"If anyone is to have a limb mutilated, burnt or sawn, he may drink half an ounce with wine, and whilst he sleeps the member may be cut off without pain or sense".
Saint Hilary's otherworldliness didn't extend to an indifference to earthly pain - fleeting as its torments might be when compared to eternal life to come. The procedure Hilary describes in preparation for limb amputation sounds akin to anaesthesia, in aspiration if not in practice.
Refs and Further Reading
Anaesthesia and Anaesthetics