Fentanyl improves analgesia but prolongs the onset of axillary brachial plexus block by peripheral mechanism

We evaluated the effects of fentanyl added to lidocaine for axillary brachial plexus block in 66 adult patients scheduled for elective hand and forearm surgery. In this double-blinded study, all patients received 40 mL of 1.5% lidocaine with 1:200,000 epinephrine, injected into the brachial plexus sheath using the axillary perivascular technique, and they were randomized into three groups. Group 1 was given lidocaine containing 2 mL of normal saline plus 2 mL of normal saline IV. Patients in Group 2 received lidocaine containing 100 mu g fentanyl plus 2 mL of normal saline IV. Group 3 patients received lidocaine containing 2 mL of normal saline plus 100 pg fentanyl IV. Sensory and motor blockade were evaluated by using a pinprick technique and by measuring the gripping force, respectively. The success rate of sensory blockade for radial and musculocutaneous nerves and the duration of the sensory blockade significantly increased in Group 2 (323 +/- 96 min) as compared with Group 1 (250 +/- 79 min). However, onset time of analgesia was prolonged in every nerve distribution by adding fentanyl to brachial plexus block. IV fentanyl had no effect on the success rate, onset, or duration of blockade. We conclude that the addition of fentanyl to lidocaine causes an improved success rate of sensory blockade but a delayed onset of analgesia, although this may be accounted for by the deceased pH caused by the fentanyl.