When seated in an aircraft nestled high up in the clouds, you may not feel any different from being on land.
What you may not realise is that the pressure of the cabin has been adjusted for your comfort. In addition, your body has an in-built mechanism to cope with the environmental changes.
No matter how high the aircraft is - even at 35,000 feet - its cabin's atmospheric pressure is kept at a level equivalent to not more than 8,000 feet above sea level, said Dr Maziah Mohamad from Flying Doctors Asia, which specialises in medical evacuation.
As atmospheric pressure is reduced the higher one goes, it causes oxygen levels to drop from 21 per cent at sea level to 15 per cent in an aircraft cabin.
This results in less oxygen being taken up by the lungs.
One's oxygen saturation therefore drops as well, said Dr Maziah. This is the extent to which the haemoglobin - iron-rich components in red blood cells - is bound to oxygen.
To correct the imbalance, several bodily reactions kick in. The heart beats faster to send more blood around the body, the respiratory system prompts more rapid and deeper breathing, and blood vessels temporarily contract to increase blood pressure - attempts to ensure life-sustaining oxygen is distributed to cells in the body, she said.
However, this self-regulation process does not work so well in sick people, who already have much lower baseline oxygen saturation due to their underlying diseases, which may also impede the body's ability to respond normally.
They include anaemic patients, those with heart or lung conditions and people with extensive brain bleeds.
FLYING AT LOWER HEIGHT
When patients require medical evacuation by air, the medical evacuation (medivac) team will assess the patient's baseline oxygen saturation level and plan for the mode and amount of oxygen to be given during the flight.
And if a patient requires invasive ventilation, where a tube has to be inserted through the mouth or nose, the procedure would be carried out prior to take-off.
Dr Maziah said that if invasive ventilation has not been planned and the patient is deemed unable to tolerate significant hypoxia (lack of oxygen), the aircraft crew may be asked to cruise at a lower height - between 25,000 and 28,000 feet - where it would be easier to maintain the right pressure in the cabin for the patient's safety.
The medivac team may also request to fly at a lower height if the patient being evacuated has trapped air in his body, said Dr Winston Jong, founder and medical director of Express Medical Assistance (EMA) Global, which carries out medical evacuation. For instance, he may have a distended gut, had recent brain surgery or ruptured lungs.
"When trapped air in the body expands by up to 25 per cent in volume and the cavity is not expandable, the increased pressure will compress surrounding tissues," explained Dr Jong.
Those who have had recent brain surgery, for example, may have air trapped under the skull. If this air expands and presses on the brain, it may lead to brain damage, he added.
EXPANDING AIR BUBBLES
Another group of patients to look out for is those with decompression sickness. This is where dissolved gases turn into bubbles in the body when one goes from a high-pressure area to a low-pressure one. It is a common problem among scuba divers.
For them, the expansion of air bubbles in the blood may prevent blood flow to vital organs, such as the heart, brain and kidneys.
If this problem is suspected, the patient will be scanned for trapped air in the specific area of the body before they are evacuated by air.
Professor Venkataraman Anantharaman, a senior consultant at Singapore General Hospital's department of emergency medicine, said that hollow organs, such as the lungs and intestines, are more likely to expand when the environmental pressure is less than that in the body.
This can pose dangers. For instance, if a patient has a tear in one of the pleural membranes - the fluid-filled spaces surrounding the lungs - air could leak into this space and affect his breathing, he said.
Medical staff need to recognise this problem early and insert a chest tube into the patient to suck the excess air out, or his lungs may collapse, he said.
This article was first published on Dec 4, 2014.
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