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selection of anaesthetic drugs: halothane, isoflurane or sevoflurane
speed of induction and recovery诱导与苏醒速度
sevoflurane is half as soluble in blood as isoflurane, which is approximately half as soluble in blood as halothane. the more insoluble an anaesthetic, the faster the anaesthetic induction, more rapid the changes in depth and faster the recovery. clinically the difference between these agents is not huge, but animals will recover fastest from sevoflurane and to a lesser degree, isoflurane, and will be more alert - owners notice this difference! speed of recovery is an important consideration for neonates (c-sections), geriatric patients and for long anaesthetics.
all inhalation anaesthetics cause a dose-dependent reduction in mean arterial pressure which is caused by a dose-dependent decrease in cardiac output and stroke volume. there is a tendency for less cardiac depression with isoflurane and sevoflurane, particularly in cats, and at deeper levels of anaesthesia. the cardiovascular system seems to be more tolerant of changes in anaesthetic depth with isoflurane or sevoflurane compared to halothane.
the arrhythmogenic potential for halothane is much greater than that for isoflurane or sevoflurane. this is especially important for any patient with cardiac disease manifested by an arrhythmia. in such patients (e.g., gastric dilatation), halothane may be contraindicated.
all inhalation anaesthetics cause a dose-dependent respiratory depression, as evidenced by a rise in arterial carbon dioxide tension with increasing anaesthetic dose. in canine patients, isoflurane tends to produce the most respiratory depression when compared to the other inhalation agents. however, in cats, isoflurane has been shown to cause less respiratory depression when compared to halothane (there is no comparative data for sevoflurane).
traditionally, it was thought that the inhalation anaesthetics were entirely taken up and eliminated by the lungs. however, of that portion of the anaesthetic taken up by the body's tissue, 25% of halothane is metabolized, compared to 3% of sevoflurane and virtually o% for isoflurane this becomes particularly important for neonatal animals and animals with liver or renal impairment.
in addition, metabolism of inhalation anaesthetics is an important concern from an occupational health aspect. since metabolism of isoflurane, and to a lesser extent sevoflurane, is minimal, it offers distinct advantages from the perspective of occupational health exposure. in veterinary practices with good waste anaesthetic gas management, occupational exposure still occurs at the time of disconnection, during recovery and when servicing equipment!
pregnant women should minimize their exposure to waste anaesthetic gases, particularly those that are highly metabolized such as halothane and nitrous oxide. aas advises pregnant woman to wear face masks that scavenge waste anaesthetic gas while working in anaesthesia.