CHAPTER 7 Anesthesia at High Altitude
Many horses live at altitudes greater than 3000 feet above sea level. Horses are also transported to high-altitude venues where they are asked to perform. These animals may require anesthesia for both routine elective and emergency procedures. Knowledge of acute and chronic physiologic adaptations and changes in functionality of anesthetic equipment is important to ensure patient safety.
Although information specific to the equine patient is limited, physiologic adaptations with changes in elevation are well documented in humans. Readers may have faced the consequences of a rapid ascent to high altitude, which in the initial stages manifests as tachypnea, tachycardia, headache, altered mentation, decreased appetite, nausea, vomiting, and insomnia. High-altitude pulmonary and cerebral edema can be severe and life-threatening complications. These changes are largely the result of hypoxia and associated hypoxic ventilatory drive, respiratory alkalemia, metabolic acidemia, and changes in the pulmonary and cerebral vascular tone. Veterinarians may also be aware of pulmonary hypertension and subsequent right-sided heart failure (so-called brisket disease) in cattle living at high altitude.
A change in elevation from sea level to 5000 feet (1523 m) above sea level, the approximate altitude at Denver, Colorado, results in a barometric pressure change from 760 mm Hg (101 kilopascals [kPa]) to 639 mm Hg (85 kPa). At 9000 feet above sea level, this decreases to 554 mmHg (73 kPa). Consequently the arterial partial pressure of oxygen (PaO2) in a normally ventilating healthy equine patient breathing ambient air decreases from 80 to 105 mm Hg at sea level to 65 to 80 mm Hg in Denver. The alveolar gas equation (Box 7-1) may be used to calculate the change in the alveolar partial pressure of oxygen (PAO2) for a given altitude and barometric pressure. This formula may also be used to determine the impact of a change in ventilatory status (PaCO2) and percent inspired oxygen (FiO2). The PaO2 differs from this by less than 10% at lower FiO2 values and up to 25% at high FiO2 in a patient with normal lungs.
Box 7-1 Alveolar Gas Equation
PAO2 = (PB − PH2O) × FiO2 − PaCO2/RQ
PAO2 is the partial pressure of oxygen in the alveolus.
PB is the ambient barometric pressure.
PH2O is the partial pressure of water vapor (approximately 47 mm Hg at 37° C).
FiO2 is the inspired oxygen fraction.
PaCO2 is the partial pressure of carbon dioxide in the arterial blood.
On the basis of the formula in text Box 7-1, the predicted PAO2