of descent in seawater increases the pressure by an additional atmosphere (14.7 psi).

The lungs and respiratory passages contain air at all times. In addition to the major air channels which include the nose, mouth, throat, larynx or voice box, and trachea, there are a number of side compartments issuing from the upper respiratory passages which are of importance in diving physiology. These include the eustachian tubes and the paranasal sinuses. During exposure of the body to pressure changes, as in diving, air contained in these cavities will undergo compression since the pressure of air delivered from the scuba is equilibrated to the surrounding hydrostatic pressure. The pressure of air breathed in and out of the lungs and respiratory passages will therefore also change in accordance with changes in the surrounding hydrostatic pressure.

2.2.1 Direct Effects of Pressure During Descent

Increased pressures can be tolerated providing they are uniformly distributed throughout the body. However, when outside pressure exceeds that inside the body air spaces, the difference in pressure may distort the shape of the involved tissues, causing injury to them. This is called barotrauma. The earliest signs are discomfort in the ears and sinuses, and swelling of tissues and blood vessels. As the pressure difference increases, small blood vessels rupture and heavy bleeding ensues.

Pressure in such spaces as the sinuses and the middle ear must be equalized on descent, or pressure differences will develop across their walls. Once equalized at a given depth, the air must also

vent freely during decrease of pressure. The effects of pressure on various parts of the body are discussed in the following paragraphs.

2.2.1.1 The Ears

Figure 2-6 illustrates the principal parts of the ear. Aerotitis (middle ear "squeeze") may be prevented by learning how to equalize pressure through the eustachian tubes, which lead from the middle ear cavity behind the eardrum to the upper expanded portion of the throat behind the nasal cavities above the palate.

Air contained within the eustachian tube is not in free communication with the throat in all individuals. This passageway is often relatively long and narrow, leaving the sides of the tube collapsed together, thereby making the passage of air in either direction, i.e., into or out of the middle ear compartment, somewhat difficult.

The passage of air from the middle ear outwards is ordinarily easier to accomplish than the admission of air into the middle ear from the throat. Consequently greater difficulty is generally experienced in equalizing the ears during descent. Air in the middle ear will tend to be compressed during descent and will require the addition of compressed air from the throat at successively greater depths in order to prevent inward rupture of the eardrum or hemorrhage from the mucous membranes lining the middle ear.

Successful methods of equalizing pressure are swallowing, yawning, or blowing against closed mouth and nostrils. When eustachian tube blockage prevents the equalization of pressure in the middle ear, causing painful inflammation with possible rupture of the eardrum, pain will be experienced in the first few feet of descent. Further descent will increase the pain, stretch the eardrum, and dilate and eventually rupture the blood vessels in the eardrum. The lining of the middle ear space is rich with blood vessels. These blood vessels may, upon serious pressure differentials between the pressure in the blood and the lower pressure in the middle ear space, expand, hemorrhage, and, in an extreme case, burst. Rupture of the eardrum may be caused by as little as three pounds of pressure differential which can take place anywhere in the water. column.

Infections including a head cold, sinusitis, or sore throat can seriously interfere with the equalization of pressure in the middle ear and sinuses. However,

a mild or slight head cold should not interfere with diving if a diver feels reasonably well and can equalize pressure without difficulty.

There are some prophylactic measures that can be employed for mild nasal congestion. A topical nasal decongestant (nose spray)-long acting type15 to 20 minutes prior to entering the water. Oxymetazoline is particularly effective. Use of a systemic decongestant, i.e., Pseudoephedrine, 15 to 20 minutes prior to entering the water may also be effective. A word of caution is necessary since it is possible that the diver may have a rare idiosyncratic reaction to the medication such as extreme drowsiness from an antihistamine, or excessive swelling of the nasal membranes shortly after using a spray. Disaster could result if this occurred in the water. For this reason all newly prescribed decongestant medications should be used on a trial basis at some period prior to diving.

Problems of equalization may also be avoided if extra care is used on descent. Some simple steps

are:

Descend feet first, preferably down the anchor

line or a drop line. Advance of the lower body portions before the eustachian tube areas permits a more gradual equalization of pressure to the middle ears.

Self-inflation of the middle ear should be carried out actively and conscientiously every two to three feet of descent. It may even be necessary to introduce the finger and thumb under the mask to squeeze the nostrils closed; this will flood the mask to adequately pressurize. It is well to inflate the middle ears prior to entering the water in anticipation of pressure changes. If pain develops, ascend a few feet and work on inflation, as outlined above. It is sometimes helpful to move the jaws or head from side to side or to move the head backward and forward. If inflation is impossible, abort the dive.

WARNING

Never Dive With an Upper Respiratory Congestion or Infection.

Cold water entering the external ear canal can upset the balance of a diver until the water in the external canal has been warmed by body heat. Pressure imbalance between the two sides of the eardrum can also be a contributing factor. Vertigo is often experienced as a result of such conditions.

Treatment of mild ear damage is symptomatic. Analgesics are indicated when pain is intense. Pain usually subsides gradually. If pain persists, thereby suggesting infection, systemic antibiotics and nasal and systemic vasoconstrictors are indicated to promote drainage and combat infection.

When the eardrum is ruptured, blood may drain through the external auditory canal but may not be visible. Retained blood is usually absorbed within a few days with no impairment of hearing. If the rupturing of the eardrum occurs in the water, vertigo may also be experienced; again, this will pass upon warming of the water in the middle ear. Local application of medication to the ear canal is ordinarily not advised, and care should be exercised to prevent water from entering the external auditory canal until healing is complete. This may take days or weeks, depending on the severity of the injury.

