In this article we will discuss about the inhaling problems faced by aviators in high altitude.
1. Anoxia:
Alveolar Po2 at different altitudes due to low barometric pressure. It has also been shown that above an altitude about 3 km or 10,000 feet the alveolar Po2 falls rapidly accompanied with desaturated of arterial blood with oxygen. At an altitude of about 12km or 40,000 feet the arterial blood is only 5% saturated with oxygen.
The fall in alveolar Po2 is due to following factors:
i. Fall in inspired air Po2 is dependent on fall in barometric pressure.
ii. Occupation of alveolar space by N2 which is an inert gas and enters the alveoli in quantities about 4 times that of oxygen during each inspiration.
iii. Co2 which is produced metabolically and exerts a tension of 24mm Hg at high altitude due to hyperpnoea.
iv. Water vapour which exerts a tension of 47mm Hg at all altitude. It will be seen, therefore, that the Pco2+ PH2O amount to 71mm Hg and is always to be deducted from the total available gas pressure in the alveoli which is equal to the ambient barometric pressure. The gas pressure remaining after deduction will be disturbed to O2 and N2 of which about only 14% will be Po2 and the remaining PN2.
Nitrogen in the alveolar air (but not the Co2 or H2O vapour) can be replaced by O2 by oxygen inhalation and thereby the alveolar PO2 can be increased at any altitude to the approximate value of combined PO2 + PN2 of the alveolar air at that altitude. Table 8.16 gives the barometric pressure, the values of alveolar PO2 and arterial O2 saturation at different altitudes with and without inhalation of oxygen.
The figure 8.50 shows the value of arterial oxygen saturation at different altitudes, without oxygen (left hand) and with O2 (right hand curves).
The data and the curve indicate that inhalation of pure O2 will keep the arterial blood 90% saturated with oxygen upto an altitude of 11.7 km or 39,000 feet. It then falls rapidly so that at an altitude of 14 km or 47,000 feet the arterial blood is only 50% saturated inspite of O2 inhalation. 50% arterial saturation is the point where consciousness is soon lost.
Without oxygen this limit is reached at 7km or 23,000 feet. The rule in aviation, therefore, is that with supply of pure oxygen one can ascend upto an altitude about 12 km or 40,000feet. Above that point a pressurised cabin will be necessary.
2. Radiation Hazards:
About 20% of the sun’s rays are filtered by the atmosphere before they reach the earth.
Brightness of the sun increases, therefore, by 1/5th at the upper atmosphere and from above the earth appears less distinct for 2 reasons:
i. One is looking at a less bright object from a more bright area.
ii. Reflection of light from the atmosphere blocks the vision of the horizon.
It has already been mentioned that most of the UV radiations and cosmic rays are absorbed by the upper atmosphere.
The cosmic particles consist mostly of electrons and protons and are aggregated in two zones, viz. Van Allen’s inner belt and outer belt respectively. The inner belt extends at an altitude from 300 to 3000 miles (about 480 to 4825 km) around the equator and consists mostly of high energy protons and electrons and since its energy level is high it is not possible to protect a spacecraft from its penetrating effect.
The cuter belt is situated between 6000 and 20,000 miles (7650 and 32200 km) around the equator and consists almost entirely of electrons.
These belts are to be avoided during space travel. For orbiting round the earth the space craft should be kept below at altitude of 300 miles or 480 km that is the inner radiation belt (Fig. 8.51) of Van Allen. For interplanatory travel, the spaceship should leave the earth through the polar escape route.
3. Dysbarism at High Altitude (Decompression Sickness):
The problem of sudden decompression has been pointed out there that dysbarism is more common in Caisson workers during rapid decompression than in aviators. It may occur in aviators during rapid ascent to an altitude of 9 km or 30,000 feet in an unpressurised aircraft.
Since now-a-days pressurised planes are always used especially during high altitude flight decompression sickness ordinarily should not occur in aviators unless the aircraft is damaged leading to sudden failure of cabin pressure at an altitude of about 9 km or 30,000 feet.
Parachute jumping from that altitude or above may also cause decompression sickness and the aviators usually wash off the N2 in their system as much as possible by taking several deep breaths of oxygen, before the jump. In fact, now-a-days parachute jump is also effected in a sealed capsule with adequate oxygen to prevent dysbarism.
