Intracranial Pressure monitoring is a crucial component of neuro-critical patient management. Clinical signs of increased intra cranial pressure can be very dubious and can be seen as a delayed cerebral decompensation. Monitoring of intracranial pressure can be invasive and non invasive. Out of which invasive techniques have advantage of reflecting real time intracranial pressures but are associated with certain complications like, infections, intracranial bleeding, occlusion of the catheter tip by blood, debris and difficulty in locating ventricle in presence of cerebral oedema. It is due to these disadvantages of invasive methods that non-invasive methods despite not being as accurate as invasive ones in predicting real time value have upper hand over the former method of intracranial pressure monitoring.

Non invasive techniques such as computerised tomographic scan or Magnetic resonance imaging are excellent tools to measure raised intra cranial pressure nowadays but ,since these techniques are time consuming, requires trained and skilled personnels and may require transportation of patients who may be unstable; Ultrasound being quick, economic and bed side procedure is rapidly becoming popular for identifying raised ICP in intensive and critical care units by monitoring optic nerve sheath diameter. Also, Ultrasonography is advantageous than clinical assessment as it helps diagnosing raised ICP, even in patients under sedation or with altered sensorium.

Rapid evaluation and management of ICP can be helpful in early detection of increased ICP and improve postoperative outcomes in neurocritically-ill patients. Ultrasonography of optic nerve sheath diameter (ONSD) is a non-invasive, indirect technique of evaluating changes in ICP and it shows a good correlation with direct ICP measurement. ONSD is well responsive to ICP change [12], despite of the association between ONSD and PaCO2, the dynamic responsiveness of ONSD, in asso­ciation with acute arterial CO2 change which can affect ICP, still has not been well investigated.

In the present study, we have hypothesized that the sonographic ONSD, as a surrogate for the ICP, can rapidly change in response to arterial CO2 change by monitoring ETCO2 reduction using short-term hyperventilation. To investigate the rapid change of ONSD, we monitor ETCO2, instead of PaCO2, to adjust systemic CO2 level constantly throughout the research. For the evaluation of acute response of ONSD, we measured ONSD in a continu­ous fashion on a single fixed transverse sonographic plane.