Blood acid-base and lactate relationships studied by ramp work tests

Abstract

The effect of work rate increase in ramp work tests was studied in six healthy subjects. Each subject exercised on a cycle ergometer with the work rate incremented by either 65.4 W·min⁻¹ and 49.0 W·min⁻¹ for the fast ramps or 8.2 W·min⁻¹ and 6.1 W·min⁻¹ for the slow ramps for male and female subjects, respectively. Gas exchange was monitored by open-circuit spirometry. Arterialized venous blood samples were obtained from a dorsal hand vein. The peak V̇o₂ was not significantly different for fast (3218 ± 602 ml·min⁻¹, X̄ ± SD) and slow (3237 ± 602 ml·min⁻¹) ramp tests. Gas-exchange anaerobic threshold, determined by multi-segment linear regression of V̇_E vs V̇o₂, was similar for fast and slow ramp tests (1742 ± 415 and 1925 ± 639 ml O₂·min⁻¹, P > 0.05). The V̇o₂ at which blood lactate increased 0.5 mM above resting levels was lower (1463 ± 259 ml·min⁻¹, P < 0.05) than the gas-exchange anaerobic threshold for the slow ramp test. The V̇o₂ at which blood lactate reached 2.0 mM was greater (2383 ± 247 ml·min⁻¹, P < 0.05) than the gas-exchange anaerobic threshold for the fast ramp test. In addition to these lactate differences, blood pH and HCO⁻₃ did not change in direct proportion to the lactate concentration in either test. Blood Pco₂ was significantly (P < 0.05) greater at the point of exhaustion in the fast ramp test (42.2 ± 2.3 mmHg) than in the slow ramp test (26.7 ± 2.1 mmHg). It is concluded that the gas-exchange anaerobic threshold can be clearly dissociated from the blood lactate threshold by altering the work rate forcing function. Other mechanisms, such as H⁺ efflux and CO₂ storage capacity, are more likely explanations for the gas-exchange anaerobic threshold.