Recently watched “Everest’. A nice movie, and it brings me to this topic.
Physiology of high altitude and human’s body adaptation (acclimatization):
- High altitude has low concentration of atmospheric oxygen. That’s what we know.
- Hypoxia leads to hypoxic stimulation of peripheral chemoreceptors (carotid bodies) which causes hyperventilation -> respiratory alkalosis.
– Hyperventilation is the most important physiologic adaptation to high altitude.
– Blood pH normalizes after 2-3 days due to renal excretion of bicarbonate.
- Hypoxia also causes polycythemia. BUT, this process takes several weeks. And in the context of acclimatization over a week or so, it does not play an important role.
What does hypoxia do to a climber?
- Increased fatigue
- Sleep disorders probably due to periodic breathing
- Mental performance also is impaired.
Misconceptions about acclimatization – it restores our body to normal sea level condition. No, it does not:
- It merely prepares our body to hypoxic condition in high altitude.
- Acclimatization by hyperventilation increases our PaO2 to merely around 55 mmHg after a few days.
- In comparison, this is the level of PaO2 patients with severe COPD are indicated to LTOT.
Although there is difference between hypoxia in healthy persons at high altitude and COPD, note that oxygen therapy in COPD (raising PaO2 to 60-80 mmHg) results in significant improvement in neuropsychological condition.
- Therefore, in pure hypoxic condition in healthy persons at high altitude, oxygen enrichment in room air is probably better than acclimatization to alleviate hypoxemia.
3 main high altitude illnesses:
- Acute mountain sickness
- High attitude pulmonary edema (HAPE)
- High altitude cerebral edema (HACE)
Acute mountain sickness
- Headache, imsomnia, N&V
- Probably due to hypoxia
- The most effective management is slow ascent to allow acclimatization.
– A rule fo thumb for climber is above 3000m, each day’s ascent should not > 300 m every 3 days.
- If rapid ascent is inevitable, use medical prophylaxis:
– Acetazolamide: produces metabolic acidosis from renal secretion of bicarbonate -> stimulate hyperventilation
– Dose : 250 mg od/bd
– Side effects: diuresis, risk of hypersensitivity (sulfa drug)
– Dexamethasone also can be used, but mechanism unknown (2 mg q6 hours).
High altitude pulmonary edema
- Dry cough, then becomes cough with frothy blood stained sputum, dyspnea
- Likely due to hypoxic pulmonary vasoconstriction causing damage to capillary wall -> leakage of protein rick edema fluid.
- RF: rapid ascent, strenuous exercise (both increases risk of hypoxia)
- Management: descent ASAP + pulmonary vasodilator (nifedipine, nitric oxide)
- If patient does not develop HAPE after 4-5 days, likely HAPE won’t happen. Probably due to vascular remodelling induced by alveolar hypoxia.
- Reascent HAPE can happen, probably because vascular remodelling undergoes involution during the time spent in low altitude.
High altitude cerebral edema
- Headache, AMS, hallucination
- Due to hypoxic cerebral vasodilation
- Management: rapid descent + dexamethasone.
Other high altitude sicknesses
- Chronic mountain sickness: probably due to viscosity of the blood due to severe polycythemia -> causing neurological symptoms (headache, fatigue, imsomnia)
– Therapeutic phlebotomy has been shown to reduce symptoms.
- Retinal hemorrhage
– Is common, asymptomatic and resolves once descent to lower altitude.
The Physiologic Basis of High Altitude Diseases
John B.West. MD, PhD
Ann Intern Med 2004;141:789-800