Background
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)
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 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 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[1]
| 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
References
- ↑ Gallagher, MD, Scott A.; Hackett, MD, Peter (August 28, 2018). "High altitude pulmonary edema". UpToDate. Retrieved May 2, 2019.
