Can a MRS save an unconscious diver?
- Michael Mutter
- 29. Juli
- 6 Min. Lesezeit
Aktualisiert: vor 23 Stunden
In recent years, the Mouthpiece Retaining Strap (MRS) has attracted increasing attention in rebreather diving. International organisations such as DAN and various rebreather training organisations now recommend the use of an MRS to reduce the risk of water aspiration in an unconscious diver.
To illustrate the benefits of an MRS, DAN recently published an account from a CCR diver. The message is clear: the MRS may well have saved the diver’s life. The story is undoubtedly fascinating – yet, on closer look, the report raises significant medical and diving-physiological questions.
What is an MRS?
A Mouthpiece Retaining Strap (MRS) is an elastic strap that is attached to the mouthpiece of a closed-circuit rebreather (CCR) and runs behind the head. Its purpose is to keep the mouthpiece in the diver’s mouth even if the diver loses consciousness or their jaw muscles relax. The aim is to reduce the risk of the mouthpiece falling out and the subsequent aspiration of water.
The story
The author reports that, after two uneventful CCR training dives with his students, he had been waiting on the surface next to the boat until they had boarded safely. Whilst waiting, he suddenly lost consciousness. It was not until about an hour later that he regained consciousness on the seabed at a depth of around 45 to 48 metres.
According to his account, the rebreather’s mouthpiece remained in his mouth throughout this time thanks to an MRS. As a result, he continued to breathe from the closed circuit despite being unconscious. His rebreather automatically regulated the oxygen partial pressure, so that, upon regaining consciousness, he was able to carry out the necessary decompression and eventually surface safely.
If the accident did indeed occur in this way, it would undoubtedly be an extraordinary case of survival. At the same time, however, the sequence of events described raises a number of fundamental questions.
The unexplained cause of the descent
Even the first part of the story seems difficult to comprehend: the diver lost consciousness at the water’s surface. At that point, the wing, drysuit and counter-lungs – or parts thereof – were filled with gas and generating considerable positive buoyancy. Nevertheless, the diver is said to have subsequently descended to a depth of around 48 metres.
For a CCR diver to sink at the water’s surface, a significant portion of his buoyancy would first have to be lost. Among other things, this would require a marked reduction in the volume of gas in the counter-lungs. However, this is not to be expected whilst spontaneous breathing is maintained via a correctly positioned mouthpiece. On the contrary: a properly functioning CCR continuously replaces the oxygen consumed by metabolism by adding oxygen, in order to maintain the set oxygen partial pressure at a constant level. As a result, gas is continuously supplied to the breathing circuit, so that the volume of the counter-lungs is essentially maintained or may even increase due to the falling oxygen partial pressure at the surface. Every CCR diver is familiar with this phenomenon from prebreathing before a dive, during which the counterlungs gradually inflate if the diver does not exhale regularly.
Combined with the buoyancy provided by the wing and drysuit, this initially suggests that spontaneous sinking at the water’s surface is unlikely. Therefore, as long as the diver continues to breathe spontaneously through the rebreather and the apparatus is functioning correctly, an unintentional sinking without additional circumstances not mentioned in the report seems physically difficult to explain.
One hour of unconsciousness
The cause of the unconsciousness also remains unclear. As a possible explanation, the author cites fatigue, stress and personal pressures. Whilst these factors can impair concentration and performance, they do not explain a loss of consciousness lasting around one hour.
Such a prolonged loss of consciousness would, rather, suggest a serious medical cause such as a cardiac arrhythmia, a neurological episode, hypopxia, a severe metabolic disturbance or another acute medical condition. Even if such a cause had been present, it remains unclear why the person concerned is said to have regained consciousness spontaneously and without any lasting damage after precisely about an hour. From a medical point of view, this aspect also appears, at the very least, extremely unusual.
The crucial point: the dry suit
However, in my opinion, the greatest inconsistency concerns the dry suit. During the descent, the gas sealed inside the suit is continuously compressed in accordance with Boyle’s and Mariotte’s law. An unconscious diver cannot operate the dry suit’s inflator and is therefore unable to equalise the pressure within the dry suit.
At a depth of almost 50 metres, the original gas volume shrinks to less than 20 per cent of its surface volume. The result would be severe suit squeeze with significant compression of the entire body, particularly the thorax and abdomen.
Thoracic compression in particular is likely to significantly impair respiratory function. The idea that an unconscious diver could ventilate spontaneously sufficiently for around an hour under these conditions seems physiologically highly questionable.
An MRS is no substitute for airway management
Provided the mouthpiece remains in the mouth, the oxygen sensor functions correctly and the solenoid reliably controls the oxygen supply, the closed circuit can, in principle, maintain a stable oxygen partial pressure over a prolonged period. However, this presupposes that the diver continues to breathe spontaneously.
This is precisely where another critical issue lies. An MRS merely prevents the mouthpiece from falling out. It does not, however, secure the airway. It prevents neither the tongue from falling back nor other forms of upper airway obstruction, such as those that can occur at any time in unconscious patients. The MRS holds the mouthpiece in place – but it neither keeps the airway open nor takes over breathing.
The often-overlooked factor: carbon dioxide
Another aspect is not mentioned at all in the story: the elimination of carbon dioxide. It is often assumed that a functioning CCR can simply continue to ‘support’ an unconscious diver. In reality, however, the rebreather merely regulates the composition of the breathing gas.
The carbon dioxide continuously produced by metabolism must still be exhaled through adequate alveolar ventilation. The absorbent removes CO₂ exclusively from the breathing gas in the circuit; it does not, however, replace breathing.
If the tidal volume is reduced as a result of severe suit squeeze or partial airway obstruction, alveolar hypoventilation inevitably develops, leading to CO₂ retention and loss of consciousness. A technically flawless rebreather cannot compensate for this problem.
Several conditions would have had to be met simultaneously
For the sequence of events described to have actually taken place in this way, numerous independent conditions would have had to be met simultaneously:
The diver would have had to have sunk spontaneously
he would have had to have continued breathing spontaneously,
the upper airways would have had to have remained open throughout the period of unconsciousness,
despite the drysuit not being inflated, the breathing mechanics should not have been significantly impaired,
alveolar ventilation would have had to be sufficient, despite suit squeeze, to continuously eliminate carbon dioxide,
and finally, a severe disturbance of consciousness would have had to have ceased spontaneously and without consequences after about an hour.
The fact that all these events are said to have occurred simultaneously makes the described sequence of events very difficult to comprehend from a medical and diving-physiological perspective, to say the least.
Conclusion
The mouthpiece retaining strap may be a useful addition to safety equipment and, in certain situations, prevent an unconscious diver from losing the mouthpiece and aspirating water. Whether this actually provides a survival advantage, however, depends on numerous other factors.
In my view, the account published by DAN is of limited value as evidence of this benefit. Its plausibility is – to put it mildly – debatable, as it presupposes a series of exceptional technical and physiological factors that are neither explained nor critically examined in the report.
An MRS holds the mouthpiece in place. However, it is no substitute for airway management, nor for adequate spontaneous breathing or ventilation. For an unconscious diver to actually continue breathing underwater, numerous conditions must be met simultaneously – far more than the mere presence of a mouthpiece retaining strap.
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