Mechanistic basis for the gas exchange threshold in Thoroughbred horses

The exercising Thoroughbred horse (TB) is capable of exceptional cardiopulmonary performance. However, because the ventilatory equivalent for O-2 ((V) over dot E/(V) over dot O-2) does not increase above the gas exchange threshold (Tge), hypercapnia and hypoxemia. accompany intense exercise in the TB compared with humans, in whom (V) over dot E/(V) over dot o(2) increases during supra-Tge work, which both removes the CO2 produced by the HCO3- buffering of lactic acid and prevents arterial partial pressure Of CO2 (Pa-CO2) from rising. We used breath-by-breath techniques to analyze the relationship between CO2 Output ((V) over dot CO2) and (V) over dot O-2 [V-slope lactate threshold (LT) estimation] during an incremental test to fatigue (7 to similar to15 m/s; 1 m.s(-1).min(-1)) in six TB. Peak blood lactate increased to 29.2 +/- 1.9 mM/l. However, as neither (V) over dot E/(V) over dot O-2 nor (V) over dot E/(V) over dot CO2 increased, Pa-CO2 increased to 56.6 +/- 2.3 Torr at peak (V) over dot o(2) ((V) over dot (2max)). Despite the. presence of a relative hypoventilation (i.e., no increase in (V) over dot E/(V) over dot O-2 or (V) over dot E/(V) over dot CO2), a distinct Tge was evidenced at 62.6 +/- 2.7% (V) over dot O-2max. Tge occurred at a significantly higher (P < 0.05) percentage Of (V) over dot (2max) than the lactate (45.1 +/- 5.0%) or pH (47.4 +/- 6.6%) but not the bicarbonate (65.3 +/- 6.6%) threshold. In addition, Pa-CO2 was elevated significantly only at a workload > Tge. Thus, in marked contrast to healthy humans, pronounced V-slope (up arrow (V) over dot CO2/(V) over dot o(2)) behavior occurs in TB concomitant with elevated Pa-CO2 and without evidence of a ventilatory threshold.