Minimum Alveolar Concentration of Sevoflurane for Maintaining BIS Below 50 in Patients Undergoing Coronary Artery Bypass Grafting During Normothermic Cardiopulmonary Bypass Phase.

Minimum alveolar concentration (MAC) is defined as the minimum alveolar concentration at sea level of inhaled anaesthetic required to prevent apparently purposeful movement in 50% of patients in response to surgical incision1,2. A similar approach to the one described to ascertain MAC has been used to determine the potency of inhaled anaesthetics for other desirable clinical end-points of general anaesthesia, such as to maintain bispectral index (BIS) below 50 (MACBIS50). Therefore, MACBIS50 is defined as the minimum alveolar concentration of inhaled anaesthetic for maintaining BIS below 50 in 50% of patients. MACBIS50 was determined in different age groups and clinical conditions of patients, but not in cardiac surgery3-5.

This study aims to determine the minimum alveolar concentration of sevoflurane for maintaining bispectral index below 50 in patients undergoing coronary artery bypass grafting during normothermic cardiopulmonary bypass phase.

Methods After obtaining approval from the institutional ethics committee and written informed consent, fifteen patients, American Society of Anestesiology physical status III or IV, aged 40-70, undergoing elective coronary artery bypass grafting, were enrolled in our study. Exclusion criteria included history of psychiatric or neurological diseases, drug abuse or medications that affect the central nervous system, pregnancy, combined surgeries, reoperation in cardiac surgery, emergency procedures and neuraxial block.

No premedication was given. Intraoperative monitoring consisted of a five-lead ECG, pulse oximetry, gas analyzer, bladder catheterization, invasive arterial pressure in the radial artery, esophageal thermometer, central venous catheter inserted through the subclavian vein, transesophageal echocardiography (when indicated), train-of-four monitoring (TOF) and BIS (BIS Vista™; Covidien). Breath-by-breath inspired/end-tidal sevoflurane and carbon dioxide (CO2) concentrations were measured with a multigas analyzer and it was calibrated before each experiment. The anaesthetic system used was a semi-closed system (Primus® - Draeger).

Anesthesia was induced with sevoflurane in 6 L.min-1 oxygen using the incremental increase technique via a face mask until the loss of consciousness and BIS value between 40-60. After then sufentanil (0.5 µg.kg-1) and pancuronium (0.1 µg.kg-1) were given and assisted ventilation was started. The T4/T1 ratio was measured using the TOF every minute until orotracheal intubation. The patient was intubated with the following parameters: BIS value between 40-60 and TOF = 0. Mechanical ventilation was initiated using controlled-volume mode (tidal volume of 6 mL.kg-1, ventilatory frequency of 12-15 bpm, PEEP of 5 mmHg and minute ventilation adjusted to maintain end-tidal carbon dioxide tension with the range of 35-40 mmHg). Neuromuscular block was maintained under deep block (TOF = 0 and PTC ≥ 1) using pancuronium up to one hour before the end of the procedure. Anesthesia was maintained using sufentanil (1 µg.kg-1 before the surgical incision) and up to 1 MAC of the end-tidal sevoflurane to maintain a BIS between 40-60. Continuous norepinephrine infusion was started if the mean arterial pressure was less than 60 mmHg.