High altitude medicine: Difference between revisions
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== Physiology of Acclimatization ==
==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
|}


=== Ventilation ===
Height of Mount Everest (tallest in world): 29,035 feet (8,850 meters)


#Increased elevation -> decreased partial pressure of O2 -> decreased PaO2
Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)
##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 ===
Conversion: 1 meter = ~3.28 feet  [https://www.metric-conversions.org/length/meters-to-feet.htm (calculator)]


#Erythropoietin level begins to rise within 2d of ascent to altitude
==Physiology of Acclimatization==
#Takes days to weeks to significantly increase red cell mass
===Ventilation===
##This adaptation is not important for the initial initial acclimatization process
*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


=== Fluid Balance ===
===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


#Peripheral venoconstriction on ascent to altitude causes increase in central blood volume  
===Fluid Balance===
##This leads to decreased ADH -> diuresis  
*Peripheral venoconstriction on ascent to altitude causes increase in central blood volume  
##This diuresis, along with bicarb diuresis, is considered a healthy response to altitude  
**This leads to decreased ADH → diuresis  
###One of the hallmarks of AMS is antidiuresis
**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]]


#SV decreases initially while HR increases to maintain CO
==Differential Diagnosis==
#Cardiac muscle in healthy pts can withstand extreme hypoxemia w/o ischemic events
{{High altitude DDX}}
#Pulmonary circulation constricts w/ exposure to hypoxia
##Degree of pulm HTN varies; a hyperreactive response is associated with HAPE


== Altitude Stages ==
==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


#Intermediate Altitude (5000-8000ft)
{{Expected SpO2 at altitude}}
##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 ==
==See Also==
*[[Commercial in-flight medical emergencies]]


#All caused by hypoxia
==References==
#All are seen in rapid ascent in unacclimatized pts
<references/>
##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


=== Acute Mountain Sickness (AMS) ===
[[Category:Environmental]]
==== Background ====
#Usually only occurs with altitude &gt;7000-8000ft
##May occur at lower altitudes in pts who are particularly susceptible (COPD, CHF)
#Associated w/ rate of ascent, sleeping altitude, strength of hypoxic vent response
##NOT associated with physical fitness, age, sex
#Pts tend to have recurrence of symptoms whenever they return to the symptomatic altitude
 
==== Clinical Features ====
#Onset
##Symptoms usually develop 1-6hr after arrival at elevation
###May be delayed for 1-2d
##Especially common after the 1st or 2nd night's sleep
#Duration
##Average duration of symptoms at 10,000ft = 15hr
##At higher elevations symptoms may last weeks / more likely to progress to HACE
#Diagnosis
##Requires HA + 1 or more of the following:
###Nausea, vomiting, or anorexia
###Fatigue or weakness
###Dizzy or lightheadedness
###Difficulty sleeping
#Ataxia and confusion heralds onset of HACE
 
==== DDX ====
#Dehydration
##AMS is not improved by fluid administration alone
##Body hydration does not influence susceptibility to AMS
#Exhaustion
#Alcohol hangover
#Hypothermia
#CO poisoning
#CNS infection
#Migraine
##Whereas supplemental O2 helps HA due to AMS in 10-15min, O2 has no effect on migraines
#TIA
 
==== Treatment ====
#Mild AMS
##Terminate ascent
###Descend to lower altitude (by 1000-3000ft) OR acclimatize for 12-36hr at same altitude
##Acetazolamide
###Mechanism: speeds acclimatization by promoting bicarb diuresis
###Indications:
####History of altitude illness
####Abrupt ascent to >9800ft
####AMS requiring treatment
####Bothersome periodic breathing during sleep
###125-250mg PO BID until symptoms resolve
###Side-effects
####Allergic reaction (if pt allergic to sulfa), paresthesias, polyuria
##Symptomatic treatment as necessary w/ analgesics and antiemetics
##Sleep-agents
###Benzos are only safe if given in conjunction with acetazolamide
###Nonbenzos are safe (zolpidem, diphenhydramine)
#Moderate-Severe AMS
##Immediate descent for worsening symptoms
##Low-flow 0.5-1 L/min O2 if available (esp nocturnal administration)
##Acetazolamide 250mg PO BID
##Dexamethasone 4mg PO q6hr
###Symptom-improvement only; unlike acetazolamide does not aid acclimatization
##Hyperbaric therapy
 
