Ask Captain Lim - How depressurization affects me in flight?

In-Flight Depressurization

Depressurization can arise as a result of a mechanical failure of the pressurization system, or if, in an unlikely event of a terrorist sabotage, an in-flight explosive decompression. It can also be caused by metal fatigue in older planes. In April 1988, an Aloha Airlines Boeing 737 in Hawaii suffered a fuselage rupture and a senior flight attendant was sucked out from the airplane. The airplane managed to land safely with 89 passengers aboard. The cause of the accident was attributed to poor maintenance of the old Boeing 737, causing the roof of the first class compartment to be ripped off.

An explosive decompression is almost similar to a balloon bursting and depending on the size of the rupture, it would most likely be accompanied by a loud bang. Loose articles and people not secured by seat belts near the hole would be sucked out. The Captain is trained to reaction to this emergency by commencing an immediate rapid descent. The aircraft would go into a dive so as to reduce the onset of hypoxia that can cause a passenger to loose consciousness within 30 seconds at about 35,000 feet without oxygen.

This is the time many passengers wished they had paid more attention to the usage of oxygen demonstration prior to departure. Remember, you are told to grab the nearest oxygen mask, pull it towards you to start the oxygen supply and don it onto your face securely. Then only would you assist an infant. Don't forget, you got only about 30 seconds at 35,000 feet!

During a normal flight at 35,000 feet, the cabin altitude is artificially maintained at 8,500 feet. As soon as depressurization starts, this cabin altitude cannot be maintained any longer and it will attempt to catch up with the actual altitude. More often, depressurization comes about in an insidious manner as pressure leak out quite slowly. In a Boeing 777, when the cabin altitude passes10,000 feet, a warning will come on and by 13,500 feet, the oxygen masks drop down automatically together with a pre-recorded emergency announcement.

Depressurization is rare. In the ten years from 1974 to 1983, there were only 355 depressurization incidents. Out of these, only about 150 or 15 incidents a year, did the oxygen masks deploy or some passengers suffer injuries as a result of the emergency descents.

Physiology of Respiration

What cause a person who is unable to don the mask in time to loose consciousness? To understand the effect of hypoxia, you need to have a clear knowledge on the physiology of respiration which is a little technical for the average person but not to a medical student! (Skip if you wish).

The human body needs a constant supply of oxygen to break down food and produce energy. The body is unable to store oxygen and nervous tissue is very sensitive to deficiency of oxygen. The brain uses 20 percent of total resting oxygen consumption. Stopping its blood supply and subjecting it to anoxia (no oxygen) produces unconsciousness in as short as 10 seconds. Brain cell damage will occur after 4 minutes.

Carriage of Oxygen in the Body

Gases are exchanged between the body and the air in the lungs. The lungs are composed of numerous tiny blind ended sacs - the alveoli.

Oxygen passes from the alveolar air into the bloodstream and is carried by the red blood cells to the tissues. Carbon dioxide is carried from the tissues to the lungs where it is breathed out. This exchange of gases depends on a pressure gradient in the lungs and the blood stream.

Atmospheric pressure at sea level is 760 mm Hg or 14.7 psi (pounds per square inch). Air is a mixture of two main gases, oxygen (14.5%) and Nitrogen (80%). The pressure of each constituent in a mixture of gases is called its partial pressure. In a mixture of gases, the sum of the partial pressures is equal to the total pressure.

Each constituent in a mixture of gases, contributes to the total pressure in proportion to its volume contribution. Therefore, the partial pressure of oxygen in dry air is 160 mm Hg (20% of 760 mm Hg). A partial pressure of 160 mm Hg is more than adequate for normal body requirements.
The volume of the lungs is large compared with the amount of air taken in with each breath so that air exchange at each breath does not nearly empty and fill the lungs. Since oxygen is constantly being removed from the gas mixture in the alveoli and the carbon dioxide added to it through the alveolar walls, it follows that the composition of the alveolar air is different from that of the atmospheric air.

The composition of dry air at sea level is approximately:

Oxygen 14.5 %
Nitrogen 80.0 %
Carbon dioxide 5.5 %

Before working out the partial pressure of the constituents of the alveolar gas mixture, a further factor must be taken into consideration. Alveolar air is always completely saturated with water vapor at body temperature and this vapor exerts its own partial pressure.

Since the partial pressure of oxygen and carbon dioxide in the blood on the other side of the alveolar wall are different, there is rapid diffusion of these gases across the wall, so that almost instantaneous equilibrium is reached and the blood leaving the alveoli has the same partial pressures of oxygen and carbon dioxide as the alveolar air.

If the pressure of the oxygen in the lungs decreases, that is when rapid depressurization occurs, less oxygen will diffuse across the alveoli into the blood stream and less oxygen will reach the tissue. If cabin altitude is increased to 47,000 feet, even breathing 100 % oxygen, oxygen will diffuse out of the blood stream into the lungs and unconsciousness will be lost within 10 to 20 seconds!

Source: CFS Aviation Medicine, UK