High altitude medicine

Altitude Stages

Stage	Altitude	Physiology
Intermediate Altitude	5000-8000ft	Decreased exercise performance without major impairment in SaO2
High Altitude	8000-12,000ft	Decreased SaO2 with marked impairment during exercise and sleep
Very High Altitude	12,000-18,000ft	Abrupt ascent can be dangerous; acclimatization is required to prevent illness
Extreme Altitude	>18,000ft	Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia Sustained human habitation is impossible RV strain, intestinal malabsorption, impaired renal function, polycythemia

Physiology of Acclimatization

Ventilation

• Increased elevation → decreased partial pressure of O2 → decreased PaO2
  • Hypoxic ventilatory response results in ↑ ventilation to maintain PaO2
  • Vigor of this inborn response relates to successful acclimatization
• Initial hyperventilation is attenuated by respiratory alkalosis
  • As renal excretion of bicarb compensates for resp alkalosis, pH returns toward normal
    • At this point ventilation continues to increase
  • Process of maximizing ventilation culminates within 4-7 days at a given altitude
    • With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
    • Completeness of acclimatization can be gauged by partial pressure of arterial CO2
    • Acetazolamide, which results in bicarb diuresis, can facilitate this process

Blood

• Erythropoietin level begins to rise within 2 days of ascent to altitude
• Takes days to weeks to significantly increase red cell mass
  • This adaptation is not important for the initial initial acclimatization process

Fluid Balance

• Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
  • This leads to decreased ADH -> diuresis
  • This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
    • One of the hallmarks of AMS is antidiuresis

Cardiovascular System

• SV decreases initially while HR increases to maintain CO
• Cardiac muscle in healthy pts can withstand extreme hypoxemia w/o ischemic events
• Pulmonary circulation constricts w/ exposure to hypoxia
  • Degree of pulmonary HTN varies; a hyper-reactive response is associated with HAPE

Differential Diagnosis

High Altitude Illnesses

• Acute mountain sickness
• Chronic mountain sickness
• High altitude cerebral edema
• High altitude pulmonary edema
• High altitude peripheral edema
• High altitude retinopathy
• High altitude pharyngitis and bronchitis
• Ultraviolet keratitis

High Altitude Syndromes

• All caused by hypoxia
• All are seen in rapid ascent in unacclimatized pts
  • Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
  • Above 10,000ft rule of thumb is to sleep no higher than 1000 additional ft/day
• All respond to O2/descent

See Also

• Commercial in-flight medical emergencies