Optic nerve sheath diameter response to end tidal carbon dioxide variation with ultrasonography in patients under general anaesthesia.

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

 Non invasive techniquessuch as computerised tomographic scan or Magnetic resonance imaging areexcellent tools to measure raised intra cranial pressure nowadays but ,sincethese techniques are time consuming, requires trainedand skilled personnels and may require transportation of patients who may beunstable; Ultrasound being quick, economic and bed side procedure is rapidlybecoming popular for identifying raised ICP in intensive and critical careunits by monitoring optic nerve sheath diameter. Also, Ultrasonography isadvantageous than clinical assessment as it helps diagnosing raised ICP, evenin patients under sedation or with altered sensorium.

Rapidevaluation and management of ICP can be helpful in early detection of increasedICP 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 correlationwith direct ICP measurement. ONSD is well responsive to ICP change [12], despiteof the association between ONSD and PaCO2, thedynamic 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 sonographicONSD, as a surrogate for the ICP, can rapidly change in response to arterial CO2 change by monitoring ETCO2 reduction using short-termhyperventilation. To investigate the rapid change of ONSD, we monitor ETCO2, instead of PaCO2, to adjust systemic CO2 level constantly throughoutthe research. For the evaluation of acute response of ONSD, we measured ONSD ina continu­ous fashion on a single fixed transverse sonographic plane.