EFFECTS OF ALTERED FIO2 ON MAXIMUM V-O2 IN THE HORSE

Although the horse is considered an elite athlete with a specific Vove r dot(O2max) some 2-4 times higher than man, maximal O-2 transport is compromised both by moderately severe arterial desaturation and by fai lure to extract all O-2 from blood perfusing exercising muscle. This p rompted the present study to ascertain whether correction of arterial desaturation would proportionally augment Vover dot(O2max) and, if so, would O-2 extraction behave in a manner predicted by diffusional tran sport limitation. Six two year old thoroughbreds were exercised to Vov er dot(O2max) on a treadmill each on three separate occasions breathin g gases of FIO2 = 0.15, 0.21 and 0.35, each used once in balanced orde r. Vover dot(O2), ventilation, arterial and pulmonary arterial blood g ases, pressures and lactate levels were measured both submaximally and maximally at each FIO2 and cardiac output was computed by mass balanc e for O-2. At FIO2 = 0.21, Vover dot(O2max) 143.9 +/- 4.8 ml kg(-1) mi n(-1), arterial saturation (Sa(O2)) was 81.6 +/- 3.3% while venous P-O 2 (PVO2) was 15.3 +/- 1.4 Torr. At FIO2 = 0.35, Vover dot(O2max) was 1 72.6 +/- 8.2 ml kg(-1) min(-1), Sa(O2) reached 97.4 +/- 0.4% and PVO2 was 23.4 +/- 0.7 Torr. Vover dot(O2max) at FIO2 = 0.15 was 109.8 +/- 4 .1 ml kg(-1) min(-1), Sa(O2) fell to 68.1 +/- 2.5% and PVO2 was 10.6 /- 1.0 Torr, all changes being significant, p < 0.01. As FIO2 was vari ed, Vover dot(O2max) changed proportionally to calculated mean capilla ry P-O2 as well as to total O-2 delivery. These data confirm substanti al O-2 supply dependence of Vover dot(O2max) in the horse, and in such a manner as to be consistent with the hypothesis of combined diffusiv e and convective transport limitation within muscle.