DCIEM decompression theory

  What is DCIEM?
 DCIEM is a major research establishment in Canada. Its mission is to enhance the safety and effectiveness of Canadian Forces personnel in the way in which they interact with their equipment and the way in which they function in difficult environments.
 DCIEM is an abbreviation for "Defense and Civil Institute of Environmental Medicine". Diving research conducted at DCIEM had its origins in 1939.

  Outline of Decompression Theory
 The compressed air, which we inhale during scuba diving, contains nitrogen and oxygen at the rate of 8:2. The nitrogen we inhale is dissolved in our tissues under high pressure.
 Nitrogen pressure in your body is referred to as nitrogen tension. There is a maximum nitrogen tension for tissues. Known as an M-value, it indicates the maximum tension before bubbles are thought to form during ascent. (M stands for maximum) These bubbles are the cause of decompression sickness.
 In short, if we ascend without outguessing the nitrogen dissolved during diving, we may get decompression sickness. The purpose of decompression theory is to determine how long and how deep you can dive without undue risk of DCS.

  DCIEM decompression theory
 The DCIEM decompression theory is based on the Kid-Stubbs model, which was made in 1962 according to the dive table of USNavy and considering multi-level and repetitive dives.
 Their approach was to dive the model and, when symptoms of DCS occurred, to change the parameters of the model making it more conservative. They went through several variations of their air decompression model, improving the safety of the model after each iteration. They came to realize that the human body is better represented by a series arrangement of tissues.
 By 1967, over 5,000 experimental dives had been conducted to validate the K-S (Kidd-Stubbs) model. In 1971, the K-S decompression model was approved in Canada as a safer alternative to the U.S. Navy tables.
 In 1979, DCIEM initiated a critical reevaluation of the K-S model using digital computers to control the dives and specially-designed Doppler ultrasonic bubble detectors to evaluate the severity of the dive profiles. Then, thousands of verification diving and many improvements of the theory have been performed and the dive table for air diving was released in 1992. The present theory is based on this dive table.

  DCIEMダイブテーブル    DCIEM dive table
No-decompression limit time (NDL) for the first dive
Depth[m] 12 15 18 21 24 27 30 33 36 39 42 45
Depth[ft] 40 50 60 70 80 90 100 110 120 130 140 150
NDL[min] 99 80 52 35 25 19 14 11 9 7 7 6

  What is a tissue?
 Your entire body absorbs nitrogen under pressure. Some areas of your body absorb gas faster than others. Decompression tissues are categorized by how fast they uptake gas.
 Although tissue divisions do not correspond one to one with anatomic tissues, they do reference existing decompression areas that behave alike. Decompression tissues might be similar structures scattered all over your body. The numbers assigned to the tissues are derived from theory and experiment. Decompression tables and computers account for what we currently hope are most of the possibilities.

 What Are Fast and Slow Tissues
 Fast tissues ongas and offgas in shorter halftimes than slow tissues. Exact halftimes are not known for every single anatomic structure in the body. Experiments and educated guestimation have led to some generalizations about which areas of the body are faster or slower than others.
 Areas well supplied by blood like lungs and abdominal organs absorb nitrogen faster than other tissues. Slower tissues are usually considered to include fat, fatty marrow and a vascular area like cartilage and certain joint structures. Because fatty tissues hold more nitrogen than watery tissues, it takes longer for nitrogen to fill and leave fatty tissue. This is a property of fat, and is true even for fatty areas with a blood supply similar to leaner tissue.

  Ongas and outgas on ascent
 It's not true that you only offgas on ascent. Slow tissues don't have time during a recreational dive to equilibrate to ambient pressure. They will still have a lower pressure than the surrounding water. Water pressure during ascent forces nitrogen into your slow tissues, while your fast tissues outgas.

  What Are M-Values?
 There is a maximum nitrogen tension for each halftime tissue. Known as an M-value, it indicates the maximum tension before bubbles are thought to form during ascent. (M stands for maximum) Faster tissues have higher M-values and will tolerate higher supersaturating ratios than slower tissues.

  What Are Serial and Parallel Models?
 In a parallel model, the tissue compartments are assumed to ongas and offgas to the blood stream independently of each other. No gas transfer is assumed between different tissues. Most dive tables are based on parallel decompression models.
 The DCIEM tables are based on a four compartment serial model. Serial decompression models assume that gas transfers from tissue to tissue during a dive. Only one tissue is assumed to be exposed to ambient pressure.
 In a serial model, the compartments outgas to each other even as they ongas from other tissues of higher nitrogen tension. These compartments do not use set halftimes. Different filling times result for each compartment depending on depth and time.
    P(0): Ambient pressure (In initial dive, P(0) = Atmospheric pressure)
    P(1) - P(4): Nitrogen pressure in compartment

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