Hypercapnia in rebreather diving – can you learn to recognise it?

Michael Mutter
vor 6 Tagen
5 Min. Lesezeit

Hypercapnia is one of the most dangerous problems associated with rebreather diving. Elevated CO₂ levels can lead to shortness of breath, cognitive impairment and disorientation. At the same time, CO₂ increases oxygen exposure to the brain through enhanced cerebral blood flow, thereby potentially raising the risk of CNS oxygen toxicity. Added to this is the narcotic effect of CO₂, which can occur independently of nitrogen narcosis or be exacerbated by it.

The real problem is that hypercapnia cannot be reliably measured underwater – and its symptoms are not always clear-cut.

A recent randomised study therefore investigated a question of interest to rebreather divers: can a controlled experience of hypercapnia teach divers to recognise a subsequent rise in CO₂ levels at an earlier stage?

Marmorera-See. Image template: Karin Aggeler

What causes hypercapnia in rebreathers?

In a CCR, exhaled gas is reused. The resulting CO₂ is absorbed by the lime in the scrubber. A malfunction or incorrect packing of the scrubber can therefore lead to CO₂ rebreathing. However, hypercapnia can also occur during diving even without a scrubber problem. Increased respiratory effort and physical exertion can mean that ventilation does not respond adequately to rising CO₂ levels. There are significant individual differences in this regard. Some divers are more prone to CO₂ retention than others. This is important because the technical systems currently available only partially address this problem.

A temperature sensor in the scrubber can provide indications of the activity or possible failure of the breathing lime. However, a practical, affordable underwater capnography system for the direct measurement of CO₂ in the breathing gas does not yet exist.

Can we rely on the symptoms?

Typical symptoms include shortness of breath, light-headedness, a feeling of warmth, dizziness and nausea. The problem is that these symptoms can also occur during physical exertion even without hypercapnia. The symptoms of early-stage hypercapnia can therefore easily be misinterpreted as a normal reaction to exertion or increased respiratory resistance. The authors therefore wanted to find out whether the conscious experience of hypercapnia makes a diver more sensitive to these symptoms later on.

The study design

The study involved 40 healthy divers aged between 18 and 55. Their diving experience ranged from just a few dives to 2,000 dives. However, only four participants were certified CCR divers. All participants were first given written information about hypercapnia and its typical symptoms. They were then randomised (assigned at random) into two groups.

The first group also underwent a controlled, consciously perceived exposure to hypercapnia. During this, the participants breathed from a modified rebreather without a CO₂ scrubber. The CO₂ concentration increased progressively until an end-tidal CO₂ partial pressure (PETCO₂, at the end of exhalation) of 8.5 kPa (64 mm Hg) was reached or the symptoms could no longer be tolerated. The second group received only the written information.

The simulated dive

At least one month later, a blinded simulated dive took place. The participants were randomised again: 30 were exposed to hypercapnia, whilst 10, as the control group, continued to breathe ambient air. None of the participants knew which group they belonged to.

The participants cycled at a light intensity on an exercise bike and were required to count orcas swimming past via a VR headset. For the hypercapnia group, the system was switched unnoticed to a closed rebreather circuit without a CO₂ scrubber. As a result, they rebreathed their own CO₂ and the CO₂ level rose slowly. As soon as the participants noticed symptoms of hypercapnia, they were instructed to switch to bailout mode independently. Anyone who failed to do so before reaching a PETCO₂ of 8.5 kPa received a warning via the virtual HUD.

The results

Of the divers with previous experience of hypercapnia, 13 out of 15 – that is, 87 per cent – carried out a bailout independently. In the group that were only familiar with hypercapnia from the information sheet, the figure was 10 out of 15 – that is, 67 per cent. However, given the small number of participants, the difference of 20 percentage points was not statistically significant (P = 0.149).

Two participants in the training group and five in the information group first required a prompt via the HUD. Afterwards, all participants carried out the bailout correctly.

The control group is also of interest: none of them carried out a bailout incorrectly. There were therefore no erroneous, unnecessary bailouts.

What was the CO₂ level at the time of the bailout?

The PETCO₂ averaged around 7.9 kPa during self-initiated bailouts. Some participants noticed the hypercapnia as early as 6.7 kPa. Those who had to be prompted to perform a bailout via the HUD had correspondingly higher CO₂ levels of around 8.5 kPa. The normal end-tidal CO₂ partial pressure (PETCO) in healthy adults at rest is typically 4.7–6.0 kPa (35–45 mmHg).

It is worth noting that there were no significant differences in the cardiorespiratory response during the bailout between the training group and the information-only group.

What does hypercapnia feel like?

During the blinded trial, 15 out of 30 participants with hypercapnia reported shortness of breath as their first symptom. This was followed by a feeling of warmth (15 per cent), light-headedness (15 per cent), visual disturbances (3 per cent) and nausea (3 per cent).

Know your lime and don’t rely on noticing hypercapnia in time!

However, the individual pattern of symptoms is by no means consistent. Of the 15 participants who actually developed hypercapnia during both the training session and the subsequent test, only 7 out of 15 – 47 per cent – exhibited a comparable pattern and intensity of symptoms. For more than half of the participants, the second episode of hypercapnia was therefore subjectively different from the first. This puts into perspective the notion of a personal ‘CO₂ warning signal’ that occurs reliably time and again.

Discussion

The study thus confirms a fundamental problem with CCR diving: the subjective perception of a CO₂ problem is only of limited reliability. This was already demonstrated by the study discussed in the article ‘CCR Prebreathing to Check the Scrubber’. During a five-minute prebreathe, only 2 out of 20 divers noticed a partially failing scrubber; even when the breathing lime was completely absent, only 15 out of 20 recognised the problem. A gradual rise in CO₂ is therefore particularly insidious.

Furthermore, a functioning scrubber does not rule out hypercapnia. High gas density, increased respiratory effort, cold, a high minute volume and physical exertion can lead to inadequate ventilation, insufficient CO₂ absorption and CO₂ retention. The best solution would be direct CO₂ measurement. However, as discussed in the article ‘CO₂ Monitoring of CCR Breathing Gas’, reliable underwater capnography remains technically challenging. The authors also emphasise that there is currently no reliable method available for detecting a rise in arterial CO₂ during a dive.

Such a warning would also need to be issued early: In the study, divers who bailed out independently did so on average at a PETCO₂ of 7.9 kPa. Earlier studies have described prolonged reaction times as early as 7.3 kPa, whilst at values just above 8.5 kPa, the inability to act may already occur. A controlled hypercapnia experience can therefore be highly instructive, but it is not a reliable warning system.

Conclusion

Until CO₂ can be monitored directly and reliably in a CCR, prevention remains crucial: correct handling of the scrubber, avoiding high respiratory effort, ensuring an appropriate gas density at depth, and, when in doubt, it is better to bailout once too often.