Hyperbaric treatment schedules or hyperbaric treatment tables, are planned sequences of events in chronological order for hyperbaric pressure exposures specifying the pressure profile over time and the breathing gas to be used during specified periods, for medical treatment. Hyperbaric therapy is based on exposure to pressures greater than normal atmospheric pressure, and in many cases the use of breathing gases with oxygen content greater than that of air.
A large number of hyperbaric treatment schedules are intended primarily for treatment of underwater divers and hyperbaric workers who present symptoms of decompression illness during or after a dive or hyperbaric shift, but hyperbaric oxygen therapy may also be used for other conditions.
Most hyperbaric treatment is done in hyperbaric chambers where environmental hazards can be controlled, but occasionally treatment is done in the field by in-water recompression when a suitable chamber cannot be reached in time. The risks of in-water recompression include maintaining gas supplies for multiple divers and people able to care for a sick patient in the water for an extended period of time.
Background Recompression of diving casualties presenting symptoms of decompression sickness has been the treatment of choice since the late 1800s. This acceptance was primarily based on clinical experience. John Scott Haldane's decompression procedures and the associated tables developed in the early 1900s greatly reduced the incidence of decompression sickness, but did not eliminate it entirely. It was, and remains, necessary to treat incidences of decompression sickness.
Hyperbaric chamber recompression During the building of the Brooklyn Bridge, workers with decompression sickness were recompressed in an iron chamber built for this purpose. They were recompressed to the same pressure they had been exposed to while working, and when the pain was relieved, decompressed slowly to atmospheric pressure.
Although recompression and slow decompression were the accepted treatment, there was not yet a standard for either the recompression pressure or the rate of decompression. This changed when the first standard table for recompression treatment with air was published in the US Navy Diving Manual in 1924. These tables were not entirely successful – there was a 50% relapse rate, and the treatment, though fairly effective for mild cases, was less effective in serious cases.
1945 series of human experiments
Field results showed that the 1944 oxygen treatment table was not yet satisfactory, so a series of tests were conducted by staff from the Navy Medical Research Institute and the Navy Experimental Diving Unit using human subjects to verify and modify the treatment tables.
Tests were conducted using the 100-foot air-oxygen treatment table and the 100-foot air treatment table, which were found to be satisfactory. Other tables were extended until they produced satisfactory results. The resulting tables were used as the standard treatment for the next 20 years, and these tables and slight modifications were adopted by other navies and industry. Over time, evidence accumulated that the success of these table for severe decompression sickness was not very good.
These low success rates led to the development of the oxygen treatment table by Goodman and Workman in 1965, variations of which are still in general use as the definitive treatment for most cases of decompression sickness.
In water recompression
Treatment of DCS utilizing the US Navy Treatment Table 6 with oxygen at 18 m is a standard of care. Significant delay to treatment, difficult transport, and facilities with limited experience may lead one to consider 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.
Applications
Treatment of decompression sickness, arterial gas embolism, and other medical applications.
Equipment
Recompression chamber
The type of chamber which can be used depends on the maximum pressure required for the schedule, and what gases are used for treatment. Most treatment protocols for diving injuries require an attendant in the chamber, and a medical lock to transfer medical supplies into the chamber while under pressure.
Monoplace chambers Outside of the diving industry, most chambers are intended for a single occupant, and not all of them are fitted with built-in breathing systems (BIBS). This limits the schedules which can be safely used in them. Some schedules have been developed specifically for hyperbaric oxygen treatment in monoplace chambers, and some hyperbaric treatment schedules nominally intended for chambers with BIBS have been shown to be acceptable for use without air breaks if the preferred facilities are not available.
Treatment gases
Originally therapeutic recompression was done using air as the only breathing gas, and this is reflected in several of the tables detailed below. However, work by Yarbrough and Behnke showed that use of oxygen as a treatment gas is usually beneficial and this has become the standard of care for treatment of DCS. Pure oxygen can be used at pressures up to 60 fsw (18 msw) with acceptable risk of CNS oxygen toxicity, which generally has acceptable consequences in the chamber environment when an inside tender is at hand. At greater pressures, treatment gas mixtures using Nitrogen or Helium as a diluent to limit partial pressure of oxygen to 3 ata (3 bar) or less are preferred to air as they are more effective both at elimination of inert gases and oxygenating injured tissues in comparison with air. Nitrox and Heliox mixtures are recommended by the US Navy for treatment gases at pressures exceeding 60 fsw (18 msw), and Heliox is preferred at pressures exceeding 165 fsw (50 msw) to reduce nitrogen narcosis. High oxygen fraction gas mixtures may also be substituted for pure oxygen at pressures less than 60 fsw if the patient does not tolerate 100% oxygen.
