title: 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 (VE/VO2) does not increase above the gas exchange threshold (Tge), hypercapnia and hypoxemia accompany intense exercise in the TB compared with humans, in whom VE/VO2 increases during supra-Tge work, which both removes the CO2 produced by the HCO 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 (VCO2) and VO2 [V-slope lactate threshold (LT) estimation] during an incremental test to fatigue (7 to approximately 15 m/s; 1 m x s(-1) x min(-1)) in six TB. Peak blood lactate increased to 29.2 +/- 1.9 mM/l. However, as neither VE/VO2 nor VE/VCO2 increased, PaCO2 increased to 56.6 +/- 2.3 Torr at peak VO2 (VO2 max). Despite the presence of a relative hypoventilation (i.e., no increase in VE/VO2 or VE/VCO2), a distinct Tge was evidenced at 62.6 +/- 2.7% VO2 max. Tge occurred at a significantly higher (P < 0.05) percentage of VO2 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 (increase VCO2/VO2) behavior occurs in TB concomitant with elevated PaCO2 and without evidence of a ventilatory threshold.
authors.affiliation: Department of Anatomy, Kansas State University, Manhattan, Kansas 66506-5802, USA. pjmcdono@vet.ksu.edu
authors.forename: Paul
authors.initials: P
authors.lastname: McDonough
authors.forename: Casey A
authors.initials: CA
authors.lastname: Kindig
authors.forename: Howard H
authors.initials: HH
authors.lastname: Erickson
authors.forename: David C
authors.initials: DC
authors.lastname: Poole
mesh_terms: D015308:Anaerobic Threshold / Q000502:physiology*; D000818:Animals; D002245:Carbon Dioxide / Q000097:blood; D006469:Hemoptysis / Q000503:physiopathology; D006736:Horses / Q000502:physiology*; D006801:Humans; D000860:Hypoxia / Q000503:physiopathology; D007773:Lactates / Q000378:metabolism; D008297:Male; D010100:Oxygen / Q000097:blood; D010313:Partial Pressure; D005082:Physical Exertion / Q000502:physiology; D011659:Pulmonary Gas Exchange / Q000502:physiology*; D015656:Respiratory Mechanics / Q000502:physiology; D013045:Species Specificity
chemical_list: D007773:Lactates; D002245:Carbon Dioxide; D010100:Oxygen