By David P. Reilly and Neal W. Pollock, Ph.D
Using a closed-circuit rebreather leads to a very different dive experience. A CCR is highly effective on deep dives, can reduce your gas usage and provides a peaceful alternative to the noise of open-circuit scuba. However, using a CCR also introduces new risks and complications.
The first thing you notice when diving a closed-circuit rebreather (CCR) is the lack of noise. Unlike open-circuit (OC) scuba, with its sounds of pressurised inspiratory air flow and its torrents of exhaled bubbles, rebreathers provide a relatively quiet way to dive. CCRs also allow a degree of gas conservation not possible with OC systems. While the added complexity of the devices puts an extra burden on divers, rebreathers are very useful for certain applications.
HOW CCRS WORK
A person breathing air extracts only a fraction of the available oxygen. An inspired breath holds 21% oxygen and an expired breath about 16%. Instead of losing the expired oxygen during exhalation, CCRs are a closed loop in which the diver’s breathing provides the motive force to push gas through the circuit. System components include the counterlung, a flexible bladder that expands during exhalation and contracts during inhalation; the scrubber, a material that removes carbon dioxide (CO2) from the expired gas; flexible hoses with one-way valves to control the direction of gas flow; sensors to analyse the oxygen content of the inspired gas; and valves for the automatic and/or manual addition of gases into the circuit. The gases are oxygen and a diluent, the latter typically a breathable gas with a lower fraction of oxygen. Depending on the planned dive, the diluent may be air, nitrox or another gas mix.
The rebreather electronics evaluate and adjust the gas inside the breathing loop to maintain gas composition and volume during the dive. Oxygen sensors, usually more than one for redundancy, continuously monitor the partial pressure of oxygen (PO2) and add oxygen, usually automatically, when the PO2 falls below a setpoint selected by either the diver or the operating system. If the PO2 rises appreciably above the setpoint, the diver can manually reduce the PO2 by adding diluent.
CO2 and water vapour in the diver’s exhaled breath react to form carbonic acid, which then reacts with the scrubber material in an exothermic (heat-releasing) reaction that removes CO2 from the gas in the loop before it is rebreathed.
BENEFITS OF CCRS
By recycling exhaled gas, rebreathers allow almost all of the oxygen in the supply to be used, dramatically reducing gas volume requirements. The magnitude of this benefit increases with depth, where OC divers lose large amounts of gas through exhalation. Even though divers need more gas molecules at depth to fill their lungs, closed-circuit systems almost entirely eliminate expired losses. Since metabolism is mostly independent of depth, a diver at 6 m of seawater would consume roughly the same amount of oxygen as he or she would with similar exertion and thermal stress at 61 m.
CCR efficiency makes extreme dives possible at a fraction of the operating cost and complexity of OC operations. Not only do rebreathers require much less gas, but they can also reduce the need for different mixtures for the travel, bottom and decompression phases of dives (although bailout supplies are still needed).
Regulating PO2 in accordance with a setpoint reduces inert gas uptake in the descent and bottom phases of modest-depth dives and increases inert gas elimination during the ascent and stop phases. This optimises decompression while controlling the risk of oxygen toxicity.
Divers tend to appreciate the warm, moist breathing gas maintained in the circuit, particularly during long dives.
Finally, rebreathers’ quiet operation makes them less obtrusive in the environment, enabling closer encounters with a variety of underwater creatures. The relatively bubble-free operation provides further benefit in overhead environments where exhaled bubbles can damage structures or compromise visibility.
CHALLENGES WITH CCRS
Compared with OC scuba, rebreathers require significantly more effort for maintenance, assembly, testing and monitoring during dives. Instead of focusing only on remaining gas pressure, closed-circuit divers must be aware of gas volume and composition in the loop.
When diving a CCR, simple actions can have great impacts on diver safety. For example, a rapid descent will transiently generate a very high PO2 (hyperoxia) due to the increase in ambient pressure and concentration of gases within the circuit. Similarly, rapid ascents can lead to transient states of marked hypoxia (insufficient oxygen) in the circuit.
