Large cargo aircraft offer major advantages as ICU-transport vehicles. They cover long distances rapidly, can move multiple patients simultaneously, and provide more space than smaller platforms. However, the environment of an aircraft in flight presents significant challenges. In contrast to a hospital, an aircraft cabin experiences rapid shifts in barometric pressure and has a marked reduction in relative humidity. Significant acceleration occurs during takeoff, landing, turbulence, and the tactical maneuvering that is required during certain military flights. Workspace is constricted and access to all parts of the patient is sometimes compromised. Accessing electrical power often requires special equipment to convert typical 400-Hz aircraft power to 50 to 60 Hz. Total available amperage, limited by aircraft design, may be insufficient for some medical configurations. Oxygen supplies are generally limited, so the team must perform a pre-flight calculation of oxygen requirements and supplies, including a safety factor that accounts for the likelihood of a change in patient status or flight plan. Supplies, medications, and equipment are limited to what is carried aboard, so it is important to carefully assemble a standardized set and test it in exercises prior to use on patients. Aircraft noise impairs communication and interferes with the ability to rely on audible alarms on medical equipment. It is also common for the aircraft environment to interfere with the normal function of medical equipment, and it is possible for the medical equipment to interfere with safe operation of the aircraft. For this reason, all medical equipment used in air transport must be tested and certified for use in flight.
The physiologic aspects of flight impart unique stresses to the patient. The most obvious change is the decrease in ambient pressure to a typical altitude equivalent of 6,000 to 8,000 feet during long-range transport. Aircraft cabin altitude can be maintained near sea level, but this increases fuel consumption and limits aircraft range. Considerations for mechanical ventilation during long-range air transport have been reviewed [23]. Gas volume increases by a factor of 1.35 between sea level and 8,000 feet, so careful attention must be paid to trapped gas within the patient and within medical devices. Contraindications to air transport at reduced ambient pressure include decompression sickness and gas trapped in the thorax, pericardium, bowel, eye, or skull unless these conditions are specifically addressed. Gas expansion in an endotracheal tube cuff increases pressure on the tracheal mucosa, so air should be replaced with sterile saline or cuff pressure should be monitored and adjusted frequently during the transport. In spontaneously breathing patients, decreased partial pressure of oxygen is reflected in decreased arterial oxygen saturation unless oxygen supplementation is increased. The effect in mechanically ventilated patients is less obvious, likely due to the effectiveness of positive-end expiratory pressure in a hypobaric environment [24]. It has been the experience of CCATTs that nearly all patients with acute respiratory distress syndrome can be adequately oxygenated during long-range air transport while following a lung-protective strategy. The low humidity experienced during air transport causes increased insensible fluid loss in patients and caregivers. Patients with burns and tracheotomies as well as children and neonates are particularly susceptible to drying, so this must be accounted for in their care.
Acceleration causes complex physiologic changes, the net effects of which are difficult to predict. Patients likely to be highly susceptible to acceleration are those with severe left ventricular failure, increased ICP, and hypovolemic shock. In the absence of specific data, it is the authors' practice to position the patient so the vector of greatest anticipated acceleration runs perpendicular to the patient's long axis. In the example of a patient with increased ICP, the patient's torso is placed as upright as possible during takeoff and landing.
The major remaining challenges derive from the fact that a flying ICU does not have access to the capability present in a medical center. Foremost is rapid access to emergency surgical, angiographic, and endoscopic interventions. Portable blood analyzers are available, allowing basic chemistry, blood gas, and hemoglobin/hematocrit evaluation at the bedside. Diagnostic imaging in flight is currently possible only with portable ultrasound. This technology has an emerging role in critical care practice [25] and could advance the level of care available in flight. Expert consultation is possible using a telephone patch through the aircraft communication system, but this is not perfectly reliable. Transfusion support is available only if the requirement is identified before the flight and blood products, which may not be used, are committed to the patient. This will often pose a difficult choice in the locations from which critically ill patients are transferred. Despite these limitations, long-range critical care air transport is frequently performed.