Gas exchange in man during combined +Gz acceleration and exercise.

Gravity and acceleration from head to foot (+Gz) handicap gas exchange by shifting blood from thorax to dependent veins and creating a ventilation/perfusion (V/Q) mismatch in the lung. At 1 G leg exercise improves V/Q matching. Gas exchange was measured in six subjects +1, +2, and +3 Gz during rest and two levels of exercise, either unloaded pedaling or 600 kpm/min. The VO2 for pedaling was clearly related to G level, but work efficiency was unaffected. Acceleration lowered resting Pao2 while raising VE/VO2, HR, AaDo2, VD, and VD/VT. Unloaded pedaling returned VE/VO2 and HR toward their 1-G values. In contrast, at 3 G each increase in VO2 caused a fall in Pao2 and a rise in AaDo2. The VD showed no further change with exercise, while VD/VT decreased at all G levels. It thus appears that only some of the effects of acceleration are counteracted by exercise, probably by the peripheral muscle pump. Any accompanying rise in VO2 adds to the stress of acceleration, due to limitations on gas transport.