Cardiac output of dogs exercising in the heat
Chapman, L.W.; Baker, M.A.
American Journal of Physiology 247(1 Pt 2): R124-R126
1984
ISSN/ISBN: 0002-9513 PMID: 6742222 Document Number: 230149
Cardiac output and central blood temperature were measured in 5 large dogs (27 kg mean body wt) running at 7.5 km/h for 30 min at 2 work loads and at low and high ambient temperature (Ta). Each animal ran on a level treadmill (O2 cost about 4 times that of resting) at 25 and 35.degree. C Ta and at a 20% slope (O2 cost about 10 times that of resting) at 25 and 35.degree. C Ta. Cardiac output (CO) was the same at 15 and 30 min of exercise at both work loads and both Tas. CO was higher at 35.degree. C Ta at both work loads. Blood temperature rose 0.6.degree. C during exercise on the level treadmill at 25.degree. C and stabilized after the 15th min of exercise. On the level treadmill at 35.degree. C, blood temperature increased by 1.9.degree. C after 30 min. During 30 min of running at 20% slope, blood temperature increased by 3.2.degree. C at 25.degree. C and by 4.6.degree. C at 35.degree. C. At these work loads the dog is able to increase CO during exercise in the heat. This response is similar to that of humans doing moderate exercise in the heat but is in contrast to that of humans doing heavy exercise in the heat, in whom CO shows a drop or no change compared with cool conditions. The stabilization of CO in dogs after 15 min of exercise when body temperature continued to rise suggests either a leveling off of thermoregulatory blood flows or a redistribution of blood flow from visceral vascular beds to peripheral regions and to respiratory muscles.