6. Prevention of Hypoxia.pdf

PREVENTION OF HYPOXIA • The physiological effects of breathing air at reduced atmospheric pressure must be prevented during flight. One method of achieving this aim is to provide an artificial pressure environment, Pressurized Cabin, so that the occupants in the airplane are not exposed to reduce...

PREVENTION OF HYPOXIA • The physiological effects of breathing air at reduced atmospheric pressure must be prevented during flight. One method of achieving this aim is to provide an artificial pressure environment, Pressurized Cabin, so that the occupants in the airplane are not exposed to reduced biometric pressure. • The alternative method is to increase the PO2 in the lungs by the use of Oxygen Equipment. In modern A/C, both methods of preventing hypoxia are employed. The Pressure Cabin • The cockpit and the passenger compartment of all modern A/C are pressurized with air. • At the beginning, the airplanes were compressed at one atmosphere (760mmHG) throughout flight. This imposed considerable penalties, with regard to the weight and the pressurization equipment, that effected the performance of the plane. Furthermore, the larger the pressure differential across the wall of A/C, the greater the risk of damage of the A/C. • So compromises were made between the physiological idea of a cabin pressure of one atmosphere with its weight and performance penalties of the high pressure differential, and the probability of explosive failure of the cabin. • The compromises which have been adopted in the design of pressure cabin. Comfort is of prime concern Probability of structure damage is very remote “The pressure in the cabin is maintained at a level at which the occupants can breath air throughout flight.” The compromises that are adopted in the design of pressure cabins produced 2 groups.  First Group. Where comfort of passengers is the prime concern. (High pressure differential)  Second Group. In compact aircraft where weight and maneuvers is the prime concern. (Low pressure differential)  Lower level of pressurization of the cabin is adopted and the occupants must breath O2 or O2/Air mixture. Ambient Altitude It is the actual altitude outside the flying aircraft. Cabin Altitude It is the artificial altitude that represent the level of pressure inside the cabin. i.e. if the cabin altitude is 6000 ft, it means that the cabin is compressed to a pressure equal to that of 6000 ft altitude. 2 Methods of Maintaining the Pressure in the A/C Higher than the (Immediate Environment Ambient Environment) I. Conventional Method “Draw air from outside A/C → compress it and deliver it into the cabin.” The desired pressure is maintained by controlling the flow of compressed air out of the cabin to the atmosphere. By the flow of the compressed air → The ventilation of the cabin → Control of the thermal environment in the cabin (Control of pressure & temperature) II.Sealed Cabin At very high altitudes, the energy required to compress the low density air become excessive. Above 80,000ft, it becomes almost impossible. In vacuum space → pressurizing the cabin must be carried within the vehicle. “The used gases are recycled (sealed cabin).” CABIN DIFFERENTIAL PRESSURE • The pressure in the cabin is generally greater than that of the atmosphere. (Positive differential) • The absolute pressure in the A/C cabin is termed Cabin Altitude (feet above sea level) • The absolute pressure in the cabin=Atmospheric pressure + Cabin Differential pressure At 25,000ft altitude 10.0Lb/in² = 5.5Lb/in² + 4.5Lb/in² equal to altitude 10,200ft What were the factors considered in deciding the magnitude of the pressurization of the aircraft’s cabin? The answer is → The physiological requirements of the occupants PHYSIOLOGICAL REQUIREMENTS 3 groups of physiological factors to consider in design of C.A.: 1. Factors which determine the maximum acceptable cabin altitude a. Hypoxia We should consider the effect of mild hypoxia on performance of aircrew and on well-being of passengers b. D.S. & c. Exp of GIT gas 2.Factors determines the maximum acceptable rate of change of cabin altitude during ascent and descent. -Ventilation M.E. & P.N.S. 3.Factors related to the magnitude of the effects of a sudden cabin failure. *At altitude greater than 10,000ft, there is significant impairment in the ability to perform flight tasks. “Maximum cabin altitude consistent with flight safety is 5,0007,000ft.” *Cruising In cabin altitude environment (8,000ft) for several hours may produce sporadic incident, of heart failure, induced probability by combination of: - Mild hypoxia - Expansion of abdominal gases - Lack of movement and the seated posture Solution: Making the maximum CA (6,000ft) will eliminate these incidents.  D.S. is very rarely to occur below 22,000ft  Expand of GIT gas For normal individuals and fit aircrew may have transient discomfort if altitude exceeds 25,000ft. For passengers suffering from CV/respiratory disorder, they may be distressed even by a small increase in the volume of GI gas that produced by ascent to levels more than 8,000ft. For planes of aeromedical evacuation, the maximum cabin altitude accepted is 6,000ft. Rate of ascent to altitude or descent to ground “5,000-20,000 ft/ minute rate of ascent are very well tolerated.” • In descent, it is a different story “Inexperienced passengers who are not trained in the techniques of inflation of the M.E. and sinuses during descent, will complain of ear discomfort if the rate of increase the cabin pressure exceeds certain limits” (the accepted rate of the decrease of the cabin altitude is 500 ft / minute). Decompression of the Pressure Cabin The effects of sudden decompression of a cabin on the occupants depend, in its severity, on the following factors: a. The size of the defects in the wall. (rate of decompression) *The doors and hatches are designed to open inwards. b. The volume of the cabin. c. The cabin pressure differential. DECOMPRESSION • Whilst pressurization has overcome, most of physiological disturbances that is associated with exposure to low environmental pressure, decompression of the cabin at high altitude is associated with hazards of its own. Causes of Failure of Cabin Pressure a. Reduced cabin air inflow b. Failure of the pressure control system c. Failure of the cabin structure *ranged from impaired sealing of a door, canopy, or escape hatch, to a gross structural failure of the wall of the cabin Thank you