High altitude medicine
Physiology of Acclimatization
Ventilation
- Increased elevation -> decreased partial pressure of O2 -> decreased PaO2
- Hypoxic ventilatory response results in incr 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 4-7d 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
- As renal excretion of bicarb compensates for resp alkalosis, pH returns toward normal
Blood
- Erythropoietin level begins to rise within 2d 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 pulm HTN varies; a hyperreactive response is associated with HAPE
Altitude Stages
- 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
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
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)
- High UV exposure can lead to corneal burns w/in 1hr
- May also see with arc welders, tanning beds
- Symptoms develop after delay of up to 6-12hr
- Ocular pain, foreign-body sensation, photophobia, tearing, conj erythema, chemosis
- Generally is self-limited and heals within 24-36hr
Source
Tintinalli