More insidious hazards can develop if assembly or operation problems allow some or all of the exhaled gas to bypass CO2 scrubbing. A build-up in CO2 can lead to a debilitating state of intoxication and reduce the threshold for oxygen-induced seizures and decompression sickness. Hypoxia, hyperoxia and hypercapnia are all of critical concern because life-threatening incapacitation can develop before the diver recognises symptoms and takes corrective actions. While manufacturers put a great deal of effort into reducing the risk of problems, CCR divers are at risk from both simple and complicated errors.
CO2 monitoring in rebreather circuits is an active area of development. Some units employ a series of temperature sensors to measure the reaction front (the peak of exothermic heat production) moving through the scrubber material. This does not measure CO2, but it does indicate when the scrubber material is active and then inactive (exhausted). It is important to realise that this technology does not detect CO2 that may leak past the scrubber to reach the inspiratory side of the circuit. Some degree of scrubber bypass can occur if critical components are left out or improperly assembled or if gas finds a path through the scrubber bed in which CO2 scrubbing is not effective (channelling).
A recent study evaluating the typical five-minute prebreathe protocol found that an absence of scrubber could not be reliably identified, nor could a partly-failed scrubber.1 Subjects may be unaware of a high-risk compromised state. In-line CO2 monitoring technologies are now available and in active development, but a standard of robust and reliable monitoring with little false signalling has not yet been achieved, primarily due to the challenges of monitoring in the high-humidity loop environment. Until truly trustworthy monitoring is in place, special care is required to minimise the risk of CO2 accumulation.
The work of breathing (WOB) in CCR diving can be greater and more dynamic than in OC diving. WOB is a result of the resistance to the flow of breathing gas caused by the circuit components and pathway and the position of the counterlung relative to the diver. Rebreather design
and scrubber material heavily influence performance characteristics. For example, a finer-grain material can be more effective at removing CO2 but at the cost of greater flow restriction.
CCR breathing is facilitated by highly flexible counterlungs that allow easy inhalation and exhalation. The position of the counterlung relative to the diver influences the static lung load (SLL), which contributes to WOB. SLL can be computed as the difference in pressure between the counterlung and a reference point within the diver’s lungs known as the centroid. A pressure imbalance occurs when the counterlung is located above or below the centroid in the water column.5
There are three classic counterlung positions: front-mounted, back-mounted and over-the-shoulder (OTS). In a prone diving position, front-mounted counterlungs will be deeper than the centroid and thus at a higher pressure because of the additional hydrostatic (water) pressure. This will have a tendency to push gas into the diver’s lungs, making inhalation easier and exhalation more difficult. This case is referred to as positive SLL. Conversely, back-mounted counterlungs on the same diver would create a negative SLL. OTS counterlungs are popular because they can minimise SLL through a range of typical diver attitude and trim positions, but they add unwelcome bulk to the front profile of the diver. Several manufacturers are exploring or offering the option of a mostly rear-mounted set of counterlungs, with small volumes wrapping over the top of the shoulders. In any case, excessive SLL, either positive or negative, will have undesirable effects. Given that inspiratory resistance is generally associated with more troublesome physiological and psychological effects than expiratory resistance, a neutral to slightly positive SLL is often preferable.
The WOB associated with increased gas density produces practical depth limits for a given rebreather design. As a rule, physical effort should be aggressively minimised at depth. An unusually well-documented fatal rebreather incident which occurred during a deep dive highlights how easily extreme exposures can exceed safety margins.3
Finally, buoyancy control is different with rebreather systems. Breathing cannot be used for small-scale vertical corrections since exhaling into or inhaling from the counterlung does not change the gas volume in the human-machine circuit. CCR divers must also remember to close the mouthpiece before removing it to keep gas from escaping the circuit. Losing loop volume creates a loss of positive buoyancy, and introducing a substantial volume of water into the circuit will interfere with scrubber function.
