Background Mannitol is used extensively in supratentorial surgeries to reduce intracranial pressure. The optimum dose of mannitol is still a matter of debate. We have done this prospective, comparative and randomized study to compare the changes in cardiac output (CO), stroke volume (SV), cardiac index (CI), central venous pressure (CVP), brain relaxation, and side effects in two doses of mannitol infusion. We have divided 60 enrolled patients randomly into two groups. We infused 0.7gm kg-1,

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... mannitol in 20 min duration in Group A and 1.4gm kg-1 of 20% mannitol in 20 min in Group B. Flotrac transducer was connected to assess SV, CO, and CI. We recorded and analyzed brain relaxation score, HR, MAP, CVP, SV, CO, and CI at predefined time intervals after mannitol infusion. Chi-square test and unpaired t test were used to analyze categorical and continuous variables between two groups. Results We have found statistically significant and better brain relaxation in Group B than Group A. Significant (p<0.05) difference in CVP was found between the groups at 10, 20 min after start of infusion, and 5, 10, and 15 min after termination of infusion. We found significant (p<0.05) difference in SV, CO, and CI between the groups at most of the time periods. Conclusions We can conclude that higher dose of mannitol provides better brain relaxation intraoperatively without significant adverse effects. We assessed more changes in hemodynamic and cardiac parameters with higher dose of mannitol that did not cause clinical deterioration.