High altitude medicine

Physiology of 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 volume
   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; a hyperreactive response is associated with HAPE

Altitude Stages

1. Intermediate Altitude (5000-8000ft)
   1. Decreased exercise performance without major impairment in SaO2
2. High Altitude (8000-12,000ft)
   1. Decreased SaO2 with marked impairment during exercise and sleep
3. Very High Altitude (12,000-18,000ft)
   1. Abrupt ascent can be dangerous; acclimatization is required to prevent illness
4. 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
2. All are seen in rapid ascent in unacclimatized pts
   1. Hypoxemia is maximal during sleep; the altitude in which you sleep is most important
   2. Above 10,000ft rule of thumb is to sleep no higher than 1000 additional ft/day
3. All respond to O2/descent

High Altitude Pulmonary Edema (HAPE)

Background

1. Noncardiogenic pulm edema d/t 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. Typical pt is strong and fit; may not have symptoms of AMS before onset of HAPE
5. Most commonly noticed on the second night at a new altitude
6. 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. 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)
2. 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)
3. ECG
   1. Right strain pattern
4. CXR
   1. Progresses from interstitial to localized-alveolar to generalized-alveolar infiltrates

Treatment

1. Immediate descent is 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 prevention

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
   1. May also see with arc welders, tanning beds
2. Symptoms develop after delay of up to 6-12hr
   1. Ocular pain, foreign-body sensation, photophobia, tearing, conj erythema, chemosis
3. Generally is self-limited and heals within 24-36hr

Source

Tintinalli