Aviation Medicine Analysed

Cast your mind back to when you applied for your student pilot licence. Who was one of the first people you had to visit? The DofA approved doctor. You were asked a multitude of seemingly unrelated and irrelevant to aviation questions; you were then put through a series of mystical manoeuvres designed by some higher authority to determine if you have two eyes, two ears, two arms and two legs, and that they were all connected in the correct sense. You then suffered the final indignity of being presented with a container and being asked to produce a ‘specimen’. You could not help coming away thinking that all doctors were somewhat bent and maybe you might be right in some circumstances. At least you didn’t have to present again for another two years.

I would like you to reflect on your medical and I will attempt to show you how relevant it is and how much aviation medicine is involved and further, how it can be related to your everyday flying. Do you remember the MO asking you to perform the Valsalve manoeuvre (that is when you pinch your nose, keep your mouth closed and blow until your ears pop). The reason is to check the patency of the tube that joins your middle ear to your throat. How many of you do that regularly on the descent? How many of you suffer pain in the ear until you effectively Valsalva? How many of you have had to abort a descent and climb until the pain eases and you can Valsalva? The symptom of pain indicates that there may be some disease of the middle ear or eustachian tube: The pain itself may be enough to create a dangerous situation by itself, particularly if you are flying a tight ILS with weather at minimums. It may also mean that you have an underlying problem in the middle ear cavity wh. ich can disturb the normal function of the inner ear and lead to severe disorientation.

Was the state of your teeth commented on? That has got nothing to do with flying, you say. I can assure you that it has. Although the treatment may be dental, in general aviation the matter is usually noted at a medical. Let me explain. In the days before improved dental filling, material became available or in the case of very old fillings, a gap filled with air may occur between the tooth and the base of the filling. Due to the expansion of a gas as a result of decreased pressure, as occurs during an ascent or a cabin depressurisation, the gas trapped between the tooth and the filling attempts to expand and assume the volume that it would occupy in the ambient atmosphere. This expansion can cause disabling pain. There is a third occasion when you may suffer from dontalgia. It occurs during aerobatics during the rapid changes of vertical direction that occur during a competition sequence. It is postulated that the rate of change of pressure in a number of directions rather than the total change of pressure is the cause of odontalgia. I could continue to discuss the rationale and the applicability of your medical, but this would be an article in itself and it is only a very small aspect of aviation medicine.

The atmosphere in which we fly is quite a stressful place to be and so are the aeroplanes that we fly when you look at it from your body’s point of view. Your body has evolved to live at sea level under the influence of an atmospheric pressure of 1013 mb, an earthward acceleration of 9. 8 m/sec/sec or 1 ‘G’, a partial pressure of oxygen of approximately 20 mmHg, little or no vibration and tolerable noise limits. ,

Leave the bonds of earth and venture into the atmosphere and these parameters change according to the laws of physics and the design of the aeroplane. As a result errors may be induced as your physiological responses adapted for sea level respond to this change in environment. If these changes are allowed to continue beyond a certain point they change from a reversible physiological response to an irreversible pathological response causing illness, or in the worst circumstance, death. I will now illustrate this this-with some examples. I would like you to consider the application of this aspect of the topic to your flying.

Every pilot knows if he ascends to high altitude without supplementary oxygen he Will eventually lose consciousness due to lack of oxygen. Depending on age, fitness, altitude attained, time at altitude, tasks to be performed at altitude and whether he or she smokes, the onset of hypoxia can occur at varying altitudes which can be as low as 8000 feet. . The dramatic onset of acute hypoxia which is demonstrated in the decompression chamber, is readily detected and dealt with. What is not appreciated is the potential dangers associated with the development of the slower onset and subtle incapacitation of relative hypoxia which occurs at much lower altitudes. This is best illustrated by describing the effect of oxygen on night vision.

The eye has two types of light-sensitive nerve endings: the rods and the cones. The rods are primarily used for night vision and are more light sensitive than the cones. These nerve endings are also more sensitive to oxygen or lack thereof. Studies have shown that a pilot’s visual acuity and more importantly his peripheral vision at night at 5000 feet are improved if the pilot uses supplementary oxygen. The conclusion to be drawn from these studies is that although in daylight the small change in partial pressure of oxygen from sea level to 5000 feet is small, its effect on vision is not apparent due to the behaviour and sensitivity of the cones, which are responsible for day vision. However, the rods, due to their increased sensitivity to low light intensity are more sensitive to smaller changes in the partial pressure of oxygen. This situation is potentially dangerous in the approach and landing at night at an airfield that has few visual cues apart from runway lighting, the ‘black hole’ effect.

The effects of reduced atmospheric pressure, fortunately, do not manifest themselves at low altitudes, although the possibility of developing the ‘bends’ by flying after scuba diving is well documented. It is well to remember that this warning applies to pressurised airline aeroplanes as well as light aircraft. This aspect of aviation physiology will become more important as examples of the pressurised cabin single with the performance of the Piper Malibu become more common. The most common effect of reduced atmospheric pressure is the one of gastrointestinal gas, which expands as ascent continues; This gas is generally passed as flatus or belched, not causing any untoward effects. However, should this process be altered as can accumulate in the intestine and cause severe and disabling pain.

