A further refinement of intravenous anesthesia was the addition of a South American drug called
curare. Natives had used it for centuries to poison the tips of their arrows, and its discovery by the
Western world solved one of the biggest problems remaining with surgery under general anesthesia.
Although patients were unconscious, their muscles were still tense and would resist the surgeon’s
actions, sometimes undergoing spasms without warning. Injected intravenously, curare relaxes all the
muscles of the body so they cannot react to pain, but it does not prevent the patient from feeling pain
or induce unconsciousness. Although curare causes asphyxia by paralyzing muscles used for breathing,
it also allows for easy intubation of the trachea for artificial respiration. Modern anesthesia is
sometimes referred to as balanced anesthesia because it combines a narcotic for pain relief with a
barbiturate to induce sleep and a synthetic muscle relaxant similar to curare to allow intubation and
create optimum operating conditions.