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SCUBA DIVING EXPLAINED

Questions and Answers on
Physiology and Medical Aspects of Scuba Diving

 

Lawrence Martin, M.D. Copyright 1997

 

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SECTION N

Medical Fitness for Diving: Guidelines Real and Imagined

ARE THERE REGULATIONS REGARDING WHO MAY SCUBA DIVE?

No. There are no local, state or federal laws regulating recreational scuba diving. Some regulations apply to certain aspects of equipment, such as tank inspection and air compressor maintenance, but there are no regulations regarding divers.

New applicants for certification may be required to obtain medical clearance if they acknowledge a problem on their health questionnaire, but there is nothing to compel an applicant to admit an active medical problem. On the other hand, relatively few physicians are familiar with scuba diving, so an encounter between diver-with-medical-question and a physician may not result in correct advice or information. Even when physicians are knowledge-able in this area, there is apt to be disagreement about what disqualifies people from diving.

WHO IS FIT FOR DIVING?

Before discussing medical conditions that might prohibit diving, it will be useful to discuss fitness in a general sense. In Section B I explained that diving requires at least a sound mind, heart and lungs. Beyond these basics, much has been written about physical fitness for diving, including the subjects of exercise, nutrition, and physical stamina.

Clearly one does not have to be an athlete or body builder to dive. On the other hand, diving is not recommended for true "couch potatoes" either. It is probably risky to be sedentary in life style overweight, no exercise, no routine physical exertion on the job or otherwise and then go diving. Diving physicians believe that the more physically fit the individual, the less risk in diving. There are several cogent reasons for scuba divers to be physically fit.

1) Diving can be strenuous. It can require sudden bursts of physical exertion such as when swimming against a current, climbing onto an unsteady boat, or rescuing a buddy. Obviously the more fit you are, the better you can handle heightened physical requirements. Many a diver has "tired out" and had to be rescued because he didn't have the stamina for unexpected physical stress.

2) Physically fit people tend to use less air than the unfit. Hyperventilation and panic stress reactions are more likely to occur in the physically unfit.

3) Physical fitness reduces the risk of heart attack, which is a major cause of diving fatality.

4) Physical fitness may reduce the risk of developing DCS. Apart from the exercise jocks, how does someone know if they are physically fit? Sophisticated exercise testing can give numbers to go by, but such tests are cumbersome, expensive and hardly necessary (with the exception of testing for underlying heart disease). In truth, if the question is just about overall physical fitness, and not about underlying heart disease, you are probably the best judge. You are probably physically fit for scuba diving if you can swim several laps in the pool without difficulty (the basic swimming test for enrollment into a scuba certification course), ride a bicycle for half an hour, or jog a half mile without collapsing. Certainly if you perform aerobic exercise regularly you are probably physically fit.

You are probably physically unfit if you don't regularly exercise, or you are short of breath with simple efforts like stair climbing or brisk walking, or you exceed 20% of your ideal body weight and/or smoke heavily. In other words, the question of physical fitness for an activity like scuba diving is mostly one of common sense. Common sense suggests that, since a scuba diver's life may depend on heavy physical exertion at some point, you should not be grossly overweight, should not smoke, and you should engage in some aerobic exercise on a frequent basis. The exercise could be bicycling, running, swimming, racquet sports, or any other aerobic activity. (Exceptions to this recommendation are physically impaired people who may take special training to go scuba diving; see below.)

CAN ONE DIVE WHILE TAKING A DECONGESTANT?

Generally, yes. Decongestants are commonly employed to clear up nasal inflammation and to help shrink mucous membranes in the head. They are not treatment for infection of ears or sinuses, but only for symptomatic relief of mild head congestion. Over-the-counter decongestants commonly employed include Dimetapp, Sudafed, and various acetaminophen (Tylenol) preparations combined with an antihistamine. Some caveats: you should feel well, not have any side effects from the medication (such as drowsiness or dizziness), and be able to clear your ears without difficulty.

