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CLINICAL REVIEW: Peter Wilmshurst ABC of oxygen: Diving and oxygen BMJ 1998; 317: 996-999 [Full text]

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Correct terminology: FiO2 or PiO2?

Pauline Wong   (21 October 1998)

Diving and Oxygen

Frank K Butler   (17 December 1998)

Correct terminology: FiO2 or PiO2? 21 October 1998
Pauline Wong, Staff anesthesiologist Veterinary Medical Teaching Hosp, Univ of Calif, Davis, CA USA Send response to journal: Re: Correct terminology: FiO2 or PiO2?	This article was brought up for discussion on GASNet's Anesthesiology email discussion group by a listmember who noted that the author stated "Prolonged breathing of a gas with an FiO2 greater than 60 kPa can lead to pulmonary toxicity...At an FiO2 greater than 160 kPa acute oxygen toxicity can occur..." and "The general rule is to try to achieve a gas mixture giving an FiO2 of about 140 kPa." The author's terminology was questioned. Skimming through the article, I found "The haemoglobin in arterial blood is virtually saturated at an inspired partial pressure of oxygen (FiO2) of 21 kPa...." It seems to me that the author & editors defined FiO2 differently from what I've seen conventionally used, namely fraction of inspired gas as oxygen. In the latter case, FiO2 can't exceed 1.0 (or 100%). I think their definition is the source of confusion since they apparently use FiO2 to describe the partial pressure of inspired O2 (PiO2) in the article. Some listmembers were surprised that BMJ would publish an article with what appears to be incorrect terminology. If the author or editors can cite a reference for their novel use of FiO2, I'd appreciate it. thanks, Pauline PL Wong, DVM, DACVA
Diving and Oxygen 17 December 1998
Frank K Butler Send response to journal: Re: Diving and Oxygen	Editor: I read with great interest the article by Peter Wilmshurst entitled "Diving and Oxygen" which was published in the BMJ on 10 October 1998. I would like to comment on some of the points made in that article. The article uses the term "FIO2" to describe the partial pressure of inspired oxygen. This is not a standard usage of the abbreviation. FIO2 is generally used to describe the fraction by volume of oxygen in the breathing mix. The term PIO2 is used to describe the partial pressure of inspired oxygen. Correctly distinguishing between the two is most important. The table entitled " Characteristics of Types of Decompression Illness" states that the neurological signs and symptoms of arterial gas embolism after pulmonary barotrauma are "mild." They can in fact be devastating and result in the death of the diver. The article states that "when a diver starts breathing from an oxygen rebreather, the fraction of inspired nitrogen is zero." This is not quite right. Despite the fact that the gas bottle in a closed-circuit oxygen rebreather contains 100% oxygen, nitrogen is present in the diver's lungs and in the breathing loop of the underwater breathing apparatus (UBA) prior to the dive. If a large amount of this inert gas is retained in the breathing loop, the breathing bag may not reach a small enough volume for the oxygen demand valve to be activated, even though the oxygen fraction in the breathing mix has dropped to a dangerously low level. To prevent this, purging procedures for the Draeger LAR V currently used by the U.S. Navy have been developed at the U.S. Navy Experimental Diving Unit (NEDU). (1) Elimination of all of the nitrogen from the UBA is not necessary; the goal of the pre-dive purge is to remove only enough nitrogen to eliminate the risk of hypoxia in UBAs which do not have an oxygen sensing and control capability. Excessive purging of the UBA beyond the prescribed procedure serves only to reduce the amount of oxygen available in the cylinder for diving and to increase the risk of CNS oxygen toxicity. Statistical analysis of toxicity episodes from experimental oxygen dives by Harabin and her colleagues at the U.S. Naval Medical Research Institute has shown that relatively small changes in the oxygen fraction of a UBA may be very important in reducing the risk of CNS oxygen toxicity. (2) The fraction of oxygen in the UBA required to prevent hypoxia is dependent upon both the gas volume in the breathing loop and the