High altitude medicine

Physiology of Altitude Acclimatization

Ventilation

1. Increased elevation -> decreased partial pressure of O2 -> decreased PaO2
   1. Hypoxic ventilatory response results in incr ventilation to maintain PaO2
   2. Vigor of this inborn response relates to successful acclimatization
2. Initial hyperventilation is attenuated by respiratory alkalosis
   1. As renal excretion of bicarb compensates for resp alkalosis, pH returns toward normal
      1. At this point ventilation continues to increase
   2. Process of maximizing ventilation culminates 4-7d at a given altitude
      1. With continuing ascent the central chemoreceptors reset to ever lower values of PaCO2
      2. Completeness of acclimatization can be gauged by partial pressure of arterial CO2
      3. Acetazolamide, which results in bicarb diuresis, can facilitate this process

Blood

1. Erythropoietin level begins to rise within 2d of ascent to altitude
2. Takes days to weeks to significantly increase red cell mass
   1. This adaptation is not important for the initial initial acclimatization process

Fluid Balance

1. Peripheral venoconstriction on ascent to altitude causes increase in central blood vol
   1. This leads to decreased ADH -> diuresis
   2. This diuresis, along with bicarb diuresis, is considered a healthy response to altitude
      1. One of the hallmarks of AMS is antidiuresis

Cardiovascular System

1. SV decreases initially while HR increases to maintain CO
2. Cardiac muscle in healthy pts can withstand extreme hypoxemia w/o ischemic events
3. Pulmonary circulation constricts w/ exposure to hypoxia
   1. Degree of pulm HTN varies and a hyperreactive resopnse is a/w HAPE

Altitude Stages

1. Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
2. Intermediate Altitude (5000-8000ft)
   1. Decreased exercise performance without major impairment in SaO2
3. High Altitude (8000-14,000ft)
   1. Decreased SaO2 with marked impairment during exercise and sleep
4. Very High Altitude (14,000-18,000ft)
   1. Abrupt ascent can be dangerous; acclimatization is required to prevent illness
5. Extreme Altitude (>18,000ft)
   1. Only experienced by mountain climbers; accompanied by severe hypoxemia and hypocapnia
   2. Sustained human habitation is impossible
      1. RV strain, intestinal malabsorption, impaired renal function, polycythemia

High Altitude Syndromes

1. All caused by hypoxia, seen in rapid ascent in unacclimatized pts, respond to O2/descent

Acute Mountain Sickness (AMS)

Background

1. Usually only occurs with altitude >7000-8000ft
   1. May occur at lower altitudes in pts who are particularly susceptible (COPD, CHF)
2. Associated w/ rate of ascent, sleeping altitude, strength of hypoxic vent response
   1. NOT associated with physical fitness, age, sex
3. Pts tend to have recurrence of sx whenever they return to the symptomatic altitude

Clinical Features

1. Symptoms usually develop 1-6hr after arrival at elevation
   1. May be delayed for 1-2d (esp after a night's sleep)
2. Average duration of symptoms at 10,000ft = 15hr
   1. At higher elevations symptoms may last weeks / more likely to progress to HACE
3. Diagnosis requires headache + one or more of the following:
   1. Nausea/vomiting
   2. Fatigue/weakness
   3. Dizzy/light-headedness
   4. Difficulty sleeping
4. Onset of ataxia and ALOC heralds onset of HACE
5. Fluid retention with facial/peripheral edema is physical hallmark of AMS

DDX

1. Hypothermia
2. CO poisoning
3. Pulmonary or CNS infection
4. Dehydration
5. Migraine
   1. Whereas supp O2 dissipates HA due to AMS in 10-15min, O2 has no effect on migraines
6. Exhaustion

