[An experimental study of the effects of ketamine on the central sympathetic system, respiration and circulation...

Cats were given basal anaesthesia (nitrous oxide-oxygen) with artificial ventilation. They were then injected intravenously with 5 mg/kilogram bodyweight of ketamine. The injection was followed by a marked ketamine. The injection was followed by a marked reduction in both the tone of the central sympathetic system and the activity of the phrenic nerve which lasted for 10 minutes and 30 minutes respectively. Doses of 10 mg/kilogram bodyweight caused a longer lasting and more pronounced lowering of sympathetic activity while phrenic activity was approximately the same as with the smaller dosage. Injection of 2.5 mg/kilogram bodyweight reduced the sympathetic tone only if it was accompanied by asphyxia (central stimulation). Administration of 5 mg and of 10 mg/kilogram bodyweight was followed by a fall in blood pressure by 15-45 per cent up to 2 minutes after the injection. The heart rate at rest was unaffected but in the presence of asphyxia there was a dose-dependent reduction in the expected bradycardia. Ketamine also induced definite central inhibition thereby preventing the circulatory activation which occurs when the drug is used for anaesthetic purposes. The observation that the bradycardic effect arising from central stimulation was weakened suggests a reduction of central vagal activity.