In-water recompression (IWR) or underwater oxygen treatment is the emergency treatment of decompression sickness (DCS) by returning the diver underwater to help the gas bubbles in the tissues, which are causing the symptoms, to resolve. It is a procedure that exposes the diver to significant risk which should be compared with the risk associated with the available options and balanced against the probable benefits. Some authorities recommend that it is only to be used when the time to travel to the nearest recompression chamber is too long to save the victim's life; others take a more pragmatic approach and accept that in some circumstances IWR is the best available option. The risks may not be justified for case of mild symptoms likely to resolve spontaneously, or for cases where the diver is likely to be unsafe in the water, but in-water recompression may be justified in cases where severe outcomes are likely if not recompressed, if conducted by a competent and suitably equipped team.
Carrying out in-water recompression when there is a nearby recompression chamber or without suitable equipment and training is never a desirable option. The risk of the procedure is due to the diver suffering from DCS being seriously ill and may become paralysed, unconscious, or stop breathing while underwater. Any one of these events is likely to result in the diver drowning or asphyxiating or suffering further injury during a subsequent rescue to the surface. This risk can be reduced by improving airway security by using surface supplied gas and a helmet or full-face mask. Risk of injury during emergency surfacing is minimised by treatment on 100% oxygen, which is also the only gas with a reliable record of positive outcomes. Early recompression on oxygen has a high rate of complete resolution of symptoms, even for shallower and shorter treatment than the highly successful US Navy Treatment Table 6.
Several schedules have been published for in-water recompression treatment, but little data on their efficacy is available. The Australian Navy tables and US Navy Tables may have the largest amount of empirical evidence supporting their efficacy.
Background
Decompression sickness (DCS) is a medical condition caused by dissolved gases emerging from solution as bubbles inside the body tissues during decompression. DCS most commonly occurs during or soon after a decompression ascent from underwater diving DCS and arterial gas embolism are collectively referred to as decompression illness. Since bubbles can form in, or migrate to, any part of the body, DCS can produce many symptoms, and its effects may vary from joint pain and rashes to paralysis and death. Individual susceptibility can vary from day to day, and different individuals under the same conditions may be affected differently or not at all. The severity of symptoms varies from barely noticeable to rapidly fatal.
The risk of DCS caused by diving can be managed through proper decompression procedures, and contracting the condition has become uncommon. Its potential severity has driven much research to prevent it, and divers almost universally use dive tables or dive computers to limit their exposure, to monitor their ascent speed, and to ensure theoretically adequate decompression stops. If DCS is suspected, it is treated by hyperbaric oxygen therapy in a recompression chamber. Where a chamber is not accessible within a reasonable time frame, in-water recompression may be indicated for a narrow range of presentations, if there are suitably skilled personnel and appropriate equipment available on site. Diagnosis is confirmed by a positive response to the treatment. Early treatment results in a significantly higher chance of successful recovery.
Treatment of DCS utilizing the US Navy Treatment Table 6 with oxygen at 18m or an equivalent alternative is a widely recognised standard of care. Ideally, treatment should take place in a chamber where there is no risk of drowning or hypothermia, and other medical problems can be managed conveniently. Significant delay to treatment, difficult transport, and facilities with limited experience may lead to consideration of on site treatment. Surface oxygen for first aid has been proven to improve the efficacy of recompression and decreased the number of recompression treatments required when administered within four hours post dive. IWR to 9 m breathing oxygen is one option that has shown success over the years IWR is not without risk and should be undertaken with certain precautions. IWR would only be suitable for an organised and disciplined group of divers with suitable equipment and practical training in the procedure.
The principle behind IWR treatment is the same as that behind the treatment of DCS in a recompression chamber: an increase in ambient pressure will reduce the volume of the bubbles allowing better blood transport downstream of the bubbles. If the casualty can breathe pure oxygen further improvements will occur because the increase in the proportion of oxygen in the blood may keep previously oxygen-starved tissues alive and the oxygen window will accelerate the removal of inert gases from the bubbles making the bubbles smaller. The unacceptable risks of oxygen toxicity convulsions can be reduced by recompressing to lower pressures and for shorter durations than otherwise preferred.
