Diving Rebreathers

Function

Rebreathers are divided into types based on how they perform the function of adding gas, specifically oxygen, to the breathing loop. There are several types: a semi-closed circuit with active or passive addition mechanisms and fully-closed circuit rebreathers with manual addition of oxygen or electronically controlled addition of oxygen. There are also hybrid units that incorporate more than one mechanism of oxygen addition.

Fundamental Rebreather Design

There are commonalities among rebreathers dictated by the physics that govern the function of the rebreather. Every rebreather has a scrubber of some design that is capable of removing exhaled carbon dioxide from the system and a counter lung, which is a flexible bag that the diver can breathe into and out of while underwater, as it is impossible to breathe into a rigid closed container. Rebreathers typically have one-way valves installed in the breathing loop to direct the flow of gas into and out of the unit, to prevent a diver from rebreathing gas that has not been scrubbed of carbon dioxide. All models of rebreathers currently available have a sensor system to monitor the partial pressure of oxygen to prevent the diver from breathing a hypoxic or hyperoxic gas mixture. Depending on the type of rebreather, they have one or more gas supply bottles to provide fresh gas to the diver. Some rebreathers simply bleed oxygen-enriched mixtures into the counter lung, while others sense the amount of oxygen available to the diver and bleed pure oxygen into the system to make up for the oxygen-depleted by the metabolic needs of the diver. As both the diver’s lungs and the counter lung on the rebreather are subject to compression with descent due to the effects described by Boyles law, all rebreathers have a mechanism to inject gas into the breathing loop to counter the effects of volume loss with increased pressure. Since pure oxygen becomes toxic at relatively shallow depths, rebreathers with pure oxygen supplies usually have a diluent gas to make up for volume loss without inducing oxygen toxicity. Various military organizations worldwide use shallow water pure oxygen rebreathers with single gas supplies due to their relatively specific needs and advantages. Many of these rebreathers employ mechanisms such as retention straps to prevent loss of airway protection in the event of a hyperoxic seizure.[4]

Semi-Closed Circuit Rebreathers (SCR)

Semi-closed circuit systems add breathing gas to the loop constantly via a constant mass flow orifice and vent exhaled gas from the loop every few exhalations. By using this passive system that is typically controlled by the diver’s breathing, the fraction of inspired oxygen (FiO2) in the breathing loop is typically maintained within 2 to 3% of a preselected amount. The system is simple to use and highly reliable, with early units sold devoid of oxygen monitoring equipment. There are, however, notable limitations as the FiO2 is designed for a diver workload within a specific range, and hypoxia can occur with unexpectedly high workloads. As most systems vent roughly one-fifth of each breath, the volume of the counter lung varies, and diver buoyancy is affected. Currently, this system is seldom used due to the availability of more advanced units with far greater depth and duration capability. Semi-closed units did have an advantage of simplicity in design and training that made them desirable to some divers in the early days of rebreather use in the nonmilitary community.

Closed-Circuit Rebreathers (CCR)

These units are available in many different designs, and they have several oxygen addition mechanisms. CCRs are usually delineated based on their oxygen addition mechanism. The most common mechanisms are the manually-closed circuit rebreather (mCCR), electronically-closed circuit rebreather (eCCR), and the hybrid-closed circuit rebreather (hCCR), which is a combination of the manually and electronically controlled rebreather. Each model has advantages and limitations.

Manual CCRs are common and rely upon the diver to maintain the partial pressure of oxygen in the breathing loop. In their most basic form, the diver simply must inject oxygen manually with the press of a button and monitor the partial loop pressure of oxygen on a series of sensors. If the diver fails to act with this system, no oxygen will be added, and the diver will eventually become hypoxic. Due to the dire consequences of failing to act, the diver is forced to pay close attention to the unit, and most units appear to have a good safety record as a result. A more common variation of this unit is one where oxygen addition is manual via a button, and oxygen bleeds into the loop at a very slow rate as well, which matches the diver’s resting metabolic rate. Because there is a small constant addition of oxygen, the diver may only need to add oxygen every 10 to 15 minutes when at rest, which decreases diver workload. The oxygen in this system is injected via a constant mass flow restricted orifice. Because the orifice is a sonic orifice, once drive pressure is set (typically about 160 psi), the mass of oxygen injected over any given period is constant, regardless of depth.  This drive pressure also limits the depth of the unit, as the unit will stop injecting oxygen at the depth of the drive pressure, which typically varies from 250 feet to 360 feet of water.  This form of passive constant oxygen injection combined with manual push-button injection makes up the majority of mCCRs on the market currently.

