the effect of ascent and descent on gas
collections within the body
Dr Dougal Watson
“Mummy my ears just went
pop!” chiming from the back seat as you drive the family in the hills.
The RAAF Doctor’s advice to aircrew “You should never fly when you’ve got
a cold, not even a hint of a cold.”
“BUUURRRRP !!!” or similar unmentionable noises denoting escape of gas
from the body as you climb towards 8,000 ft.
A Royal Flying Doctor Service speaker advising a group of doctors
concerning air ambulance usage Always x-ray penetrating injuries, where
air may have entered the body part involved, before you consider Air
Ambulance transport. When in doubt insist on a sea level cabin altitude.
This is especially important for open skull fractures and penetrating eye
What do these four, seemingly unrelated, events have in common? You’ve
bothered to read this far, so I have little doubt that you’ve read the
title and now realise that these are all manifestations of the expansion
of gas within cavities of the human body during ascent in altitude.
Most aviators will remember from their license studies that as you climb
in our atmosphere the air density and the air pressure fall. This occurs
in the real atmosphere as well as the ICAO International Standard
Atmosphere that your texts mention. The drop in pressure is such that at
18,000ft. the air pressure is around half that of sea level, and at about
33,000 ft. it is a quarter of it’s sea level value. This reduction in air
pressure has a number of possible adverse effects on aviators, gas
expansion within body cavities is covered here.
The laws of gas behaviour dictate that as the pressure falls a given
amount of gas will expand (mass and temperature remaining constant). There
are a number of collections of gas within the body. So as a person within
an aeroplane, is subjected to progressively higher altitude (and therefore
progressively less air pressure) the collections of gas within his body
will expand. This is not a problem if the collection of gas communicates
freely with the outside (as with the mouth, nose, and to a degree the
sinuses and the middle ear cavity). The gas merely expands and escapes to
the outside atmosphere. But if the gas doesn’t communicate freely with the
outside it’s expansion will cause a build up in pressure on the cavity
walls which can cause discomfort and even impair the function of the organ
involved (consider gas trapped in tooth abscesses or fillings, or the
sinuses when you’ve got a cold).
Several areas of the body normally contain gas, these include the lungs
and air-passages, the sinuses of the face, the stomach and bowel, and the
middle ear cavity. Gas may be present in other areas of the body under
abnormal circumstances such as the gas produced when a tooth abscess
forms, or sometimes a small amount of gas will occur under a tooth’s
filling and after an accident gas may be able to enter the skull, the
chest or abdomen, or even the eye. The effects that gas expansion has on
each area of the body will be considered individually.
The Middle Ear Cavity
The ear drum separates the middle ear cavity from the outside. The soft
walled Eustachian tube connects the middle ear cavity to the back of the
throat. Figure 1 shows the normal middle ear at sea level, you will notice
that the ear drum is not bulging in either direction and that the pressure
within the middle ear equals the outside air pressure.
Diagram of the normal ear at sea level showing the flat drum and middle
ear pressure equal to the outside air pressure.
During ascent the gas
(air) in the middle ear cavity expands and a small amount of pressure
builds up against the ear drum causing them to bulge outwards ever so
slightly (that ‘fullness’ you feel in your ears just before they ‘Pop’).
This pressure builds until it is sufficient to vent gas through the floppy
walled Eustachian tubes. As the gas vents the ear drum settles quickly
back into it’s normal position. This venting of gas and movement of the
drum is the ‘popping’ feeling we all know so well (see figure 2.). Popping
of the ears is completely normal and usually occurs every 500 to 1,000 ft.
of ascent, you may not even be aware that it is happening. Yawning
swallowing, and chewing tend to make it easier for the Eustachian tube to
open and so can assist in popping the ears.
Conversely, during descent, as the air pressure increases the gas within
the ear cavity will contract producing a slight vacuum. Normally the ear
drum will bulge inwards slightly with a similar ‘fullness’ as before and
then air will vent into the middle ear cavity, again with a ‘pop’. Because
of the floppy walls of the Eustachian tube the air tends to have more
difficulty moving in this direction than in the previous example.
The effects of increasing altitude on the middle ear. Note the drum
bulging outward and the increased pressure compared to the outside. The
‘pop’ is the ear drum moving quickly back to its normal position as the
pressure vents through the Eustachian tube.
