One hour before the surgery, patients are premedicated with lorazepam (per os, 2.5 mg). Induction of anaesthesia is achieved by iv midazolam (30 μg/kg), sufentanyl (0.4-0.5 μg/kg), and propofol (0.3-0.5 mg/kg), and iv propofol (50 mg/kg/min) is administered to maintain anaesthesia. Intravenous boluses of rocuronium (0.6 mg/kg for induction and 0.2 mg/kg every 30 minutes for maintenance) is administered iv to ensure neuromuscular blockade. A cuffed tracheal tube (internal diameter of 7, 8, or 9 mm) is used for tracheal intubation, and patients are mechanically ventilated (Dräger Zeus, Lübeck, Germany) in volume-controlled mode with decelerating flow. A tidal volume of 7 ml/kg and a positive end-expiratory pressure of 4 cmH2O are applied, and the ventilation frequency is adjusted to 12-14 breaths/min to maintain end-tidal CO2 partial pressure of 36 38 mmHg. Mechanical ventilation is performed with a fraction of inspired oxygen of 0.5 before CPB, and it is increased to 0.8 after CPB. As a standard part of the cardiac anaesthesia procedure, oesophageal and rectal temperature probes are introduced, and a central venous line is inserted into the right jugular vein. The left radial artery is also cannulated to monitor systolic, diastolic and mean arterial (MAP) blood pressures and arterial blood gas samples.
The membrane oxygenator is primed with 1,500 ml lactated Ringer's solution prior to CPB. Intravenous heparin (300 U/kg) is injected into the patient, to achieve an activated clotting time of 400 s during CPB procedures. During CPB, mild hypothermia is allowed, the mechanical ventilation is stopped, and the ventilator is disconnected without applying positive airway pressure. Before restoring ventilation, the lungs are inflated 3-5 times to a peak airway pressure of 30 cmH2O to facilitate lung recruitment.
A flow meter and a mainstream capnometer is to be inserted into the breathing circuit between the endotracheal tube and the Y-piece of the breathing circuit to record capnogram curves. a T-piece with 2 collapsible segments is attached to the distal tracheal tube, with one end connected to the respirator and the other end to a loudspeaker-in-box system (FOT system) to measure respiratory mechanics by the use of forced oscillations. During normal ventilation the FOT system is not communicating with the patient, however, during measurements of respiratory mechanics the ventilator is to be stopped at end-expiration and the breathing circuit is blocked in a way that allows a connection between the FOT system and the patient. During a 15-s long apnoeic period respiratory mechanical parameters are to be measured with the FOT system and then normal ventilation is to be restored. Capnogram curves are going to be recorded and respiratory mechanical parameters are going to be measured a) after anaesthesia induction with the chest wall still intact, b) following chest wall opening before CPB initiation, c) following CPB with the chest wall open and d) following sternal closure before end of surgery in all groups of patients. Arterial and venous blood gas samples are also going to be collected at these time points.
Sample sizes are estimated to enable the detection of a 15% difference in the primary outcome parameter, the lung tissue elastance. Accordingly, sample-size estimation based on an ANOVA test with two groups of patients indicated that 155 patients were required in each group to detect a significant difference between the protocol groups (the assumed variability of 30%, power of 80% and the significance level of 5%).
Two-way repeated measures ANOVA with the inclusion of an interaction term is used for all measured variables with the protocol stage as within-subject factor (protocol stages) and group allocation as between-subject factor to establish the effects of T2DM and the surgical procedure on the measured respiratory mechanical and capnography indices. At half-way of the data collection, an interim analyses will be performed and the further data collection will be proceeded based on the results of this analysis.
