The result of the subgroup analysis showed that the opioid consumption was slightly different between the two groups, but a decreasing trend was found in the SPI group (SMD − 0.31, 95% CI − 0.63 to 0.00, p = 0.05) when using entropy or BIS under general anesthesia.
The result of the subgroup analysis showed that the SPI group significantly had decreased opioid consumption (SMD − 0.45, 95% CI − 0.83 to − 0.06, p = 0.02) when the opioid administration in the control group was adjusted according to “inadequate anesthesia.” Opioid consumption was slightly different between the two groups, but an increasing trend was noted in the SPI-guided group (SMD 0.61, 95% CI − 0.07 to 1.28, p = 0.08) when opioid administration in the control group was adjusted according to the individual clinical practice by the anesthesiologist.