High altitude medicine: Difference between revisions
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== Altitude Stages == | == 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 | |||
== Physiology of Acclimatization == | == Physiology of Acclimatization == | ||
=== Ventilation === | === 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 | |||
=== Blood === | === 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 === | === 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 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 | |||
==Differential Diagnosis== | ==Differential Diagnosis== | ||
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== High Altitude Syndromes == | == 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 | |||
==See Also== | ==See Also== | ||
Revision as of 04:29, 5 March 2015
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
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
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 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
See Also
Source
- Tintinalli