Low flow breathing systems in diving

An alternative method of providing the diver with breathing gas is to use a re-breathing system. These have been extensively used for military purposes from the 1940s and are currently a growth area for recreational use. These systems are akin to “low flow breathing systems in anaesthesia”. The operating principle of a diving re-breather is that the diver may conserve gas by continuously exhaling and inhaling the same gas. Prior to re-inhaling the gas, any previously exhaled carbon dioxide is removed and oxygen added to maintain the inspired partial pressure of oxygen (PO₂) at a physiologically acceptable level. Although diving re-breathers have many different configurations, all have the same basic components. The fundamental building block of a re-breather is the breathing circuit or breathing loop comprising: facepiece and breathing hoses (typically gas is forced to flow in one direction round the breathing circuit by non-return valves positioned either side of the facepiece), counterlung and absorbent canister. To complete a simple system attached to the breathing circuit are: relief valve, gas supply system and harness. The gas supply and associated inspired PO₂ control systems are discussed