The effect of speed of injection on the potency of anaesthetic induction agents.

The effect of speed of injection on the hypnotic actions of thiopentone, methohexitone, propanidid and Alfathesin was studied in elective surgical patients. The sequential up-and-down technique was used to determine ED50 values. A significant reduction in ED50 was observed for doses injected over five seconds when compared with doses injected over fifteen seconds for thiopentone (64%, P=.001) and methohexitone (83%, P=.05). In the case of Alfathesin, relatively wide confidence limits were observed at all injection speeds and reduction in ED50 at fifteen seconds (63%, P=.001) was noted when compared with the ED50 at five seconds. No significant differences were noted with any of the drugs studied, for variations in infusion time between fifteen, thirty and sixty seconds. It was concluded that only at very rapid infusion rates does the speed of injection influence the potency of the commonly used induction agents.