WHAT CONDITIONS SHOULD PROHIBIT DIVING?

Table 1 lists conditions that physicians involved in dive medicine generally regard as permanently prohibitive of scuba diving. Table 2 lists conditions that are self-limiting or treatable, and that would prohibit diving only until resolved or adequately treated. Table 3 lists chronic conditions that might or might not prohibit diving, depending on a medical assessment. In reviewing these lists, keep in mind the following:

As explained above, scuba diving can be a physically demanding activity when everything goes well; it can become more so when there are adverse conditions such as waves, current, poor visibility, faulty equipment, etc. People not in good health should not dive. The problem is in defining "good health." From the diver's point of view, he or she should feel well, not be fatigued, and have no medical problem that might affect diving.


TABLE 1. Conditions that should prohibit scuba diving

Any significant, exercise-limiting problem. Examples: angina [heart pain], chronic asthma, heart failure, cardiomyopathy [heart muscle weakness], cardiac arrhythmia that limits exercise, pulmonary insufficiency, high blood PCO2 or low PO2 (an indication of respiratory insufficiency), vertigo or other nervous system instability.

Presence of non-ventable air spaces, e.g., bullous emphysema, blebs.

History of air embolism from ruptured lung

History of spontaneous pneumothorax

Serious ear problems, such as: permanent perforation of tympanic membrane; history of otosclerotic surgery; chronic middle or inner ear infection; Meniere's disease

Psychiatric or emotional instability

* Epilepsy requiring drug treatment for prevention of seizures

* Diabetes requiring insulin for control of blood sugar levels

* Asthma requiring drug treatment for control of symptoms

Alcoholism or other dependency on mind-altering drugs

Sickle cell disease

Hemophilia or other severe clotting disorders

Diffuse scarring in the lungs from any cause (e.g., idiopathic pulmonary fibrosis, sarcoidosis, pneumoconiosis)

From the physician's point of view, good health means at least a good heart and lungs, a sound mind, and no significant problem with ears or sinuses. Any impairment of heart or lung function, any mental impairment, or any blockage of ears or sinuses could be disastrous underwater. There are other medical problems, of course, which can affect safe diving, but the main concerns are heart, lung or mental impairment, and blockage of air passages in the head.

Someone physically disabled from a musculoskeletal problem, such as leg or arm paralysis, might safely scuba dive under certain conditions. There are programs to train physically-disabled divers, who then can dive with unimpaired buddies. The Handicapped Scuba Association trains scuba instructors who wish to certify disabled people for diving.

TABLE 2. Conditions or situations that should delay scuba diving until resolved or treated

Anemia (low blood count)

Sinusitis or sinus infection

Ear infection (any type)

Head or chest cold

Continuous cough

Pregnancy

Acute orthopedic or musculoskeletal problems that impair underwater movement (e.g., a broken arm or leg)

Use of anti-anxiety or sedative medications

Any acute impairment in vision (e.g., eye infection, need for eye patch)

A non-diver with any of the above conditions will invariably be advised not to take up the sport. However, experienced divers who develop any of these conditions often keep on diving. There is documentation that some asthmatics who use medication, some insulin-dependent diabetics, and some epileptics taking anti-seizure medication, do dive. If these people feel fit and secure with their skills, they may never ask anyone "for clearance." Alternatively, some experienced divers with these and other medical problems are concerned enough to consult a diving specialist. Depending on the problem, the specialist may do an evaluation and then offer advice based on the specific findings.

This dual standard between new and experienced divers is not as illogical as it may first appear. For most divers it takes at least 50 dives to feel fully comfortable in the water. Some skills like buoyancy control only come with experience and much trial and error. A specialist in diving medicine will be more inclined to allow an experienced diver to return to scuba after, say, a heart operation, than for a similar patient who has never been under water to take up the sport. Whether medically condoned or not, the fact is that many people with some of the medical conditions listed above do scuba dive.

