High altitude medicine: Difference between revisions
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== | ==Background== | ||
===Altitude Stages=== | |||
{| class="wikitable" | |||
| align="center" style="background:#f0f0f0;"|'''Stage''' | |||
| align="center" style="background:#f0f0f0;"|'''Altitude''' | |||
| align="center" style="background:#f0f0f0;"|'''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 [https://www.metric-conversions.org/length/meters-to-feet.htm (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 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 === | ===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 [[High altitude pulmonary edema|HAPE]] | |||
==Differential Diagnosis== | |||
{{High altitude DDX}} | |||
== Altitude | ==High Altitude Syndromes== | ||
[[File:Altitude flow sheet.png|thumb|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 at altitude}} | |||
== | ==See Also== | ||
*[[Commercial in-flight medical emergencies]] | |||
==References== | |||
<references/> | |||
[[Category:Environmental]] | |||
[[Category: |
Revision as of 16:57, 7 April 2020
Background
Altitude Stages
Stage | Altitude | Physiology |
Intermediate Altitude | 5,000 - 8,000 ft
|
|
High Altitude | 8,000 - 12,000 ft
|
|
Very High Altitude | 12,000-18,000 ft
|
|
Extreme Altitude | >18,000 ft
|
|
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 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
- 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.