2.6. Inclusion and exclusion criteria
RCTs that matched the following criteria were considered eligible: (1) anesthesia was induced and maintained with opioids; (2) pharmacological interventions were given to patients at any dose before or during the operative period; and (3) a comparison was conducted between pharmacological interventions and placebo.
Articles were excluded based on the following considerations: (1) combination with regional nerve block during the anesthesia induction or maintenance period, and (2) data from healthy volunteer or children’s studies, abstracts, letters, or reviews.