Objectives To investigate the influence of cardiac output reduction on volatile agent concentration required for maintenance of the targeted anaesthesia depth during cardiac surgery.
Methods 36 patients who had signed informed consent form and had been scheduled for CABG with cardiopulmonary bypass (CPB) were included into the study. Age of the enrolled subjects ranged from 50 to 65 years of age (mean 58.5 ± 4.6). 30 subjects were males and 6 were females. In all cases patients underwent general anaesthesia on the base of sevoflurane and fentanyl. Anesthetic concentration (from 0.75 to 2.5 vol.%.) was adjusted according to spectral EEG analysis data (Entropy, General Electric) to ensure target anaesthesia depth, corresponding to Entropy index not exceeding 40. Analgesia was provided by fentanyl infusion 5 mcg/kg/hour. To assess the influence of the cardiac output on sevoflurane pharmacodynamics we measured cardiac index (CI) simultaneously with anaesthetic concentration in the end tidal gas mixture (ETsev) and anaesthesia depth based on Entropy monitor data. Studied variables were measured at the following time points: 1 – 5 minutes after sternotomy, 2 – during internal mammary artery harvesting, 3 – during pericardiotomy. 95 sets of variables (Entropy index, ETsev, CI) were obtained at 3 time points. Since we proposed that maintenance of the target level of anaesthesia (Entropy index not more then 40) in patients with reduced cardiac index may be accomplished with lower ETsev, further analysis included 65 sets of variables measured in patients with target Entropy index.
Results We did not found linear correlation between ETsev and CI in patients with target Entropy index (correlation coefficient = 0.18, p = 0.14). It was assumed existence of a non-linear correlation between CI and ETsev variables with potential raise in anaesthetic efficacy of the sevoflurane in patients with reduced CI. To test this hypothesis we divided all the data on ETsev obtained at the target anaesthesia level into 2 groups depending on the measured CI: CI ≤ 2.2 l/min/m2, n = 19 (Gr I) and CI > 2.2 l/min/m2, n = 46 (Gr II). The lowest sevoflurane concentration necessary to maintain target level of anaesthesia was observed in patients with reduced CI. ETsev in Gr I was 1.15 ± 0.28% vs 1.37 ± 0.31% in Gr II, p = 0.01.
Conclusions Relationship between CI and ETsev required for maintenance of the target level of anaesthesia is non-linear. Patients with CI ≤ 2.2 l/min/m2 are characterised by lowering of the ETsev required for maintenance of the target level of anaesthesia.