Decompression Theory & Treatment Tables
Despite the acceptance in the 19th century that there was a problem with hyperbaric air decompression, early divers continued to suffer the effects. In the early 1900's, the Royal Navy employed the services of the scientist John Haldane to devise a way of preventing the loss of their divers to the condition.
Haldane carried out many experiments on goats and finally developed a simplified mathematical model of how nitrogen gas molecules behave in the body's various tissues. He hypothesised that the body could be represented as five 'compartments', each of which absorbed and released the gas at different rates. The relationship between time and tissue 'loading' is based on the principle of half-times. For example, tissue compartment T5 will absorb half the maximum possible amount of inert gas in five minutes. After the next five minutes, the same compartment will have absorbed half of the remaining total amount of gas, and so on. Compartment T10 will acquire half of its potential maximum in ten minutes.
Another way to describe the half-time principle is by expressing an example of a volume of gas absorbed.
If the total volume of an inert gas that can be absorbed by compartment T10 is 80ml, then:
- 40ml will have been absorbed after 10 minutes
- 20ml will be absorbed during the next 10 minutes
- 10ml after another 10 minutes
- 5ml following the next and so on
For T75, it would take 75 minutes for each amount to be absorbed. When the maximum possible amount of inert gas is absorbed, the tissue is said to be 'saturated'. Haldane's hypothesis also stated that during decompression the inert gas in each tissue compartment is 'off-gassed' based on the same half-time principle. E.g. T10 will lose half its total amount after 10 minutes, another half after another 10, etc.
Haldane's second hypothesis was that it was actually possible for divers to have tissues 'supersaturated' with nitrogen without the gas forming bubbles. This was thought to be true so long as the ratio of environmental pressure (i.e. the gas breathed) to tissue gas tension (pressure exerted by the nitrogen) was 2:1 or less. For example, this meant that it was considered safe for a diver to ascend from a depth of 10msw (pressure of 2ATA) to the surface (1ATA) without developing DCS. Another example would be an ascent from 50msw (6ATA) to 20msw (3ATA).
Although his model remains the basis for modern decompression tables, Haldane's first decompression tables proved to be far from ideal. Further study on the subject of inert gas tissue loading led to the addition of many more theoretical compartments to the original model and the development of 'M' values in place of the over simplistic 2:1 ratio. These 'M' values, developed by Robert Workman in 1965, are the maximum allowable amount of inert gas supersaturation for each tissue compartment at a given depth. This was later modified by Albert Buhlmann in 1983, resulting in the Buhlmann algorithm, for diving at altitude rather than from sea level.
The body consists of various types of tissue. The rate at which an inert gas is absorbed (loaded) by each tissue during hyperbaric exposure, and subsequently released (off-loaded or off-gassed) during decompression, depends on several factors. These include the blood perfusion in the tissue and the solubility of the gas in each particular tissue type. A simplified description of tissues is that they can be fast or slow at absorbing and releasing inert gas. The table below gives examples of the 'speed' at which this process can occur for several tissue types exposed to both nitrogen and helium - the two most commonly used inert gases in diving.
|Skin, Muscle||37 - 79||14 - 30|
|Inner Ear||146 - 238||55 - 90|
|Joints, Bones||304 - 635||115 - 240|
|Edmonds, Lowry and Pennefather (1991)|
As discussed in the diving section, rate of ascent to the surface (in water or in a chamber) must be controlled to eliminate the risk of developing decompression sickness as far as reasonably possible.
A dive table (or profile) enables decompression to be conducted safely by graphically showing the depth and rates of ascent against a time scale. Although modern wrist worn dive computers can calculate this information more accurately for recreational SCUBA divers, printed tables (and the ability to interpret them) should always be available as a back up.
Over the past few decades, the United States Navy (USN) and offshore diving companies have gathered a large amount of information from their own diving operations. This has led to the development of tables for various circumstances. Because of the extensive experience used to formulate these tables, several of them have been modified for use by the hyperbaric medicine unit. These are shown below.
Click on a table for a larger image.
Modified USN Table 5Depth: 18msw
Atmosphere: air mix
Use: Treatment of CO poisoning
Comment: Can be extended if patient's condition indicates requirement for more prolonged treatment.
Modified USN Table 6
Use: Treatment of decompression sickness
Comment: Can be extended if required, or changed to saturation dive
Modified Comex Treatment Table TT12
Atmosphere: air mix
Use: Treatment for wounds and infections
Comment: Usually carried out five times per week for four to six weeks
Modified Comex 30
Use: Treatment of decompression sickness
Comment: Can be extended to several days
When it is considered appropriate to extend treatment for decompression sickness, the table may be changed to a 'saturation dive' by increasing (or maintaining) the depth to 30msw using a helium / oxygen mix called Heliox (HeO2). A 'sat dive' will normally last for 3 to 4 days. One interesting (but perfectly harmless) side effect of breathing Heliox is an increase in voice pitch that results in 'Donald Duck' speech or 'Helium voice'.
Tables 5, 6 and TT12 are restricted to a depth of 12 to 18msw and are carried out in a chamber atmosphere consisting of 21% oxygen and 79% nitrogen (air mix). However, under hyperbaric conditions, nitrogen acts as a narcotic on the brain and the effects are pronounced at depths greater than 18msw. Simply increasing the percentage of oxygen to reduce the amount of nitrogen is unsafe as it would increase the risk of fire and becomes toxic to the body in high concentrations over time. To allow people to reach greater chamber depths without suffering from nitrogen narcosis or oxygen toxicity, helium is used in place of nitrogen.
As each table shows, compression to the treatment depth takes only a few minutes but decompression is carried out more gradually.
The green areas on the graphs represent the time spent by the patient breathing pure oxygen using a hood and neck seal. The red areas on the Comex 30 table represent the patient breathing a 50/50 mix of oxygen and helium - the patient cannot breath 100% oxygen at depths greater than 18msw due to the increased risk of oxygen toxicity.
To further reduce the potential for oxygen toxicity, regularly breaks in oxygen therapy are scheduled - shown as the blue areas on the tables. When decompressing from an air dive, everyone in the chamber including the patient(s) and staff breathe pure oxygen to help flush nitrogen out of the body's tissues - thereby reducing the risk of decompression illness.