DISORDERS CAUSED BY ALTITUDE - Medicine for Mountaineering and Other Wilderness Activities - page 338

Pulse Rate and Cardiac Output
During a climb the pulse rate rises with the workload but subsides during rest. The in-
crease and speed with which it returns to normal are a function of altitude and, to a certain
degree, an indicator of acclimatization. A slow resting pulse that increases little during
work and rapidly returns to resting level is a sign of physical fitness at sea level and also
indicates that anindividual isadjusting well toaltitude, buttheresting pulse increases with
increasingaltitudeandishigherataltitudethanatsealevel.Maximumheartratewithmax-
imum exercise decreases at high altitude ( Fig. 24-2 ) because the lower amount of oxygen
available results in lower maximum exercise capacity. The maximum achievable heart rate
at sea level decreases with age and is roughly 220 minus a person's age.
For a short while after reaching altitude, the volume of blood pumped by the heart (the
cardiac output) at any level of exercise is greater than normal. However, it very soon de-
creases to normal and remains normal at rest, or for a given level of exercise, throughout a
stay at high altitude.
A crude but useful indication of the load on the heart is the double product: the systolic
blood pressure (the top number in a blood pressure reading) multiplied by the pulse rate.
Obviously these increase with work and with altitude. The double product can estimate the
combined effect and can be helpful in deciding whether a person with heart disease can
safely go to altitude.
Blood Volume
Rapid ascent to high altitude is accompanied by a prompt decrease in blood volume be-
causefluidisexcretedintheurineduringnormalacclimatization. Acutely,fluidmaymove
out of the blood vessels into the tissues and cells, resulting in tissue swelling (edema). The
severity of fluid retention is related to risk of altitude illness. During acclimatization the
decrease in blood plasma volume is 5 to 10 percent of the sea level blood volume, which is
equivalent to removing about a pint of blood plasma. This loss of fluid causes an increase
in the concentration of red blood cells, although the actual number of circulating red cells
does not increase for many days. Inadequate fluid intake at altitude may further decrease
blood volume and result in a decreased maximal exercise capacity.
Sleep Hypoxia
During sleep at altitude, ventilation is often decreased and wide fluctuations in the res-
piratory rate—periodic breathing—may occur. Sometimes alarming periods (ten to twelve
seconds) of not breathing (apnea) are followed by a period during which the depth and rate
ofbreathingincrease.Oftenthedepthandraterapidlyincreasetoalevelgreatlyabovenor-
mal but then subside until apnea intervenes again.
The generally accepted explanation for periodic breathing is that the brain's respiratory
control centers become less sensitive during sleep so that respiration decreases or stops.