Problems will develop if
the air cannot vent Inwards or outwards through the Eustachian tube to
equalise pressure with the outside. If for some reason the Eustachian tube
is blocked air will not be able to move into or out of the middle ear to
Pressure will build up in the middle ear cavity during ascent (or
conversely a vacuum on descent) causing the drum to bulge further than
before and cause pain. if the Eustachian blockage is not relieved and the
descent (or ascent) continued the drum will bulge further and further
until it ruptures or tears, causing sudden short lived severe pain
followed quickly by some relief of discomfort and impaired hearing (Fig.3)
The most frequent cause of blockage to the Eustachian tubes is the common
cold. The membranes lining the Eustachian tube become swollen effectively
blocking the tube. So now you can appreciate why flying with a cold is
dangerous, and in fact why an ordinary cold is sufficient to ‘ground’ our
Air Force pilots. Normal Eustachian tubes are often partially blocked
during descent and many people may get milder versions of the problems
Descent with a blocked Eustachian tube will cause a progressive increase
in pressure on the ear drum. Eventually, if the pressure is not relieved,
the drum will rupture (Otic barotrauma).
Interestingly children and
babies have shorter straighter Eustachian tubes than adults and tend to
have less difficulty equalising the middle ear pressure (“popping”).
Some susceptible individuals can suffer from severe dizziness (vertigo)
when the pressure in their middle ear cavity isn’t equalised with the
outside. A pilot can be rapidly and completely disabled by even mild
Otic Barotrauma, literally meaning “ear-pressure-damage”, is the medical
term for the problems mentioned above.
It’s well enough to know about such problems, but it’s just as important
to know what you, as the pilot, can do about them. First, and foremost,
it’s downright foolish to fly when you’ve got a cold, the flu, sinusitis
or any similar problem. Air Force pilots are not permitted to fly with any
of the above problems and usually ground themselves without necessarily
seeing a Doctor.
The CAA regulations are
not as rigid as those of the RAAF and many GA pilots do fly when suffering
from colds etc. Most of these pilots do not suffer major problems, but do
they realise the risk they are taking (and the risk that their poor
unsuspecting passengers are taking)?
Air Force pilots are not permitted to fly with any of about problems and
usually ground themselves without necessarily seeing a Doctor. The CAA
regulations are not as rigid as those of the RAAF and many G.A. pilots do
fly when suffering from colds etc. Most of these pilots do not suffer
major problems, but do they realise the risk they are taking and the risk
that their poor unsuspecting passengers are taking?
Sudden perforation of an ear drum can be quite a disabling occurrence.
Chewing gum or sucking sweets can assist the Eustachian tubes in opening
and may help avoid discomfort or even ear drum damage.
Decongestant tablets such as Sudafed and sprays such as Drixene act by
reducing the swelling around the Eustachian tubes thus making it easier
for them to open. These medications are fine for passengers in aircraft
but should not be used by pilots. Although not specifically banned by the
CAA, I would suggest that if you need these medications to fly you are
putting yourself and your passengers at unnecessary risk.
The other main methods of relieving pressure in the middle ears and
avoiding otic barotrauma are active manoeuvres such as swallowing and
chewing already mentioned, and the Valsalva Manoeuvre. This involves
increasing the pressure in your nose and throat by trying to blow your
nose while keeping your mouth and nose tightly closed. This increase in
pressure will often be enough to force the Eustachian tube open and
relieve the pressure, or vacuum within. If you are not familiar with the
Valsalva I suggest you contact your DME (CAA Designated Medical Examiner)
for advice and training.
If you do, for example, run into such ear problems on descent and are
unable to clear your ears using the Valsalva manoeuvre you can always
climb again (fuel permitting) until your ears are comfortable then begin a
slower descent profile clearing your ears more frequently. The reverse
applies to ear discomfort during ascent where relief will be afforded by
Diagram showing some of the sinuses around the nose.
The sinuses are air filled
cavities situated within the bones of the face and skull. They open into
the back of the nose through thin tunnels in the bone (See Figure 4).
As with the middle ear cavity, the air within the sinuses expands on
ascent and contracts on descent. Normally the sinuses vent to the back of
the nose easily and little more than a tickling sensation is felt. If,
however, the linings of the sinuses are swollen and their tiny vent tunnel
blocked the air will not be easily able to move to and fro to equalise
pressure with the outside. This can be caused by inflammation of the
sinus’ membranes during a cold or sinusitis. When pressure (or a vacuum)
builds up in the sinuses it is felt as pain in an area of the face
corresponding to the sinus concerned. Pain may be felt in the forehead,
the cheek, or deep within the head and can be quite severe and accompanied
by profuse watering of the eyes. you don’t need me to tell you that severe
pain and constant profuse tears in your eyes is going to impair your
aircraft control and decision making.
To avoid this sinus barotrauma (sinus-pressure-damage) ids common sense
and a little bit of knowledge that we need. As before, flying with a head
cold or sinusitis puts you and your passengers, at an unnecessary risk
Don’t do it! If you, or your passengers, do get some sinus problems during
ascent or descent treatment is much the same as with middle ear cavity
problems. Firstly try nasal decongestants (e.g. Drixene) if you have them,
this may reduce the swelling enough to let the air vent. The Valsalva
manoeuvre doesn’t tend to work as well with sinuses as with the ear, but
it still worth trying this procedure to equalise the pressures. If these
efforts are unsuccessful and the symptoms persist, or worsen you should
return to the altitude you were at prior to problems (again, fuel
permitting). The subsequent descent should be taken at a slower rate
allowing the sinuses to equalise pressure as you go.
