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Dr Oli Firth

No doubt most of us have, at one time or another, cast a worried glance at one of our larger diving brethren and wondered how on earth they manage to kit up, let alone dive safely. Itís certainly one of the commoner comments made to me in the consulting room. There are many reasons that diving attracts those of, ah, more expansive girth, not least the temporary removal of gravitational pull on their abdomen. However, one of the consequences of obesity is heart disease, which is the third most common cause of death whilst diving. In this article i want to highlight a few aspects of this often overlooked risk factor in diving accidents.

Firstly, letís consider some of the hazards created by the underwater environment that apply to the heart in particular. These include:

  • Increased demands from physical exertion, due to water drag, cumbersome equipment, increased work of breathing and other factors
  • Increased adrenaline production, from unpredictable stresses (e.g. equipment problems)
  • Thermal stress (usually cold, but sometimes overheating as well)
  • Blood vessel constriction in the arms and legs (this causes a shift of up to 700ml blood into the central vessels, which distends and stretches the heart, causing it to contract harder)

All of these factors whack up the workload on the heart substantially. In terms of physical effort, a stately bimble in calm conditions is a world away from a thunderous workout in a strong current. But as we all know, one can change to the other in no time at all Ė certainly during a single dive. A heart that is diseased, or just managing to cope in a land- based environment, can easily be pushed to (or over) its limits, simply by submerging its owner in water. Ask it to power even mild exercise underwater, and it may not be able to cope. The consequences could be breathlessness, panic, chest pains from angina or a heart attack, or abnormal rhythms which can be equally harmful. Whilst the end result is recorded by the coroner as drowning, there are often pre- disposing cardiac risk factors when one studies the accident reports. As the age of the diving population increases, with more people continuing to dive into their 70ís and 80ís, the risk of injury from heart problems rises too.

So how should we assess divers to minimize these risks? Questionnaires are a good start, to pick out those more likely to have hidden heart disease Ė the smokers, the diabetics, the over 40ís, the males (yes, itís a risk factor in its own right), those with a family history of heart attacks or strokes and so on. By looking at these factors and incorporating cholesterol levels and blood pressure, we can estimate the likelihood of cardiovascular disease. Lower than 10% would mean the diver is at low risk of an event under- water; higher risk individuals should be evaluated further, and generally this is done by trying to estimate their exercise capacity.

Some of you may have had the dubious pleasure of tack- ling the step test. Not, as some might suspect, a nefarious method of satisfying your diving docís more sadistic tenden- cies, but one of the methods by which we try to assess the subjectís capacity for physical work underwater. Of course, the extrapolation from leaping athletically on a box in a warm, dry, well-lit office to finning furiously in the cold, wet, inky blackness of an inland quarry can be questioned. However, short of dunking everyone in a pool, itís the best test we can currently muster.

Putting numbers on exercise capacity is the difficult bit. Iíd like to introduce you to the concept of oxygen consump- tion (or VO2 in scientific parlance). This is a measure of how much oxygen muscles can use in a given time, and is expressed in units of ml/kg/minute. The other notation I need to explain is the MET, or Metabolic Equivalent of Task. This is another way of expressing energy expenditure, and is ap- plicable to people of different weight. By convention, 1 MET is what we consume at rest, i.e. sitting quietly, and equates to about 3.5 ml/kg/min in VO2 terms. Walking at 3 miles an hour takes 3.3 METs, and running about 8 METs.

The American physiologist David Pendergast and his col- leagues spent some time trying to estimate the oxygen con- sumption demands of swimming with scuba gear at various speeds. They suggested that a recreational diver should be able to reach a steady state VO2 of 20ml/kg/min (or about 6 METs), which they can sustain for 50-60 minutes. Sustain- able exercise capacity is usually thought to be about 50% of peak capacity, so this translates to a peak exercise capacity on stress testing of 12 METs.

How should we measure these values in a diver though? The best measure we have of fitness is maximal oxygen consumption (or VO2 max), which is generally done through incremental exercise tests (on treadmills or bikes, whilst measuring oxygen and carbon dioxide levels in the inhaled and exhaled air). An average untrained male would have

a VO2 max of around 45ml/kg/min. Cross country skiers top the human charts with figures of 90ml/kg/min or more; Lance Armstrong recorded 84. Sled-dog huskies manage an incredible 240ml/kg/min! However, these tests are a bit too long-winded and impractical to do on every sport diver, so a myriad of surrogate tests have evolved to try to estimate the VO2 max. Some of these are the step tests so beloved of us diving docs. So now you know why we put you through the humiliation.

Ultimately, then, what would actually reduce the number of deaths from cardiac events in divers? A suggested ap- proach appeared in the Undersea and Hyperbaric Medical Society journal last year:

  • Medical clearance for individuals with known cardiac disease (the highest risk subgroup), using exercise tests of some sort
  • Training of diving personnel to elicit possible cardiac symptoms, and restriction of symptomatic individuals from diving until cleared. In one study of 45 subjects who died suddenly, the main symptoms reported in the week before their death were chest pain/angina, increasing fatigue, and indigestion / heartburn
  • Cardiac emergency training and scheduled drills for diving supervisors.

We also need to emphasise the importance of regular re-evaluation of health risks. A one-off assessment is not enough. If a diverís health changes, or if they are unwell, they need re-assessing; and some suggest that everyone should have a check-up at age 45 (males) or 55 (females), and at regular intervals thereafter.

So, a hurried and somewhat simplified spotlight on some aspects of heart disease and their relevance to divers, but one that has hopefully given you food for thought. I doubt weíll be seeing BodyPump classes on the shores of Stoney anytime soon, but next time you feel the bacon fat oozing through your fingers, spare a thought for your arteries...

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