Oxygen window in technical diving

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In diving, the oxygen window is the difference between the partial pressure of oxygen (ppO2) in arterial blood and the ppO2 in body tissues. It is caused by metabolic consumption of oxygen. [1] The term oxygen window was first used by Albert R. Behnke in 1967 [2]. Behnke refers to early work by Momsen on "partial pressure vacancy" (PPV) [3] where he used partial pressures of O2 and He as high as 2-3 ATA to create a maximal PPV [4]. Behnke then goes on to describe "isobaric inert gas transport" or "inherent unsaturation" as termed by LeMessurier and Hills [5] and separately by Hills[6] [7] [8] who made their independent observations at the same time. Van Liew et. al. also made a similar observation that they did not name at the time [9]. The clinical significance of their work was later shown by Sass [10].

The oxygen window effect in decompression is described in diving medical texts[1] and the limits reviewed by Van Liew et. al. in 1993 [11].

The following passage is quoted from their Technical Note [11]:
— When living animals are in steady state, the sum of the partial pressures of dissolved gases in the tissues is usually less than atmospheric pressure, a phenomenon known as the "oxygen window", "partial pressure vacancy" or "inherent unsaturation" [2] [7] [10] [12]. This is because metabolism lowers partial pressure of O2 in tissue below the value in arterial blood and the binding of O2 by hemoglobin causes a relatively large PO2 difference between tissues and arterial blood. Production of CO2 is usually about the same as consumption of O2 on a mole-for-mole basis, but there is little rise of PCO2 because of its high effective solubility. Levels of O2 and CO2 in tissue can influence blood flow and thereby influence washout of dissolved inert gas, but the magnitude of the oxygen window has no direct effect on inert-gas washout. The oxygen window provides a tendency for absorption of the gas quantities in the body such as pneumothoraces or decompression sickness (DCS) bubbles [9]. With DCS bubbles, the window is a major factor in the rate of bubble shrinkage when the subject is in a steady state, modifies bubble dynamics when inert gas is being taken up or given off by the tissues, and may sometimes prevent the transformation of bubble nuclei into stable bubbles[13].

Van Liew et. al. describe the measurements important to evaluating the oxygen window as well as simplify the "assumptions available for the existing complex anatomical and physiological situation to provide calculations, over a wide range of exposures, of the oxygen window"[11].

[edit] Background

Oxygen is used to decrease the time needed for safe decompression in diving, but the practical consequences and benefits need further research. Decompression is still far from being an exact science, and divers when diving deep must make many decisions based on personal experience rather than scientific knowledge.

In technical diving, applying the oxygen window effect by using decompression gases with high ppO2 increases decompression efficiency and allows shorter decompression stops. Reducing decompression time can be important to reduce time spent at shallow depths in open water (avoiding dangers such as water currents and boat traffic), and to reduce the physical stress imposed on the diver.

[edit] Application

Use of 100% oxygen is limited by oxygen toxicity at deeper depths. Convulsions are more likely when the pO2 exceeds 1.6ATAs. Technical divers use gas mixes with high ppO2 in some segments of the decompression schedule. As an example, a popular decompression gas is 50% nitrox on decompression stops starting at 21 meters.

Where to add the high ppO2 gas in the schedule depends on what limits of ppO2 are accepted as safe, and on the diver's opinion on the level of added efficiency. Many technical divers have chosen to lengthen the decompression stops where ppO2 is high and to push gradient at the shallower depths of the decompression curve, thus creating an S-shaped curve.[citation needed]

Nevertheless, much is still unknown about how long this extension should be and the level of decompression efficiency gained. At least three variables of decompression are relevant in discussing how long high ppO2 decompression stops should be:

  • Time needed for circulation and elimination of gas through the lungs;
  • The vasoconstrictor effect (reduction of the size of blood vessels) of oxygen, reducing decompression efficiency when blood vessels start contracting;
  • The relative threshold where the body starts on-gassing rather than off-gassing.

[edit] References

  1. ^ a b Brubakk, Alf O. & Neuman, Tom S. (2004). Bennett and Elliott's Physiology and Medicine of Diving. Philadelphia, USA.: Saunders. 
  2. ^ a b Behnke, A. R. (1967), “The isobaric (oxygen window) principle of decompression.”, The New Thrust Seaward. Trans. Third Marine Tech. Soc. Conf., San Diego: Marine Tech. Soc. 
  3. ^ Momsen, C. (1942). "Report on Use of Helium Oxygen Mixtures for Diving.". US Naval Experimental Diving Unit Technical Report (42-02). 
  4. ^ Behnke, A. R. (1969). The Physiology and Medicine of Diving. Baltimore, USA.: The Williams & Wilkins Company, 234. 
  5. ^ LeMessurier, D. H.; Hills B. A. (1965). "Decompression Sickness. A thermodynamic approach arising from a study on Torres Strait diving techniques.". Hvalradets Skrifter 48: 54–84. 
  6. ^ Hills, B. A. (1966). "A thermodynamic and kinetic approach to decompression sickness.". PhD Thesis. Adelaide, Australia: Libraries Board of South Australia. 
  7. ^ a b Hills B. A. (1977). Decompression Sickness: The biophysical basis of prevention and treatment. 1. New York, USA.: John Wiley & Sons. 
  8. ^ Hills, B. A. (1978). "A fundamental approach to the prevention of decompression sickness.". South Pacific Underwater Medicine Society journal 8 (4). ISSN 0813-1988. OCLC 16986801. 
  9. ^ a b Van Liew, H. D.; Bishop, B.; Walder, P.; Rahn, H. (1965). "Effects of compression on composition and absorption of tissue gas pockets.". J. Appl. Physiol. 20 (5): 927–33. ISSN 0021-8987. PMID 5837620. OCLC 11603017. 
  10. ^ a b Sass, D. J. (1976). "Minimum <delta>P for bubble formation in pulmonary vasculature.". Undersea Biomed. Res. 3 (Supplement). ISSN 0093-5387. OCLC 2068005. 
  11. ^ a b c Van Liew, H. D.; Conkin, J.; Burkard M. E. (1993). "The oxygen window and decompression bubbles: estimates and significance.". Aviat Space Environ Med. 64 (9): 859–65. ISSN 0095-6562. PMID 8216150. 
  12. ^ Vann, R. D. (1982). The Physiology and Medicine of Diving 3rd ed.. London: Bailliere Tindall, 352-82. 
  13. ^ Van Liew, H. D. (1991). "Simulation of the dynamics of decompression sickness bubbles and the generation of new bubbles.". Undersea Biomed. Res. 18 (4): 333–45. ISSN 0093-5387. PMID 1181708. OCLC 2068005. 

[edit] Additional Reading

  • Hills; LeMessurier (1969). "Unsaturation in living tissue relative to the pressure and composition of inhaled gas and its significance in decompression theory.". Clin Sci. 36 (9): 185–95. PMID 5772098. 
  • Yount; Lally (1980). "On the use of oxygen to facilitate decompression.". Aviat Space Environ Med. 51: 544–50. ISSN 0095-6562. PMID 6774706. 

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