Association Between Fresh Gas Flow Rate and Optic Nerve Sheath Diameter During Laparoscopic Cholecystectomy: A Prospective Observational Study

Background/Objectives: Carbon dioxide pneumoperitoneum during laparoscopic surgery may alter intracranial dynamics, while the influence of fresh gas flow on these changes remains incompletely understood. This prospective observational study investigated the association between fresh gas flow rate and perioperative optic nerve sheath diameter (ONSD) trajectories during desflurane anesthesia for laparoscopic cholecystectomy. Methods: Sixty-four adults undergoing elective laparoscopic cholecystectomy were classified according to the fresh gas flow rate received during routine anesthetic management as low flow (1 L/min; n = 32) or high flow (3 L/min; n = 32). Bilateral ONSD was measured ultrasonographically at six predefined time points (T0–T5) and analyzed using linear mixed-effects models. Results: ONSD changed significantly over time (p < 0.001), increasing after pneumoperitoneum and decreasing after desufflation. No significant overall group effect (p = 0.337) or group × time interaction (p = 0.356) was detected, and between-group contrasts were nonsignificant at all individual time points after Holm correction. Covariate-adjusted sensitivity analysis yielded consistent findings. No significant associations between ONSD and regional cerebral oxygen saturation were detected after Holm correction. Minimum alveolar concentration (MAC) showed a significant group × time interaction, with higher values in the high-flow group at T2 and T3, whereas bispectral index (BIS) showed no significant group or group × time effect. Conclusions: Under the studied conditions, no statistically significant difference in perioperative ONSD trajectories was detected between fresh gas flow rates of 1 L/min and 3 L/min during laparoscopic cholecystectomy. These findings provide prospective observational data on the relationship between fresh gas flow and perioperative ONSD but should not be interpreted as evidence of equivalence or physiological safety.