Aviation medicine. Problems of altitude I: hypoxia and hyperventilation.

Abstract

a 5 _c V C) n -50 L-60 FIG 1-Relation between altitude and pressure and between altitude and temperature. Conversion: traditional to SI units-atmospheric pressure: 100 mm HgI 13-3 kPa. (Altitude: 5000 ft 1500 m.) Altitude (x 1000 ft) c a) x 0 -c E a) c--c a a 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Partial pressure of alveolar oxygen (kPu) FIG 2-Relation between partial pressure of alveolar oxygen, percentage saturation of haemoglobin with oxygen, and altitude. Conversion: SI to traditional units-partial pressure of alveolar oxygen: 1 kPaz8 mm Hg. (Altitude: 5000 ft 1500 m.) ded at normal operating altitudes. In practice the degree of pressurisation increases linearly with, but at a slower rate than, actual altitude, from ground level to a high maximum differential pressure (fig 3). Thereafter, cabin altitude increases at the same rate as aircraft altitude along a line determined by the pressure characteristics of the hull, the maximum differential pressure line. The problems of altitude become manifest if and when cabin Cabin differential pressure (lb/in2) 0 2 4 / , 6 5 10 15 20 25 30 40 50 Aircraft altitude (x lOOOft) FIG 3-Typical subsonic passenger aircraft pressurisation profile. In this ex-ample the maximum differential pressure line is reached when aircraft altitude is 39 500 feet (12 040 m) and cabin altitude is 6000 ft (1929 m). Conversion: traditional to SI units-cabin differential pressure: 1 lb/in2' 0-07 kg/cm2. (Altitude: 5000 ft-1500 m.) pressurisation fails. Such failures still occur in both civil and military aircraft, large and small.4 6 In the case of a slow loss of pressurisation-for example, as a result of malfunction of a con-trol system-the cabin altitude will increase slowly; this is usually rapidly recognised and dealt with by the crew. Hypoxia in susceptible subjects is the main danger. Effects are more drama-tic when a rapid decompression occurs-for example, when a window is lost or the pressure hull is ruptured by an explosion. In these cases there is a massive movement of air out through the defect, which will carry with it any loose articles and even pas-sengers close to the breach who are not wearing their seat belts.4 The air movement creates considerable noise and the sudden cooling causes condensation and misting, thus making both communication and vision difficult. These initial events are rapidly succeeded by the problems of hypoxia, perhaps compli-cated by cold injury (frostbite and frostnip), and decompression sickness. The rate, duration, and effects of rapid decompression depend on the external and cabin altitudes at the moment of decompression, the volume of the cabin, and the size of the defect. Combat aircraft are pressurised to a lesser degree so as to achieve a lower structural weight and improve the power to weight ratio. In addition, the risk of sudden loss of cabin pressurisation in these aircraft is increased as a result of enemy action. The degree of pressurisation depends on actual altitude, as in commercial aircraft. The oxygen deficit when cabin altitudes exceed 10 000 ft (3125 m) in military aircraft is compensated for by oxygen enrichment of the air as altitude increases, until at about 30 000 feet (9144 m) 100% oxygen is being delivered. The supply is fed individually to each crew member through a regulator and mask. Hypoxia PHYSIOLOGICAL FACTORS The earliest feature of hypobaric hypoxia is often a subtle personality change perhaps coupled with euphoria, lack of judgment, loss of short term memory, and mental incoordina-tion. This combination is not unpleasant and resembles the early stages of alcoholic intoxication, but its insidiousness is of the greatest danger to the victim. In a crew member it may be disastrous since he is unaware of his failing performance. Subsequent features reflect the stimulation of cardiovascular and respiratory compensatory mechanisms. In moderate hypoxia-for example, air breathing at 25 000 ft (7620 M)6-cardiac output and heart rate are increased but overall peripheral resistance is reduced, so that mean arterial blood pressure is unchanged. Cerebral blood flow is increased, though the 1409