Determining medical suitability for diving is often a matter of opinion. Given the existence of a medical problem about which there are differing opinions, many physicians will opt for the safe route and rule against diving. It is up to the patient to take any medical advice, of course. An alternative water activity that can carry great pleasure with little risk is snorkeling using mask, fins, and snorkel to swim on the surface while viewing underwater marine life.

Boyle's law explains why patients with wheezing, sinus congestion, colds and ear infections should not dive until fully recovered; and why patients with bullous lung disease (abnormal pockets of air within the lungs) should never dive. Any air space that cannot be vented or equalized presents serious potential hazard to the scuba diver. Divers with prohibitive medical problems may rationalize that they can avoid trouble by staying shallow, e.g., diving to less than 35 feet. While shallow diving will prevent nitrogen narcosis and decompression sickness, the risk of barotrauma is actually greater near the surface, because of the larger percentage change in air space volume.

Any doubtful case should be discussed with, or referred to, a diving medicine specialist.

WHY ARE SOME CONDITIONS CONTROVERSIAL FOR DIVING?

Controversy results from disagreement among specialists about who should not dive. These disagreements usually appear in journal articles, in public forums such as scuba diving conferences, and when physicians and scuba instructors talk among themselves. There are five reasons for controversy about medical fitness for diving; these reasons are listed in the box and then explained in detail.


MEDICAL FITNESS FOR DIVING: REASON FOR CONTROVERSY

1. Objections for a given medical condition are largely theoretical

2. Lack of good evidence to support theoretical objection

3. Wide variability for each condition

4. Affected people with the condition do dive

5. Differences in philosophy about participation

1. For many conditions there are sound theoretical objections to diving, and these objections are often put forth by acknowledged medical experts. Thus it "makes sense" to write in a medical journal that anyone who might have an asthma attack under water, or who might develop a heart attack while diving, should not participate. It also makes sense that, if a diver's lungs trap air, there is a high risk of lung rupture and air embolism; or that, if a diver ever had a seizure before diving, he might have another seizure at depth and drown. Since scuba diving is voluntary and recreational, it is hard to argue against such seemingly plausible, albeit theoretical, objections.

2. On the other hand, someone who really wants to dive has a right to say, "show me why not." And guess what? Typically, there is no good evidence to prove that more accidents occur among divers with theoretically risky conditions such as asthma, diabetes or epilepsy. This does not mean that diving with these and other medical conditions is safe, only that an increased risk can't be proved or disproved.

So the diver legitimately asks: if my condition is supposed to be so risky for diving, how come you can't prove it? There are several possible reasons why proof may be lacking.

First, diving accidents are relatively rare, so it's hard to show any trend among specific causes. Less than 1000 diving accidents are reported to DAN each year out of hundreds of thousands or perhaps millions of divers (among North American residents); of the accidents, only about half are confirmed as meeting criteria for DCS or AGE. Thus, there may not be enough injured divers with a specific condition (e.g., asthma) to prove anything statistical.

A second reason is that most scuba diving deaths are either due to an unforeseen event that could occur anywhere (such as sudden death from heart attack), to some catastrophe like "out of air," to entrapment in an overhead environment (e.g., a cave), or to panic; when these deaths are examined closely (including autopsies) there is no evidence to show that asthma, diabetes, epilepsy (or most other conditions considered at risk' for diving) played a significant role. The one major underlying condition often found on close examination of diving deaths is heart disease.

Third, the theoretical objections may be correct, but only for the extremes of a given condition. People with Condition X who scuba dive may have the most stable state of that condition. Patients with the more severe form of Condition X may have selected themselves out of the diving population, either because they were advised against diving or because they experienced problems and dropped out on their own. This happened to one physician with asthma, who wrote about his experience in a medical journal (Martindale 1990).

Whatever the reasons, doctors and scientists like to prove things with studies, but studies proving an increased risk are lacking for all conditions listed in the Tables. Common sense prevails, of course, when there is obvious impairment. Only a fool would demand a "study" to prove that someone short of breath from emphysema or heart failure should not scuba dive.

