"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 was a noble-born pagan who converted to Christianity. He spent most of his later years struggling to secure the eternal life of souls. Hilary believed that their salvation was at risk from Arianism, a rampant heresy which denied the divinity of the Son of God.
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