Effects of low-dose morphine and fentanyl infusions on urinary and plasma catecholamine concentrations during scoliosis surgery.

Plasma and urinary catecholamines were measured in 20 patients during scoliosis surgery to determine whether low dose fentanyl (1.5-2.5 micrograms . kg-1 . min-1) and morphine (150-250 micrograms . kg-1 . min-1) affect the catecholamine stress response to surgery differently. In all patients epinephrine (1.6-11.4 micrograms/kg) was injected locally at the operative site for hemostasis. This was an advantage because exogenous and endogenous epinephrine undergo the same fate and the increase in epinephrine might enhance otherwise subtle effects. The data indicate that both narcotics have similar effects on catecholamine metabolism but that even low doses of fentanyl are more effective than morphine in obtunding the catecholamine response to painful stimuli. Also, postoperative differences in plasma epinephrine indicate that recovery of awareness, and thus the onset of postoperative pain, is more rapid in patients receiving fentanyl.