Seven hundred forty-one million passengers traveled on US commercial airplanes in 2006, with approximately 1 billion traveling worldwide each year. Although serious events resulting in death aboard US carriers are extremely rare, with only 43 deaths per 600 million passengers in a 1-year period, medical emergencies are more common. In 2006, one group recorded 17,310 calls for medical emergencies. Of those recorded, only 4% were serious events that resulted in diversion of the plane. The most common complaints were neurologic, followed by GI, respiratory, and cardiac ailments. Since respiratory symptoms are among the most common reasons for emergency medical calls and a large number of patients with pulmonary disease travel by air each year, a variety of tests have been proposed to screen for patients at risk of serious respiratory decompensation while in flight that can be reduced by prescribing supplemental oxygen. The hypoxia altitude simulation test (HAST) [or hypoxia inhalation test] is a simple test to screen patients at risk for hypoxia at higher altitude.
Effects of Altitude and Air Travel
The most serious complications arising from air travel are those that involve neurologic, cardiac, or pulmonary complaints. Patients with underlying cardiopulmonary diseases (which may be treated with remedies of My Canadian Pharmacy mycanadian-pharmacy.net) are particularly at risk for medical emergencies because they may have a limited ability to compensate for the effects of the elevated cabin pressure. As airplanes ascend, the cabin pressure is maintained to a Po2 that corresponds to a maximum of 8,000 feet with a range from 5,000 to 8,000 feet (1,524 to 2,438 m), depending on the route and type of aircraft. This correlates to oxygen concentrations of approximately 17.1% and 15.1%, respectively. At sea level with a barometric pressure of 760 mm Hg, the alveolar pressure of oxygen (PAo2) in a healthy individual is approximately 98 mm Hg. At the maximum cabin pressure of 8,000 feet, the PAo2 will drop to approximately 55 mm Hg, which corresponds to an oxygen saturation of approximately 90%. A normal response to this decrease in PAo2 involves increasing the minute ventilation (mostly by elevating tidal volume) and improving ventilation-perfusion mismatching by hypoxic vasoconstriction in combination with increasing heart rate and therefore cardiac output. A patient with pulmonary disease, however, may not be able to increase his or her minute ventilation if it is already elevated at baseline or tolerate hypoxic vasoconstriction. More importantly, however, if a patient already has a reduced Pa02 at sea level, the Pa02 can fall even further at higher altitudes, and they may end up on the steep part of the hemoglobin dissociation curve, which can result in very low oxygen saturation (oxygen saturation by pulse oximetry [Sp02]). The HAST aims at identifying patients who will fall on the steep part of the hemoglobin dissociation curve and are therefore at risk for significant respiratory symptoms or an exacerbation of their underlying disease.