High altitude medicine

Physiology of Acclimatization

Ventilation

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

Blood

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

Fluid Balance

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

Cardiovascular System

1. SV decreases initially while HR increases to maintain CO
2. Cardiac muscle in healthy pts can withstand extreme hypoxemia w/o ischemic events
3. Pulmonary circulation constricts w/ exposure to hypoxia
   1. Degree of pulm HTN varies; a hyperreactive response is associated with HAPE

Altitude Stages

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

High Altitude Syndromes

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

DDX

• Acute Mountain Sickness (AMS)
• High Altitude Cerebral Edema (HACE)
• High Altitude Pulmonary Edema (HAPE)
• High Altitude Peripheral Edema
• High Altitude Retinopathy
• High Altitude Pharyngitis and Bronchitis
• Chronic Mountain Sickness
• Ultraviolet Keratitis (Snow Blindness)

Source

Tintinalli