apex vs base of lung dead space and ahunt

1– 3 The effects of the prone posture on gas exchange are complex and related to alterations in the distribution of alveolar ventilation and blood flow, improved matching of local ventilation and perfusion, and reduction in regions of low ventilation/perfusion ratios. The prone posture has been used for more than 40 years to treat patients with severe ARDS. Other benefits include a more uniform distribution of alveolar stress, relief of left-lower-lobe lung compression by the heart, enhanced secretion clearance, and favorable right-ventricular and systemic hemodynamics. Because both ventilation and perfusion heterogeneity decrease in the prone posture, gas exchange improves. The prone posture results in more uniform pulmonary blood flow when compared with the supine posture, due to an anatomical bias for greater blood flow to dorsal lung regions. Ventilation heterogeneity is reduced in the prone posture due to more uniform alveolar size secondary to a more uniform vertical pleural pressure gradient. Improvements in gas exchange occur via several mechanisms: alterations in the distribution of alveolar ventilation, redistribution of blood flow, improved matching of local ventilation and perfusion, and reduction in regions of low ventilation/perfusion ratios. Clinical studies have consistently demonstrated improvements in oxygenation, and a multi-center randomized trial found that, when implemented within 48 h of moderate-to-severe ARDS, placing subjects in the prone posture decreased mortality.

The prone posture is known to have numerous effects on gas exchange, both under normal conditions and in patients with ARDS.

Apex vs base of lung dead space and ahunt