## Special considerations of altitude in air medical transport

Several special considerations must be taken into account when caring for patients being transported at altitude. Dalton's law states that the total pressure of a gas mixture is the sum of the individual partial pressures of all the gases in the mixture:

Breathing air contains 21 per cent oxygen. At sea level, where the barometric pressure is 760 mmHg, the partial pressure of oxygen is 160 mmHg (21 per cent of 760 mmHg is 160 mmHg). It is even lower (150 mmHg) if the partial pressure of water vapor (47 mmHg) is considered. As altitude increases, the barometric pressure decreases while the percentage of oxygen in breathing air remains the same. Thus at an altitude of 10 000 ft (3000 m), where the barometric pressure is 523 mmHg, the partial pressure of oxygen is 21 per cent of 523 mmHg or 110 mmHg. Simply put, the higher the altitude, the lower is the partial pressure of oxygen ( Table 1).

Although most helicopters operate below 8000 ft (2400 m), all critically ill patients being transported must receive supplemental oxygen.

HUpVhTIf

Table 1 Dalton's law and hypoxia at altitude

Higher levels of ventilatory support (positive end-expiratory pressure etc.) may be required at altitude with pulse oximetry monitoring and ideally arterial blood gas analysis in flight if the transport is over long distances. The pilot can also be requested to make a slow ascent and descent to minimize the rate of pressure changes. A request can also be made to pressurize the cabin to sea level. Although this reduces the fixed-wing aircraft's range, it may be necessary in a patient with poor oxygenation. Altitude can also potentiate the effects of drugs, particularly sedatives.

The following simple equation can be used to determine the amount of supplemental oxygen required by a patient at a given altitude to maintain the level of supplemental oxygen received in the referring hospital:

(■'in. (jt rcfcrrin.ji hirtpiuLj^h.uxiGiitlric pnaHjrc (il Ti'fcrririg Iih^mIjI)

Thus a patient who was on 40 per cent oxygen in a referring hospital at sea level being transported at an altitude of 8000 ft (2400 m) would need at least 50 per cent oxygen ((0.40 x 760)/609 = 0.50) during transport (BJumen.efa/ 1992).

Boyle's law states that the pressure of a gas varies inversely with its volume (P p 1/V). Since atmospheric pressure decreases as altitude increases, gas contained within a fixed confine expands as altitude increases. In the clinical setting, Boyle's law can affect any body cavity or piece of equipment that contains an enclosed air space. Helicopters rarely ascend to very high altitudes, and so the volume changes are small. Fixed-wing aircraft should be pressurized at 5000 to 8000 ft (1500-2400 m). The endotracheal tube cuff will expand with increasing altitude, placing the patient at risk of tracheal pressure necrosis. Endotracheal tube cuff rupture may also occur which will result in loss of tidal volume and inability to ventilate or maintain positive pressure. The risk of aspiration also increases when the cuff is ruptured. The air medical crew must release gas from the cuff manually as altitude increases, or it can be filled with water rather than gas. Only a modest increase in volume (a factor of 1.2) is experienced in helicopters operating at altitudes of less than 5000 ft (1500 m). Volumes will double in fixed-wing aircraft operating at altitudes above 18 000 ft (5400 m). Similar attention should be given to Foley catheters if the balloon has been inflated with gas rather than water.

Charles' law states that when pressure is constant, the volume of a gas is nearly proportional to its absolute temperature. As the temperature increases, gas molecules will move faster and the volume of gas will increase:

where V1 is the initial volume, V2 is the final volume, T1 is the initial absolute temperature, and T2 is the final absolute temperature (absolute temperature is defined as temperature in degrees celsius plus 273). Thus it is important that patients remain warm for hemodynamic stability and to maintain perfusion, and that they are not exposed to extreme temperature variations during transport. However, hyperthermia can lead to gas expansion.

Most of the marked pathophysiological changes occur in flight at an altitude over 8000 ft (2400 m). Sudden loss of cabin pressurization from a large defect results in rapid movement of air towards the hole. There is often a rapid drop in temperature, flying debris, noise, and fog formation due to condensation. Hypoxia rapidly ensues, and so supplemental oxygen should be applied immediately as the pilot attempts to bring the aircraft below 10 000 ft (3000 m). All catheters, chest tubes, and nasogastric tubes should be unclamped to combat the rapid gas expansion that may occur ( Rodenberg and Blumen 19.9.4).

Patients with pneumothoraces should be kept on suction drainage if possible (either continuous or intermittent). The tube thoracostomy must never be clamped during transport. If a suction system is not available, a simple one-way flap valve (Heimlich device) is useful. Patients with significant pneumocephaly from sinus fractures or a basilar skull fracture require special attention. Neurosurgical consultation should be obtained if possible. Consideration should be given to intracranial pressure monitoring or ventriculostomy if the patient has significant intracranial gas or will ascend to an altitude where gas expansion will occur. When possible, the patient should be placed in the conveyance head first and at a 10° to 15° head-up tilt since this permits better toleration of sudden decelerations.

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