Fig. 4. Relations between diastolic arterial pressure and muscle sympathetic activity (MSA) burst frequency in response to a hypotensive challenge by sodium nitroprusside (SNP) in the awake state and during anesthesia with S  (+)-ketamine. Similar increases in MSA in response to an evoked decrease in diastolic arterial pressure were obtained in the awake state (1) and during anesthesia with S  (+)-ketamine (3). Mean ± SD from six participants. Thus, the muscle sympathetic response to a hypotensive challenge is maintained during anesthesia with S  (+)-ketamine.