ALTITUDE AND COMMON MEDICAL CONDITIONS - Medicine for Mountaineering and Other Wilderness Activities - page 364

THE EFFECT OF ALTITUDE ON CARDIOVASCULAR PROBLEMS
During ascent and the first few days of residence at altitude, the cardiovascular systems of
healthy individuals make important adjustments. These physiologic changes may be lim-
ited in those with underlying disease, or these adjustments may stress the compromised
cardiovascular system and aggravate illness. The principal initial adjustments are increases
in resting and exercise heart rate, increases in blood pressure and systemic vascular res-
istance, increases in cardiac output and velocity of contraction, along with contraction of
the veins in the skin, muscles, and intestines resulting in increased central blood volume.
Largely mediated by increased sympathetic nerve activity, the net effect is an increase in
cardiac work, cardiac muscle oxygen consumption, and coronary blood flow. After four to
eight days at altitude most of these changes diminish, and with prolonged exposure, cardi-
ac output, coronary blood flow, and eventually blood pressure may fall to below sea-level
values. An elevated pulmonary blood pressure, however, persists.