Living in thin air [Archives:2009/1226/Health]

January 19 2009

Salma Ismail
High-altitude sickness, also known as mountain sickness, can cause some people to get sick at high altitudes. Sitting at almost 2300 meters above sea level, Sana'a is definitely at a high altitude. The highest altitude in Yemen is Jabal an-Nabi Shu'ayb, which is located in Sana'a Governorate; this mountain is 3,760 meters above the sea. Regions which are around 1500 meters above sea level are generally considered to be at high altitude.

There are no specific factors such as age, gender, or physical condition that correlates with susceptibility to altitude sickness. Some people get it and some people don't, and some people are more susceptible than others.

Less oxygen in the lungs

As altitude increases, the air becomes “thinner,” which means there is less oxygen in the atmosphere. With each breath you take, you get less oxygen in your lungs, and the amount of oxygen in your blood declines. This is also known as hypoxia. Symptoms of hypoxia include headaches, vomiting, sleeplessness, impaired thinking, and an inability to sustain long periods of physical activity. All people can experience mountain sickness, but it may be more severe in people who have heart or lung problems. The atmospheres at high altitudes have a drastically different composition and temperature than the atmosphere at sea level. These differences can affect humans.

Why less oxygen?

At high altitude, atmospheric pressure is lower as compared to the pressure at sea level. This is due to two competing physical effects: gravity, which causes the air to be as close as possible to the ground, and the heat content of the air, which causes the molecules to bounce off each other and expand.

Physiological response to less oxygen

When we travel to high altitudes, our bodies initially develop inefficient physiological responses. There is an increase in breathing and heart rate to as much as double the natural heart rate, even while resting. Pulse rate and blood pressure go up sharply as our hearts pump harder to get more oxygen to the cells. These are stressful changes, especially for people with weak hearts.

At high altitude your blood thickens and your blood pressure rises. This may increase your risk of a stroke because it can cause hypertension or high blood pressure.

Adapting to less oxygen

Acclimatization or adjustment to high altitude is complex and varies greatly between individuals and in different trips for the same individual.

In general, the body becomes approximately 80 percent acclimatized after 10 days at a higher altitude, and approximately 95 percent acclimatized by six weeks. The respiratory rate peaks in about one week and then slowly decreases over the next few months, although it tends to remain higher than its normal rate at sea level. After 10 days, the heart rate starts to decrease.

When we descend, we begin losing our hard-won adaptations at approximately the same rate at which we gained them; 10 days after returning to sea level, we have lost 80 percent of our adaptations.

At a later stage, a more efficient response normally develops as acclimatization takes place. Initially, after arriving at a higher altitude, the rate of breathing will increase to deliver more oxygen to the body as more red blood cells and capillaries are produced to carry more oxygen. The lungs increase in size to facilitate the osmosis of oxygen and carbon dioxide. There is also an increase in the vascular network of muscles which enhances the transfer of gases.

The heart rate also increases in order to help deliver more oxygen to the body. Fluids redistribute; more blood flows to the brain to provide oxygen, and pulmonary blood pressure increases. If blood pressure gets too high, pulmonary edema, which is the swelling or fluid accumulation in the lungs, can result.

On returning to sea level after successful acclimatization to high altitude, the body usually has more red blood cells and greater lung expansion capability than needed. Since this provides athletes in endurance sports with a competitive advantage, many nations train their athletes at higher altitudes. However, the physiological changes that result in increased fitness are short term at low altitude. In a matter of weeks, the body returns to a normal fitness level.

Life threatening conditions at high altitudes

There is no precise definition of high altitude. However, many people feel lightheaded and have other symptoms if they ascend from near sea level to 3000 meters. Some individuals are affected at as low as 2000 meters. Nearly 140 million people worldwide live at altitudes above 2500 meters.

Serious altitude sickness generally develops at elevations higher than 2,400 meters above sea level or when the rate of ascent exceeds 300 meters per day. This problem usually happens to mountain climbers or skiers. The more serious life threatening conditions brought on from exposure to very high altitudes include:

– HACE – High Altitude Cerebral Edema, or fluid buildup in the brain. As the brain swells with fluid, the person's mental state changes. Loss of coordination, coma, and death can result unless the problem is recognized and treated immediately.

– HAPE – Altitude Pulmonary Edema, or high blood pressure in the lungs. This illness occurs when fluid builds up within the lungs, a condition that can make breathing extremely difficult. Usually this happens after the second night spent at a high altitude, but it can also happen earlier or later. HAPE often comes on quickly. If left untreated, it can progress to respiratory collapse and ultimately lead to death. It is the number one cause of death from altitude sickness.

– HARH – High Altitude Retinal Hemorrhage, or small areas of bleeding in the back of the eye.

Coping with high altitude

The simplest cure for altitude sickness is to return to sea level conditions. A person suffering from altitude sickness should drink large amounts of water to rehydrate their blood and reduce the concentration of red blood cells. If possible, individuals should avoid ascending too rapidly, overexertion within 24 hours of ascent, inadequate fluid intake, and the consumption of alcohol or other sedatives.