Halothane dissolved in fat as an intravenous anaesthetic to rats

The anaesthetic properties of a halothane-in-fat solution given either as a single i.v. dose or as a continuous i.v. infusion were investigated in rats. 0.3 ml of a 5% solution of halothane in fat emulsion was injected i.v. into 15 awake rats. At the end of the 30 s injection, all rats had collapsed from the upright position and showed no response to a firmly applied tail clamp. Breathing usually became shallow and irregular just after injection. Two rats died. In the surviving rats, movement in response to clamping of the tail reappeared after some 30 s (range 15-90 s). The rats regained the upright position after about 100 s, and appeared fully awake about 3 min (range 2-5 min) after injection. Surviving rats behaved normally after the experiment, and gained in weight. They were killed 1-29 days later. The lungs, kidneys, heart, brain and liver had a normal macroscopic and microscopic appearance. In a second set of experiments (n = 9), a 10% solution of halothane was continuously infused i.v. (3.75 microliters min-1). The anaesthetic depth, as well as the mean arterial pressure, heart rate, respiratory rate and arterial PCO2 and PO2 were similar to values observed during inhalation of halothane in air at an inspired concentration of 1.1%. By doubling the infusion rate, MAP was reduced by 23%. It was easy to adjust anaethestic depth by changing the infusion rate and recovery was fast.