Incident Details
A passenger with a medical history of coronary artery disease and prior coronary surgery lost consciousness while aboard a non-pressurized twin-engine airplane during a sightseeing flight at 21,000 feet. All passengers on the flight were using supplemental oxygen from the airplane's oxygen system.
Medical Response
The passenger was supported in an upright, seated position as the flight returned to the departure airport. Upon landing, emergency medical personnel met the aircraft and transported her by ambulance, lying down on a stretcher. She regained consciousness en route to a hospital. Initial diagnostic studies were consistent with a heart attack and congestive heart failure. Although the initial diagnosis included possible high altitude pulmonary and cerebral edema, these conditions were considered unlikely because the flight lasted less than one hour; such symptoms typically occur only after a day or more of exposure to high altitude.
Oxygen System
According to the pilot's operating handbook, the FAA-approved passenger oxygen mask for the incident airplane is a partial rebreather mask equipped with a reservoir bag at a flow rate of 1.5 or 2.0 liters per minute. However, on the incident flight, the passengers were using a disposable, hospital-type oxygen mask without a reservoir bag, at a flow rate of 1.5 liters per minute. FAA oxygen system certification standards, at 21,000 feet, require a minimum flow rate of about 1.8 liters per minute, and the recommended upper altitude limit for use of an oxygen mask without a reservoir bag is 15,000 feet.
Post-Incident Actions
Following the incident, the operator changed the passenger oxygen masks to an FAA-approved mask with a reservoir bag, at a flow rate of 3.0 liters per minute. The passenger was subsequently released for in-home rehabilitation two months later with a diagnosis of an anoxic brain injury.