The work-rate-dependent effect of carbon monoxide on ventilatory control during exercise.

The effect of low levels of carbon monoxide (CO) was studied during exercise in order to determine the work-rate dependent effect of CO breathing on exercise ventilation (VE). Ten normal subjects (aged 32.8 +/- 7.1 years) were studied during air breathing and air with added CO to bring carboxyhemoglobin (COHb) to approximately 11% and 20%. During the incremental exercise test, VE was not affected by CO breathing at work rates below the lactic acidosis threshold (LAT), defined as the O2 uptake above which CO2 is excreted by the lungs consequent to buffering of metabolic acid (not hyperventilation) (Beaver et al. (1986) J. Appl. Physiol. 60: 2020-2027). However, VE was increased above the LAT as work rate and COHb saturation were increased. At the end of constant work rate exercise, the increase in VE caused by increased COHb was positively correlated (r = 0.83, P less than 0.0001) with the increase in venous lactate sampled 2 min into recovery. This was complemented by a decrease in end-tidal PCO2 versus lactate (r = 0.76, P less than 0.0001). We conclude that the increase in exercise VE resulting from COHb levels up to 20%, is restricted to work rates above the LAT, and is proportionately higher, the greater the lactic acidosis.