Chapter 22: High-Altitude Anesthesia

INTRODUCTION

Decreased atmospheric pressure at altitude has profound effects on human physiology and anesthetic delivery. Approximately 140 million people worldwide live at altitudes above 2500 m (8000 ft). These residents have the usual needs for surgical and anesthesia care in their native localities. Additionally, millions of people transiently visit high altitudes. Anesthesiology providers are asked to support military operations, aviation, mountain climbing expeditions, and humanitarian missions at extreme altitudes. Safe and effective anesthesia care at high altitude requires an understanding of the normal and pathological effects of altitude and an appreciation for the anesthetic challenges in this environment.

PHYSIOLOGY

Atmospheric pressure decreases nonlinearly with increase in altitude. Half of the atmosphere is below 18,000 ft and the remainder is up to >100,000 ft. Increased altitudes are often categorized to denote their increasing effect of physiology:

• “High altitude” begins at 1500 m (~5000 ft) above sea level.

• “Very high altitude” begins at 3500 m (~11,500 ft) above sea level.

• “Extreme altitude” begins at 5500 m (~18,000 ft) above sea level.

Anesthetics delivered between 5000 and 11,500 ft require only modest changes from standard sea-level practices. Anesthetic care above 11,500 ft. becomes increasingly problematic. Fractional atmospheric oxygen concentration remains approximately 21% at all altitudes. As atmospheric pressure decreases with elevation, the absolute partial pressure of oxygen in air declines according to Dalton’s Law, which states the sum of all partial pressures equals the total pressure. The alveolar gas equation defines a partial pressure for alveolar oxygen based on barometric pressure:

where P_AO₂ is the partial pressure oxygen in the alveolus (mmHg), F_iO₂ is the fractional concentration of oxygen in inhaled gas, P_atm is the barometric (atmospheric) pressure (mmHg), P_H2O is the vapor pressure of water at body temperature (47 mmHg), P_aCO₂ is the partial pressure of CO₂ in the alveolus (mmHg), and RQ is the respiratory quotient (moles CO₂ produced per moles O₂ consumed, which is typically about 0.82 for normal diet).

Using the alveolar gas equation, the effects of altitude and CO₂ on alveolar (P_AO₂) and arterial oxygen (P_aO₂) become very evident (Table 22-1; Figure 22-1).