Built in breathing system
Treatment gases are generally oxygen or oxygen rich mixtures which would constitute an unacceptable fire hazard if used as the chamber gas. Chamber oxygen concentration is limited due to fire hazard and the high risk of fatality or severe injury in the event of a chamber fire. US Navy specifications for oxygen content of chamber air allow a range from 19% to 25%. If the oxygen fraction rises above this limit the chamber must be ventilated with air to bring the concentration to an acceptable level. To minimize the requirement for venting, oxygen-rich treatment gases are usually provided to the patient by built in breathing system (BIBS) masks, which vent exhaled gas outside the chamber. BIBS masks are provided with straps to hold them in place over the mouth and nose, but are often held in place manually, so they will fall away if the user has an oxygen toxicity convulsion.
BIBS masks provide gas on demand (inhalation), much like a diving regulator, and use a similar system to control outflow to the normobaric environment. They are connected to supply lines plumbed through the pressure hull of the chamber, valved on both sides, and supplied from banks of storage cylinders, usually kept near the chamber. The BIBS system is normally used with medical oxygen, but can be connected to other breathing gases as required. Chamber gas oxygen content is usually monitored by bleeding chamber gas past an electro-galvanic oxygen sensor cell.
Units of measurement used in hyperbaric treatment
The commonly used units of pressure for hyperbaric treatment are metres of sea water (msw) and feet of sea water (fsw) which indicate the pressure of treatment in terms of the height of water column that would be supported in a manometer. These units are also used for measuring the depth of a surface supplied diver using a pneumofathometer and directly relate the pressure to an equivalent depth. The pressure gauges used on diving chambers are often calibrated in both of these units. Elapsed time of treatment is usually recorded in minutes, or hours and minutes, and may be measured from the start of pressurisation, or from the time when treatment pressure is reached.
Hyperbaric chamber treatment schedules
The schedules listed here include both historical procedures and schedules currently in use. As a general rule, more recent tables from the same source have a greater success rate than the superseded schedules. Some of the older procedures are now considered to be dangerous.
US Navy 1943 100-foot Air Treatment Table
Use: Treatment of decompression sickness where relief is obtained at or less than 66 fsw. Obsolete Oxygen is not used Maximum pressure 100 fsw (30 msw) Run time 3 hours 37 minutes
US Navy 1943 150-foot Air Treatment Table
Use: Treatment of decompression sickness where relief is obtained at or less than 116 fsw. Obsolete Oxygen is not used Maximum pressure 150 fsw (46 msw) Run time 4 hours 55 minutes
US Navy 1943 200-foot Air Treatment Table
Use: Treatment of decompression sickness where relief is obtained at or less than 166 fsw. Obsolete Oxygen is not used Maximum pressure 200 fsw (61 msw) Run time 5 hours 58 minutes
US Navy 1943 250-foot Air Treatment Table
Use: Treatment of decompression sickness where relief is obtained at or less than 216 fsw. Obsolete Oxygen is not used Maximum pressure 250 fsw (76 msw) Run time 6 hours 46 minutes
US Navy 1943 300-foot Air Treatment Table
Use: Treatment of decompression sickness where relief is obtained at or less than 266 fsw. Obsolete Oxygen is not used Maximum pressure 300 fsw (91 msw) Run time 7 hours 29 minutes
US Navy 1944 Long Air Recompression Treatment Table
Use: Treatment of moderate to severe decompression sickness when oxygen is not available or the patient cannot tolerate the elevated oxygen partial pressure. Oxygen is not used Maximum pressure 165 fsw (50 msw) Run time 5 hours 39 minutes
US Navy 1944 Long Air Recompression Treatment Table with Oxygen
Use: Treatment of moderate to severe decompression sickness when oxygen is available. Oxygen is used Maximum pressure 165 fsw (50 msw) Run time 3 hours 0 minutes
US Navy 1944 Short Air Recompression Treatment Table
Use: Treatment of mild decompression sickness when oxygen is not available or the patient cannot tolerate the elevated oxygen partial pressure. Oxygen is not used Maximum pressure 100 fsw (30 msw) Run time 5 hours 5 minutes
US Navy 1944 Short Oxygen Recompression Treatment Table
Use: Treatment of mild decompression sickness. Oxygen is used Maximum pressure 100 fsw (30 msw) Run time 2 hours 17 minutes
US Navy Recompression Treatment Table 1
Use: Treatment of pain only decompression sickness. Pain is relieved at less than 66 fsw (20 msw) Oxygen is available Maximum pressure 100 fsw (30 msw) Run time 2 hours 21 minutes Omitted from the US Navy Diving Manual since Revision 6
US Navy Air Treatment Table 1A
Table 1A is included in the US Navy Diving Manual Revision 6 and is authorized for use as a last resort when oxygen is not available. This table has been revised by decreasing the ascent rate from 1 minute between stops to 1 fsw per minute since the original was published in 1958.
Use: For treatment of pain only decompression sickness. Pain is relieved at less than 66 fsw. (20 msw) Air only, No oxygen. Maximum pressure 100 fsw (30 msw) Run time 7 hours 52 minutes
US Navy Recompression Treatment Table 2
Use: Treatment of pain-only decompression sickness. Pain is relieved at greater than 66 fsw (20 msw) Oxygen available