“An ab-initio military pilot on a familiarisation sortie to the sector with his Instructor, was finding it difficult to see the defined waypoints. This was due to thin clouds below, through which the Instructor could see and point out the features, but the rookie pilot, flying his first sortie in that sector, just could not visualise it. His anxiety of sector briefing sortie was compounded by apprehension of inability to seeing what he was supposed to see on ground. He started to feel numbing sensations at his fingers and toes with wet palms and dry tongue. He informed the Instructor of his not feeling ‘so well’. Instructor found him breathing rapidly without any reason. The sortie was aborted. The trainee narrated his plight, which was corroborated by the instructor, who informed the flight surgeon about the sortie profile, weather conditions and the state of the trainee pilot. The trainee was hyperventilating. He was explained about hyperventilation in detail by the flight surgeon. The reassured trainee pilot successfully earned his wings a few months later.”
“In another instance, though tragic, accident inquiry team found it difficult to ascertain about an experienced pilot’s inability to take correct actions to save his life in an aircraft crippled by fire. Unfolding sequence of events in this accident were such that, the ill-fated pilot was faced with several emergencies one after the other. A careful analysis of voice recorder revealed that pilot was breathing rapidly – hyperventilating – when faced with series of critical events.”
Hyperventilation is a state of either an excessive rate or depth of breathing, or both, than the requirement of the body. This results in excessive removal of Carbon dioxide than that required to maintain an equilibrium within the body.
Whereas the normal blood PaCO2 ranges between 36 and 44 mm Hg, hyperventilation leads to abnormally low levels of carbon dioxide in the blood (<36 mm Hg). This lowering of PaCO2 and increase in the pH results in alkalosis within the body. This results in dilatation of the blood vessels and a fall in blood pressure.
Effects of Hyperventilation The lower the PaCO2, the worse are the symptoms. The most common symptoms of hyperventilation are numbness, tingling of the hands and feet, muscular in-coordination, giddiness visual disturbances and fainting spell. The common signs observed in those hyperventilating are:-
- Increased rate and depth of breathing
- Muscle twitching and tightness
- Cold clammy skin
- Muscle spasms
There are several causes that may lead to hyperventilation in aviation, which the pilots must be aware of. This includes:-
- Breathing through mask in the initial stages, till the combat pilot gets accustomed to such breathing
- Air Sickness
- High ambient temperature
- Vibrations and air turbulence
- Positive Pressure Breathing. The mechanics of respiration gets reversed during positive pressure breathing where inspiration becomes passive and expiration becomes active. The force that the individual has to exert in exhaling against the applied pressure results in increase in rate and depth of breathing.
- Psychological stress. Fear, anxiety or unusual stress leads to hyperventilation. This is frequently observed amongst trainee pilots, in combat flying and during hypoxia indoctrination runs in decompression chamber.
Another important thing to remember is that the signs and symptoms of hyperventilation can easily be confused with those of hypobaric hypoxia. Diagnosis of hyperventilation in aircrew is invariably determined by:-
- Narrative of individual/colleague/instructor (QFI) in a training sortie wherein the pilot is noted to breathe rapidly before manifesting classical signs and symptoms
- Observing the Oxygen flow rates by Doll’s Eye of the Oxygen regulator in the cockpit
- Post sortie urine sample may be alkaline.
Though the mechanism of hypoxia and hyperventilation are distinct, the symptoms are very similar. Thus it is difficult to distinguish between the two. There are however a few distinctive differences between the two devils of aviation. In hyperventilation, the onset is gradual, with the presence of pale, cold, clammy skin and the development of muscle spasm and tetany. In hypoxia, the onset of symptoms is usually altitude-dependent, and the individual has flaccid muscles and cyanosis. The symptoms of hyperventilation may persist even after switching over to 100% oxygen while that of hypoxia will get relieved.
In-flight actions in case of HyperventilationHypoxia and hyperventilation are very similar and both can rapidly lead to incapacitation. Hence the in-flight approach is to correct both problems simultaneously. There are five steps for treatment in-flight in:
- Switch over to 100 percent oxygen if not already on it
- Check oxygen equipment to ensure proper functioning
- Control breathing – reduce the rate and depth
- Descend below 10,000 feet where hypoxia is an unlikely problem
- Communicate the problem to ground control.
1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.
2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.
3. Human Performance & Limitations – JAA ATPL Theoretical Knowledge Manual. 2nd Edition. Jeppesen GmbH, Frankfurt 2001
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