Pressure Effects On The Body

Hyperbaric oxygen therapy is limited by toxic oxygen effects to a maximum pressure of 300kPa (3 bar). For some indications mixed gases are used for treatments up to 600kPa (6 bar). Within this pressure range, all solid

and liquid parts of the body are practically incompressible and there are no known clinical effects caused by the pressure per se. At therapeutic pressures, effects may only be noticed in relation to gas filled cavities1.

1.1 Ear

Provided that the auditory canal is not occluded by any means, the middle ear is the only gas filled cavity of the ear which may be affected by pressure changes. Together with the mastoid cells with which it communicates, it forms the only gas filled cavity in the head without a permanent open connection to the rhino-pharynx.

The inner ear with cochlea and vestibular organ is completely filled with perilymphatic and endolymphatic fluid and is thus not directly affected by pressure changes.

Figure 1.3-1. The human ear (Welslau2)

1.1.1 Pressure equalization

The Eustachian tube, which connects the middle ear and rhino-pharynx, is normally closed. When chewing, swallowing, or yawning mm. tensores veli palatini & levatores veli palatini are activated and open the pharyngeal orifice of the Eustachian tube, thus allowing different gas pressures in rhino-pharynx and middle ear to equalize. Without conscious actions the tube is opened once every one to four minutes. During rapid pressure changes tubes must be opened actively.

This may be done either by chewing, swallowing, or yawning where pressure is allowed to equalize passively. It may also be done by a Valsalva manoeuvre: mouth and nose are closed and a short forced exhalation attempt against the closed nose causes a rise of rhino-pharyngeal gas pressure, which opens the Eustachian tube orifice.

All these methods for pressure equalization depends on normal mucosa situation in the rhino-pharynx and Eustachian tube. When the mucosa is swollen during an upper respiratory tract infection, it may become impossible to equalize pressure in the middle ear.

1.1.2 Alternobaric vertigo

Vertigo is caused by mismatch of information deriving from right and left vestibular organs. In general, vertigo is connected with spontaneous nystagmus, and it may be accompanied by symptoms like nausea, vomiting, sweating and cardiovascular reactions.

In contrast with other less common cause of vertigo in hyperbaric therapy, alternobaric vertigo is caused by a difference of pressure between the two middle ear cavities. If vertigo is noticed during compression, it is called "alternobaric vertigo of descent", an expression which derived from diving medicine. The mechanism of alternobaric vertigo seems to be that different pressures in the left and right middle ear lead to different pressures against the round and oval windows on each sides, causing a differential sensitivity of the two vestibular organs3.

Vertigo may also happen when a forced Valsalva manoeuvre is effective only on one side. It is not clear whether this is a result either of a unilateral sudden change of pressure on the inner ear fluid or of unilateral inner ear liquid oscillation due to rapid movement of the oval and round windows. This cause of vertigo is described as being caused by a hypermobile stapes.

If vertigo becomes apparent during decompression, it is called "alternobaric vertigo of ascent". The mechanism here is unilateral blocking of the Eustachian tube with delayed pressure equalization on that side. Animal experiments have shown that increased lymphatic pressure in the inner ear causes increased electric activity of n. vestibulo-cochlearis3.

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