Barotrauma and Decompression Sickness

The human body contains gas in two forms, and a change of pressure affects each differently. Gas trapped in the middle ear, the sinuses, the gut and occasionally the teeth expands as the aircraft climbs and contracts as it descends. When it cannot escape or be replaced, it presses on the surrounding tissue and causes barotrauma. Gas dissolved in the blood and tissues, mainly nitrogen, can come out of solution as bubbles if the pressure falls far enough, causing decompression sickness (DCS).

For most pilots barotrauma is the everyday problem: a painful ear in the descent after flying with a cold. Decompression sickness is rarer, but becomes a real risk after a loss of cabin pressure at high altitude or after scuba diving. This article describes the physiology as it is taught for licences. Questions about an individual's fitness to fly belong with an aeromedical examiner.

Gas laws and trapped gas

Boyle's law states that, at constant temperature, the volume of a given mass of gas is inversely proportional to its pressure. Halve the pressure and the gas doubles in volume. In the ICAO standard atmosphere the pressure is about half its sea-level value at around 18,000 ft (500 hPa) and about a quarter at around 34,000 ft (250 hPa). A pocket of gas trapped at sea level would therefore double in volume by FL180 and quadruple by FL340. Even at the 8,000 ft maximum cabin altitude of a pressurised airliner in normal operation, where the standard pressure is about 753 hPa, it is about 35 per cent larger than on the ground (1,013 ÷ 753 ≈ 1.35).

Pressure changes fastest with height near the ground, so a given change of altitude produces the biggest pressure change at low level. That is why a descent into the circuit often troubles the ears more than the first part of the descent from cruise. Airliner cabins are normally limited to climbing at about 500 ft per minute and descending at about 300 ft per minute, largely to spare passengers' ears.

Henry's law states that, at equilibrium, the amount of a gas dissolved in a liquid is proportional to the pressure of that gas above it. At sea level the body's tissues hold dissolved nitrogen in balance with the air. Lower the pressure and the excess must leave; if it leaves too quickly, it forms bubbles. This is the basis of decompression sickness.

Ear barotrauma and the Eustachian tube

The outer ear is open to the air through the ear canal. The middle ear, behind the eardrum, is also an air space; it contains the three small bones, the malleus, incus and stapes, that carry sound to the fluid-filled inner ear. It is vented to the back of the nose and throat by the Eustachian tube, which lets the pressure on the inner side of the eardrum match the pressure outside.

The tube behaves like a flap valve. In the climb, the expanding air in the middle ear pushes it open and escapes almost without the pilot noticing, apart from a click or pop. In the descent the outside pressure rises, and air has to be driven back up the tube into the middle ear. The tube tends to stay closed unless it is actively opened. If the pressure difference builds up, it can hold the tube shut altogether.

The result is ear barotrauma, also called otic or otitic barotrauma, and known to pilots as an ear block. It occurs mainly in the descent. The eardrum is pushed inwards, causing pain and a loss of hearing that can last from hours to days. The eardrum can rupture, in flight or after landing, and fluid can collect in the middle ear and become infected. The usual cause is congestion around the tube from a cold, a sore throat or a nasal allergy.

Pressure changes can also disturb the balance organs of the inner ear, next to the middle ear. Pressure vertigo, a sudden sensation of spinning or tumbling, can follow a blocked Eustachian tube or a violent Valsalva manoeuvre (see spatial disorientation).

Clearing the ears

In the descent the Eustachian tube has to be opened regularly, before a large pressure difference builds up. The gentle methods come first:

• swallowing, yawning, or moving the lower jaw from side to side
• tensing the muscles of the throat
• swallowing with the nose pinched.

If these fail, the Valsalva manoeuvre is used: close the mouth, pinch the nose and blow gently against the closed nostrils, forcing air up the tubes into the middle ears. It is done gently, because a violent Valsalva can itself cause pressure vertigo. The Frenzel manoeuvre, a movement similar to stifling a sneeze, is a less forceful alternative.

If an ear still will not clear, the answer is to stop the pressure difference growing. Level off or climb back a little until it eases, clear the ear, then continue the descent more slowly. Pressing on with a fast descent risks a ruptured eardrum.

Prevention is simpler than cure. The FAA advises against flying with an upper respiratory infection or a nasal allergy. Decongestant sprays and drops usually do not give adequate protection, and oral decongestants have side effects that can impair performance. Under both EASA and FAA rules, a pilot must not fly while aware of a decrease in medical fitness, and FAA material treats a cold that blocks the ears and sinuses as such a condition. An ear block that does not clear soon after landing needs a doctor.

Sinus, dental and gut barotrauma

Sinus barotrauma, or sinus block, affects the air-filled sinuses in the skull, which vent to the nose through small openings. The frontal sinuses lie above the eyebrows and the maxillary sinuses in the upper cheeks. A cold, sinusitis or an allergy can block an opening. The FAA's AIM notes that a sinus block occurs most often in the descent, but unlike an ear block it can also occur in the climb, and EASA texts note that the pain may then be just as acute. The pain is often excruciating and can be severe enough to incapacitate. It usually spreads from around the eyes to the temples, a maxillary block can make the upper teeth ache, and bloody mucus may come from the nose. The remedy is to return towards the altitude at which the pain began and then descend slowly.

