Correlation of cardiac output and sevoflurane required to maintain anesthetic depth targeted with entropy index

AIMS AND OBJECTIVES: We investigated the correlation of reduced cardiac output on required sevoflurane to maintain targeted anesthesia depth. MATERIALS AND METHODS: 36 patients undergoing coronary artery bypass grafting with cardiopulmonary bypass were prospectively included in the study. Inspired sevoflurane concentration was adjusted to ensure state entropy index < 40. Analgesia was provided by either boluses of fentanyl 200 μg or continuous infusion of fentanyl 5 μg/kg/h; the total dose of fentanyl administered in the patients was not different (fentanyl boluses 6.5 ± 0.3 μg/kg/h vs. fentanyl infusion 5 μg/kg/h). Cardiac-index (CI), end tidal sevoflurane (ETsev) and entropy index were measured simultaneously at 1-5 min after sternotomy, during internal mammary artery harvesting and during pericardiotomy. 108 sets of variables (entropy index, ETsev, CI) were recorded from 36 subjects at three time points; 13 sets were excluded due to technical drawbacks in measurements. 95 data sets were eligible for analysis. Sixty-five data sets measured in patients with target state entropy index were analyzed to establish the relationship between CI and ETsev. RESULTS: We did not find a linear correlation between ETsev and CI in patients with target entropy index (correlation coefficient = 0.18, P = 0.14). The ETsev necessary to maintain the target level of anesthesia was lower in patients with CI ≤ 2.2 l/min/m2 (1.15% ± 0.28%) than patients with CI > 2.2 l/min/m2 (1.37% ± 0.31%), P = 0.01. CONCLUSION: Relationship between CI and ETsev required for maintaining target level of anesthesia is non-linear. Patients with CI ≤ 2.2 l/min/m2 need lower levels of the ETsev for maintenance of the target anesthesia at an entropy index < 40.