THE RESPIRATORY SYSTEM

Effects of High Altitude

You and some friends decide to journey to the top of a mountain that is 16,000 feet above sea level. Assume

that you reside, and start your trip, at sea level, and that it takes 2 days to reach the summit. Describe how

each of the following parameters will change during your journey, and explain each change.

1. FIO2

2. Barometric pressure

3. PaO2

4. PaCO2

5. P50

6. Pulmonary artery pressure

Near the summit, at 14,000 feet, you meet villagers who have lived at this altitude all their lives. Compare

the following parameters in the mountain dwellers to people who live near sea level:

7. Hemoglobin content

8. Hypoxic ventilatory response

9. Hypercapnic ventilatory response

At the summit, one member of the climbing party becomes very short of breath and complains of severe

headache.

10. Assume you have only a basic first aid kit, which does not contain any supply of oxygen. What is

the treatment of choice?

ANSWERS

1. It is a common misconception that FIO2 falls with altitude. In fact, the FIO2 is the same throughout the

atmosphere (0.21).

2. Barometric pressure falls with altitude. Barometric pressure reflects the weight of the atmosphere; the

less atmosphere above a certain point, the lower the barometric pressure at that point. At sea level, the

barometric pressure averages 760 mm Hg. An altitude of 16,000 feet is above almost half of the earth's

atmosphere, and barometric pressure is about 420 mm Hg. (The halfway point for barometric pressure,

380 mm Hg, is at about 18,000 feet altitude. On the summit of Mt. Everest, the world's highest point at

29,028 feet, barometric pressure is only 253 mm Hg.)

3. PaO2 falls because PAO2 falls. The fall in PaO2 is predicted by the alveolar gas equation.

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4. PaCO2 falls because hypoxic stimulation of the peripheral chemoreceptors (mainly the carotid bodies)

stimulates breathing and raises the alveolar ventilation above the level of CO2 production. Although

hyperventilation effectively raises PAO2 and PaO2 (approximately 1 mm Hg increase in PaO2 for every

mm Hg fall in PaCO2), the increase is not enough to overcome the fall in PAO2 that results from

decreased barometric pressure. Consequently, PaO2 will always fall as one ascends in altitude

(assuming ambient air is breathed and one is not in a pressurized cabin).

5. P50 rises, reflecting the shift of the oxygen equilibrium curve to the right. This occurs because of a rise in

2,3-diphosphoglycerate. Although a right shift of the curve modestly decreases uptake of oxygen in the

pulmonary capillaries, it unloads a greater amount of oxygen in the systemic capillaries; the net effect is

enhanced oxygen transport to the tissues.

6. Alveolar hypoxia constricts pulmonary arterioles and small muscular arteries, thereby increasing

pulmonary vascular resistance and pulmonary artery pressure. The pressure increase would be modest

and reversible on descent to sea level.

7. Hemoglobin content is higher in mountain dwellers, because of continued hypoxic stimulation of bone

marrow by erythropoietin.

8. The ventilatory response to hypoxia is less than that of sea level dwellers, also because of continued

exposure to low PO2 conditions.

9. The ventilatory response to hypercapnia is unaffected by altitude.

10. The climber has developed acute mountain sickness, manifested by severe headache and high-altitude

pulmonary edema. The cause is hypoxia, and the treatment is to increase his blood oxygen content. He

should immediately receive 100% oxygen, if available. If it is not available, he must be taken to a lower

altitude. This is an emergency situation and mandates immediate descent (even if he has to be carried). He

might improve dramatically after descending only a few thousand feet.

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