Gastric POCUS, 3-Part Episode

The physiologic effects of anesthesia care at increased altitude are due to reductions in inspired PO2, ambient pressure and gas density. Acute altitude exposure will cause decrease in PaO2 and a compensatory hyperpnea, leading to reduction in PCO2. Chronic changes with high altitude are adaptive and include polycythemia (improves tolerance to hypoxia), reduction in plasma bicarbonate (to offset the respiratory alkalosis) and overall adaptation to physical exertion at lower barometric pressures (higher altitudes). MAC refers to the concentration of vapor that prevents movement to surgical stimulus in 50% of patients and it is measured as the partial pressure of the gas at 1 atmosphere. At an altitude of 8,000 feet above sea level (approximately 2,400 meters above sea level), a variable-bypass vaporizer will deliver an increased concentration or volume percent of isoflurane; however, the concentration or volume percent is not an important factor. The partial pressure of a volatile anesthetic is what determines anesthetic potency. Thus, because the atmospheric pressure is lower, a higher concentration or volume percent is needed to reach the same partial pressure. At higher altitudes, setting the vaporizer dial to deliver 1.1% will result in the same anesthetic depth as it does at sea level.