PubMed · 9382212
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P Conzen. 1997. [Is halothane "out"?].. https://doi.org/10.1007/s001010050460
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The effects on intramuscular blood flow and cardiopulmonary parameters of changing from anaesthesia with halothane to isoflurane and vice versa were investigated in six ponies (small horses). Anaesthesia was induced with xylazine, ketamine and diazepam, maintained for one hour with halothane at an end tidal concentration of 1 per cent and then with isoflurane at 1.5 per cent for a further hour (halo/iso). On another occasion the order in which the volatile agents were administered was reversed (iso/halo). After one hour of anaesthesia the mean (sd) arterial blood pressure (MAP) and cardiac output (CO) of the ponies on the two occasions did not differ significantly (iso/halo, MAP 43 [5] mmHg, CO 10.9 [2.4] litre/min; halo/iso, MAP 53 [8] mmHg, CO 8.9 [2.3] litre/min). On changing the anaesthetic, MAP rose similarly in both groups. In the halo/iso group CO remained stable (8.64 [1.4] litre/min after the hour of isoflurane), but in the iso/halo group, CO decreased significantly on the administration of halothane (6.16 [1.3] litre/min after the second hour). When halothane replaced isoflurane, the intramuscular blood flow in both the upper and lower triceps brachii decreased significantly by 23 to 35 per cent, but when isoflurane replaced halothane the changes were not significant. It is concluded that CO and intramuscular blood flow both deteriorated when isoflurane was replaced by halothane. When isoflurane replaced halothane, cardiopulmonary function did not deteriorate further, but any improvement was not statistically significant.
The chemical and pharmacological properties of the current fluorinated inhalation anaesthetics, halothane, enflurane, isoflurane, sevoflurane and desflurane, are surveyed with implications to toxicity. Analytical methods, especially gas chromatography with head space, purge and trap, or pulse heating extraction, are reviewed in forensic toxicological and occupational/therapeutic monitoring contexts.
1. We studied the effects of inhalation anaesthetics on the membrane properties of hypoglossal motoneurones in a neonatal rat brainstem slice preparation. 2. In current clamp, halothane caused a membrane hyperpolarization that was invariably associated with decreased input resistance; in voltage clamp, halothane induced an outward current and increased input conductance. Qualitatively similar results were obtained with isoflurane and sevoflurane. 3. The halothane current reversed near the predicted K+ equilibrium potential (EK) and was reduced in elevated extracellular K+ and in the presence of Ba2+ (2 mM). Moreover, the Ba2+-sensitive component of halothane current was linear and reversed near EK. The halothane current was not sensitive to glibenclamide or thyrotropin-releasing hormone (TRH). Therefore, the halothane current was mediated, in part, by activation of a Ba2+-sensitive K+ current distinct from the ATP- and neurotransmitter-sensitive K+ currents in hypoglossal motoneurones. 4. Halothane also inhibited Ih, a hyperpolarization-activated cationic current; this was primarily due to a decrease in the absolute amount of current, although halothane also caused a small, but statistically significant, shift in the voltage dependence of Ih activation. Extracellular Cs+ (3 mM) blocked Ih and a component of halothane-sensitive current with properties reminiscent of Ih. 5. A small component of halothane current, resistant to Ba2+ and Cs+, was observed in TTX-containing solutions at potentials depolarized to approximately -70 mV. Partial Na+ substitution by N-methyl-D-glucamine completely abolished this residual current, indicating that halothane also inhibited a TTX-resistant Na+ current active near rest potentials. 6. Thus, halothane activates a Ba2+-sensitive, relatively voltage-independent K+ current and inhibits both Ih and a TTX-insensitive persistent Na+ current in hypoglossal motoneurones. These effects of halothane decrease motoneuronal excitability and may contribute to the immobilization that accompanies inhalation anaesthesia.