design of the oxygen addition mechanism. Because these may vary somewhat from UBA to UBA, it is important that the diver use a purge procedure developed specifically for the closed-circuit oxygen UBA from which he is breathing. The article further states that "the diver's body contains several liters of dissolved nitrogen and the pressure gradient causes this nitrogen to pass back into the lung and the counterlung. The oxygen is consumed, carbon dioxide is removed, and nitrogen accumulates, gradually reducing the percentage of oxygen in the counterlung. This can lead to unconsciousness. Flushing the system with pure oxygen periodically overcomes this problem…" Research performed at NEDU has clearly shown that this does not occur. The oxygen fraction in a closed-circuit UBA does undergo changes throughout the dive. As the diver descends from the surface to the desired dive depth, additional oxygen will need to be added to the UBA to compensate for the increased pressure. This has been found to cause the oxygen fraction in the Draeger LAR V to increase from a mean of 71% on the surface to 82% at a depth of 6.1m (20 fsw). (1) Theoretically, the oxygen fraction in a closed-circuit UBA after the initial descent might indeed be expected to decrease during the dive because of nitrogen off-gassing from body tissues. This consideration resulted in the previous U.S. Navy practice of re-purging the UBA every 30 minutes during the dive. The human body, however, contains only approximately one liter of nitrogen when saturated at 100kPa (1 ATA). (3) This is a relatively small amount of gas in comparison to the volume contained in the breathing loop of the Draeger LAR V at a depth of 6.1m. No decrease in the oxygen fraction of this UBA was found on a series of 18 two-hour experimental dives at NEDU in which additional purging during the dive was not performed. (1) The U.S. Navy discontinued the practice of repurging the Draeger LAR V periodically throughout the dive in 1984. The oxygen fraction might well increase during the dive if the diver has a significant leak from his face mask or changes depth frequently, since the nitrogen-oxygen gas mix lost from the breathing loop during these events is replaced with 100% oxygen from the cylinder. The article states that "breathing 100% oxygen there is a risk of convulsion at only 6m." In 686 experimental oxygen dives performed at NEDU during the development of the current USN closed-circuit oxygen exposure limits, the shallowest depth at which a convulsion occurred in the absence of a preceding excursion to a greater depth was 7.6m (25 fsw). The article goes on to state that " on the deepest working dives, at depths greater than 600m, ambient pressure is greater than 6100 kPa and the divers breathe gas mixtures containing about 2% oxygen to avoid acute oxygen toxicity." Use of a breathing mix with an FIO2 of 2% at 600msw would result in a PIO2 of 121 kPa (1.21 ATA). The approximate threshold for pulmonary oxygen toxicity is 0.55 ATA (55 kPa); exposures to a partial pressure of oxygen of 0.75 ATA (75 kPa) for only 24 hours have been shown to produce pulmonary oxygen toxicity. (4) The PIO2 for saturation dives is 0.44 to 0.48 ATA (44 - 48 kPa) in the US Navy. (5) Use of a PIO2 of 44 kPa calls for an FIO2 of 0.73% at 600m. Frank K. Butler, Jr. CAPT MC USN Biomedical Research Director, Naval Special Warfare Command Phone: 850-505-6754, Fax: 850-438-1123, e-mail: fkbutler@med.navy.mil References 1. Butler FK. Purging procedures for the Draeger LAR V Underwater Breathing Apparatus. NEDU Report 5-84; March 1984. 2. Harabin AL, Survanshi SS, Homer LD. A model for predicting central nervous system toxicity from hyperbaric oxygen exposure in man; effects of immersion, exercise, and old and new data. NMRI Report 94-03; March 1994. 3. U.S. Navy Diving Manual. Commander Naval Sea Systems Command Publication 0994-LP-001-9010. Washington D.C., U.S. Government Press, 1993; Revision 3, Vol 2, Chapter 3. 4. Clark JM, Thom SR. Toxicity of Oxygen, Carbon Dioxide, and Carbon Monoxide. In Bove AA, (ed): Bove and Davis' Diving Medicine. Philadelphia, WB Saunders, 1997; 131-145. 5. U.S. Navy Diving Manual. Commander Naval Sea Systems Command Publication 0994-LP-001-9010. Washington D.C., U.S. Government Press, 1993; Revision 3, Vol 2, Chapter 12.