Cardiovascular and respiratory responses in thoroughbred horses during treadmill exercise

Abstract

Six detrained Thoroughbred horses performed incremental treadmill tests. Maximal oxygen uptake (V̇(O2max)) was measured during exercise by analysis of expired gas. The relationships between oxygen consumption (V̇(O2)) and work rate, heart rate (HR), cardiac output (Q̇), stroke volume (SV) and arteriovenous oxygen content difference [C(a-v̄(O2))] were examined during submaximal and maximal exercise. The relative contributions of blood flow and extraction of oxygen from muscle capillaries were assessed during exercise at V̇(O2max). Mean V̇(O2max) was 129.7 ± 2.9 (mean ± S.E.M.) ml kg-1 min-1, which occurred at a mean speed of 8.0 m s-1, with the treadmill set at a slope of 10%. At V̇(O2max) the mean HR was 222 ± 7 beats min-1. Maximal Q̇ was 534 ± 22 ml kg-1 min-1 and mean SV was 2.4 ± 0.1 ml kg-1. Mean C(a-v̄)(O2) was 24.5 ± 1.2 ml 100 ml-1 blood. Linear relationships were found between V̇(O2) and work rate, HR, Q̇ and C(a-v̄)(O2). Stroke volume did not increase significantly during exercise. Increasing metabolic rate during exercise was associated with linear increases in arterial haemoglobin concentration and oxygen content, and decreases in arterial partial pressures and haemoglobin saturation. We conclude that the relatively high V̇(O2max) in the detrained Thoroughbred racehorse is dependent on the generation of a large C(a-v̄)(O2), despite development of hypoxaemia and haemoglobin desaturation, during strenuous exercise.