Aerodontalgia, dental barotrauma, is toothache at altitude. A healthy tooth contains no gas, but a small pocket trapped under an old, poor or recent filling, or in an abscess, expands in the climb and presses on the nerve. The pain typically comes on in the climb and eases in the descent. Good dental care prevents it, and a maxillary sinus block can produce similar pain.

Gastrointestinal barotrauma comes from gas in the stomach and intestines, which expands in the climb. Gas in the small intestine has no easy exit and causes the most trouble: discomfort, severe pain and occasionally fainting. Gas-forming food and drink, such as beans, cabbage, raw apples, beer and curries, rushed or large meals and chewing gum before flight make it worse.

Types of cabin decompression

A pressurised aircraft normally keeps its cabin at or below 8,000 ft, and certification rules require a warning when the cabin altitude exceeds 10,000 ft. A failure lets the cabin altitude climb towards the aircraft's actual altitude. Three types are recognised (see decompression):

• Slow decompression takes minutes, typically from a leaking seal or a system fault. The first signs are often pressure felt in the ears, sinuses, teeth or gut. Hypoxia can creep up unnoticed.
• Rapid decompression takes a few seconds, slowly enough for the lungs to vent faster than the cabin does.
• Explosive decompression takes less than about half a second, faster than the lungs can vent, and risks lung injury. The breath must never be held during any decompression.

The most obvious sign of a rapid or explosive decompression is a sudden mist as the cooling air's moisture condenses, with a bang or rush of air, debris, cold and ear pain. Every trapped-gas problem above then arrives at once, with hypoxia and a risk of decompression sickness.

Time of useful consciousness

The time of useful consciousness (TUC) is the time from the loss of an adequate oxygen supply to the point at which a person can no longer take effective action to help themselves. It ends well before unconsciousness. The FAA uses effective performance time (EPT) as another name for it; EASA teaching uses EPT for the shorter, highly individual period of unimpaired performance, which always lies within the TUC.

These FAA figures, from AC 61-107B, apply to a person at rest. EASA training material gives the same order of magnitude, such as 1 to 2 minutes at 30,000 ft and 15 to 20 seconds at 40,000 ft, although some European texts give 30 to 90 seconds at 35,000 ft. Activity shortens the time sharply: at 20,000 ft, EASA texts give about 30 minutes at rest but only about 5 minutes with moderate activity. After a rapid decompression the TUC should be assumed to be roughly halved, because the sudden fall in pressure reverses the oxygen gradient in the lungs. Smoking, illness and fatigue shorten it further. The only safe response is oxygen first (see hypoxia and hyperventilation).

Decompression sickness

Decompression sickness (DCS) is caused by nitrogen coming out of solution in the body as bubbles when the surrounding pressure falls. It is unlikely below about 14,000 ft and unusual below 18,000 ft in people who have not been diving. Above 18,000 ft it becomes more likely the higher the altitude and the longer the exposure, and exposure above 25,000 ft is normally associated with a risk of DCS. Age, obesity, cold and hypoxia increase the risk, and recent diving increases it greatly.

The symptoms are traditionally grouped into four types:

• The bends: pain in and around the joints, such as the shoulders, elbows, wrists, knees and ankles, the classic symptom.
• The creeps: bubbles in the skin, felt as a crawling sensation, as if ants were moving under it.
• The chokes: bubbles in the small blood vessels of the lungs, causing breathing difficulty.
• The staggers: bubbles affecting the brain and nervous system.

Symptoms may appear during the exposure or develop after descent and landing, and collapse can follow some hours later. The treatment is 100 per cent oxygen, descent, a landing as soon as possible and urgent medical attention, which may include recompression in a hyperbaric chamber. Anyone with joint pain, skin symptoms or neurological signs after a decompression should be assessed on landing.

Flying after scuba diving

A diver breathing compressed air at depth absorbs extra nitrogen: every 10 m (about 33 ft) of seawater adds roughly one atmosphere of pressure. After surfacing, the surplus takes many hours to leave the body. Flying lowers the pressure below its sea-level value, so bubbles can form at altitudes that would be harmless to someone who had not dived. DCS after diving has been reported at altitudes as low as 6,000 ft.

The FAA's AIM (8-1-2) recommends:

• at least 12 hours after a dive that did not require a controlled ascent (a no-decompression-stop dive) before flying at altitudes up to 8,000 ft
• at least 24 hours after a dive that required a controlled ascent (decompression stops)
• at least 24 hours after any dive before flying above 8,000 ft.

These are actual flight altitudes, not cabin altitudes, to allow for a possible decompression. EASA human performance teaching gives the same basic figure of 12 hours, extended to 24 hours after dives requiring decompression stops. Some European texts instead give 24 hours after any dive deeper than 30 ft.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.