3. Each diagnosis pertains to a diverse group of patients with a wide range of symptoms. Some asthmatics have symptoms only once every 10 years, whereas others have trouble every day. The statement that "asthma prohibits diving" will seem illogical if it is applied equally to both extremes.

Similarly, some patients with diabetes require meticulous control of blood sugar with twice daily insulin injections, and other patients can be well controlled by diet alone. To state that "diabetics" should not dive is far too broad a prohibition, and sure to engender disagreement and controversy. In the real world, such broad and unqualified prohibitions serve no useful purpose.

4. It is a fact that many people with asthma, diabetes, epilepsy and other medical conditions do dive. Arguments against diving with a history of these and other "risky" conditions will not mean much to people who have been diving with the disease for years, without a mishap. This is certainly one reason for the controversy about asthma and diving (Section O).

5. Finally, controversy arises because of simple differences in philosophy about participation. Some physicians have a liberal attitude, exemplified by the statement that patients "should lead full lives without any personal restrictions" (Dreifuss 1985). Other physicians are more cautious, as exemplified by the statements that a patient "should take up another sport less risky for him and others" (Millington 1988). This variation in philosophy about risk-taking is prominent in opinions about asthma and diving (Section O).

The bottom line is that no one should be excluded from diving because of a diagnostic label, whether it be "asthma" or "diabetes" or whatever. Exclusion for a medical problem should take into account the person's current state of health, recent medical history, and a careful assessment of what is likely to occur under ordinary diving conditions. Sometimes evaluation is easy, sometimes difficult. Here are two examples.

Easy Decision

A 22-year-old man was referred by a disabled diving group for an evaluation. His diagnosis was Duchenne muscular dystrophy, a condition that weakens the muscles of the arms and legs. The diving instructor was prepared to begin training him in scuba, provided that he was otherwise fit. The request for evaluation was not unreasonable, since at the time the young man was driving a car and living independently. On exam he had some mild weakness of his upper arms, and a few other motor abnormalities related to muscular dystrophy. However, most disturbing was his recent history of shortness of breath. On further evaluation I found that he also had weakened heart muscle and heart failure, a complication of his basic disease. This finding made him physically unfit to scuba dive.

Difficult Decision

A 27-year-old man came for evaluation because "I foolishly checked asthma on my health form" (for a scuba certification course). He had a history of mild asthma and admitted to using an asthma inhaler "a couple of times" during the year. I explained that anyone needing an asthma inhaler should not scuba dive. This didn't concern him because, he said, he had already made several dives in resort courses and had experienced no problem. Furthermore, a buddy of his "uses an inhaler and has no trouble diving." Lung exam revealed no wheezing but a breathing test showed what doctors call a slight obstruction to air flow. However, he claimed to feel normal and thought that any prohibition to his diving was "ludicrous" because "I already dive." He just wanted to be certified, he said, and made it clear that he "could just start all over again with another dive shop." I agreed to reserve judgment until some more tests were done, but he did not return as scheduled. Presumably he went elsewhere.

WHY ARE SOME CONDITIONS CONSIDERED PROHIBITIVE FOR SCUBA DIVING?

Tables 1 through 3 include many common medical conditions, some of which warrant further discussion. (Asthma poses one of the most difficult decisions regarding fitness for scuba diving, and is discussed in Section O.) In all cases, of course, the individual with a medical situation should seek out personal medical advice.

Massive Obesity.

There are two major concerns with massive obesity. One is the limitation of physical fitness. Massive obesity, which can be defined as either 50 pounds or 30% over one's ideal body weight, limits exercise, imposes a strain on the heart, and makes any physical activity riskier. Massively obese patients are simply not in "good shape" compared to normal weight people, and can be presumed to have increased risk for scuba diving.

