In diving and decompression, the oxygen window is the difference between the

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...

of oxygen (P_O₂) in arterial blood and the P_O₂ in body tissues. It is caused by

metabolic Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...

consumption of oxygen.

Description

The term "oxygen window" was first used by

Albert R. Behnke Captain Albert Richard Behnke Jr. USN (ret.) (August 8, 1903 – January 16, 1992) was an American physician, who was principally responsible for developing the U.S. Naval Medical Research Institute. Behnke separated the symptoms of Arterial Ga ...

in 1967. Behnke refers to early work by

Momsen Momsen may refer to: * Charles Momsen (1896–1967), American pioneer in submarine rescue for the United States Navy * Richard Paul Momsen (1891–1965), American/Brazilian lawyer, US Consul General in Rio de Janeiro *Taylor Momsen (born 1993), Am ...

on "partial pressure vacancy" (PPV) where he used partial pressures of oxygen and helium as high as 2–3 ATA to create a maximal PPV. Behnke then goes on to describe "isobaric inert gas transport" or "inherent unsaturation" as termed by LeMessurier and Hills and separately by Hills, 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. The clinical significance of their work was later shown by Sass. The oxygen window effect in decompression is described in diving medical texts and the limits reviewed by Van Liew et al. in 1993. 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".

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 P_O₂ 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.

Mechanism

The oxygen window does not increase the rate of offgassing for a given concentration gradient of inert gas, but it reduces the risk of bubble formation and growth which depends on the total dissolved gas tension. Increased rate of offgassing is achieved by providing a larger gradient. The lower risk of bubble formation at a given gradient allows the increase of gradient without excessive risk of bubble formation. In other words, the larger oxygen window due to a higher oxygen partial pressure can allow the diver to decompress faster at a shallower stop at the same risk, or at the same rate at the same depth at a lower risk, or at an intermediate rate at an intermediate depth at an intermediate risk.

Application

Use of 100% oxygen is limited by oxygen toxicity at deeper depths. Convulsions are more likely when the P_O₂ exceeds . Technical divers use gas mixes with high P_O₂ in some sectors of the decompression schedule. As an example, a popular decompression gas is 50% nitrox on decompression stops starting at . Where to add the high P_O₂ gas in the schedule depends on what limits of P_O₂ 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 P_O₂ is high and to at the shallower decompression stops. Nevertheless, much is still unknown about how long this extension should be and the level of decompression efficiency gained. At least four variables of decompression are relevant in discussing how long high P_O₂ decompression stops should be: * Time needed for

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and elimination of gas through the

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s; * The vasoconstrictor effect (reduction of the size of blood vessels) of oxygen, reducing decompression efficiency when blood vessels start contracting; * The threshold depth where the critical tissue compartments start off-gassing rather than in-gassing. * Cumulative effect of acute oxygen toxicity.

References

External links

* * good in-depth article * {{DEFAULTSORT:Oxygen Window In Technical Diving Underwater diving safety Diving medicine