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  • AutorenbildMichael Mutter

"... and at the end of the dive we shoot a buoy."

Who does not know this sentence from a dive briefing? No doubt, delayed surface marker buoys (DSMB) perform important functions when diving. It is therefore no wonder that great importance is layed on their handling during diver training. Among the various techniques, inflating with the mouth is considered the supreme discipline. There are many videos in the www whose protagonists demonstrate this skill under perfect buoyancy control. But does it also make sense from a pathophysiological point of view?

PFO and diving

Approximately 25% of the population has a patent foramen ovale (PFO). PFO is a risk factor for arterial gas embolism (AGE) during diving. There are widely accepted behavioral measures for diving with a PFO, and the SUHMS guidelines have set the worldwide standard in this regard. In principle, all types of pressing maneuvers or related actions at the end of or after a dive are to be refrained from, since a PFO is opened in this way and venous inert gas embolie (VGE) can pass from the right to the left atrium bypassing the lung filter and from there as arterial gas emboli (AGE) into the systemic circulation and thus trigger a stroke, for example.

A case report illustrates the problem

A case report shows that this is true. After a stroke, a PFO was sought in a 45-year-old female patient and found by echocardiography (cardiac ultrasound) (Figure A). Initially, it was not possible to demonstrate the transfer of contrast from the right to the left atrium (Figures B and C). Only when the patient was asked to inflate a party balloon during the examination (pressure approx. 20 mm Hg = 26 mbar) did the PFO open, resulting in arterial embolization with a massive transfer of contrast medium into the left atrium (image E, compare also video).

Akihisa Kataoka et al. J Am Coll Cardiol Case Rep 2022; 4:102-104

Inflating a DSMB by mouth corresponds exactly to this and thus represents nothing more than a provocation maneuver which will open any PFO, and this in the diving phase with the highest supersaturation and the greatest risk of VGE towards the end of a dive at a relatively shallow depth. It is obvious, that such a maneuver promotes the occurrence of AGE.

Pressing maneuvers open any PFO.

It is thus incomprehensible that many diving organizations, which claim "safety" as their banner, still train the inflation of DSMBs by the mouth as a skill. On the contrary, from a diving medical point of view, this technique must be discouraged. This applies not only to divers with PFO, for whom this maneuver is absolutely forbidden, but in principle to all. Or do you know, whether you have got a PFO or not? This deserves particular attention in technical diving, where DSMBs are an essential tool for controlled decompression stops in open water, and where the risk of VGE after deep, long dives is greater than in recreational diving. For this reason, alternative methods for setting signal buoys that do not require pressing maneuvers should be favored.

Why DCI after AGE is very rare, but AGE is more likely to cause DCI in technical divers than in recreational divers, is explained in an other article in this blog.

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