posted by Ern Campbell, MD, on 10footstop

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June 2, 2006 — An echocardiographic study done on healthy
volunteers in actual scuba diving conditions found an important
bubble grade in all, according to a report in the May issue of
Chest [Chest. 2006;129:1337-1343]. The hemodynamic changes
suggested that hypovolemia [low blood volume] is an important
factor and that restoration of fluid balance should be
considered after all dives.

"During a scuba dive, subjects undergo environmental constraints
such as immersion, exposure to cold, and increased ambient
pressure," write Alain Boussuges, MD, PhD, from the Institut de
Médecine Navale du Service de Santé des Armées in Marseille,
France, and colleagues. "Intravascular gas bubbles are carried
from the venous circulation to the pulmonary vessel, where they
are eliminated through the lungs. The formation of bubbles is
recognized as the basis for decompression illness, but such
bubbles are also commonly detected in venous circulation of
asymptomatic divers."

Ten divers were studied and one hour after the dive,
microbubbles were detected in the right-heart chambers of all
subjects, and left atrial and left ventricular diameters were
significantly decreased. Cardiac output measured by aortic blood
flow remained unchanged, but heart rate increased and stroke
volume decreased after the dive. Left ventricular filling, which
was assessed on transmitral profile, was partially increased by
atrial contraction. Right cavity diameters were unchanged, but
there was an increase of the right ventricular/right atrial
gradient pressure.

"The diving profile studied promotes a rather important bubble
grade in all volunteers," the authors write. "Two factors can
explain these results: low volemia secondary to immersion, and
venous gas embolism induced by nitrogen desaturation.
Consequently, restoration of the water balance of the body
should be considered in the recovery process after diving."

Clinical Context
Scuba diving invilves immersion in water and the use of
self-contained breathing units known to produce some interesting
effects on the circulation, including a reduction in peripheral
blood flow that concentrates blood in the central circulation.
This effect, in turn, stimulates increased release of atrial
natriuretic peptide and diuresis. In addition, the increase in
ambient pressure associated with diving increases the arterial
concentrations of oxygen and nitrogen, and the higher levels of
oxygen may impair cardiac relaxation.

Previous studies have suggested possible cardiac changes
associated with scuba diving. The current study follows up 10
healthy male volunteers with echocardiography before and after
diving to document these changes.

Study Highlights
*Study subjects underwent echocardiography at baseline and 1
hour following a 25-minute dive at a mean depth of 34.3 m.
Participants breathed a standard mixture of nitrogen and oxygen
and wore neoprene diving suits.
*Circulating bubbles following the dive were detected with 2-D
echography.
*The main study outcomes were differences in echocardiogram data
before and after the experimental dive.
*All participants had evidence of circulating bubbles, and 7 of
the 10 divers had grade 3 bubbles (the majority of cardiac
periods contained bubble signals singularly or in group).
*The authors conclude that a relative hypovolemia and venous gas
embolism help explain the changes in echocardiographic findings
following diving, and they recommend oral rehydration following
diving. Rehydration might be especially important in the setting
of repeated diving.

Take home stuff:
Scuba diving can reduce peripheral blood flow, increase the
release of atrial natriuretic peptide and diuresis, and increase
the arterial partial pressures of oxygen and nitrogen.
The current study suggests that oral rehydration following
recreational scuba diving may reduce echocardiographic changes
associated with diving.