Eleven RCTs (605 patients) were included in the review. All trials were free of selective reporting and addressed incomplete outcome data, six described blinding, four had adequate allocation concealment and three had adequate sequence generation.
Compared with high dose bupivacaine, low dose bupivacaine decreased the incidence of intraoperative hypotension (RR 0.64, 95% CI 0.42 to 0.96; significant heterogeneity; five trials). There was no evidence of a significant difference in analgesic efficacy (RR 1.50, 95% CI 0.95 to 2.37; significant heterogeneity; three trials) but the trend favoured high dose bupivacaine.
Compared with high dose bupivacaine, low dose bupivacaine combined with opioids decreased the incidence of intraoperative hypotension (RR 0.52, 95% CI 0.33 to 0.82; significant heterogeneity; six trials). There was no evidence of a difference in the proportion of participants requiring additional analgesics (RR 2.56, 95% CI 0.77 to 8.48; no significant heterogeneity; four trials). The low dose bupivacaine combination regimen was significantly associated with more pruritis (RR 16.13, 95% CI 3.39 to 76.74), less vomiting (RR 0.41, 95% CI 0.23 to 0.74) and a smaller dose of ephedrine (SMD -0.88, 95% CI -1.28 to -0.49) than high dose bupivacaine.
Sensitivity analyses suggested that the heterogeneity could be explained by the dosage of bupivacaine. The method of assessing intraoperative analgesia also appeared to vary. There was no evidence of publication bias using the Egger or Begg tests of funnel plot asymmetry.