Halothane detection

Dozens of compounds have been used in in vivo fluonne NMR and MRI studies, chosen more for their commercial availability and established biochemistry than for ease of fluonne signal detection [244] Among the more common of these are halothane and other fluormated anesthetics [245, 246], fluorodeoxyglucose [242 243], and perfluormated synthetic blood substitutes, such as Fluosol [246], a mixture of perfluorotnpropylamine and perfluorodecahn Results have been Imut-ed by chemical shift effects (multiple signals spread over a wide spectral range) and long acquisition times... 

In the preceding eqnation, the primary anion-radical gives the l-chloro-2,2,2-trifluoroethyl radical. In vivo, this radical was detected by the spin-trapping method (Poyer et al. 1981). Ahr et al. (1982) had presented additional evidence for the formation of the radical as an intermediate in halo-thane metabolism and identified l-chloro-2,2-difluoroethene as a product of radical stabilization. Metabolytic transformations of l-chloro-2,2-difluoroethene lead to acyl halides, which are relevant to halothane biotoxicity (Guengerich and Macdonald 1993). 

Enflurane is metabolised by the cytochrome P-450 series, specifically P-450 2E1, but the agent is much less extensively metabolised than halothane (see above). Metabolites include trifluoroacetic acid (TEA) and inorganic fluoride ion. A small number of cases of enflurane hepatitis have been reported but the overall incidence of liver damage following enflurane anaesthesia is estimated to be 1 in 800000. Clinical studies have failed to detect any significant effects of enflurane on liver function even when given repeatedly. 

Some of this free radical covalently binds to specific liver protein and elicits an immune response. This scenario is supported by data obtained in human subjects. Serum samples from patients suffering from halothane hepatitis contain antibodies directed against neoantigens formed by the trifluroacetylation of liver proteins. In addition, the administration of radiolabeled halothane to a patient undergoing a transplant operation resulted in the detection of covalent binding in liver protein. 

The principal antigenic proteins seem to be associated with the smooth endoplasmic reticulum. This is not particularly surprising since it is the probable site of metabolite formation via P450. Trifluoroacyl adducts have also been detected on the outer surface of hepatocytes, although it is not clear how they arrive there. In halothane-induced hepatitis the number of exposures does seem to be important, with about four being optimum. 

Blood concentrations ranging from 50.5 to 106.5 pg/ml were reported in 8 patients following anaesthesia with 1.5% halothane for 20 minutes. At recovery 10 minutes later the range was 22 to 30 pg/ml traces were still detectable after 44 hours bromide concentrations increased during the post-anaesthetic period (M. M. Atallah and I. C. Geddes, Br. J. Anaesth., 1973, 45, 464 70). 

Although the concentrations of practically all the impurities in the market product are too low to detect by infrared absorption, it has been used to analyze for 1-bromo-2-chloro-1,1,2-trifluoroethane at the 0.1 percent level, in halothane made by isomerizing the former material (3, 25). 

The changes in GST observed post-halothane anesthesia are small and it is possible that they may reflect circadian changes in the enzyme levels. To study this we measured plasma B, concentrations in 30 healthy volunteers after an overnight fast and subsequently at 3, 6, and 24 hr, i.e., the sampling intervals used for the study on the effects of halothane on GST. We could detect no signiflcant increase in B, subunits over this time period. 

More recently, the reductive activation of halothane (CFaCHBrCl), which is a hepatotoxic anaesthetic molecule, by human hemoglobin results in the modification of the prosthetic heme [57]. The inhibition of the reaction by adding exogeneous CO or the spin trapping agent N-t-butyl-a-phenyl nitrone to the incubation mixture indicated that (i) a reduced and free heme iron is required by Hb to activate the halogenated substrate and (ii) the formation of free radical species is responsible for Hb inactivation. However, no carbene species were detected in these reactions. The mechanism is shown in Scheme 7. 

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