==== Prevention ====
#Graded ascent w/ adequate time for acclimatization is the best prevention
#Acetazolamide prophylaxis
##Indicated for pts w/ history of altitude illness or forced rapid ascent to altitude
##Start 24hr before ascent and continue for the first 2d at altitude
##Can be restarted if illness develops
##Reduces symptoms of AMS by 75% in pts ascending rapidly to altitudes &gt;8200ft
#Dexamethasone
##Start day of ascent and continue for first 2d at altitude
##4mg PO q12hr
##Prevents and treats cerebral edema
#Ginkgo biloba
##Controversial if effective; safe
 
=== High Altitude Cerebral Edema (HACE) ===
 
==== Background ====
 
#Progressive neurologic deterioration in someone with AMS or HAPE (due to incr ICP)
#Almost never occurs at &lt;8000ft
 
==== Clinical Features ====
 
#Altered mental status, ataxia, stupor
##Progresses to coma if untreated
#Headache, nausea, and vomiting are not always present
#Focal neuro deficits may be seen (3rd/6th CN palsies)
 
==== Treatment ====
 
#Immediate descent is the treatment of choice
#If descent not possible use combination of:
##Supplemental O2
##Dexamethasone 8mg initially, then 4mg q6hr
##Hyperbaric bag if available
 
=== High Altitude Pulmonary Edema (HAPE) ===
 
==== Background ====
#Noncardiogenic pulm edema d/t increased microvascular pressure in the pulm circulation
#Most lethal of the altitude illnesses
#Occurs in <1/10,000 skiers in Colorado; 2-3% of Mt. McKinley climbers
#Typical pt is strong and fit; may not have symptoms of AMS before onset of HAPE
#Most commonly noticed on the second night at a new altitude
#Risk Factors:
##Heavy exertion
##Rapid ascent
##Cold
##Excessive salt ingestion
##Use of a sleeping medication
##Preexisting pulmonary HTN
##Preexisting respiratory infection (children)
##Previous history of HAPE
 
==== Clinical Features ====
#Early
##Dry cough, decreased exercise performance, dyspnea on exertion, localized rales
##Resting SaO2 is low for the altitude and drops markedly w/ exertion (aids in the dx)
#Late
##Dyspnea at rest, marked weakness, productive cough, cyanosis, generalized rales
##Tachycardia and tachypnea correlate with the severity of illness
##Altered mental status and coma (from severe hypoxemia)
#ECG
##Right strain pattern
#CXR
##Progresses from interstitial to localized-alveolar to generalized-alveolar infiltrates
 
==== Treatment ====
#Immediate descent is treatment of choice
##While pt is descending attempt to limit exertion as much as possible
#If cannot descend use combination of:
##Supplemental O2
###Can completely resolve the pulmonary edema within 36-72hr
##Hyperbaric bag
##Keep pt warm (cold stress elevates pulm artery pressure)
##Use expiratory positive airway pressure mask
##Consider the medications listed below that are usually used for prevention
 
==== Disposition ====
#Admission
##Warranted for severe illness that does not respond immediately to descent
#Discharge
##Progressive clinical and X-ray improvement and a PaO2 of 60mmHg or SaO2>90%
 
==== Prevention ====
#Nifedipine 20mg q8hr while ascending is effective prophylaxis in pts who had HAPE before
#Tadalafil 10mg BID 24hr prior to ascent
#Salmeterol inhaled BID
 
=== High Altitude Peripheral Edema ===
 
#Swelling of face and distal extremities is common (20% of trekkers at 14,000ft)
#Often associated with AMS but not in all cases
#Resolves spontaneously with descent
 
=== High Altitude Retinopathy ===
 
#Retinal hemorrhages are common at sleeping altitudes &gt;16,000ft
##Not considered an indication for descent unless vision changes are present
 
=== High Altitude Pharyngitis and Bronchitis ===
 
#Dry, hacking cough is common at &gt;8000ft
#Purulent bronchitis/painful pharyngitis common w/ prolonged periods at extreme altitude
#Severe coughing spasms can result in cough fx of ribs
#Treatment
##Alubterol
##Breathing steam, sucking on hard candies, forcing hydration
##Abx are not helpful
 
=== Chronic Mountain Sickness ===
 
#Excessive polycythemia for a given altitude (Hb &gt;20
##Occurs in pts living at high-altitude who have COPD, sleep apnea or impaired resp drive
#Head ache, difficulty thinking, impaired peripheral circulation, drowsiness
#Treatment
##Phlebotomy
##Relocation to lower altitude
##Home O2 use
 
=== 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
 
[[Category:Environ]]

Revision as of 16:57, 7 April 2020

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)

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

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

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