diving Diving most often refers to: * Diving (sport), the sport of jumping into deep water * Underwater diving, human activity underwater for recreational or occupational purposes Diving or Dive may also refer to: Sports * Dive (American football), ...

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 g ...

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

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

metabolic Metabolism (, from ''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 cellular processes; the ...

consumption of oxygen.

Description

The term "oxygen window" was first used by Albert R. Behnke in 1967. Behnke refers to early work by Momsen on "partial pressure vacancy" (PPV) where he used partial pressures of oxygen and

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

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 A hill is a landform that extends above the surrounding terrain. It often has a distinct summit, and is usually applied to peaks which are above elevation compared to the relative landmass, though not as prominent as mountains. Hills fall und ...

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

, 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 Technical diving (also referred to as tec diving or tech diving) is scuba diving that exceeds the List of diver certification organizations, agency-specified limits of recreational diving for non-Professional diver, professional purposes. Technica ...

, 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 Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lung ...

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 Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. It is usually used for mixtures that contain less than 78% nitrogen by volume. In the usual application, underwater diving, nitrox is normally distinguished ...

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 circulation and elimination of gas through the

lung The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...

s; * The

vasoconstrictor Vasoconstriction is the narrowing of the blood vessels resulting from contraction of the muscular wall of the vessels, in particular the large arteries and small arterioles. The process is the opposite of vasodilation, the widening of blood vesse ...

effect (reduction of the size of blood vessels) of oxygen, reducing decompression efficiency when

blood vessel Blood vessels are the tubular structures of a circulatory system that transport blood throughout many Animal, animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the Tissue (biology), tissues of a Body (bi ...

s start contracting; * The threshold depth where the critical tissue compartments start off-gassing rather than in-gassing. * Cumulative effect of acute oxygen toxicity.

Extended oxygen window

The concept of partial unsaturation in tissues has been extended to saturation decompression. At the beginning of decompression from saturation the amount of unsaturation in the tissues allows a faster rate of decompression through the oxygen window, which is proportional to the partial pressure of oxygen in the breathing gas. Kot et al. have hypothesised that the oxygen window originally defined by Behnke as 60 torr (in air) can be extended by about 90 torr contributed by water vapour tension, as it does not contribute to bubble creation, and part of the carbon dioxide partial pressure, as it is highly soluble and chemically active. After the initial decompression through the oxygen window, the safe rate of decompression is limited by the slowest tissue, generally taken to be 360 minutes half time. This concept has been used in calculating the Polish saturation decompression tables.

References

External links

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