High altitude medicine

Background

Relationship between total atmospheric pressure and altitude above sea level.

Altitude Stages

Stage	Altitude	Physiology
Intermediate Altitude	5,000 - 8,000 ft (1,524 - 2,438 meters)	Decreased exercise performance without major impairment in SaO2
High Altitude	8,000 - 12,000 ft (2,438 - 3,658 meters)	Decreased SaO2 with marked impairment during exercise and sleep
Very High Altitude	12,000-18,000 ft (3,658 - 5,487 meters)	Abrupt ascent can be dangerous; acclimatization is required to prevent illness
Extreme Altitude	>18,000 ft (>5,500 meters)	Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia Sustained human habitation is impossible RV strain, intestinal malabsorption, impaired renal function, polycythemia

Height of Mount Everest (tallest in world): 29,035 feet (8,850 meters)

Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)

Conversion: 1 meter = ~3.28 feet (calculator)

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 respiratory alkalosis, pH returns toward normal
• 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 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 patients can withstand extreme hypoxemia without ischemic events
• Pulmonary circulation constricts with exposure to hypoxia
  • Degree of pulmonary hypertension 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

High altitude management algorithm.

• All caused by hypoxia
• All are seen in rapid ascent in unacclimatized patients
  • 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 1,000 additional ft/day
• All respond to O2/descent

Expected SpO₂ and PaO₂ levels at altitude

Altitude	SpO2	PaO2 (mm Hg)
1,500 to 3,500 m (4,900 to 11,500 ft)	about 90%	55-75
3,500 to 5,500 m (11,500 to 18,000 ft)	75-85%	40-60
5,500 to 8,850 m (18,000 to 29,000 ft)	58-75%	28-40

See Also

• Commercial in-flight medical emergencies