The effect of large-dose intrathecal opioids on the autonomic nervous system

Decreases in blood pressure after the spinal injection of opioids suggest t hat intrathecal (IT) opioids may have a sympatholytic effect similar to tha t of local anesthetic drugs. We compared two groups of patients aged 10-16 yr (n = 10 in each group). Group One (IT group) received IT opioids. Group Two (Epidural group) received 0.5% bupivacaine epidurally. The sympathetic effects of IT opioids and epidural bupivacaine were monitored by the change s in toe relative to calf temperature and by the changes in pulse wave grad ients with digital plethysmography. Changes in temperature gradients compar ing calf to toe and increases in pulse amplitude indicate vasodilatation ca used by sympathetic blockade in this model. Calf to toe temperature gradien ts (Delta calf-Delta toe) were evaluated by subtracting the two measurement s. Pulse wave plethysmography was recorded before and after spinal and epid ural injection at intervals of 10 min for 40 min. All patients demonstrated changes in their calf to toe gradients after IT and epidural injections (- 3.2 +/- 1.6). Systolic blood pressure decreased from a mean of 70 +/- 15 mm Hg to 55 +/- 10 mm Hg. Pulse wave plethysmography amplitude increased afte r the intrathecal opioid and epidural bupivacaine injection similarly. We c onclude that the increases in pulse wave amplitude and decreases in calf-to e gradients indicate a sympatholytic effect after IT opioids similar to tha t of local anesthetics.