Effect of dexamethasone on the duration of interscalene nerve blocks with ropivacaine or bupivacaine

Background. Pain after shoulder surgery is often treated with interscalene nerve blocks. Single-injection blocks are effective, but time-limited. Adjuncts such as dexamethasone may help. We thus tested the hypothesis that adding dexamethasone significantly prolongs the duration of ropivacaine and bupivacaine analgesia and that the magnitude of the effect differs among the two local anaesthetics. Methods. In a double-blinded trial utilizing single-injection interscalene block, patients were randomized to one of four groups: (i) ropivacaine: 0.5% ropivacaine; (ii) bupivacaine: 0.5% bupivacaine; (iii) ropivacaine and steroid: 0.5% ropivacaine mixed with dexamethasone 8 mg; and (iv) bupivacaine and steroid: 0.5% bupivacaine mixed with dexamethasone 8 mg. The primary outcome was time to first analgesic request after post-anaesthesia care unit discharge. The Kaplan-Meier survival density estimation and stratified Cox's proportional hazard regression were used to compare groups. Results. Dexamethasone significantly prolonged the duration of analgesia of both ropivacaine [median (inter-quartile range) 11.8 (9.7, 13.8) vs 22.2 (18.0, 28.6) h, log-rank P<0.001] and bupivacaine [14.8 (11.8, 18.1) and 22.4 (20.5, 29.3) h, log-rank P<0.001]. Dexamethasone prolonged analgesia more with ropivacaine than bupivacaine (Cox's model interaction term P=0.0029). Conclusions. Dexamethasone prolongs analgesia from interscalene blocks using ropivacaine or bupivacaine, with the effect being stronger with ropivacaine. However, block duration was longer with plain bupivacaine than ropivacaine. Thus, although dexamethasone prolonged the action of ropivacaine more than that of bupivacaine, the combined effect of dexamethasone and either drug produced nearly the same 22 h of analgesia.