Lower pulmonary diffusing capacity in the prone vs. supine posture

We evaluated the effect of prone positioning on gas-transfer characteristics in normal human subjects. Single-breath (SB) and rebreathing (RB) maneuvers were employed to assess carbon monoxide diffusing capacity (DlCO), its components related to capillary blood volume (Vc) and membrane diffusing capacity (Dm), pulmonary tissue volume (Vti), and cardiac output (Q̇c). Alveolar volume (Va) was significantly greater prone than supine, irrespective of the test maneuver used. Nevertheless, DlCO was consistently lower prone than supine, a difference that was enhanced when appropriately corrected for the higher Va prone. When adequately corrected for Va, diffusing capacity significantly decreased by 8% from supine to prone [SB: DlCO,corr supine vs. prone: 32.6 ± 2.3 (SE) vs. 30.0 ± 2 ml·min-1·mmHg-1 stpd; RB: DlCO,corr supine vs. prone: 30.2 ± 2.2 (SE) vs. 27.8 ± 2.0 ml·min-1·mmHg-1 stpd]. Both Vc and Dm showed a tendency to decrease from supine to prone, but neither reached significance. Finally, there were no significant differences in Vti or Q̇c between supine and prone. We interpret the lower diffusing capacity of the healthy lung in the prone posture based on the relatively larger space occupied by the heart in the dependent lung zones, leaving less space for zone 3 capillaries, and on the relatively lower position of the heart, leaving the zone 3 capillaries less engorged.