The other concern with obesity is the increased risk of DCS. Fat takes up more nitrogen than other tissues and releases it more slowly, and many experts feel the massively obese person is at greater risk for DCS. This may well be true but, like most issues relating to actual occurrence of DCS, there are no hard data to prove the point.

One rule of thumb is that an obese person should reduce bottom time in proportion to the extra weight. For example, a 20% overweight diver should shorten bottom time by 20%, so a 45 minute bottom time is reduced to 36 minutes. Obviously, this rule would mean that someone two to three times ideal body weight will have very little bottom time! A diver triple his ideal body weight at, say, 450 lbs., a 45 minute bottom time drops to 15 minutes!

Diabetes.

Diabetes is an imbalance of blood sugar due to insufficient insulin, the hormone that regulates blood sugar. Diabetics are prone not only to swings in blood sugar, but also to vascular disease that can affect the heart, kidneys, eyes, and other organs.

Concerns about diabetics and diving (Dembert 1986, Dembert 1987, Brouhard 1987, Wedman 1987) are similar to those for asthma. Whereas the decision point for asthma and diving often revolves around whether the diver takes medication to control symptoms, the decision point for diabetes often revolves around the need for insulin, a drug that can be used only by injection. Those requiring insulin to control blood sugar tend to have more severe diabetes, and "insulin-dependent" diabetics are more prone to large swings in blood sugar. The main concern is that an insulin-dependent diver might develop an "insulin reaction" under water. An insulin reaction is the body's response to a very low blood sugar (hypoglycemia). Hypoglycemia, which occurs from too much insulin for the amount of carbohydrate in the blood, may arise from omitting a meal, from exercising, or from injecting too much insulin at any one time.

The effects of an insulin reaction, which can range from mild sweating to dizziness to coma, could be fatal under water. Most physicians recommend that insulin-dependent diabetics not scuba dive. However, many insulin-dependent diabetics do dive, and without apparent difficulty.

A survey by DAN (Dear, et. al. 1994) among readers of Alert Diver was responded to by 116 diabetics, of whom 84 were insulin-dependent; 35% had been diving longer than 10 years and the median number of dives per diver was 100. Only one person had experienced decompression illness (AGE).

Seventy-two per cent of the 116 diabetic divers checked their blood sugar before a dive and 52% checked it after a dive. Although the survey found "a significant incidence of hypoglycemia," the authors concluded that "Some diabetics can scuba dive safely," but "bias in the sample undoubtedly exists because the sample does not include diabetics who have discontinued diving because of complications."

Interestingly, 12% of divers reported low blood sugars while diving, but 44% reported experiencing hypoglycemia during exercise. Dr. Guy Dear, the senior author of the study, noted in Alert Diver: "...most divers told us that they carried out a variety of techniques to maintain a normal or elevated blood glucose while diving. These included eating extra candy, altering the dose of insulin or other drugs, and carefully regulating both the time of the dive and their meals of the day. The sample size of our survey was too small to make any meaningful comment about whether diabetes is a risk factor for decompression illness" (Dear 1994).

This information is similar to the asthma situation (Section O) ; many asthmatics use an inhaler and dive without apparent difficulty, but you will find neither condition (diabetes or asthma) sanctioned for diving. As Dr. Dear notes, "The decision of whether a new diver who has diabetes should be allowed to dive ultimately remains between the diver and his or her physician. This applies also to an established diver who is newly diagnosed as having diabetes."

Non-insulin-dependent diabetics may control their blood sugar with diet alone, or with a pill. For both insulin and non-insulin users, a decision about diving should be based not only on the theoretical risk of a hypoglycemic reaction, but on overall health. Diabetes is a major cause of heart, kidney and eye disease, so these areas should also be checked before the patient begins any underwater activity.

Epilepsy.

