Transition from exercise to rest. Ventilatory and arterial blood gas responses.

The mechanisms leading to rapid changes in arterial blood gas values soon after exercise ends have not been well established. To further study these phenomena, we exercised seven normal male volunteers to exhaustion on a cycle ergometer with a 25-W/min ramped protocol measuring arterial blood gas values, and breath-by-breath gas exchange from rest to exercise and through 15 minutes of recovery. Arterial PO2 (PaO2) increased from 108 mm Hg at peak exercise to 125 mm Hg at 2 minutes of recovery. There was a smaller rise in calculated alveolar PO2 (PAO2) from 121 to 128 mm Hg over the same period. Arterial PCO2 (PaCO2) fell from 35.0 mm Hg to 31.9 mm Hg. The gas exchange ratio R rose from 1.21 to 1.52, after having peaked at 1.68 at 1 minute. The alveolar-arterial O2 gradient (P[A-a]O2) fell from 12.3 mm Hg at peak exercise to 3.2 mm Hg at 2 minutes. Following exercise, the rise in R is related to a more rapid fall in O2 uptake than in CO2 output, and the fall in P(A-a)O2 is probably related to improved V/Q relationships and to a rise in mixed venous PO2. We conclude that the rise in PaO2 in the recovery period after progressive nonsteady state exercise is due to several factors, including a fall in P(A-a)O2 and a rise in PAO2 due primarily to an elevation of R and also to a fall in PaCO2.