Wilderness and Rescue Medicine 8th Edition

Chapter 19: Altitude Illness

As you climb in elevation the atmosphere becomes less dense which decreases available oxygen, reduces water vapor, and allows greater ultraviolet penetration. It is the decrease in available oxy- gen that causes the most serious altitude-related symptoms.

Initially, the body compensates with mild hyperventilation and increased cardiac output. This is observed as an increased respiratory rate, increased pulse rate, and elevated blood pres- sure. This allows for a person to ascend, within limits, without a significant reduction in cellular oxygenation. One side effect of the body’s compensatory effort is respiratory alkalosis; a rise in blood pH due to blowing off too much carbon dioxide. Respiratory alkalosis produces some of the com- monly felt altitude symptoms as well as a peri- odic depression in respiratory drive that results in episodes of hypoxia and sleep apnea. Carbon dioxide is a waste product of cellular metabolism and is transported as a dissolved gas in the blood plasma. Getting rid of more of it might seem ben- eficial, but carbon dioxide has an important role in maintaining the acid/base balance in the blood, normally kept at a pH of 7.43. Under normal conditions, your brain monitors changes in pH as the primary method of control- ling respiratory effort. The brain responds to a rise in pH by reducing the rate and depth of respira- tion to retain more carbon dioxide. But at altitude, this response conflicts with the need to extract more oxygen from thinner air. The result is often a disturbance in the breathing pattern, particularly

General Principles

Altitude Illness

Mechanism: • Less available

5486 m (18000’) 50% of sea level O 2

oxygen to breathe • Reduced cellular oxygenation • Capillary leakage • Swelling

*

2438 m (8000’) 75% of sea level O 2

© 2018 WMA

As altitude increases normal oxygen satura- tion decreases. At sea level, a healthy respiratory system will fill the hemoglobin in the red blood cells with oxygen, yielding an oxygen saturation measurement of 98–100%. At an altitude of 3,000 meters, oxygen saturation in a healthy individual typically measures 90–96%. For most people from sea level, this represents mild hypoxia that can result in a noticeable decrease in performance and at least minimal symptoms of altitude illness.