Muscle RQ and Lactate Accumulation from Analysis of the VCo2-Vo2Relationship During Exercise

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Abstract
We analyzed the Vco2-Vo2 relationship derived from 1-min incremental (15 W/min) exercise tests of 10 normal subjects. The curve was quite linear below the anaerobic threshold (AT). We deduce that the slope must equal the respiratory quotient (RQ) of the exercising muscles, with a mean value for these subjects of 0.97‡0.06, indicating that the metabolic substrate is essentially glycogen. Beyond the AT, respiratory CO2 output rises at a faster rate above as compared to below the AT, reflecting the rate of HCO3-buffering of lactic acid. Projecting the straight line of Vco2 versus Vo2 below the AT into the region beyond the AT provides an estimate of the Vco2 due to continuing aerobic metabolism. The difference between the actual Vco2 and the aerobically produced Vco2 (excess Vco2) describes the rate of CO2 generated from HCO3- buffering of lactic acid. The integrated excess CO2, corrected for any hyperventilation, provides a measure of the quantity of HCO3- depletion and thus lactate accumulation. Since our measurements are non-steady state, a dynamic simulation model of total lactate accumulation and arterial lactate concentration, based on excess CO2 output and compartmental blood flows and volumes, was developed and found to predict experimental results of lactate concentration increase. Thus, the excess CO2 output can be a useful measure of lactate accumulation and, with the developed model, serve to describe the rise in arterial lactate concentration during a progressively increasing work rate test.

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