Chemistry of halothane-enflurane mixtures applied to anesthesia

Halothane and isoflurane have additive minimum alveolar concentration (MAC) effects in rats

Studies suggest that at concentrations surrounding MAC (the minimum alveolar concentration suppressing movement in 50% of subjects in response to noxious stimulation), halothane depresses dorsal horn neurons more than does isoflurane. Similarly, these anesthetics may differ in their effects on various receptors and ion channels that might be anesthetic targets. Both findings suggest that these anesthetics may have effects on movement in response to noxious stimulation that would differ from additivity, possibly producing synergism or even antagonism. We tested this possibility in 20 rats. MAC values for halothane and (separately) for isoflurane were determined in duplicate before and after testing the combination (also in duplicate; six determinations of MAC for each rat). The sum of the isoflurane and halothane MAC fractions for individual rats that produced immobility equaled 1.037 +/- 0.082 and did not differ significantly from a value of 1.00. That is, the combination of halothane and isoflurane produced immobility in response to tail clamp at concentrations consistent with simple additivity of the effects of the anesthetics. These results suggest that the immobility produced by inhaled anesthetics need not result from their capacity to suppress transmission through dorsal horn neurons.

IMPLICATIONS

Despite differences in their capacities to inhibit spinal dorsal horn cells, isoflurane and halothane are additive in their ability to suppress movement in response to a noxious stimulus.

[Monochromatic infrared halogenated gas analyzer and handling errors]

Monochromatic infrared gas spectrometers cannot identify volatile agents. Such an analyser (Capnomac, Datex) was tested while performing two errors: a) erroneous selection of the agent on the analyser, the vaporizer being filled with the correct agent; b) total or partial filling of the vaporizer (Vapor 19, Dräger) with an incorrect agent, the analyser being set for the agent the vaporizer was specified for. Three agents were studied, halothane (H), enflurane (E) and isoflurane (I). Each experiment was made in triplicate, the vaporizer being dried out between each. In case of erroneous selection on the analyser, differences between E and I were very small. When H was erroneously selected, the concentration displayed was six times higher, and when E or I was selected instead of H, the concentration displayed was six times lower than expected. In the 3.3-3.5 m band, H transmittance was six times higher than those of E and I. The agent selection control changes the gain so as to correct for the selected agent (gain for H: 12.0, E: 2.24, I: 1.89). In case of erroneous filling of the vaporizer, the concentration displayed was always different from that expected. When E or I was delivered with an H vaporizer, the analyser being set on H, the concentration displayed was 3 to 9 times higher than the concentration that had been set. On the other hand, when H was delivered with an E or I vaporizer, the analyser being set to E or I, the concentration displayed was 4 to 8 times lower.(ABSTRACT TRUNCATED AT 250 WORDS)

[A mistake in the filling of a vaporizer detected by an infrared analyser of halogenated anesthetic agents]

An anaesthetic pitfall related to an incorrectly filled vaporizer, without harmful effects on the patient, is reported. A halothane specific vaporizer has been accidentally partially filled with enflurane. The incident was suspected when the Datex Normac infrared analyser, calibrated for halothane, displayed an inspired concentration of 0.83% v/v, whereas the Dräger Vapor 19 vaporizer dial was set to deliver 0.4% v/v with a fresh gas flow of 2.7 l.min-1 to a circle system. The analyser uses infrared gas spectrometry, in the 3.3 to 3.5 microns band which contains the absorption peaks of halothane, enflurane, isoflurane and methoxyflurane. The four agents have different transmittances, with halothane the greatest. The agent selection control changes the gain to correct for the selected agent. The measurements are only accurate when only one agent is present at a time. In the reported case, the displayed concentration was very high because the high gain of halothane (12.0) was applied to enflurane (enflurane gain: 2.24). An in vitro experiment measuring the values of different enflurane/halothane mixtures, carried out with the involved vaporizer, showed that it contained, at the time of the incident, a mixture of 20% enflurane and 80% halothane. It may therefore be possible to detect a vaporizer filling error when the values "measured" by the analyser are not in concordance with those set on the vaporizer. Filling an enflurane vaporizer with halothane is more dangerous, as it results in a high halothane output with a Normac "enflurane" inspired concentration remaining very low. The indexed pin safety system remains the best means of avoiding wrong vaporizer filling.