Epilepsy poses some of the same concerns as diabetes and asthma. Instead of an asthma attack or insulin reaction under water, the concern is for a seizure (Dreifuss 1985, Hill 1985, Meckelnburg 1985, Millington 1985, Green 1992). A seizure under water will almost always lead to drowning, and for this reason most physicians would rule against diving for anyone who is at risk. Practically speaking, this means anyone who requires medication to suppress seizures.

A remote history of one seizure (perhaps related to some acute medical problem such as fever or infection), that has not required any medication, should not prohibit diving. Any potential diver with a seizure-related question should either not dive or else seek evaluation by a neurologist first.

Anemia (low blood count).

There are many causes and types of anemia and it is impossible to generalize about fitness for diving. The major problem from anemia is lack of oxygen carrying capacity and limitation of exercise tolerance. Mild anemia, often present in menstruating women, may pose no problem, but severe anemia can be life threatening in any exertional situation.

Sickle cell anemia is a genetic disease characterized by "sickling" or distortion of the oxygen-carrying red blood cells. Present almost exclusively in blacks, the disease is characterized by severe anemia (low blood count) and episodes of severe joint pains. These painful "crises" are thought to arise from increased sickling of the red blood cells, which can occur from stress, heavy exertion, hypoxia (low oxygen), etc. Each crisis requires treatment with strong pain medication and, occasionally, hospitalization. (A sickle crisis after diving would be indistinguishable from a severe case of the bends.) Physicians universally recommend that patients with sickle cell disease not scuba dive.

A disease genetically similar to sickle cell disease, but vastly different clinically, is sickle cell trait. People with the trait have part of the abnormal hemoglobin found in sickle cell disease, but don't have the anemia, the painful crises or other problems associated with the disease. (Diagnosis requires a special blood test.) People with the trait are advised not to risk extremes of stress or any low-oxygen situation (e.g., mountain climbing), since under extreme conditions the "trait" cells can sickle (become distorted in shape). However, recreational scuba diving (where the oxygen levels are actually increased during a dive) does not pose any apparent increased risk to trait carriers. It is possible that an episode of decompression sickness may be made worse in someone with the trait (as there may be local tissue hypoxia), but this is theoretical only. As with most issues in this section, data are lacking to prove increased risk.

If someone with sickle trait wishes to scuba dive, I recommend they first consult a physician to confirm the diagnosis and assure there is no accompanying blood disorder. If the only condition is sickle cell trait then there is no clear reason to prohibit recreational scuba diving. To be on the safe side, I recommend that such individuals dive very conservatively in order to minimize risk of developing DCS (e.g., 60 feet maximum depth, no more than two dives a day, wait 24 hours to fly after any diving).

Thoracotomy (operation in which the chest has been opened).

Although some authors feel that thoracotomy is a contraindication to diving because of undetectable air trapping from scars or adhesions in the lungs, there are no studies on the subject. (It is interesting that Jacques Cousteau suffered an accident in 1936 that broke several ribs and punctured his lung; see Diving Odds N' Ends, Section H.) A post-thoracotomy patient who insists on diving should have a thorough physical exam, plus chest x-ray and CT scan; the presence of any air pocket should preclude diving (Mellem 1990). If the tests are equivocal, and the patient insists on diving, a hyperbaric chamber trial might be arranged to observe the effects of pressure changes under controlled conditions (Millington 1988).

Sarcoidosis and pneumoconiosis.

These are medical terms for two different diseases that can affect the lungs in similar fashion. The cause of pneumoconiosis is inhaled dust, such as coal mine dust or silica dust. The cause of sarcoidosis is unknown. Both diseases can lead to permanent scarring of the lungs.

Although it has been stated that these diseases preclude recreational scuba diving (Hickey 1984), the extent of disease rather than the diagnosis should be the governing factor. Sometimes sarcoidosis doesn't involve the lungs, or involves the lungs and then disappears altogether; in other words, the natural history of sarcoidosis is variable. Anyone with a history of sarcoidosis who wishes to dive should consult with a lung disease specialist and, if questions remain, a diving medicine specialist. A chest CT scan and lung function tests would be most helpful in determining the overall extent of any lung involvement. Any breathing impairment should rule out scuba diving.

