The influence of a respiratory acidosis on the exercise blood lactate response

Summary

The purpose of the present study was to examine the influence of a respiratory acidosis on the blood lactate (La) threshold and specific blood La concentrations measured during a progressive incremental exercise test. Seven males performed three step-incremental exercise tests (20 W · min⁻¹) breathing the following gas mixtures; 21% O₂ balance-nitrogen, and 21% O₂, 4% CO₂ balance-nitrogen or balance-helium. The log-log transformation of La oxygen consumption (VO₂) relationship and a 1 mmol ·1⁻¹ increase above resting values were used to determine a La threshold. Also, theVO₂ corresponding to a La value of 2 (La2) and 4 (La4) mmol · 1⁻¹ was determined. Breathing the hypercapnic gas mixtures significantly increased the resting partial pressure of carbon dioxide (PCO₂) from 5.6 kPa (42 mm Hg) to 6.1 kPa (46 mm Hg) and decreased pH from 7.395 to 7.366. During the incremental exercise test,PCO₂ increased significantly to 7.2 kPa (54 mm Hg) and 6.8 kPa (51 mm Hg) for the hypercapnic gas mixtures with nitrogen and helium, respectively, and pH decreased to 7.194 and 7.208. In contrast, bloodPCO₂ decreased to 4.9 kPa (37 mm Hg) at the end of the normocapnic exercise test and pH decreased to 7.291. A blood La threshold determined from a log-log transformation [1.20 (0.28) 1·min⁻¹] or as an increase of 1 mmol·1⁻¹[1.84 (0.46) 1·min⁻¹] was unaffected by the acid-base alterations. Similarly, theVO₂ corresponding to La2 and La4 was not affected by breathing the hypercapnic gas mixtures [2.12 (0.46) 1·min⁻¹ and 2.81 (0.52) 1·min⁻¹, respectively]. Blood La values were reduced significantly at maximal exercise while breathing the hypercapnic gas mixtures (5.72±1.34 mmol ·1⁻¹) compared with the normocapnic test (6.96±1.14 mmol·1⁻¹). It is concluded that respiratory-induced acid-base manipulations due to the inspiration of 4% CO₂ have a negligible influence on the blood La response during a progressive exercise test at low and moderate power outputs. Lower blood La values are observed at maximal exercise with an induced respiratory acidosis but this negative influence is less than what has been reported for an induced metabolic acidosis.