Cardiovascular and metabolic responses of dogs to exercise at high altitude.

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Abstract
Four chronically catheterized dogs were studied initially at 5,2000 ft. during a 26-day exposure to 11,400 ft., and again upon return to the initial altitude. Heart rate, intra-arterial blood pressure, cardiac output (Q), PaO2 [arterial O2 tension], PVO2 , PaCO2, PVCO2, PHa and PHv, blood glucose, NEFA (non-esterified fatty acids], and lactate were determined at rest and periodically or at the end of 45 min. of treadmill exercise at an approximate O2 uptake of 40-50 ml/ min kg wt. Q was significantly elevated both at rest (28%) and during exercise (49%) on the 1st day at high altitude and generally remained elevated during the entire sojourn. Heart rate was unchanged while stroke volume increased proportionately to Q. Peripheral resistance fell significantly on day 1 at 11,400 ft. but not thereafter. After the initial narrowing of the Pa-VO2 gradient, no further changes of PO2 occurred with time at high altitude. PaCO2 and PvCO2 exhib-ited early declines followed by partial recovery during the exposure period, while pH showed no definite trend. No pronounced changes were observed in either blood glucose or lactate with change in altitude in contrast to blood NEFA level which was significantly elevated at 11,400 ft. both during rest and exercise. These data sug- gest that an enhanced Q is an early and adequate response to moderate, chronic hypoxemia. No harmful effects of high altitude were noted on the heart or circulation. An increased norepinephrine release indicated by elevated NEFA may account for these cardiovascular responses.

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