Title: To evaluate the effect of change in end tidal carbon dioxide on optic nerve sheath diameter under general anaesthesia: A prospective study

Abstract

Objectives:Primary objective: To evaluate the change in optic nerve sheath diameter (ONSD) with change in end tidal carbon dioxide (EtCO2). Secondary objective: To evaluate if the change in ONSD in response to change in end tidal carbon dioxide is immediate (real time). To evaluate if the change in ONSD is reversible with restoration of end tidal carbon dioxide to previous level. Scope:Elevated intracranial pressure (ICP) is a common and potentially life-threatening condition. The trans-orbital ultrasonography, is a non-invasive method which detects raised ICP by evaluating increases in the optic nerve sheath diameter (ONSD). Dynamic responsiveness of ONSD, in association with acute change in EtCO2, (both hypercapnia and hypocapnia) which can affect ICP, still has not been well investigated. We hypothesized that there would be dynamic changes in ONSD in response to corresponding changes in EtCO2. Results:A strong correlation was found between the corresponding EtCO2 and ONSD values with Pearson’s correlation coefficient of 0.89(fig -1).There was no statistical difference between ONSD values at 0 and 5 minutes after attaining the desired EtCO2 level. The change in ONSD were reversible with EtCO2 reversibility. METHODS: 56 patients, with age between 18-60 years, either sex, and admitted to undergo elective spine surgery under general anaesthesia were included. All patients were exposed to both hypercapnia and hypocapnia and the ONSD values were recorded. However, for the purpose of the study, after induction of GA the respiratory rate was adjusted to attain the desired EtCO2 value (normocapnia, hypocapnia or hypercapnia). ONSD values were obtained at normo-, hyper- and hypo-capnia in all the patients. CONCLUSION:ONSD has a strong correlation with EtCO2 and it changes directly and significantly in response to changes in EtCO2.