In this study we monitored the effect of three different EtCO2 levels ( 40mmHg,30mmHg and 50mmHg ). In these healthy patients we observed rapid response in ONSD with changes in EtCO2. This again highlights the fact that optic nerve being in direct communication with the brain ,the pressure changes in the latter are reflected in the ONSD. The alteration in ONSD was immediate in response to EtCO2 changes, and moreover,changes in ONSD were parallel to the EtCO2 changes. Since CBF and cerebral blood volume change in response to changes in PaCO2, the ONSD also changes accordingly. Over the years ,advancements in monitoring of ICP has enabled the diagnosis of elevated ICP reliably by non-invasive techniques.Optic nerve sheath diameter measurement using bedside ultrasound has been shown to correlate with clinical and radiologic signs and symptoms of raised ICP. Despite the association between ONSD and PaCO2 , there is scanty literature on ONSD responsiveness to a more dynamic surrogate of PaCO2, that is EtCO2. The pertinent advantages of EtCO2 over PaCO2 measurement is that the former is continuously monitored under anesthesia and avoids time consuming process of arterial blood gas sampling. Moreover, the sensitivity of ICP to even small fluctuations in EtCO2 has been reported in literature. Animal studies have estimated that the rate of this increase in ONSD by 0.0034mm/mmHg increase in ICP.