Cardiac output of dogs exercising in the heat.

We measured cardiac output and central blood temperature in five large dogs (27 kg mean body wt) running at 7.5 km/h for 30 min at two 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 degrees C Ta and at a 20% slope (O2 cost about 10 times that of resting) at 25 and 35 degrees 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 degrees C Ta at both work loads. Blood temperature rose 0.6 degrees C during exercise on the level treadmill at 25 degrees C and stabilized after the 15th min of exercise. On the level treadmill at 35 degrees C, blood temperature increased by 1.9 degrees C after 30 min. During 30 min of running at 20% slope, blood temperature increased by 3.2 degrees C at 25 degrees C and by 4.6 degrees C at 35 degrees 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.(ABSTRACT TRUNCATED AT 250 WORDS)