Another concern for the diver is ear infection. The prevention and treatment of this problem is found in Paragraph 17.2.5.

2.2.1.2 The Sinuses

Figure 2-7 shows the location of the sinus cavities.

Ordinarily, air in the sinuses behind the cheek and the brow and within the smaller sinuses situated in the roof of the nasal passages has free access to the respiratory passages through openings into the nose. However, in some instances the sinus openings may be abnormally small, chronically inflamed or acutely inflamed during a head cold. If any of these conditions exist, changes in depth will be marked by feeling tension, pressure, or pain in the region of the sinus involved. Vasoconstrictors may be helpful, as in barotitis, but if infection develops, as indicated by persistent pain, systemic antibiotics may be required. See Paragraph 2.2.1.1 for use of decongestants.

2.2.1.3 The Lungs

As long as normal breathing takes place with an ample breathing supply, the lungs and airways will equalize pressure without difficulty. If the breath is held during pressure increase, no difficulty arises until the total volume of air in the lungs is compressed to less than the residual volume. Pulmonary congestion, swelling, and hemorrhage of lung tissue then occurs in what is generally called "thoracic squeeze." Figure 2-8 shows pressure effects on lung volume.

In breath hold diving, no high pressure air is available to the lungs. Pressure compresses the diver's chest and raises his diaphragm: pressure equalization results from the fall in lung volume, i.e., Boyle's Law (P1V1 = P2V2). Lung volume limits the extent of tolerable compression. Descending to 33 feet will cut lung volume in half. Compression. down to residual volume (the amount of air in the lungs after forceful expiration) can be tolerated. When chest compression exceeds this limit, tissue trauma results. Fluid from capillaries and the tissues then enters the alveoli and the air passageways; gross hemorrhaging may occur. Mild lung barotrauma will cause only pain and slight exudation, which is quickly reabsorbed upon ascent. In dire cases, the lungs may be seriously damaged. Blood from the lungs may be coughed up after the dive. This form of trauma generally responds well to conservative treatment consisting of general supportive care, prevention of infection, and intermittent positive-pressure inhalation therapy (Schaefer et al. 1968).

Spraying of bronchodilators and aerosols, with gravitational drainage if hemorrhage or bruising has been severe, may prove beneficial.

Pulmonary edema (swelling) due to fluid in the lungs may follow the use of breathing apparatus with high inspiratory resistance. In an effort to maintain adequate lung ventilation during moderate activity, high negative pressure within the lungs may result in damage to small veins, seepage of fluid through membranes, and rupture. Coughing and shortness of breath are symptoms. X-rays of the chest may show patchy pulmonary infiltration, which clears within 24 hours without specific therapy.

Trauma to the lungs caused by compression is possible in pressure chambers if an individual stops breathing during compression, either voluntarily by breath holding, or involuntarily by unconsciousness, windpipe or tracheal obstruction, or convul

sions.

2.2.1.4 The Teeth

Pain in the mouth associated with an increase or decrease of pressure is seen infrequently. Whatever pain does occur is usually in the teeth of the upper jaw and associated with adjacent sinus changes evident on X-ray. Freedom from dental abnormality has been frequent enough in these cases to suggest that maxillary-sinus squeeze is the causative factor. It is possible that the presence of small gas bubbles in the tooth pulp may permit the soft tissue to be squeezed during pressure decrease. However, this theory has not been confirmed by dental examination.

2.2.2 Direct Effects of Pressure During Ascent

WARNING

A Diver Experiencing Blowup, or Overpressure Accident, Must Immediately Be Examined by a Physician to Rule Out Gas Embolism.

During pressure decrease, the air contained in body cavities expands. Normally, the air vents freely, and there are no difficulties. If breathing is normal during ascent, the expanding lung air is exhaled freely. However, if the breath is held, or there is a localized airway obstruction, the expand

ing air is retained causing overinflation and overpressurization of the lungs. For example, the air in the lungs at a depth of 66 feet gradually expands to three times its volume during ascent to the surface. The air volume can expand to the point of maximum inspiration safely, assuming the absence of airway obstruction. With further pressure decrease, overexpansion and, later, overpressurization of the lungs results in progressive distension of the alveoli. Such overdistension may be generalized with breath holding or insufficient exhalation, or localized because of partial or complete bronchial obstruction due to bronchial lesions, mucus, or bronchiospasm. Problems of lung overinflation can occur during ascent from depths as shallow as 7-10 feet if the breath is held. Several of the most commonly encountered physiological difficulties associated with pressure during ascent are included in the following paragraphs. Each may be prevented by breathing normally during ascent providing there is no localized airway obstruction.

WARNING

Do Not Hold Breath While Ascending.

Figure 2-9 shows the possible consequences of overinflation of the lungs.

2.2.2.1 Pneumothorax

Distended alveoli or temporarily enlarged fluidfilled blisters (emphysematous blebs) may rupture the membrane lining the chest (parietal pleura), causing pneumothorax.

Under pressure, this is extremely dangerous, because trapped intrapleural gas expands with continuing pressure decrease as the diver surfaces causing increased pressure in the chest cavity. The lungs may be collapsed by this pressure, and the heart and other vital organs may be pushed out of their normal position. Symptoms include sudden severe pain, reduction of breathing capability, and coughing of frothy blood.

The rapidity of development can cause sudden respiratory and circulatory difficulty, impaired cardiac function, or death from shock. Early diagnosis and prompt treatment with thoracentesis are essential. If recompression is required for concomitant conditions, the pneumothorax must be vented or released by a chest tube or other device before ascent is accomplished.