Effects of exercise on the physiologic responses to hypoxia in the conscious dog

Changes in physiologic responses to acute hypoxia were determined in six conscious dogs following eight weeks of treadmill training. Three, nonexercised dogs served as a control group. Physical conditioning was evaluated at pretraining, midtraining, and posttraining time intervals by monitoring heart rate (HR) during a 7-stage, submaximal exercise test. Physiologic responses, during 15 min exposures to 10% 0₂ in N₂ and 8% O₂ in N₂ and 10 min exposures to 6% O₂ in N₂, were determined at the above time intervals. Decreases in HR in relation to the level of physical conditioning were evident at the midtraining time interval, and were most pronounced in the later stages of the exercise test where the work intensity was greatest. The HR during the recovery portion of the exercise test was also significantly reduced following four weeks of physical training. An additional four weeks of training did not result in further significant reductions in HR. Comparable changes in HR were not measured in the control group of dogs. Pretraining, systemic circulatory and cardiac responses to acute hypoxia included significant increases in cardiac output (Q), HR, and systemic arterial blood pressure (Pa). The increase in HR was directly related to the severity of hypoxia. Total systemic vascular resistance (Rs) was nonsignificantly reduced. No apparent change in stroke volume (SV) was noted during exposure to mild (10% 02) and moderate (8% 0₂) hypoxia, but a nonsignificant decrease was measured during inhalation of 6% O₂. Pretraining, respiratory and pulmonary circulatory responses to hypoxia were characterized by respiratory alkalosis and significant increases in pulmonary arterial blood pressure (Ppa). Increases in the minute volume of expiration (Ve) were variable in significance and appeared to be directly related to the severity of hypoxemia. No significant changes were observed in O₂ consumption (V0₂) during altitude exposures.