Effect of end‐tidal carbon dioxide level on the optic nerve sheath diameter measured by transorbital ultrasonography in anesthetized pediatric patients: A randomized trial

Intraoperative hypercapnia and hypocapnia are common during pediatric anesthesia, and the cerebral blood flow and intracranial pressure may be affected by the partial pressure of arterial carbon dioxide. Transorbital ultrasound measurement of the optic nerve sheath diameter is a simple and non-invasive method for intracranial pressure assessment. The objective of this study was to evaluate the effect of end-tidal carbon dioxide (ET CO2 ) on optic nerve sheath diameter in a healthy anesthetized pediatric population.Pediatric patients scheduled for elective surgery under general anesthesia and mechanical ventilation in the supine position were enrolled and divided into four subgroups; age <2 years, 2-6 years, 6-12 years, and 12-18 years. Mechanical ventilation was adjusted to achieve target ET CO2 levels in a randomized sequence (40-35-45-40 mmHg or 40-45-35-40 mmHg). Three minutes after reaching each target ET CO2 level, transorbital ultrasound images of optic nerve sheath diameter were obtained and analyzed. The primary outcome was the optic nerve sheath diameter at each ET CO2 level.Sixty-four pediatric patients were enrolled and analyzed. At ET CO2 = 40 mmHg, the optic nerve sheath diameter was 5.6 ± 0.6 mm, 6.4 ± 0.5 mm, 6.8 ± 0.6 mm, and 7.1 ± 0.5 mm in children aged <2 years, 2-6 years, 6-12 years, and 12-18 years, respectively. The overall percent decreases in the optic nerve sheath diameter was -5.6 ± -4.3% (95% CI; -6.7 to -4.5%) at ET CO2 = 35 mmHg while the overall percent increases of optic nerve sheath diameter (ONSD) was 4.9 ± 5.1% (95% CI; 3.6 to 6.1%) at ET CO2 = 45 mmHg compared with those at ET CO2 = 40 mmHg. Spearman rank correlation analysis indicated that there were weak to moderate correlation between ET CO2 and the optic nerve sheath diameter (correlation coefficient [p-value] = .355 [.004], .318 [.014], .373 [<.001], and .420 [<.001] in children aged <2 years, 2-6 years, 6-12 years, and 12-18 years, respectively).The optic nerve sheath diameter measured by transorbital ultrasound showed rapid reactivity from ET CO2 35 to 45 mmHg in healthy pediatric patients under inhalation general anesthesia.