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High altitude medicine
From WikEM
(Redirected from High Altitude Medicine)
Contents
Background
Altitude Stages
Stage | Altitude | Physiology |
Intermediate Altitude | 5000-8000ft |
|
High Altitude | 8000-12,000ft |
|
Very High Altitude | 12,000-18,000ft |
|
Extreme Altitude | >18,000ft |
|
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 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 with out 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
- 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 1000 additional ft/day
- All respond to O2/descent