The Effect of End-tidal Carbon Dioxide Level on the Optic Nerve Sheath Diameter in Pediatric Patients

Because the optic nerve is surrounded by the cerebrospinal fluid, increase / decrease in the intracranial pressure can be examined by an increase / decrease in the optic nerve sheath diameter (ONSD). Non-invasive trans-orbital ultrasonography has been used to measure the ONSD and shown to be useful in differentiating patients with increased intracranial pressure (\> 20 cmH2O). Arterial carbon dioxide partial pressure (PaCO2) is closely related to cerebral blood flow. According to the textbook, an increase in PaCO2 in the range of PaCO2 = 25 mmHg to PaCO2 = 75 mmHg can cause a change in intracranial pressure by proportionally increasing cerebral blood flow. Because end-tidal CO2 partial pressure (ETCO2) reflects PaCO2, a significant correlation may be expected between the ETCO2 and the ONSD. Previous studies in adult patients showed that ONSD was sensitively increased / decreased according to short-term hypercapnia (ETCO2 = 50mmHg) or hypocapnia (ETCO2 = 30mmHg) for 10 minutes and normalized at normal ETCO2 (40mmHg). Therefore, the reactivity of the ONSD according to the change of the ETCO2 was confirmed in adult patients. In normal pediatric patients, ONSD is expected to respond to changes in ETCO2. However, no previous study has confirmed this. In addition, the reactivity of ONSD was not studied in neonates and infants with open fontanelle. In this study, the investigators will performed trans-orbital ultrasonography in pediatric patients with normal intracranial intracranial pressure aged 18 years or younger scheduled for surgery under general anesthesia. The ONSD will be measured at three ETCO2 levels; 35mmHg, 40mmHg, and 45mmHg.