Acute myocardial infarctions (commonly known as "heart attacks") that are brought on by exertion — such as while swimming against a current, in heavy waves or under conditions of excessive negative buoyancy — are likely involved in some dive-provoked fatalities. Heart attacks are caused by an insufficient blood supply to the muscles of the heart; diving-related heart attacks typically occur in middle-aged males with undiagnosed coronary artery disease.
Diving (or just immersion) may also provoke acute arrhythmias, or disturbances of the heart's rhythm, that can likewise result in sudden death. Arrhythmias are more likely to cause death in older divers. As Dr. Carl Edmonds explains in his book Diving and Subaquatic Medicine, and DAN data confirms, "The victim often appeared calm just before his final collapse. Some were unusually tired or resting, having previously exerted themselves, or were being towed at the time — suggesting some degree of exhaustion. Some acted as if they did not feel well before their final collapse. Some complained of difficulty in breathing only a few seconds before the collapse, whereas others underwater signaled that they needed to buddy breathe but rejected the offered regulator. Explanations for the dyspnea include psychogenic hyperventilation, autonomic induced breathing stimulation and pulmonary edema — the latter being demonstrated at autopsy. In all cases there was an adequate air supply available, suggesting that their dyspnea was not related to equipment problems. Some victims lost consciousness without giving any signal to their buddy, whereas others requested help in a calm manner."
The incidence of sudden cardiac death (SCD) also increases with age. Patterns of SCD are similar among divers and among the general population; nevertheless, it is important that divers not dismiss the possibility of a causative relationship between diving and SCD. Cases of SCD where there was no obvious external provoking factor occur more frequently in older divers. Postmortem examinations of SCD victims are more likely to reveal signs of previously unsuspected heart disease than a specific precipitating event. The best way to prevent SCD is thus to prevent heart disease and to maintain physical fitness and wellness as you age.