Electronic closed-circuit rebreathers inject oxygen via a magnetic solenoid and a computer that senses the amount of oxygen in the loop and then automatically injects a bolus of oxygen to make up for diver usage.  The system has the advantage of being fully automated, depth unlimited as the oxygen first stage does not have to be preset at a specific pressure, and highly efficient in oxygen usage.  Some argue that the fully automatic systems allow the diver to become complacent about monitoring his or her oxygen levels, and thereby delay time to detection of critical gas supply problems, and there is some evidence to support this theory, especially early in the introduction of eCCRs to the civilian market.[5] Recent evidence suggests that the accident rate between the two is identical.[6] What is not in debate is that complex electronics immersed in saltwater have a failure rate higher than the simple mechanical push-button valves found on mCCRs, and that solenoids and computers require electricity to operate, whereas mechanical valves do not. Of note, some divers do dive eCCRs manually while using a slightly lower oxygen set point on the computer, which then serves as a “reserve parachute” and kicks in if the diver fails to add oxygen. This system presumes that it would be even less likely that a diver would become distracted at the same exact moment the electronics failed.

Some divers, who were committed to running eCCRs manually, subsequently added the constant mass flow orifice to their eCCRs, thereby greatly extending the time they have before they are required to add additional oxygen. Thus, the hybrid closed circuit rebreather (hCCR) was born.  The hCCR is now available as a variant of several commercial eCCRs.

Issues of Concern

Rebreathers add complexity and potential for error over open circuit dive equipment, but they allow additional operational benefits to the diver, such as decreased gas utilization, increased bottom time, and stealthy excursions. In the underwater exploration world, unexpected events often add time to a dive, sometimes at the maximum depth of a dive. Traditionally, this unexpected time has resulted in the depletion of emergency gas supplies and resulted in both near misses and fatalities. A rebreather can substantially reduce the time pressure on the diver as it can literally extend dive time several hours[7] if needed to allow for the diver to deal with problems like being lost, clearing an entanglement, finding a lost buddy, or dealing with unexpected environmental challenges. In reality, the use of mixed gas rebreathers has been documented to allow divers to deal with these problems but are still associated with higher fatality rates than traditional open circuit SCUBA.[6] This apparent disparity is most easily explained by the types of dives undertaken with this apparatus. Rebreather-equipped divers are able to undertake dives that either would not have been possible on traditional open circuit SCUBA or would have been logistically challenging, requiring multiple set-up dives to stage safety gas bottles prior to the dive.  This may be somewhat analogous to the introduction of the jet aircraft: higher and faster flights became possible with cities connected in hours instead of days, but the crashes were more spectacular initially as well. Additional safety advances such as the introduction of carbon dioxide (CO2) monitors that are capable of reliable long-term function in a 100 percent humidity environment and scrubber bed monitoring technology may help reduce risks associated with rebreathers.[8][9] Currently, rebreathers most certainly offer advantages in depth and duration over traditional open circuit equipment.

Clinical Significance

Divers using rebreathers are subject to additional risks for oxygen toxicity, hypoxia, and hypercarbia above and beyond those associated with traditional open circuit SCUBA.[8] Divers using rebreathers are also capable of getting all forms of decompression illness, including decompression sickness and overexpansion injuries, that an open circuit diver can get. Carbon dioxide absorbents are also limited in their ability to bind CO2 and this limitation also functionally limits the time the rebreather can be used without replenishment.  The finite scrubbing capacity creates a situation where a diver may have gas, but the rebreather may no longer be able to effectively scrub carbon dioxide from the loop once the sorb is exhausted. While there are temperature sensors that can be used to monitor the effective life of a scrubber, they are imperfect and only an estimation of time remaining based on the current workload of the diver.[7] In addition, the material used for removing carbon dioxide from the breathing loop is strongly caustic (the pH can approach 14 as the material contains sodium hydroxide) and forms a strong base when mixed with water. While most modern commercial rebreathers have water traps of various designs to help prevent this caustic material from being ingested or inhaled if the breathing loop is inadvertently flooded, this can not usually be completely prevented. If a diver inhales or ingests this caustic material (known as a "caustic cocktail" to divers), the mouth should be thoroughly irrigated with freshwater, and the diver should be evaluated for a caustic burn. The diver should never be instructed to drink acidic fluids as the resultant exothermic reaction can be clinically significant. Airway compromise can occur from the ingestion of a caustic cocktail.[10] Consultation of a toxicologist and or hyperbaric physician trained in undersea medicine is recommended in these cases. 

Enhancing Healthcare Team Outcomes

Divers using rebreathers are subject to additional risks for oxygen toxicity, hypoxia, and hypercarbia above and beyond those associated with traditional open circuit SCUBA. Divers using rebreathers can also get all forms of decompression illness, including decompression sickness and overexpansion injuries, that an open circuit diver can get. Carbon dioxide absorbents are also limited in their ability to bind CO2, and this limitation also functionally limits the time the rebreather can be used without replenishment. The key to preventing these injuries is education. Healthcare workers, including the primary care physician, sports physicians, hyperbaric specialists, and nurse practitioners, should educate patients on the dangers of deep-sea diving, such as decompression sickness, as well as potential dangers from the use of rebreathers. This interprofessional team can help improve diving safety. [Level 5]