Basic HTML Version
older divers. In weightless conditions of immersion, muscular
work is less punishing and divers easily exert themselves
beyond their cardio-respiratory limits. Most divers regard
swimming against a 0.5-knot current as a challenge. Heart
rates measured in trained military divers during shallow
underwater swimming increased to an average of 101 beats per
minute (bpm) in a 0.6-knot current and to an average of 141
bpm in a 1.2-knot current. For many divers this would exceed
a sustainable level (which is usually considered to be less than
80 percent of the maximal heart rate). Even more important,
divers who do not regularly practice swimming and do not
master the technique of underwater swimming with fins as
well as those who do not streamline their gear and especially
those whose buoyancy is a bit off, would probably reach their
maximal heart rate while swimming even more slowly. Some
may reach their limit just trying to stay afloat.
What are the risks?
Fatality statistics indicate at least one-third of all dive
fatalities are related to an acute cardiac event. The risk
of cardiac-related death in divers is continuous, steadily
increasing with age; divers older than 50 have a risk 10 times
that of divers under 50. While some suspected cardiac events
may be provoked by specific dive effects, some may be not
related to diving at all, as sudden cardiac deaths occur in
swimming, land-based sports, at rest and during sleep.
An acute myocardial infarction (heart attack) due to
exertion while swimming against current, waves or excessive
negative buoyancy is probably quite common among dive-
provoked fatalities. It is caused by insufficient blood supply to
working heart muscle. This occurs most commonly in middle-
aged male divers unaware of their coronary artery disease.
It is also reasonable to expect diving could provoke an
acute arrhythmia, which might result in sudden death. The
arrhythmia is a more likely cause of death for older divers. As
Carl Edmonds, M.D., describes and DAN
®
data confirm, “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 ventilatory 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.”
SCD occurs at comparable rates and a nearly identical age-
related pattern in diving and in the general population, but a
causative relationship between diving and SCD should not be
dismissed. SCD cases without an obvious external provocative
factor are more common in older divers. Medical examinations
in such cases reveal signs of heart disease rather than identifying
a specific event that caused SCD. Outcomes of these diving
fatalities might not be different than SCD cases in the general
population except the divers usually do not have a chance of
78
|
SPRING 2012
I MA G E S T H I S S P R E A D : I S T OC K P HO T O . COM