High altitude medicine: Difference between revisions
 
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==Physiology of Altitude Acclimatization==
==Background==
===Ventilation===
[[File:Altitude and air pressure & Everest.jpg|thumb|Relationship between total atmospheric pressure and altitude above sea level.]]
#Increased elevation -> decreased partial pressure of O2 -> decreased PaO2
===Altitude Stages===
##Hypoxic ventilatory response results in incr ventilation to maintain PaO2
{| class="wikitable"
##Vigor of this inborn response relates to successful acclimatization
| align="center" style="background:#f0f0f0;"|'''Stage'''
#Initial hyperventilation is attenuated by respiratory alkalosis
| align="center" style="background:#f0f0f0;"|'''Altitude'''
##As renal excretion of bicarb compensates for resp alkalosis, pH returns toward normal
| align="center" style="background:#f0f0f0;"|'''Physiology'''
###At this point ventilation continues to increase
|-
##Process of maximizing ventilation culminates 4-7d at a given altitude
| Intermediate Altitude ||5,000 - 8,000 ft
###With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
:(1,524 - 2,438 meters)
###Completeness of acclimatization can be gauged by partial pressure of arterial CO2
||
###Acetazolamide, which results in bicarb diuresis, can facilitate this process
*Decreased exercise performance without major impairment in SaO2
===Blood===
|-
#Erythropoietin level begins to rise within 2d of ascent to altitude
| High Altitude||8,000 - 12,000 ft
#Takes days to weeks to significantly increase red cell mass
:(2,438 - 3,658 meters)
##This adaptation is not important for the initial initial acclimatization process
||
===Fluid Balance===
*Decreased SaO2 with marked impairment during exercise and sleep
#Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
|-
##This leads to decreased ADH -> diuresis
| Very High Altitude ||12,000-18,000 ft
##This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
:(3,658 - 5,487 meters)
###One of the hallmarks of AMS is antidiuresis
||
*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
|}


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


==Altitude Stages==
Height of Mount Whitney (tallest in contiguous US): 14,505 feet (4,421 meters)
#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
#Intermediate Altitude (5000-8000ft)
##Decreased exercise performance without major impairment in SaO2
#High Altitude (8000-14,000ft)
##Decreased SaO2 with marked impairment during exercise and sleep
#Very High Altitude (14,000-18,000ft)
##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==
Conversion: 1 meter = ~3.28 feet  [https://www.metric-conversions.org/length/meters-to-feet.htm (calculator)]
#All caused by hypoxia
#All are seen in rapid ascent in unacclimatized pts
#All respond to O2/descent


===Acute Mountain Sickness (AMS)===
==Physiology of Acclimatization==
====Background====
===Ventilation===
#Usually only occurs with altitude >7000-8000ft
*Increased elevation → decreased partial pressure of O2 → decreased PaO2
##May occur at lower altitudes in pts who are particularly susceptible (COPD, CHF)
**Hypoxic ventilatory response results in ↑ ventilation to maintain PaO2
#Associated w/ rate of ascent, sleeping altitude, strength of hypoxic vent response
**Vigor of this inborn response relates to successful acclimatization
##NOT associated with physical fitness, age, sex
*Initial hyperventilation is attenuated by respiratory alkalosis
#Pts tend to have recurrence of sx whenever they return to the symptomatic altitude
**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


====Clinical Features====
===Blood===
#Symptoms usually develop 1-6hr after arrival at elevation
*Erythropoietin level begins to rise within 2 days of ascent to altitude
##May be delayed for 1-2d (esp after a night's sleep)
*Takes days to weeks to significantly increase red cell mass
#Average duration of symptoms at 10,000ft = 15hr
**This adaptation is not important for the initial acclimatization process
##At higher elevations symptoms may last weeks / more likely to progress to HACE
#Diagnosis requires headache + one or more of the following:
##Nausea/vomiting
##Fatigue/weakness
##Dizzy/light-headedness
##Difficulty sleeping
#Onset of ataxia and ALOC heralds onset of HACE
#Fluid retention with facial/peripheral edema is physical hallmark of AMS


====DDX====
===Fluid Balance===
#Hypothermia
*Peripheral venoconstriction on ascent to altitude causes increase in central blood volume
#CO poisoning
**This leads to decreased ADH → diuresis
#Pulmonary or CNS infection
**This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
#Dehydration
***One of the hallmarks of AMS is antidiuresis
#Migraine
##Whereas supp O2 dissipates HA due to AMS in 10-15min, O2 has no effect on migraines
#Exhaustion


====Treatment====
===Cardiovascular System===
#Mild AMS
*SV decreases initially while HR increases to maintain CO
##Terminate ascent
*Cardiac muscle in healthy patients can withstand extreme hypoxemia without ischemic events
###Descend to lower altitude (by 1000-3000ft) OR
*Pulmonary circulation constricts with exposure to hypoxia
###Acclimatize for additional 12-36hr at same altitude
**Degree of pulmonary hypertension varies; a hyper-reactive response is associated with [[High altitude pulmonary edema|HAPE]]
##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 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====
==Differential Diagnosis==
#Graded ascent w/ adequate time for acclimatization is the best prevention
{{High altitude DDX}}
#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 >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)===
==High Altitude Syndromes==
====Background====
[[File:Altitude flow sheet.png|thumb|High altitude management algorithm.]]
#Progressive neurologic deterioration in someone with AMS or HAPE (due to incr ICP)
*All caused by hypoxia
#Almost never occurs at <8000ft
*All are seen in rapid ascent in unacclimatized patients
 
**Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
====Clinical Features====
**Above 10,000ft rule of thumb is to sleep no higher than 1,000 additional ft/day
#Altered mental status, ataxia, stupor
*All respond to O2/descent
##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 cannot descend use combination of:
##Supplemental O2
##Dexamethasone 8mg initially, then 4mg q6hr
##Hyperbaric bag if available
 
===High Altitude Pulmonary Edema (HAPE)===
====Background====
#Noncardiogenic edema 2/2 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
#Pts who live at high altitude, leave high altitude for few weeks and return are at risk
#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====
#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
#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 the 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 >16,000ft
##Not considered an indication for descent unless vision changes are present
 
===High Altitude Pharyngitis and Bronchitis===
#Dry, hacking cough is common at >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===
{{Expected SpO2 at altitude}}
#Excessive polycythemia for a given altitude (Hb >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)===
==See Also==
#High UV exposure can lead to corneal burns w/in 1hr
*[[Commercial in-flight medical emergencies]]
#Ocular pain, foreign-body sensation, photophobia, tearing, conj erythema, chemosis
#Generally is self-limited and heals within 24hr


==Source==
==References==
Tintinalli
<references/>


[[Category:Environ]]
[[Category:Environmental]]

Latest revision as of 21:32, 1 May 2024

Background

Relationship between total atmospheric pressure and altitude above sea level.

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 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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