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

We investigated the correlation of reduced cardiac output on required sevoflurane to maintain targeted anesthesia depth.

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.

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.

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.