It is perfectly normal for our stomach and intestines to contain around a
litre of gas. This is a mixture of swal lowed air and other gasses
produced by digestion. From the principles discussed earlier it’s not
going to be too difficult to deduce what will happen to this gas on
ascent. Of course, with ascent this gas will expand and when it has
expanded sufficiently it will usually vent Itself from the body via the
mouth or the anus.
Gas expansion in this way does not generally cause problems. However some
people, often relatively inexperienced aircrew, do have difficulty venting
this expanding gas during ascent and suffer a bloated feeling and some
abdominal pains. On the rare occasion this pain can be severe.
Abdominal distension with altitude will be increased if there is more gas
in the stomach and intestines to begin with. This can occur with
intestinal infections (e.g gastro’), increased air swallowing (anxiety, or
rushing a meal and not chewing enough prior to swallowing your food), and
certain gas causing foods (cabbage, cauliflower, Brussels sprouts,
turnips, raw apple, dried peas and beans, fizzy drinks and I’ve even heard
some people mention that baked beans can cause increased gas production).
People who frequently fly at high altitude soon become quite expert at
venting the expanding gas from their stomach and intestines and avoid
foods and drinks that disagree’ with them prior to flight.
The lungs and wind-pipe are full of air yet virtually never are there
problems as the air expands. The airways leading from the lungs are quite
large and usually allow the expanding gas to vent easily without causing a
pressure buildup. There has never been a case of lung damage due to gas
expansion at the ascent rates normally encountered in General Aviation,
and to my knowledge none have occurred even with explosive cabin
decompression (e.g. if a 747 loses an emergency door at altitude).
Lung damage would be highly likely if an individual were to hold their
breath during such a rapid decompression, the expanding gas would not be
able to vent as usual and pressure would build up. The only situation
where this is likely is in a practice Decompression Chamber run, and all
participants are thoroughly briefed prior to such runs.
Large changes in altitude can cause toothache (Barodontalgia =
pressure-tooth-pain). The reason this occurs is either the presence of
small pockets of gas in deep (usually unlined) fillings, or collections of
gas in areas of decay, gum inflammation, or root abscesses The pain can be
There is little you can do, except descend, to treat this pain. A high
degree of dental hygiene and care is mandatory for aviators. Regular
checkups with your dentist avoid this unnecessary risk of incapacitation.
The above are all (with the exception of the tooth filling) natural
collection of gas within the body but items worth realising that gas can
be introduced into the body in a variety of abnormal circumstances. These
situations are really the realm of a doctor but it is certainly worthwhile
that GA pilots are, at least, aware of them (You may have flown for the
Flying Doctor or performed a Medical Evacuation and wondered why the
Medical staff insisted on “sea level cabin altitude”).
Consider a bullet wound to the chest or abdomen, or a badly fractured
skull with a large cut over it, or even an eye injured when a small sliver
of metal flew into it. All of these are potentially circumstances where
air may have. abnormal entered and be trapped within the body. Consider
again the fractured skull where a small amount of air has entered the
skull through the cut and fracture. What happens if this patient ascends
in altitude (e.g.. aerial ambulance transport where sea level cabin is not
maintained)? The small quantity of gas trapped within the skull will
expand and put pressure on the brain, not a desirable situation in someone
who already has a head injury.
This doesn’t mean you should refuse to carry people with these injuries,
the medical staff involved decide whether the patient is fit for aerial
transport or not. If the facilities are available they will usually x-ray
such an injury to see if there are any abnormal collections of gas. Still,
it doesn’t hurt for your to understand “why”, you probably won’t curse as
much when the nurse insists on a sea level cabin next time.
Changing barometric pressure, especially if rapid, can cause discomfort
and damage unless the pressure within the body’s gas containing cavities
is able to equilibrate with the outside air pressure. With practice and
experience aircrew are able to avoid, or treat, these pressure related
* THE FALL IN AIR PRESSURE DURING ASCENT CAUSES
EXPANSION OF GAS WITHIN THE BODY (AND VICE-VERSA). EUSTACHIAN TUBES ARE
ESSENTIALLY FLOPPY VALVES AND SHOULD BE OPENED REGULARLY DURING DESCENT.
* PREVENTION IS FAR BETTER THAN CURE SO DON’T FLY WITH A COLD, WATCH WHAT
FOODS YOU EAT, AND VISIT YOUR DENTIST REGULARLY.
* IT MAY BE WORTHWHILE HAVING A BOTTLE OF NASAL DECONGESTANT SPRAY IN THE
CABIN TO HELP IF PROBLEMS ARISE (YOURSELF OR PASSENGERS)