation therapy include the following3,7,23,26,31,32: