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. Descend to lower altitude (by 1000-3000ft) OR
      2. Acclimatize for additional 12-36hr 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 Cerebral Edema (HACE)

Background

1. Progressive neurologic deterioration in someone with AMS or HAPE (due to incr ICP)
2. Almost never occurs at <8000ft

Clinical Features

1. Altered mental status, ataxia, stupor
   1. Progresses to coma if untreated
2. Headache, nausea, and vomiting are not always present
3. Focal neuro deficits may be seen (3rd/6th CN palsies)

Treatment

1. Immediate descent is the treatment of choice
2. If cannot descend use combination of:
   1. Supplemental O2
   2. Dexamethasone 8mg initially, then 4mg q6hr
   3. Hyperbaric bag if available

High Altitude Pulmonary Edema (HAPE)

Background

1. Noncardiogenic edema 2/2 increased microvascular pressure in the pulm circulation
2. Most lethal of the altitude illnesses
3. Occurs in <1/10,000 skiers in Colorado; 2-3% of Mt. McKinley climbers
4. Pts who live at high altitude, leave high altitude for few weeks and return are at risk
5. Risk Factors:
   1. Heavy exertion
   2. Rapid ascent
   3. Cold
   4. Excessive salt ingestion
   5. Use of a sleeping medication
   6. Preexisting pulmonary HTN
   7. Preexisting respiratory infection (children)
   8. Previous history of HAPE

Clinical Features

1. Typical pt is strong and fit; may not have symptoms of AMS before onset of HAPE
2. Most commonly noticed on the second night at a new altitude
3. Early
   1. Dry cough, decreased exercise performance, dyspnea on exertion, localized rales
   2. Resting SaO2 is low for the altitude and drops markedly w/ exertion (aids in the dx)
4. Late
   1. Dyspnea at rest, marked weakness, productive cough, cyanosis, generalized rales
   2. Tachycardia and tachypnea correlate with the severity of illness
   3. Altered mental status and coma (from severe hypoxemia)
5. ECG
   1. Right strain pattern
6. CXR
   1. Progresses from interstitial to localized-alveolar to generalized-alveolar infiltrates

Treatment

1. Immediate descent is the treatment of choice
   1. While pt is descending attempt to limit exertion as much as possible
2. If cannot descend use combination of:
   1. Supplemental O2
      1. Can completely resolve the pulmonary edema within 36-72hr
   2. Hyperbaric bag
   3. Keep pt warm (cold stress elevates pulm artery pressure)
   4. Use expiratory positive airway pressure mask
   5. Consider the medications listed below that are usually used for prophylaxis

Disposition

1. Admission
   1. Warranted for severe illness that does not respond immediately to descent
2. Discharge
   1. Progressive clinical and X-ray improvement and a PaO2 of 60mmHg or SaO2 >90%

Prevention

1. Nifedipine 20mg q8hr while ascending is effective prophylaxis in pts who had HAPE before
2. Tadalafil 10mg BID 24hr prior to ascent
3. Salmeterol inhaled BID

High Altitude Peripheral Edema

1. Swelling of face and distal extremities is common (20% of trekkers at 14,000ft)
2. Often associated with AMS but not in all cases
3. Resolves spontaneously with descent

High Altitude Retinopathy

1. Retinal hemorrhages are common at sleeping altitudes >16,000ft
   1. Not considered an indication for descent unless vision changes are present

High-Altitude Pharyngitis and Bronchitis

1. Dry, hacking cough is common at >8000ft
2. Purulent bronchitis/painful pharyngitis common w/ prolonged periods at extreme altitude
3. Severe coughing spasms can result in cough fx of ribs
4. Treatment
   1. Alubterol
   2. Breathing steam, sucking on hard candies, forcing hydration
   3. Abx are not helpful

Chronic Mountain Sickness

1. Excessive polycythemia for a given altitude (Hb >20
   1. Occurs in pts living at high-altitude who have COPD, sleep apnea or impaired resp drive
2. Head ache, difficulty thinking, impaired peripheral circulation, drowsiness
3. Treatment
   1. Phlebotomy
   2. Relocation to lower altitude
   3. Home O2 use

Ultraviolet Keratitis (Snow Blindness)

1. High UV exposure can lead to corneal burns w/in 1hr
2. Ocular pain, foreign-body sensation, photophobia, tearing, conj erythema, chemosis
3. Generally is self-limited and heals within 24hr

Source

Tintinalli