Diving Rebreathers

Function

Rebreathers are categorized based on how they add gases, particularly oxygen, to the breathing loop. There are various types of rebreathers, including semi-closed circuit rebreathers with active or passive addition mechanisms and fully closed circuit rebreathers with manual or electronically controlled oxygen addition. Some hybrid units incorporate multiple oxygen addition mechanisms.

Fundamental Rebreather Design

All rebreathers share fundamental similarities dictated by the physical principles that govern their function. Each unit includes a scrubber to remove exhaled carbon dioxide and a counterlung—a flexible reservoir that allows the diver to inhale and exhale underwater, as breathing into a rigid, sealed container is not possible. Rebreathers typically have one-way valves installed in the breathing loop to direct gas flow into and out of the unit, preventing a diver from rebreathing gas that has not been scrubbed of carbon dioxide. All commercially available rebreathers have sensor systems that monitor the partial pressure of oxygen to prevent divers from breathing hypoxic or hyperoxic gas mixtures. Depending on the model, rebreathers have 1 or more gas supply bottles to supply fresh gas to the diver. Some rebreathers bleed oxygen-enriched mixtures into the counterlung, whereas others monitor the oxygen levels available to the diver and bleed pure oxygen into the system to compensate for the oxygen depleted by the diver's metabolic needs. As both the diver's lungs and the counterlung on the rebreather are subject to compression with descent due to the effects described by Boyle's law, all rebreathers have a mechanism to inject gas into the breathing loop to counter the effects of volume loss with increased pressure. As pure oxygen becomes toxic at relatively shallow depths, rebreathers containing pure oxygen typically include a diluent gas to compensate for volume loss without risking oxygen toxicity. Various military organizations worldwide use shallow-water pure oxygen rebreathers with single-gas supplies due to their specific needs and advantages. Many rebreathers use retention straps to maintain airway protection during hyperoxic seizures.

Semi-Closed Circuit Rebreathers: Semi-closed circuit rebreathers continuously add breathing gas to the loop through a constant mass flow orifice and vent exhaled gas from the loop every few exhalations. Using this passive system, typically controlled by the diver's breathing, the fraction of inspired oxygen (FiO₂) in the breathing loop is generally maintained within 2% to 3% of a preselected amount. The system is simple and highly reliable; early units were sold without oxygen monitoring equipment. However, there are notable limitations; the FiO₂ is designed for a diver workload within a specific range, and hypoxia can occur with unexpectedly high workloads. As most systems vent approximately one-fifth of each breath, the volume of the counterlung 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 had simpler designs and training, making them desirable to some divers during the early days of rebreather use in the nonmilitary community.

Closed-Circuit Rebreathers: Closed-circuit rebreathers are available in various designs and use multiple oxygen addition mechanisms. These rebreathers are typically delineated based on their oxygen addition mechanism. The most common types include the manually controlled closed-circuit rebreather, the electronically controlled closed-circuit rebreather, and the hybrid closed-circuit rebreather, which combines features of both manually and electronically controlled rebreathers. Each model has advantages and limitations.

Manual closed-circuit rebreathers 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 must manually inject oxygen with a button and monitor the partial loop pressure of oxygen using a series of sensors. If the diver fails to act with this system, oxygen is not supplied, and the diver eventually becomes hypoxic. Due to the dire consequences of failing to act, the diver is forced to pay close attention to the unit, and as a result, most commercial units appear to have a good safety record. A more common variation of this unit involves manual oxygen addition through a button, where oxygen bleeds into the loop at a very slow rate to match 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 at rest, reducing the diver's workload. The oxygen in this system is injected through a constant mass flow restricted orifice. Because the orifice is sonic, once drive pressure is set, typically around 160 psi, the mass of oxygen injected over any given period is constant, regardless of depth. This drive pressure also limits the diving depth, as the unit stops injecting oxygen at a depth corresponding to the drive pressure, which typically ranges from 250 to 360 feet of water. The combination of passive constant oxygen flow and manual injection represents the design of most manually controlled closed-circuit rebreathers currently available.

