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Failure of hyperoxic gas to alter the arterial lactate anaerobic threshold.

PURPOSE
Oxygen-enriched gases enable patients and healthy individuals to exercise submaximally with reduced lactate concentration, lower minute ventilation (VE), and less subjective stress compared to normoxia. These findings suggest that hyperoxia may raise the lactate accumulation threshold, also known as the anaerobic threshold (AT).


METHODS
This study measured the anaerobic threshold by gas exchange (Gx-AT) and arterial lactate (Lac-AT) methods in normoxia (FIO2 = 0.209) and the Lac-AT in hyperoxia (FIO2 = 0.40). Eight healthy males (age = 30.6 +/- 3.5 years; weight = 73.4 +/- 5.2 kg; VO2max = 41.3 +/- 6.6 mL/kg/min) worked incrementally (25 Watts [W] x 2 minutes) on a cycle ergometer with the legs on three occasions: once in normoxia, twice in hyperoxia. The latter situation enabled a reliability analysis of hyperoxic anaerobic threshold by arterial lactate methods that yielded a correlation coefficient (r) of 0.94 and nonsignificant paired t-ratio. Gas exchange and arterial lactate methods of detecting the anaerobic threshold in normoxia yielded nearly identical VO2 (21.8 +/- 5.4 mL/kg/min vs 21.5 +/- 5.5 mL/kg/min) with an r of 0.98.


RESULTS
Contrary to the study's hypothesis, the normoxic Lac-AT (134.4 +/- 35.2 W), expressed in power output at which the lactate threshold occurred, was not significantly different with hyperoxic gas (128.1 +/- 32.7 W). Furthermore, arterial lactate concentration at the breakpoint in normoxia (1.74 +/- 0.50 mmol.l-1) was not significantly affected by hyperoxia (1.68 +/- 1.03 mmol.l-1) nor was it different between the two hyperoxic tests. No significant differences in VE, HR, or CO2-ventilation equivalent at Lac-AT were found between the two FIO2 conditions.


CONCLUSIONS
The elevation of estimated PaO2 to 200 mm Hg does not alter the Lac-AT, compared to the normoxic condition, nor does it affect the arterial lactate concentration at its systematic break point in incremental cycling. Lac-AT is a reliable measurement and it can be estimated accurately using the VE/VO2 in conjunction with VE/VCO2.