Fig. 3. Arterial pressure, heart rate, and epinephrine and norepinephrine plasma concentrations in the awake state, during ketamine anesthesia, and during ketamine anesthesia with arterial pressure adjusted to baseline value by infusion of sodium nitroprusside (SNP). Mean values ± SD from six healthy subjects. Arterial pressure and heart rate significantly increased during ketamine anesthesia, as did epinephrine and norepinephrine plasma concentrations. Note that at the end of the study, arterial pressure was decreased by SNP during ketamine anesthesia and did not differ from values in awake subjects. Although norepinephrine plasma concentration further increased during infusion of SNP, in parallel to an increase in muscle sympathetic activity, epinephrine plasma concentration as well as heart rate did not change further. *P < 0.05 vs.  baseline awake; ⁺P < 0.05 vs.  ketamine.