According to available literature, the upper limit of ONSD used to define ICP more than 20mmHg (raised ICP ) ranges from 0.48 to 0.57 cm. In our study ,the upper limit of ONSD at EtCO2 level 50mmHg was average of 0.40cm which implies that even at EtCO2 50mmHg, intracranial hypertension is a remote possibility in healthy non-neurosurgical patients with normal brain compliance. We kept constant the factors such as position of patients, time of measurement after achieving target EtCO2, measuring ONSD in a single plain ( transverse) and involving same experienced operator,,thereby, avoiding any confounding factors.