Treatment

1. Mild AMS
   1. Terminate ascent
      1. Either descend to lower altitude (by 1000-3000ft) or acclimatize at same altitude
   2. Acetazolamide
      1. Mechanism: speeds acclimatization by promoting bicarb diuresis
      2. Indications:
         1. History of altitude illness
         2. Abrupt ascent to >9800ft
         3. AMS requiring treatment
         4. Bothersome periodic breathing during sleep
      3. 125-250mg PO BID until symptoms resolve
      4. Side-effects
         1. Allergic reaction (if pt allergic to sulfa), paresthesias, polyuria
   3. Symptomatic treatment as necessary w/ analgesics and antiemetics
   4. Sleep-agents
      1. Benzos are only safe if given in conjunction with acetazolamide
      2. Nonbenzos are safe (zolpidem, diphenhydramine)
2. Moderate AMS
   1. Immediate descent for worsening symptoms
   2. Low-flow 0.5-1 L/min O2 if available (esp nocturnal administration)
   3. Acetazolamide 250mg PO BID
   4. Dexamethasone 4mg PO q6hr
      1. Symptom-improvement only; unlike acetazolamide does not aid acclimatization
   5. Hyperbaric therapy

Prevention

1. Graded ascent w/ adequate time for acclimatization is the best prevention
2. Acetazolamide prophylaxis
   1. Indicated for pts w/ history of altitude illness or forced rapid ascent to altitude
   2. Start 24hr before ascent and continue for the first 2d at altitude
   3. Can be restarted if illness develops
   4. Reduces symptoms of AMS by 75% in pts ascending rapidly to altitudes >8200ft
3. Dexamethasone
   1. Start day of ascent and continue for first 2d at altitude
   2. 4mg PO q12hr
   3. Prevents and treats cerebral edema
4. Ginkgo biloba
   1. Controversial if effective; safe

High Altitude Pulmonary Edema (HAPE)

1. definition: two symptoms: dyspnea at rest, cough, weakness, chest tightness or congestion.
2. And
3. two signs: central cyanosis, crackles or wheezes, tachypnea, tachycardia.
4. most common medical cause of altitude related death.
5. >2500m, young males, usually second night of altitude or after 3- 4 days ascent.
6. recent URI predisposes
7. highest risk in mountain dweller who descends to sea level and then reascends- possibly due to pulm art muscle remodeling.
8. is noncardiogenic pulmonary edema with pulm hypertension and inflammation of capillaries and transepithelial water and sodium transport. Caused by combination of both pulm hypertension and increased cap permeability.
9. Nitric oxide (NO) inhalation decreases pulm art pressures and can improve oxygenation. Dz possibly due to NO deficiency?

Prevention

1. limit exercise for first 1- 2 days. Also limit ascent when over 2500m to 300- 350m/day.
2. Nifedipine 20mg TID or 30- 6- mg extended release qd- prevents HAPE but not pulm edema of exercise of AMS or HACE.

Treatment

1. descend, oxygen, nifedipine 10 mg po, CPAP mask, diuretics, GAMOW bag.
2. Can reascend in 2- 3days in needed but at increased risk for reoccurence.

High Altitude Cerebral Edema (HACE)

1. Acute Mountain Sickness plus altered mental status or ataxia. Of if mountain sickness not present, is ataxia with mental status changes.
2. occurs >4000m
3. due to increased brain water, not just volume. Get increased intracranial pressure.
4. initially get vasogenic edema- fluid and protein crosses BBB, Get reversible changes in white matter, especially corpus callosum.. Later get cytotoxic edema by toxins and ischemia. Mostly of gray matter and has poorer px.
5. Theories: angiogenesis model- hypoxemia causes macrophages to release cytokines and vascular endothelium growth factor. Basement membranes of capillaries are dissolved causing leaks and petechial hemorrhages. Inhibited by dexamethasone.
6. Other theory is due to unexpandable cranial vault. As brain volume increases buffering ability of CSF overcome and brain swells in closed nonexpanding space.
7. Prevent as with AMS

Treatment

1. descend, oxygen, dex 4- 8mg IV, then 4mg q6hr. If GAMOW bag available- 4-8 hr recompression may allow pt to walk down mountain (big help).

Source

Tintinalli