Pneumoconiosis may be evaluated similarly. Sometimes the diagnosis is a mistake, from over interpreting a chest x-ray. The disease may be so minimal that it affects lung function not at all. As with sarcoidosis, a chest CT scan and lung function tests should be done to determine overall lung involvement. Any impairment should rule out scuba diving.

Pulmonary fibrosis.

This is the medical term for diffuse scarring of the lungs. It can occur as the result from either of the two diseases mentioned above, but can also occur spontaneously, from no known cause; in the latter case it is called idiopathic pulmonary fibrosis, IPF. Diffuse lung scarring can be fatal. Anyone with this disease should not dive. Fortunately IPF is rare.

Chronic obstructive pulmonary disease (COPD).

This is the umbrella term for both "chronic bronchitis" and "emphysema," two chronic lung conditions that are almost always due to long term cigarette smoking. Diagnosis is confirmed by the patient's history (usually shortness of breath on exertion and history of heavy smoking), an abnormal pulmonary function study, and chest x-ray. The chest x-ray may show some "air trapping" or even bullae ( holes') in the lungs. In some cases the chest x-ray can be normal, but the breathing tests will still show air flow obstruction. Unlike asthmatics, where the air flow obstruction is episodic and largely unpredictable, patients with COPD have permanent air flow obstruction. Anyone with COPD should not scuba dive.

Coronary artery disease (CAD).

This type of heart disease is the most common in the U.S. and is the cause of most heart attacks and "sudden deaths." CAD is a major cause of mortality in divers. Since sudden death from CAD occurs in half a million Americans yearly, it is to be expected to cause some in-water accidents, particularly in people over 50. Sudden death from heart attack seems to occur under conditions of hypothermia or stress, and has been attributed to an irregularity of the heart beat (Eldridge 1979).

Victims of sudden cardiac death could just as easily die while bicycling, jogging or hang gliding, so scuba diving per se is not the actual cause (the deaths are not from air embolism or DCS, but from drowning). Even so, deaths from heart attack while diving are included in DAN's annual statistics. In its 1992 compilation of mortality statistics DAN noted: "Cardiovascular disease is a prominent immediate cause of death" (DAN 1994). This finding was in contrast to other medical conditions, such as diabetes and asthma.

A patient with known coronary artery disease who requires medication, or for which surgery has been recommended, should not dive. One author has suggested that a history of heart attack does not preclude diving if: 1) more than a year has passed since the heart attack; 2) there is no chest pain or arrhythmia on maximal exercise; 3) the patient has a normal stress test; and 4) the physical exam is normal (Millington 1988). Doubtful cases (when the patient insists on diving) should be referred to a cardiologist for a more complete evaluation, which might include stress testing or coronary angio-graphy (injection of dye into the coronary vessels).

Smoking.

Organizations promoting scuba diving are united in their no-smoking policy. One editorialist wrote: "There is no choice for the diver. He or she should quit smoking now or consider giving up diving." (Tzimoulis 1986) Apart from the likelihood of developing airway disease, which increases the risk of pulmonary barotrauma from air trapping, the greatest immediate hazard from smoking is excess blood carbon monoxide and decrease in arterial oxygen content (see Section I) .

Diving can be quite stressful and any physiologic impairment increases its inherent risks. Based on half-life of excess CO in the blood (about six hours), and typical CO-hemoglobin levels of smokers (5%-10%), scuba divers who cannot break the smoking habit should abstain at least 12 hours before any dive. However, many divers do smoke, and sometimes just before a dive. Sadly, it is not uncommon to see dive professionals divemasters and instructors smoke during the surface interval between a two-tank dive.

(For discussion of other conditions see one or more of several reviews: Becker 1983; Hickey 1984; Davis 1986; Dembert 1986; Millington 1988; Davis 1990; Linaweaver 1990, Mebane 1993.)

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