Effect of 60 degrees head-down tilt on peripheral gas mixing in the human lung.

The phase III slope of sulfur hexafluoride (SF6) in a single-breath washout (SBW) is greater than that of helium (He) under normal gravity (i.e., 1G), thus resulting in a positive SF6-He slope difference. In microgravity (microG), SF6-He slope difference is smaller because of a greater fall in the phase III slope of SF6 than He. We sought to determine whether increasing thoracic fluid volume using 60 degrees head-down tilt (HDT) in 1G would produce a similar effect to microG on phase III slopes of SF6 and He. Single-breath vital capacity (SBW) and multiple-breath washout (MBW) tests were performed before, during, and 60 min after 1 h of HDT. Compared with baseline (SF6 1.050 +/- 0.182%/l, He 0.670 +/- 0.172%/l), the SBW phase III slopes for both SF6 and He tended to decrease during HDT, reaching nadir at 30 min (SF6 0.609 +/- 0.211%/l, He 0.248 +/- 0.138%/l; P = 0.08 and P = 0.06, respectively). In contrast to microG, the magnitude of the phase III slope decrease was similar for both SF6 and He; therefore, no change in SF6-He slope difference was observed. MBW analysis revealed a decrease in normalized phase III slopes at all time points during HDT, for both SF6 (P < 0.01) and He (P < 0.01). This decrease was due to changes in the acinar, and not the conductive, component of the normalized phase III slope. These findings support the notion that changes in thoracic fluid volume alter ventilation distribution in the lung periphery but also demonstrate that the effect during HDT does not wholly mimic that observed in microG.