The use of rebreathers has markedly increased in the past 20 years. Though activity data are limited, the number of fatalities involving rebreathers has also increased. While the specifics of accident cases vary, the relative complexity of the equipment appears to impose a significant risk.
Not all dive operators support rebreather diving. There can be a core incompatibility between the typically short dive profiles of OC divers and the often longer profiles of CCR divers. While the efficiency of closed-circuit rigs can make them favourable for remote operations, difficulties in locating scrubber material and high-pressure oxygen stores can be an issue. The need for gas cylinders different from the OC diving norm and the difficulties of transporting gas cylinders by air can be substantial impediments.
The ability to maintain a fixed PO2 is a powerful capability. The risk of oxygen toxicity is controlled by limiting PO2 to safe levels. Inert gas elimination is improved by keeping the PO2 elevated during ascent. CCR dives can produce substantial oxygen exposures. The National Oceanic and Atmospheric Administration (NOAA) central nervous system (CNS) oxygen toxicity exposure limits provide guidance. The single-dive limit is 45 minutes at a PO2 of 1.6 ATA, 150 minutes at 1.4 ATA, 210 minutes at 1.2 ATA and 300 minutes at 1.0 ATA. While the selection of optimal PO2 is complicated and evolving, it is important to appreciate that running a high PO2 at the bottom during a deep dive offers little benefit in reducing inert gas uptake since it will still be a small fraction of the total gas pressure, but this will increase the total oxygen exposure dose. It can make more sense to run with a modest PO2 at depth and increase it during ascent, where the high PO2 can have a much greater impact on inert gas elimination. The best choices for PO2 are influenced by the specifics of the dive.
Situations may develop that require a CCR diver to “get off the loop”. The diluent gas supply is usually a breathable mix that can be used for bailout. Since this source is limited, CCR divers usually carry additional supplies to ensure safe ascent. Switching to OC bailout can create or dramatically increase a decompression obligation since the optimised mix provided by the rebreather during ascent is gone. Bailout options will depend on the situation and available equipment configurations. Preparedness and response strategies can be complex for more extreme dives. Groups will sometimes rely on team bailout, in which a pooled supply of bailout gas is necessary to complete the bailout obligation of a single diver. This poses additional risk for team separation or multiple unit failures.
The capital investment required for rebreather purchase, training and maintenance is considerable. Regular use is required both to justify the expense and to maintain critical skills.
CCR OR OC?
Rebreathers provide another tool for diving and, like any tool, should be used thoughtfully. The capabilities of closed-circuit systems make them a valuable alternative to open-circuit, but divers must appreciate the increased complexity and weigh the risks before adopting this mode of diving.
1. Deng, C., Pollock, N.W., Gant, N., Hannam, J.A., Dooley, A., Mesley, P. & Mitchell, S.J. 2015. The five minute prebreathe in evaluating carbon dioxide absorption in a closed-circuit rebreather: a randomized single-blind study. Diving Hyperb Med, 44(1):16-24.
2. Fock, A.W. 2013. Analysis of recreational closed-circuit rebreather deaths 1998-2010. Diving Hyperb Med, 43(2):78-85.
3. Mitchell, S.J., Cronje, F.J., Meintjes, W.A.J. & Britz, H.C. 2007. Fatal respiratory failure during a “technical” rebreather dive at extreme pressure. Aviat Space Environ Med, 78(2):81-6.
4. Vann, R.D., Pollock, N.W. & Denoble, P.J. 2007. Rebreather fatality investigation. In: Pollock, N.W. & Godfrey, J.M., eds. 2007. Proceedings of the American Academy of Underwater Sciences 26th Symposium: Diving for Science 2007, Dauphin Island, Alabama, 9-10 March 2007:101-110.
5. Warkander, D.E., Nagasawa, G.K. & Lundgren, C.E.G. 2001. Effects of inspiratory and expiratory resistance in divers’ breathing apparatus. Undersea Hyperb Med, 28(2):63-73.