Sunday, July 09, 2006

Bubble Grade Study in Actual Scuba Diving Conditions

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.