Effect of acute reduction in oxygen transport on parameters of aerobic function during exercise

The binding of haemoglobin by carbon monoxide reversibly decreases the blood O2 carrying capacity, providing a useful model of impaired circulatory O2 transport. We evaluated noninvasive parameters of aerobic function during exercise to detect small changes in O2 transport, using carbon monoxide. Ten normal subjects performed both symptom-limited incremental and two levels of constant work rate on a cycle ergometer while breathing air and air with added carbon monoxide to cause carboxyhaemoglobin (COHb) to be approximately 11% (level of heavy cigarette smoker) and 20%. Maximal O2 uptake (VO2 max), the anaerobic threshold (AT) determined from the plot of CO2 output as a function VO2 (V-slope, the ratio of increase in VO2 to work rate increment (delta VO2/delta WR) and the upper slope of the V-slope analysis were measured while progressively increasing work rate. These changed in approximately the same percent as the increase in COHb. For the constant work rate tests, the time constant of VO2 and the difference in VO2 at six minutes as compared to three minutes of exercise (delta VO2 (6-3)) were significantly increased when COHb was increased. These noninvasive parameters of aerobic function, determined from the cardiopulmonary response to incremental and constant work rate exercise, particularly when used in combination, proved to be sufficiently sensitive to objectively detect small changes in O2 transport to the working muscles during exercise.