Abstract: We included 22 studies in this updated analysis, including six new trials with 584 additional participants, resulting in a total of 1428 participants. The risk of bias was low in 11 studies, high in one study and unclear in the remaining studies, due primarily to failure to report methodology for randomisation, and allocation concealment or blinding, or both. Fourteen studies examined intraoperative core temperatures among heated and humidified insufflation cohorts and core temperatures were higher compared to cold gas insufflation (MD 0.31 °C, 95% CI, 0.09 to 0.53, I2 = 88%, P = 0.005) (low-quality evidence). If the analysis was limited to the eight studies at low risk of bias, this result became non-significant but remained heterogeneous (MD 0.18 °C, 95% CI, -0.04 to 0.39, I2= 81%, P = 0.10) (moderate-quality evidence). In comparison to the cold CO2 group, the meta-analysis of the heated, non-humidified group also showed no statistically significant difference between groups. Core temperature was statistically, significantly higher in the heated, humidified CO2 with external warming groups (MD 0.29 °C, 95% CI, 0.05 to 0.52, I2 = 84%, P = 0.02) (moderate-quality evidence). Despite the small difference in temperature of 0.31 °C with heated CO2, this is unlikely to be of clinical significance. For postoperative pain scores, there were no statistically significant differences between heated and cold CO2, either overall, or for any of the subgroups assessed. Interestingly, morphine-equivalent use was homogeneous and higher in heated, non-humidified insufflation compared to cold insufflation for postoperative day one (MD 11.93 mg, 95% CI 0.92 to 22.94, I2 = 0%, P = 0.03) (low-quality evidence) and day two (MD 9.79 mg, 95% CI 1.58 to 18.00, I2 = 0%, P = 0.02) (low-quality evidence). However, morphine use was not significantly different six hours postoperatively or in any humidified insufflation groups. There was no apparent effect on length of hospitalisation, lens fogging or length of operation with heated compared to cold gas insufflation, with or without humidification. Recovery room time was shorter in the heated cohort (MD -26.79 minutes, 95% CI -51.34 to -2.25, I2 = 95%, P = 0.03) (low-quality evidence). When the one and only unclear-risk study was removed from the analysis, the difference in recovery-room time became non-significant and the studies were statistically homogeneous (MD -1.22 minutes, 95% CI, -6.62 to 4.17, I2 = 12%, P = 0.66) (moderate-quality evidence). There were also no differences in the frequency of major adverse events that occurred in the cold or heated cohorts. These results should be interpreted with caution due to some limitations. Heterogeneity of core temperature remained significant despite subgroup analysis, likely due to variations in the study design of the individual trials, as the trials had variations in insufflation gas temperatures (35 ºC to 37 ºC), humidity ranges (88% to 100%), gas volumes and location of the temperature probes. Additionally, some of the trials lacked specific study design information making evaluation difficult. While heated, humidified gas leads to mildly smaller decreases in core body temperatures, clinically this does not account for improved patient outcomes, therefore, there is no clear evidence for the use of heated gas insufflation, with or without humidification, compared to cold gas insufflation in laparoscopic abdominal surgery.

Plain-language summary: We searched the medical literature for randomised controlled trials (where people are allocated at random to one of two or more treatment groups) that compared heated and cold CO2. We analysed data from the trials for changes in core temperature. We also compared post-operative pain scores and pain medication requirements, length of hospital stay, length of surgery and fogging of the surgical video camera lens. Evidence is current to September 2016. We identified and included 22 trials. There was an increase of 0.31 °C in the heated, humidified CO2 group compared to the cold CO2 group but the data were heterogeneous (highly variable). However, if the analysis was limited to the eight low-risk-of-bias studies that reported core temperatures, no significant difference was found. Also, there was no temperature difference for heated and non-humidified gas compared to cold gas. There was no difference in postoperative pain with heated or cold insufflation. However, pain medication use was higher in only the heated, non-humidified group on postoperative days one and two. Heated gas apparently did not change length of hospitalisation, lens fogging or length of operation. Recovery room stay was shorter with heated gas but the data was heterogeneous (highly variable). When we only included studies at low risk of bias, the data became homogeneous (less variable) and the recovery room time was not significantly different between the heated and cold gas groups. While heated, humidified gas leads to slightly smaller decreases in core body temperatures, this does not account for improvement in any patient outcomes. Therefore, there is no clear evidence for the use of heated gas insufflation, with or without humidification, in laparoscopic abdominal surgery.