Halothane, sold under the brand name Fluothane among others, is a general anaesthetic.[5] It can be used to induce or maintain anaesthesia.[5] One of its benefits is that it does not increase the production of saliva, which can be particularly useful in those who are difficult to intubate.[5] It is given by inhalation.[5]
Side effects include irregular heartbeat, respiratory depression, and hepatotoxicity.[5] It appears to be safe in porphyria.[6] It is unclear whether use during pregnancy is harmful to the baby, and it is not generally recommended for use during a C-section.[7]
In rare cases, repeated exposure to halothane in adults was noted to result in severe liver injury. This occurred in about one in 10,000 exposures. The resulting syndrome was referred to as halothane hepatitis, immunoallergic in origin,[18] and is thought to result from the metabolism of halothane to trifluoroacetic acid via oxidative reactions in the liver. About 20% of inhaled halothane is metabolized by the liver and these products are excreted in the urine. The hepatitis syndrome had a mortality rate of 30% to 70%.[19] Concern for hepatitis resulted in a dramatic reduction in the use of halothane for adults and it was replaced in the 1980s by enflurane and isoflurane.[20][21] By 2005, the most common volatile anesthetics used were isoflurane, sevoflurane, and desflurane. Since the risk of halothane hepatitis in children was substantially lower than in adults, halothane continued to be used in pediatrics in the 1990s as it was especially useful for inhalation induction of anesthesia.[22][23] However, by 2000, sevoflurane, excellent for inhalation induction, had largely replaced the use of halothane in children.[24]
Halothane sensitises the heart to catecholamines, so it is liable to cause cardiac arrhythmia, occasionally fatal, particularly if hypercapnia has been allowed to develop. This seems to be especially problematic in dental anesthesia.[25]
Like all the potent inhalational anaesthetic agents, it is a potent trigger for malignant hyperthermia.[5] Similarly, in common with the other potent inhalational agents, it relaxes uterine smooth muscle and this may increase blood loss during delivery or termination of pregnancy.[26]
Halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) is a dense, highly volatile, clear, colourless, nonflammable liquid with a chloroform-like sweet odour. It is very slightly soluble in water and miscible with various organic solvents. Halothane can decompose to hydrogen fluoride, hydrogen chloride and hydrogen bromide in the presence of light and heat.[34]
Attempts to find anesthetics with less metabolism led to halogenated ethers such as enflurane and isoflurane. The incidence of hepatic reactions with these agents is lower. The exact degree of hepatotoxic potential of enflurane is debated, although it is minimally metabolized. Isoflurane is essentially not metabolized and reports of associated liver injury are quite rare.[36] Small amounts of trifluoroacetic acid can be formed from both halothane and isoflurane metabolism and possibly accounts for cross sensitization of patients between these agents.[37][38]
The main advantage of the more modern agents is lower blood solubility, resulting in faster induction of and recovery from anaesthesia.[39]
History
An advertisement for Fluothane, published in various American medical journals between 1961 and 1962.
Halothane was first synthesized by C. W. Suckling of Imperial Chemical Industries in 1951 at the ICI Widnes Laboratory and was first used clinically by M. Johnstone in Manchester in 1956. Initially, many pharmacologists and anaesthesiologists had doubts about the safety and efficacy of the new drug. But halothane, which required specialist knowledge and technologies for safe administration, also afforded British anaesthesiologists the opportunity to remake their speciality as a profession during a period, when the newly established National Health Service needed more specialist consultants.[40] In this context, halothane eventually became popular as a nonflammable general anesthetic replacing other volatile anesthetics such as trichloroethylene, diethyl ether and cyclopropane. In many parts of the world it has been largely replaced by newer agents since the 1980s but is still widely used in developing countries because of its lower cost.[41]
A meter for measuring halothane. This was used to measure the amount of halothane as flow of inspired gas during anesthesia.
