EVIDENCE THAT THE METABOLIC-ACIDOSIS THRESHOLD IS THE ANAEROBIC THRESHOLD

We evaluated maximal O2 uptake (V̇O(2 max)), the metabolic acidosis threshold determined by the V-slope analysis [plot of CO2 output (V̇CO2) as a function of oxygen uptake (V̇O2)], the ratio of increase in V̇O2 to work rate increment (ΔV̇O2/ΔWR), the upper slope (S2) of the V-slope analysis, and the V̇O2 for work below and above the metabolic acidosis threshold to determine whether the changes in O2 transport caused by increased carboxyhemoglobin (HbCO) affected these parameters and variables. Ten normal subjects (aged 32.8 ± 7.1 yr) performed symptom-limited incremental exercise tests in a ramp pattern on a cycle ergometer while breathing air and air with added carbon monoxide to cause HbCO to be ~11% and 20%. V̇O(2 max) decreased by 11.6 and 19.3%, the metabolic acidosis threshold decreased by 11.9 and 19.6%, ΔV̇O2/ΔWR decreased by 8.9 and 14.0%, and S2 increased by 13.6 and 21.8% when HbCO was increased to 11 and 20%, respectively. Most importantly, V̇O2 was unchanged related to work rate below the metabolic acidosis threshold during the tests with increased HbCO but was reduced at the work rates above the metabolic acidosis threshold. These findings are consistent with the concept that the metabolic acidosis threshold is synonymous with an anaerobic threshold, i.e., the latter demarcating the V̇O2 above which the contracting muscles are not adequately supplied with O2 but below which they are.