The accelerations, to which the body is exposed during ‘normal’ flight manoeuvres, are usually no more than 2½-3 G. However, in aerobatic flight, you can be exposed to a range of – 2 to + 6 in a Decathlon and – 7 to + 9 or + 1Oare not uncommon in a Pitt’s or Laser. The pilot is subjected to this stress without the protection of a G-suit or supplementary oxygen. These accelerations will affect the pilot as long as the force is applied and will decrease as the force is taken off, usually by releasing pressure on the stick. The symptoms that you experience are very individual and their rate of onset and severity depend very much on the same factors that influence your response to hypoxia. The danger of the effects of and the illusions caused by ‘G’ are minimised if you are operating at a safe altitude. But an intimate knowledge of these effects on you as a pilot becomes nothing short of life-saving if you fly in air shows or high-level competition at altitudes down to 500 feet.

The effects of the aviation environment on the pilot discussed so far are all preventable, and reversible and have no lasting effect detrimental to the pilot’s health unless they have not been recognised and the appropriate action taken to avoid danger. If allowed to progress to their potential endpoint, they are, however, potentially fatal. There are aspects of the aviation environment which are less dramatic in their efficacy but are just as important from the point of view of a pilot’s health and productivity. These are the modalities of noise and vibration.

The problem of noise is well documented and straightforward, as noise-induced deafness is well recognised. The danger of deafness in aircrew is obvious. You need to be able to hear radio messages, identify aids and perceive engine sounds (e. g. detect an overspeed) for the safe operation of your aircraft. A less common aspect of exposure to noise is the development of ringing in the ears (tinnitus) which, if severe enough, can cause vertigo. A Pitt’s Special developing full power can generate a noise level of 120 dB or greater which is above pain threshold. Therefore, any method which is known to suppress sound from approved ear plugs, through a good head-set and boom mike to a flying helmet, is to be recommended for use from the beginning of your flying career. It would be disastrous if you were to lose your license to a preventable condition.

The problem of vibration and its effect on a pilot’s health is a vastly different situation and the subject of a great deal of current research. Recent studies have shown that frequencies of vibration of helicopter airframes can cause ‘Jack pain. A study in the USA used an Iroquois airframe and simulated the vibration of the airframe in all modes of flight and found that the subjects complained of quite severe cervical neck pain as a result. Whether this will have any long term effects such as the development of arthritis of such severity to cause cessation of flying will be of interest to all professional helicopter pilots. Some frequencies of vibration are implicated in causing damage to solid organs such as the liver and spleen, causing illness. This again could be of significance depending upon the characteristics of the aeroplane you are flying.

The previous discussion has revolved around aviation medicine as it applies to the individual pilot and his health. I now wish to look at an area of applied aviation medicine where a pilot may well have to apply his knowledge to a situation as pilot in command. The area under consideration is aerial ambulance services, whether they be fixed wing, rotary wing or in extreme circumstances the declaration of a ‘mercy flight’. Why should we know about the transport of sick people? That’s the doctor’s domain: True and false. It really is a team effort between you and the doctor to transport a patient safely. Let me illustrate this by some examples. And remember as you read this, with respect to my colleagues, there are very few doctors who have skills in AME techniques outside the military. In the first example, you are the pilot of an air ambulance which has been tasked to fly a patient with a penetrating eye injury from the country to the city. A penetrating eye injury is one in which a piece of glass or metal has totally or partially entered the eye. What specific action is taken depends upon whether your aeroplane is pressurised or not. Let us spoil ourselves and say the aeroplane is pressurised and fully AME-equipped. The strip is dry, short grass, and conditions are such that you will need a performance take-off. The doctor will ask you if you can fulfil the following conditions. 1. A sea level cabin pressure. Can you provide it? 2. A turbulence-free and vibration-free trip. Where is the best place to load the patient’s head to minimise in flight vibration and turbulence? Can you place the patient’s head at the C of G and still remain within weight and balance limits? 3. In case of depressurisation, does your oxygen system have enough contents to provide you, the doctor and nurse with 100% oxygen for a cruisedescentto10,000feet? The patient should be on his own circuit, independent from the aircraft with enough for his requirements for the trip. If your aeroplane is unpressurised the situation changes and you will have to be able to discuss these points with the MO, just as he will have to understand the operational restrictions placed on you in such an aeroplane.

In the second example; you are pilot of an air ambulance that has to tiy a patient with a spinal injury to a major centre. The basic criteria discussed above apply to this patient also, plus the consideration of the effect of the acceleration of the aircraft during the take-off run on the patient’s injury.

In this article I have attempted to describe the areas of aviation medicine which are directly related to the health and welfare of a general aviation pilot as well as describe an area of employment in aviation in which a pilot has to apply his knowledge of aviation medicine to his work. There are other areas in general aviation where aviation medicine has a role and these are in flight instruction the area of air sickness, hyperventilation and disorientation; in accident investigation, aviation psychology and the investigation of medical aspects of accidents. I hope you have been stimulated to find out more about this subject as it applies to your day-to-day flying activity.