Mechanistic basis for the gas exchange threshold in Thoroughbred horses

Abstract

The exercising Thoroughbred horse (TB) is capable of exceptional cardiopulmonary performance. However, because the ventilatory equivalent for O2(V˙e/V˙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˙e/V˙o 2 increases during supra-Tge work, which both removes the CO2 produced by the HCO[Formula: see text] buffering of lactic acid and prevents arterial partial pressure of CO2 (PaCO2 ) from rising. We used breath-by-breath techniques to analyze the relationship between CO2 output (V˙co 2) andV˙o 2 [V-slope lactate threshold (LT) estimation] during an incremental test to fatigue (7 to ∼15 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˙e/V˙o 2 norV˙e/V˙co 2 increased, PaCO2 increased to 56.6 ± 2.3 Torr at peakV˙o 2(V˙o 2 max). Despite the presence of a relative hypoventilation (i.e., no increase inV˙e/V˙o 2 orV˙e/V˙co 2), a distinct Tge was evidenced at 62.6 ± 2.7%V˙o 2 max. Tge occurred at a significantly higher ( P < 0.05) percentage ofV˙o 2 max than the lactate (45.1 ± 5.0%) or pH (47.4 ± 6.6%) but not the bicarbonate (65.3 ± 6.6%) threshold. In addition, PaCO2 was elevated significantly only at a workload > Tge. Thus, in marked contrast to healthy humans, pronounced V-slope (↑V˙co 2/V˙o 2) behavior occurs in TB concomitant with elevated PaCO2 and without evidence of a ventilatory threshold.