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Re: Unconsciousness after diving in Osler-Weber-Rendu disease

Dear Editor,

The article published by Dr. Hsu and colleagues [1] appears to furnish a new mechanism for neurological decompression illness, related to intrapulmonary right-to-left shunting. However, the authors’ explanation of the mechanism of injury in their patient is not supported by a more detailed analysis.

During a scuba dive, the tissue partial pressure of inert gas (nitrogen if the diver is breathing air) increases. During decompression, if the tissue gas tension exceeds atmospheric pressure, gas supersaturation can occur, causing in situ bubble formation. After scuba diving, venous gas bubbles are frequently detectable using ultrasound, but rarely cause symptoms because they are removed by the pulmonary capillary network. As pointed out in Dr. Hsu’s article, several authors have observed a statistical relationship between neurological decompression illness and the presence of inter-atrial shunts (e.g. a patent foramen ovale, PFO). A PFO is presumed to allow entry of venous bubbles into the arterial circulation, where they can cause systemic vascular occlusion or injury. In order for this process to occur, two factors must be present: (a) a right-to-left shunt and (b) venous gas embolism. If this mechanism is correct, the same phenomenon should occur with intrapulmonary shunts, such as in Osler-Weber-Rendu disease. Indeed, the sudden loss of consciousness during ascent from the dive in a patient with Osler-Weber- Rendu disease appears to support such a mechanism.

However, the dive depth was only to a depth of 5 feet (1.15 atmospheres absolute). Thus, the tissue PN2, even after an infinitely long dive, could have been only 690 mmHg, hence rendering supersaturation impossible. The threshold depth for in situ bubble formation is believed to be around 12 feet [2]. Assuming gas bubbles to have been the cause of unconsciousness in Dr. Hsu’s patient, a more plausible explanation is pulmonary barotrauma caused by breath-holding or regional pulmonary gas trapping.

References:

1. Hsu Y-L, Wang H-C, Yang P-C. Desbaric air embolism during diving: an unusual complication of Osler-Weber-Rendu disease. Br J Sports Med 2004;38:e6.

2. Eckenhoff RG, Olstad CS, Carrod G. Human dose-response relationship for decompression and endogenous bubble formation. J Appl Physiol 1990;69:914-8.