Oxygen window in technical diving
Encyclopedia
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.
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
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".
, 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.
at deeper depths. Convulsions are more likely when the pO2 exceeds 1.6 bar (160,000 Pa). 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 metres (68.9 ft).
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.
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:
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
(ppO2) in arterial blood and the ppO2 in body tissues. It is caused by metabolic consumption of oxygen.
The term "oxygen window" was first used by Albert R. Behnke
Albert R. Behnke
Captain Albert Richard Behnke Jr. USN was an American physician, who was principally responsible for developing the U.S. Naval Medical Research Institute...
in 1967. Behnke refers to early work by Momsen
Charles Momsen
Charles Bowers Momsen , nicknamed "Swede", was born in Flushing, New York. He was an American pioneer in submarine rescue for the United States Navy, and he invented the underwater escape device later called the "Momsen lung", for which he received the Distinguished Service Medal in 1929...
on "partial pressure vacancy" (PPV) where he used partial pressures of oxygen and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
as high as 2–3 ATA
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
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
Diving medicine
Diving medicine, also called undersea and hyperbaric medicine , is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment...
texts and the limits reviewed by Van Liew et al. in 1993.
| This passage is quoted from Van Liew's technical note: |
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 divingUnderwater diving
Underwater diving is the practice of going underwater, either with breathing apparatus or by breath-holding .Recreational diving is a popular activity...
, 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
Technical diving is a form of scuba diving that exceeds the scope of recreational 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.
Application
Use of 100% oxygen is limited by oxygen toxicityOxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...
at deeper depths. Convulsions are more likely when the pO2 exceeds 1.6 bar (160,000 Pa). 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 metres (68.9 ft).
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.
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 circulationCirculatory systemThe circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
and elimination of gas through the lungLungThe lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s; - The vasoconstrictor effect (reduction of the size of blood vessels) of oxygen, reducing decompression efficiency when blood vesselBlood vesselThe blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s start contracting; - The relative threshold where the body starts on-gassing rather than off-gassing.