Halothane was given to many millions of people worldwide from its introduction in 1956 through the 1980s.[42] Its properties include cardiac depression at high levels, cardiac sensitization to catecholamines such as norepinephrine, and potent bronchial relaxation. Its lack of airway irritation made it a common inhalation induction agent in pediatric anesthesia.[43][44]
Its use in developed countries has been mostly replaced by newer anesthetic agents such as sevoflurane.[45] It is not commercially available in the United States.[7]
It is the only inhalational anesthetic containing bromine, which makes it radiopaque.[47] It is colorless and pleasant-smelling, but unstable in light. It is packaged in dark-colored bottles and contains 0.01% thymol as a stabilizing agent.[20]
Greenhouse gas
Owing to the presence of covalently bonded fluorine, halothane absorbs in the atmospheric window and is therefore a greenhouse gas. However, it is much less potent than most other chlorofluorocarbons and bromofluorocarbons due to its short atmospheric lifetime, estimated at only one year vis-à-vis over 100 years for many perfluorocarbons.[48] Despite its short lifespan, halothane still has a global warming potential 50 times that of carbon dioxide, although this is over 100 times smaller than the most abundant fluorinated gases, and about 800 times smaller than the GWP of sulfur hexafluoride over 500 years.[49] Halothane is believed to make a negligible contribution to global warming.[48]
Ozone depletion
Halothane is an ozone depleting substance with an ODP of 1.56 and it is calculated to be responsible for 1% of total stratospheric ozone layer depletion.[12][13] Unlike most ozone depleting substances, it is not governed under the Montreal Protocol.[50]
^ abWorld Health Organization (2023). The selection and use of essential medicines 2023: web annex A: World Health Organization model list of essential medicines: 23rd list (2023). Geneva: World Health Organization. hdl:10665/371090. WHO/MHP/HPS/EML/2023.02.
^Habibollahi P, Mahboobi N, Esmaeili S, Safari S, Dabbagh A, Alavian SM (January 2018). Halothane. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. National Institute of Diabetes and Digestive and Kidney Diseases. PMID31643481. NBK548151.
^ abGyorfi MJ, Kim PY (2022). Halothane Toxicity. StatPearls. Treasure Island (FL): StatPearls Publishing. PMID31424865. NBK545281.
^Hankins DC, Kharasch ED (9 May 1997). "Determination of the halothane metabolites trifluoroacetic acid and bromide in plasma and urine by ion chromatography". Journal of Chromatography B: Biomedical Sciences and Applications. 692 (2): 413–8. doi:10.1016/S0378-4347(96)00527-0. ISSN0378-4347. PMID9188831.
^Sakai EM, Connolly LA, Klauck JA (December 2005). "Inhalation anesthesiology and volatile liquid anesthetics: focus on isoflurane, desflurane, and sevoflurane". Pharmacotherapy. 25 (12): 1773–88. doi:10.1592/phco.2005.25.12.1773. PMID16305297. S2CID40873242.
^Patel SS, Goa KL (April 1996). "Sevoflurane. A review of its pharmacodynamic and pharmacokinetic properties and its clinical use in general anaesthesia". Drugs. 51 (4): 658–700. doi:10.2165/00003495-199651040-00009. PMID8706599. S2CID265731583.
^"Halothane". NIOSH Pocket Guide to Chemical Hazards. (NIOSH) National Institute for Occupational Safety and Health, Centers for Disease Control. Archived from the original on 8 December 2015. Retrieved 3 November 2015.
^Lewis, R.J. Sax's Dangerous Properties of Industrial Materials. 9th ed. Volumes 1-3. New York, NY: Van Nostrand Reinhold, 1996., p. 1761
^Suckling et al.,"PROCESS FOR THE PREPARATION OF 1,1,1-TRIFLUORO-2-BROMO-2-CHLOROETHANE", US patent 2921098, granted January 1960 , assigned to Imperial Chemical Industries
^Halogenated Anesthetics. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury. National Institute of Diabetes and Digestive and Kidney Diseases. January 2018. PMID31644158. NBK548851.
^Ma TG, Ling YH, McClure GD, Tseng MT (October 1990). "Effects of trifluoroacetic acid, a halothane metabolite, on C6 glioma cells". Journal of Toxicology and Environmental Health. 31 (2): 147–158. Bibcode:1990JTEH...31..147M. doi:10.1080/15287399009531444. PMID2213926.
^Eger EI (1984). "The pharmacology of isoflurane". British Journal of Anaesthesia. 56 (Suppl 1): 71S –99S. PMID6391530.
^Mueller LM (March 2021). "Medicating Anaesthesiology: Pharmaceutical Change, Specialisation and Healthcare Reform in Post-War Britain". Social History of Medicine. 34 (4): 1343–65. doi:10.1093/shm/hkaa101.