Experimental work on verification of decompression tables by the US Navy Experimental Diving Unit with outcomes of symptomatic decompression sickness treated divers displaying symptoms by immediate recompression on oxygen, with 100% resolution of symptoms, and almost all cases resolved during the first treatment on Table 6, most of them during repressurisation or within the first 10 minutes at treatment depth. During the experimental work of developing the standard US Navy treatment tables 5 and 6, some experimentation was done with shorter treatment to shallower depths before standardising on 60 fsw (18 msw). This included immediate recompression on oxygen to 33 fsw (10 msw) for 30 minutes, which provided complete resolution in the majority of cases, but was not as effective as the full Table 6. Compression to 60 fsw (18 msw) would follow if symptoms did not resolve during initial treatment at 33 fsw. This previously unpublished data indicates that immediate recompression on oxygen to shallower depths for shorter periods does positively affect the probable long term outcome of decompression sickness, and compares favourably with outcomes for delayed hyperbaric treatment, and has been used as supporting evidence for new (2022) recommendations on in-water recompression. Outcomes for in-water recompression on other gas mixtures are mostly anecdotal and unsuitable for informing medical policy.(Mitchell et al. Sept 2022 in press)
The use of in-water oxygen at 6 meters has been shown to reduce bubble loads in divers compared to divers breathing oxygen on the surface.
Risks Any potential benefits of using IWR for earlier recompression should be balanced against the risks. These risks are well known, and their potential mitigations are fairly well understood.
The Royal Australian Navy School of Underwater Medicine was charged to supervise the then non-sanctioned practice of IWR, in response to the very long delays that often occurred between the presentation of DCS and recompression treatment. The issues identified include:
Inappropriate cases for treatment, Oxygen toxicity, Emergency termination of treatment, Hypothermia Adequacy of equipment in remote areas, Seasickness, Operator expertise and training, Safety of the diving attendant and the boat tenders, Requirement for medical supervision, Transport availability, Misuse of equipment, Pulmonary barotrauma cases.
Risk management In 2018, a group of diving medical experts issued a consensus guideline on pre-hospital decompression sickness management and concluded that IWR is only appropriate in groups that have been trained and are competent in the skills required for IWR and have appropriate equipment.
Mitigation of a CNS oxygen toxicity seizure focuses on protecting the airway to prevent drowning. A full-face mask or mouthpiece retaining strap is reasonably effective though not guaranteed. Tethering the diver to prevent sinking, and providing a safety diver to accompany the diver under treatment at all times, who can recover the diver to the surface immediately in the event of a seizure are also recommended, following the recommended procedures for recovery of a convulsing diver.
Factors influencing a decision to use IWR Recompression and hyperbaric oxygen administered in a recompression chamber are recognised as the definitive treatment for DCI, but when there is no readily available access to a suitable hyperbaric chamber, and if symptoms are significant or progressing, in-water recompression with oxygen is an option where a group of divers, including the symptomatic diver, already have the necessary equipment and relevant training and knowledge that provides a sufficient understanding of the associated risks to allow the involved parties to collectively accept responsibility for a decision to proceed with IWR.
The decision of whether or not to attempt IWR depends on identifying the diver whose condition is serious enough to justify the risk, but whose clinical condition does not indicate that the risk is unacceptable. The risk may not be justified for mild DCI, if spontaneous recovery is probable whether the diver is recompressed or not, and surface oxygen is indicated for these cases. However, in these cases the risk of the recompression is also low, and early abandonment is also unlikely to cause further harm. There are no firm guidelines regarding expected delay to access of a decompression chamber, but divers generally consider a predicted delay of more than 8 hours from symptom onset to be reason to consider in-water recompression.
Environmental conditions Water conditions, such as low temperature and rough sea state are relative contraindications, and the team must take these factors into account. Divers undergoing IWR risk becoming cold or hypothermic. They may already have done long dives in cold water, but the use of dry suits is common for such conditions, and the use of active heating systems in dry suit undergarments may be available. IWR requires a stable platform that can remain in one place for three hours. These factors should be considered when deciding whether to undertake IWR. Deteriorating condition of the diver or environmental conditions may make it necessary to abort or shorten treatment, or terminate it before full resolution. There is no known reason to assume that this would be inherently worse than not providing what treatment is possible at reasonable risk.
Indications Indications are based on symptoms and signs of decompression illness where expected benefits are likely to outweigh risk. There is a tension between conditions serious enough to justify the risks and clinical states which indicate excessive risk. Risk may not be considered justified for cases where the symptoms suggest a spontaneous recovery is likely without recompression, although the actual risk in these cases is likely to be relatively low. As of 2018 there is no widely accepted method for objectively selecting divers suitable for in-water recompression, and although remote consultation with a diving medical practitioner is advised by the Doolette and Mitchell 2018 protocol, it is likely that remote advice will vary significantly depending on who is contacted and how willing they are to accept responsibility for giving advice on a procedure with which they may have little personal experience and for which there is no clear decision tree.