4. Explosive Decompression:
Sudden decompression from sea-level pressure to low barometric pressure of an altitude of 15 km or 50,000 feet or over do not cause any damage to the system and with little practice human beings can tolerate this explosive decompression without much discomfort.
Air in the cavities, e.g., the middle ear is expelled through the Eustachian tube, the air in the colon is expanded and is expelled out as flatus, air in the stomach is belched out, air in lungs escapes through the open glottis. Subsequent X ray examination of the chest reveals partial atelectasis in some parts of the lungs in a few subjects.
Boiling of body fluids occurs on exposure to an altitude of 63,000 feet where the barometric pressure is less than the water vapour pressure in the lungs (47 mm Hg). About 2% of the body weight is lost during the 3-minute time the subject remains alive.
5. Acceleratory Forces:
1. Linear Acceleration:
At the commencement of flight simple linear acceleration and the termination of flight linear deceleration occurs.
2. Angular Acceleration:
During turning on plane or ‘loop’ formation or dive bombing operation.
The force of centrifugal acceleratory force (F) during turning of an aeroplane is given by the formula:
F = mv2/r,
where, m mass of the plane, v is the velocity of turning and r is the radius of curvature of the turn.
It may be noted that the force of angular acceleration increases with the square of the velocity i.e., if the velocity is doubled the force will increase 4 times. The force also varies directly as the sharpness of the turn (1 /r).
3. Acceleratory Force-‘G’ Unit:
When a man is sitting on his chair the intensity of the force exerted on his seat by the weight of his body resulting from the pull of gravity called 1 ‘G’. If the force with which he presses against his seat becomes 4 times his normal weight such as during pullout from a dive-the force acting on his seat and body is equal to 4 G.
During outside loop formation the pilot is to be held down to his seat by belt and a force of negative G acts upon his body from foot to head direction. The magnitude of this force may be – 1G or – 2G, etc. depending on its intensity.
6. Effect of Centrifugal Acceleratory Forces:
Circulamion:
Blood by virtue of its fluid nature can be easily translocated along the long axis of the body. When the human body is subjected to positive ‘G’ force acting from head to foot direction, the blood is translocated to lower parts of the body resulting in rise of pressure of veins of the lower limb and stagnation of blood in that situation.
When the centrifugal acceleratory force is +4G or over the venous pressure in the lower limbs in the standing position is approximately 400 mm Hg and stagnation of the blood in the lower parts of the body results in fall of inflow to the heart followed by diminished cardiac output and consequently the blood pressure falls to a very low level.
Fig. 8.52 shows the relationship between the ‘G’ force and blood pressure. It may be seen that with a force of +4G acting on the body the systemic blood pressure falls to 40 mm Hg. This causes at first retinal anaemia leading to ‘blackout’ followed almost immediately by cerebral anaemia leading to unconsciousness.
Such a situation is faced by pilots during ‘pullout’ of the dive bombing operations and can be avoided by adopting appropriate posture (knee and hip bent position) and by wearing proper type of suits called anti-G suits which maintains a firm pressure on the lower limbs and abdomen preventing accumulation of blood in these areas. However dive bombing the consciousness is soon regained and the vision is restored to normal. Mental confusion persists for some time during which the plane may go out of control.
Extremely high positive acceleratory forces, e.g., +20G lasting for fraction of a second will cause fracture of vertebrae.
Since negative G forces act from foot to head direction blood is translocated towards the brain and as shown in Fig. 8.50 the blood pressure at the heart level may be as 400 mm Hg with a force of – 12G acting for few seconds. This causes extreme bradycardia from baroreceptor reflexes.
Rupture of the subarachnoid vessels in spite of severe hypertension occurs much less frequently than expected because translocation of c.s.f. also occurs towards the head region which buffers the effect of hypertension on cerebral blood vessels. Extreme congestion of the blood vessels of the eye with temporary ‘red-out’ is a common feature. Mental confusion is likely to last for some time after the negative G forces have ceased to act.
Transverse G forces acting through the antero-posterior axis of the body will not produce any ill effect ordinarily. Thus a force of 15G to 25G acting for many seconds or a force of 100G acting for fraction of a second can be tolerated easily. When very large acceleratory forces are to be faced the astronauts should place himself in a semi-reclining or lying position.
7. Protection of Body against Centrifugal Acceleratory Forces:
1. Posture:
If the aviator compresses his abdominal muscles to an extreme degree and then leans forwards with the knees’ flexed and drawn up pooling of blood in the large vessels of the abdominal and legs can be prevented to some extent and onset of blackout delayed.
