High Altitude Illness

1. High Altitude Illness David Gonzales, MD

2. Medicine You Will Probably Never Use in Texas • Guadalupe Peak, 8,749 feet • Might as well be in New Mexico

3. Outline • Challenges of High Altitude • Physiologic Response to Hypobaric Hypoxia • High Altitude Syndromes • Acute Mountain Sickness/ High Altitude Cerebral Edema • High Altitude Pulmonary Edema

4. Oxygen = Good • Amount of oxygen available to breathe is a function of the percentage of oxygen in the air and barometric pressure. • Earth’s atmosphere is 21% oxygen • Barometric pressure at sea level = 760 mm Hg • Pressure of inspired oxygen =149 mm Hg

5. Less oxygen = bad • Denver = 5000 feet • PiO2 = 124 mm Hg • Santa Fe = 7000 feet • PiO2 = 115 mm Hg • Highest human habitation = 18,000 ft. • PiO2 = 73 mm Hg • Mt. Everest = 29,528 ft • PiO2 = 42 mm Hg (about ¼ that of sea level)

6. Oxygen saturation does not decrease until PaO2 reaches approximately 60 torr • Corresponds to an altitude of 10,000 ft.

7. Physiologic Response to Hypoxia • Acclimatization • A gradual process (days to weeks) whereby individuals respond to hypoxia in order to adapt and increase performance • Rate varies among individuals • Mediated through sympathetic nervous system

8. Ventilatory Response • Carotid body senses decreased PaO2; signals medulla to increase ventilation • Respiratory alkalosis ensues, decreasing ventilation • Subsequent HCO3 diuresis occurs through the kidney and ventilation subsequently increases again • This process stabilizes after 4-7 days, provided altitude does not change

9. Cardiovascular Response • Heart rate increases, leading to a moderate rise in cardiac output • Pulmonary artery pressure increases secondary to hypoxic vasoconstriction • Cerebral blood flow increases • These last 2 adaptations may become pathologic (more on this later)

10. At moderate altitude, curve does not shift • Extreme altitude leads to severe alkalosis and a leftward shift • PCO2 may decrease to 10 torr

11. Pathologic Syndromes • Acute Mountain Sickness (AMS) • A headache + (any of the following) • Nausea/vomiting • Fatigue • Dizziness • Sleep disturbance

15. Diagnosis • Suspect in non-acclimatized persons above 8,200 feet • Rapid ascent

16. AMS Pathophysiology • Not so much hypoxia, rather your body’s response to it • Lag time between onset of symptoms; acclimatization cures

17. Pathophysiology of AMS • Low ventilatory response increases risk • Fluid retention • Evidence suggests vasogenic cerebral edema plays a central role, however cellular mechanisms not yet elucidated • Big brain, small skull

18. Treatment of AMS • Prevention is best treatment • Avoid abrupt ascent to sleeping altitudes >10,000 feet • Don’t increase sleeping altitude by more than 2000 ft. per night • Climb high, sleep low philosophy • Acetazolamide (Diamox) • 125 to 250 mg po bid • Carbonic anhydrase inhibitor • Diuresis • Metabolic acidosis  increased breathing • Decreases CSF production

19. Treatment of AMS • Supportive analgesics, antiemetics • Diamox to hasten acclimatization • Minimize exertion • Low flow oxygen if available • Consider dexamethasone • Failure of symptoms to improve with treatment or progression of symptoms despite 24 hours of acclimatization is an indication to descend.

20. High Altitude Cerebral Edema(HACE) • A progression of AMS to a severe, life-threatening condition • AMS + • Ataxia • Altered consciousness • Severe lassitude • Cerebral edema is cytotoxic rather than vasogenic

21. High Altitude Cerebral Edema(HACE) • Cellular swelling thought to be caused in part by NMDA-receptor mediated calcium influx. • Trial using magnesium infusion (an NMDA blocker) were clinically unsuccessful in treating AMS; prophylaxis with Mg citrate only caused diarrhea

22. Treatment of HACE • Early recognition is key • Oxygen 2-4 liters • Dexamethasone • Immediate Descent

23. Gamow Bag • An impermeable bag that can be inflated to simulate a lower altitude • Patient placed inside but reassessed periodically • HAPE = 2 to 4 hours of treatment • HACE = 4 to 6 hours of treatment

24. Gamow Bag

25. Portable Altitude Chamber • Zipper placement makes it easier to use than Gamow • Low, low price of $1,200

26. High Altitude Pulmonary Edema (HAPE) • Most common cause of high-altitude related death • Easily treated/prevented with prompt recognition • <1 in 10,000 in Colorado skiers • 1 in 50 in climbers on Mt. McKinley • Risk factors include individual susceptibility, rapid ascent, exertion, altitude reached

27. Manifestations of HAPE • Decreased exercise performance • Dyspnea at rest; often occurs during sleep • AMS (50%) • Dry cough • Cyanosis • RLL crackles • Pink, frothy sputum (late sign)

28. Manifestations of HAPE • Temperature >38.5 • Ulcers on tongue • Sinus tachycardia • Other signs of acute pulmonary hypertension • RBBB • RAD • RVH voltage

29. Manifestations of HAPE • Respiratory Alkalosis • Severe hypoxemia • Fluffy infiltrates • Autopsy consistent with noncardiogenic pulmonary edema

30. Pathophysiology of HAPE • Pulmonary Hypertension-A fact of life at high altitude • Global hypoxic pulmonary vasoconstrictor response • When is it pathologic? • Increased Capillary Permeability • Shear forces vs. endothelial dysfunction • Decreased HVR • Role in nighttime hypoxia

31. Treatment of HAPE • Early recognition should lead to evacuation/descent • This will limit severity and hasten recovery • O2 if available; Gamow bag • Vasodilators as adjuncts • Nifedipine • Salmeterol • Ounce of prevention

32. Summary • Altitude acclimatization is a highly individualized process • Mild AMS is best treated supportively • HACE and HAPE require more aggressive treatment • Common sense and adequate preparation go a long way