Closed-circuit rebreather (CCR) diving relies on scrubber systems to remove carbon dioxide (CO2) from exhaled air. CO2 monitoring in CCR diving is critical because high CO2 levels (hypercapnia) can be life-threatening. While scrubber monitoring systems are currently used to predict scrubber depletion, they do not provide direct CO2 concentration measurements in the breathing loop, which can lead to false alarms but also to missed CO2 breakthroughs. The last blog post explored the challenges of developing accurate and reliable CO2 scrubber monitoring for CCRs, highlighting the trade-off between sensitivity and specificity in detecting CO2 levels. Direct CO2 concentration measurements in the breathing gas would be crucial for improving CCR safety and is the subject of this blog post. Read to the end, where a real life calculation is waiting for you.
End-tidal CO2
What we really want to know is the level of CO2 in the inhaled gas, the so called end-tidal CO2 (etCO2). EtCO2 refers to the concentration of carbon dioxide (CO2) in the air exhaled at the end of a breath. When we exhale, the air in the mouth and airways includes recently inhaled gas. To measure alveolar gas, which reflects the CO2 levels in the blood, we wait until the very end of the exhalation. This ensures the reading is from deep within the lungs, not mixed with fresh inhaled gas. CO2-concentrations in the alveoli and blood equlibriate immediately. So etCO2 provides a close estimate of the CO2 levels in a person's bloodstream and reflects the effectiveness of ventilation, gas exchange, and blood flow in the lungs. Monitoring etCO2 is crucial in medical settings, such as during anesthesia, to track a patient’s respiratory function. In diving, etCO2 monitoring could help identify dangerous CO2 buildup (hypercapnia) and prevent accidents due to poor ventilation, excessive CO2-production due to (physical) stress or faulty equipment, e.g. scrubber malfunction or saturated lime.
The problem of gas density
Another cause of an excessive build-up of CO2 is an increase in the density of the respiratory gas. This leads to a higher work of breathing, which may no longer ensure that the alveoli are sufficiently ventilated to exhale CO2. This can be the case with very deep dives under unfavorable breathing gas mixtures. But even under trimix there is a depth limit, beyond which the work of breathing becomes too great for sufficient ventilation. This applies in particular to physically challenging dives, which is why DPVs must be used in these situations, and is one of the reasons for the first CCR dives with the very easy-to-breathe hydrogen. Several fatal diving accidents during cave diving are known, in which this mechanism of inadequate breathing caused hypercapnia.
The challenges and pitfalls of etCO2-measurement
The measurement of etCO2 would provide an in-time warning. However, technical and practical challenges make its implementation difficult. These comprise:
1. Humidity and Sensor Sensitivity:
Mouthpieces in rebreathers are extremely humid environments (probably the most humid in a CCR at all). Infrared sensors, which are commonly used for CO2 monitoring, struggle in such conditions because moisture can interfere with the accurate detection of CO2 levels.
For etCO2 monitoring to be safe and useful in diving, the readings must be accurate and reliable. Any errors could have severe consequences, as divers often rely on precise data for decision-making underwater. Here, we meet the problem of sensitivity and specificity again, in an even more delicate environment than in the scrubber.
2. Sampling and Placement:
Placing the analyzer directly in the mouthpiece could cause discomfort and add to the bulk, making it less practical for divers.
An alternative approach is to take a gas sample from the mouthpiece to an analyzer located elsewhere on the rebreather. This setup would require a pump to pull the dense gas through a small tube, which could lead to gas loss and would demand significant power.
3. Technological Limitations:
Current CO2 sensors used in diving have slow response times, making them unsuitable for the rapid detection of changes in inhaled and exhaled gas content.
The sensors need to be compact and lightweight, but current technology struggles to meet these criteria without compromising performance.
4. Cost and Market Demand:
Developing accurate and reliable CO2 sensors for diving is expensive. Creating such sensors would require significant investment to transition from prototype to mass production.
The diving market is relatively small, making it difficult to justify the investment needed for developing and producing these sensors at a cost that would be accessible to most divers.
As an example, Open Safety Equipment Ltd. claims to have developed a device capable of reliably measuring end-tidal CO2 on divers. However, the high cost ($14,000) and lack of availability (out of stock) indicate that there are still significant barriers to widespread adoption.
Know your lime - again and again
Instead of relying on complicated measurement equipment, we should concentrate on the correct use of lime and its replacement intervals. The best protection against CO2 breakthroughs is still a well-maintained scrubber and knowing when it is reaching its limits. Divers who know their lime, know exactly when it is time to change it - and are safe from hypercapnia without the need for expensive and complex technology.
The real life example
2.68 kg of Sofnolime 797 show a CO2 breakthrough after 202 minutes under a simulated metabolic rate of 6 METS (metabolic equivalents). For a relaxed dive, no more than 4 METS can be assumed. This means that a only 2/3 of the CO2 is produced in the same time. This extends the service life by a third. If the standard scrubber of the JJ-CCR (2.5 kg) is packed correctly, CO2-breakthrough can be expected as late as 250 minutes - much later than the manufacturer specifies. So if you stick to the JJ-manual, you will be diving well within the safe range.
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