Electronic closed-circuit rebreathers inject oxygen through a magnetic solenoid and a computer that monitors the oxygen levels in the loop, automatically injecting a bolus of oxygen to compensate for diver usage. These systems are fully automated; support unlimited depth, as the oxygen first stage does not require presetting at a specific pressure; and are highly efficient in oxygen usage. Some argue that fully automatic systems can lead to the diver becoming complacent about monitoring their oxygen levels, potentially delaying the detection of critical gas supply issues. There is evidence supporting this theory, especially when electronically controlled closed-circuit rebreathers were first introduced to the civilian market.[4] Recent evidence suggests that the accident rates between electronically controlled closed-circuit rebreathers and manual systems are comparable.[5] What is not in debate is that complex electronics immersed in saltwater have a higher failure rate than the simple mechanical push-button valves found on manually controlled closed-circuit rebreathers, and solenoids and computers require electricity to operate, whereas mechanical valves do not. Some divers use electronically controlled closed-circuit rebreathers 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 assumes that the likelihood of a diver becoming distracted at the exact moment the electronics fail is minimal. Some divers who preferred to operate electronically controlled closed-circuit rebreathers manually added a constant mass flow orifice to their systems, thereby significantly extending the time before needing to add additional oxygen. This innovation led to the development of the hybrid closed-circuit rebreather, which is now available as a variant of several commercial electronically controlled closed-circuit rebreathers.

Issues of Concern

Rebreathers are associated with increased complexity and potential for error compared to open-circuit dive equipment; however, they offer additional benefits for operational divers, including reduced gas utilization, increased bottom time, and the ability to perform stealthy excursions. In underwater exploration, unexpected events can prolong a dive, particularly at its maximum depth. Traditionally, this unexpected time has resulted in the depletion of emergency gas supplies and an increased incidence of near misses and fatalities. A rebreather can significantly reduce the time pressure on the diver, as it can extend dive time by several hours if needed, allowing the diver to address problems such as getting lost, clearing an entanglement, finding a lost buddy, or dealing with unexpected environmental challenges.[6] In reality, the use of mixed-gas rebreathers has been documented as enabling divers to address these issues. However, it is still associated with higher fatality rates compared to traditional open-circuit SCUBA.[5] This apparent disparity is most easily explained by the types of dives undertaken with this apparatus. Rebreather-equipped divers undertake dives that either are not possible on traditional open-circuit SCUBA or are logistically challenging, requiring multiple setup dives to stage safety gas bottles before the dive. This situation may be somewhat analogous to the introduction of jet aircraft—higher and faster flights became possible, connecting cities in hours instead of days, but the crashes were initially more spectacular as well. Additional safety innovations, such as the development of carbon dioxide (CO₂) monitors that function reliably in 100% humidity environments and advancements in scrubber bed monitoring technology, may help mitigate the risks associated with rebreather use.[7][8] Rebreathers offer advantages in terms of increased depth and duration compared to traditional open-circuit equipment.

Clinical Significance

Divers using rebreathers are subject to additional risks for oxygen toxicity, hypoxia, and hypercarbia beyond those associated with traditional open-circuit SCUBA.[7] These divers are also susceptible to 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 CO_2, 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 CO₂ from the loop once the sorb is exhausted. Although temperature sensors can be used to monitor the effective life of a scrubber, they are imperfect and only provide an estimation of time remaining based on the current workload of the diver.[6] In addition, the material that removes CO₂ from the breathing loop is highly caustic (sodium hydroxide has a pH of nearly 14) and forms a strong base when mixed with water. Although 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 typically be wholly 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.[9] Consultation with a toxicologist, pulmonologist, or hyperbaric physician trained in undersea medicine is recommended.

Enhancing Healthcare Team Outcomes

Divers using rebreathers are subject to additional risks for oxygen toxicity, hypoxia, and hypercarbia beyond those associated with traditional open-circuit SCUBA. These divers are also susceptible to 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 professionals, including diving medical technicians, hyperbaric nurses, and physician hyperbaric specialists, should educate patients on the dangers of deep-sea diving, such as decompression sickness, and potential risks associated with using rebreathers.