2. ‘Anti-G’ Suit:
Prevents pooling of blood to lower part of the body. Theoretically, a pilot with a suit of water can withstand large acceleratory forces both positive and negative because the movement of blood. Suits have been devised which prevents pooling of blood to the lower part of the body by applying positive pressure to the legs and abdomen. It has been said that the Germans lost the last Great War because they failed to devise appropriate ‘anti-G’ suits.
3. Effect of Liner Acceleratory Forces on the Body:
The problem of toleration of tremendous linear acceleratory forces develops during blast-off acceleration and landing deceleration of the spaceships. A force of about + 9G develops during the first stage booster and +8G during the second stage.
It is not possible for human body to withstand these acceleratory forces in the standing position. Astronauts, therefore, assume a semi-reclining position transverse to the direction of acceleration. It is possible to tolerate this force for as long as 5 minutes or longer the G force acts through the transverse axis of the body.
Deceleration during landing poses another difficulty. It must be accomplished much more slowly from high velocities and deceleration should commence at a distance of 10,000 miles (about 16,000 km) for a space craft travelling at a speed of 100 Mach (a speed possible in interplanetary space travel).
8. Sense of Position and Equilibrium:
The utricle and saccule and the 3 pairs of semicircular canals are primarily concerned in perception of the sense of position of the head in relation to space and trunk and also in angular rotation of the head in difference direction.
These organs reflexly adjusts posture and equilibrium by altering the position of the body in relation to the position of the head through their elaborate connection with vestibular nuclei, flocculonodular lobe of the cerebellum and the red nucleus, reticular formation and medial longitudinal fasciculus.
The sensory epithelium of the utricle and saccule is known as macula and consists of rows of sensory hair cells embedded in gelatinous mass and pulled by calcareous particles in different directions depending on different position of the head. They are, therefore, typical gravity receptors and fail to function in zero-G state.
Further, they are not stimulated unless the head leans forwards by at least 10 degree or backwards by 25 degrees. These respectors, therefore, are very inefficient and their evolution lags far behind the requirement of rapid change in position demanded by modern aircrafts. Aircrafts, therefore, are provided with appropriate instruments to keep the aviator informed about the rapid ascent and descent, during fight. This is, of course, more important during night flight.
The sensory epithelium of the semicircular canals is simulated during angular rotation of the head due to inertia of the endolymph. They, however, are stimulated at the beginning and at the termination of rotatory movement and remain inactive during rotation.
Further, the rotational movement must exceed 2° per second to stimulate effectively the semicircular canal mechanism. These organs, therefore, are also very inefficient organs for perception of rotatory movement demanded by modern aircraft manoevers. The aviator again must depend on his instruments for proper appreciation of this movement during flight.
9. Parachute Jump:
When an aviator jumps with a parachute he falls with an accelerated velocity of 16 feet per second per second. However as his velocity of fall increases the air resistance also increases because the atmosphere becomes denser and denser.
After about 12 seconds the decelerator forces due to atmosphere counter balances the acceleratory forces due to gravity and by the time the aviator has fallen by about 4.25 km or 1400 feet, he acquires a constant rate of fall, of about 55 metres or 175 feet per second. This is called terminal velocity (Fig. 8.52).
By this time either the parachute opens spontaneously or is opened voluntary by the aviator by pulling on the ripcord. With usual size parachute the terminal velocity is reduced to about 6 m or 20 feet per second and force of impact is actually the same as experienced by one jumping from a height of 7 feet and in untrained subjects is severe enough to cause fracture of the leg bones and the pelvis.
An aviator jumping from a very high altitude will certainly be unconscious immediately after fall due to acute anoxia and will regain his consciousness when he has descended to an altitude of about 7 km or 23,000 feet. His mental confusion will persist for a few seconds but if he is provided with a parachute with automatic opening device he will have safe landing on the earth.
Another hazard of parachute landing is injury from the damaged plane. Usually the tail-end of the plane hits the head of jumper causing serious injury. To avoid this hazard now-a-days pilots of military planes are provided with a device so that they enclose themselves in a sealed capsule filled with oxygen.
The capsule is fitted with the seat of the pilot which is ejected upwards by pressing a lever so that the pilot lands safely after the damaged plane has cleared off and also is protected from anoxia and cold.
