The ONSD was smallest (0.29cm) at EtCO2 30mmHg, and biggest (0.40cm ) at EtCO2 50mmHg while it was intermediate ( 0.34cm) at EtCO2 40 mmHg. These changes in ONSD are direct representation of changes in ICP brought about by changes in CBF due to PaCO2 changes. Based on results of Bland Altman plots, the calculated 95% confidence interval (CI) for the difference of two measures( EtCO2 40mmHg and 30mmHg ) on ONSD was 0. 009 to 0.102 and the calculated CI for the difference of other two measures ( EtCO2 40mmHg and 50mmHg ) on ONSD was 0.152 to 0. 29 and thus were observed to be statistically insignificant.