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)

1. Usually only occurs with altitude >7000-8000ft
   1. May occur at lower altitudes in pts who are particularly susceptible (COPD, CHF)

Clinical Features

1. Diagnosis of AMS requires headache plus 1 or more of the following symptoms:
   1. Headache
      1. No headache - 0pts
      2. Mild headache — 1pt
      3. Moderate headache — 2pts
      4. Severe headache - 3pts
   2. GI symptoms
      1. No symptoms — 0pts
      2. Poor appetite or nausea — 1pt
      3. Moderate nausea or vomiting — 2pts
      4. Severe nausea and vomiting - 3pts
   3. Fatigue/weakness
      1. Not tired or weak at all — 0pts
      2. Mild fatigue or weakness — 1pt
      3. Moderate fatigue or weakness — 2pts
      4. Severe fatigue or weakness - 3pts
   4. Dizzy/light-headedness
      1. No dizziness/light-headedness — 0pts
      2. Mild dizziness/light-headedness — 1pt
      3. Moderate dizziness/light-headedness — 2pts
      4. Severely light-headed/fainting — 3pts
   5. Difficulty sleeping
      1. Slept well — 0pts
      2. Did not sleep as well as usual — 1pt
      3. Woke many times, poor night's sleep — 2pts
      4. Could not sleep at all — 3pts
2. Mild AMS: score of 2–4
3. Moderate AMS: score of 5–9
4. Severe AMS: score of 10–15

1. acetazolamide
   1. Start day before ascent 125- 150 mg BID or qhs for 2- 3 days while at altitude and then stop. Peds dose is 5mg/kg/day. Watch for sulfa allergy, paresthesias, diuresis.
2. Dexamethasone
   1. prevents as well as treats cerebral edema. 4mg BID- QID, day of ascent and taper off over several days. Can combine with acetazolamide.

Treatment

1. rest, descend 500- 1000m, acetazolamide 250- 500mg, dex 4mg with taper, Gamow bag (portable hyperbaric chamber)

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