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

Department of Medicine, Division of Physiology, University of California, San Diego, La Jolla, California 92093

Submitted 22 December 2003 ; accepted in final form 15 April 2004

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 (µG), 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° head-down tilt (HDT) in 1G would produce a similar effect to µG 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 µG, 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 µG.

phase III slope; single-breath washout; multiple-breath washout; helium; sulfur hexafluoride

This article has been cited by other articles:

				A. C. Henderson, D. L. Levin, S. R. Hopkins, I. M. Olfert, R. B. Buxton, and G. K. Prisk Steep head-down tilt has persisting effects on the distribution of pulmonary blood flow J Appl Physiol, August 1, 2006; 101(2): 583 - 589. [Abstract] [Full Text] [PDF]