Comparison of Isoflurane Anaesthesia by Closed Loop Controlled Administration Versus Manually Controlled Administration Using Bispectral Index in Open Heart Surgery

The basic components in a closed loop anesthesia delivery system are: (1) a system under control- patient; (2) a controlled variable that measures depth of anesthesia, (3) a set point for this variable specified by the user, (4) an actuator which comprises an algorithm to translate a measured value of the controlled variable to a particular action or the actuator to steer the controlled variable closer to the target variable.

In the proposed study, the controlled variable will be BIS. The control actuator is a patented (2158/DEL/2007) pharmacodynamic- pharmacokinetic model based adaptive infusion system termed "Improved Anesthetic Agent Delivery System (IAADS"). An IBM compatible with PENTIUM 2 or higher processor personal computer (2 GHz) is used to implement the control algorithm, provide a user interface and control communication through serial ports (RS 232) with the infusion system (Pilot- C, Frasenius, France) and a Datex vital sign monitor. We will use Avance ventilator (Datex Ohmeda, software version 5.0) with carbon di oxide absorber and ascending bellows to provide constant volume ventilation. The amount of acceptable leak in the circuit will be reduced to ≤ 150 ml min-1 determined during ventilation of a test lung. Low fresh gas flows will be used during the study. Isoflurane will injected into the expiratory limb of circle system via 20 ml Plastipak (polyethylene BD) syringe with a 100 cm long polyethylene tubing (Vygon, India) using a Pilot C Infusion pump (Fresenius vial SA, France). The stated volumetric accuracy of the infusion pump is ±2 %.

After obtaining written informed consent, patients will be randomly allocated (by closed envelope technique using computer generated random numbers) to one of the two groups- manual and automatic. The investigator will be present during the procedure for data collection purpose only and will not be involved in the conduct of anaesthesia. All patients will receive 5- 10mg oral diazepam as per institutional protocol the night before and on the morning of surgery as pre-medication. After shifting the patient to the operation table, patients will be started on normal saline 100 ml/hr (1-2 ml/kg/hr). Routine physiological monitoring will be commenced (pulse oximetry, electrocardiography, non-invasive blood pressure monitoring). BIS (Bispectral index)will be obtained by disposable sensors (Aspect medical system Inc. MA USA) attached to the forehead of the patient. Patients will receive 0.15 mg/kg of morphine sulphate before induction in aliquots of 3-5mg, during which indwelling arterial cannula, central venous line and pulmonary artery catheters (if deemed to be necessary) will be inserted.

Before induction, patients shall receive fentanyl 3μg/kg over three minutes. In automatic mode, IAADS automatically will calculate and titrate the initial and subsequent propofol infusion rate according to the weight of the patient, risk status and target BIS value, which will be set at 50 for all cases with a range of 40-60. In manual group, the propofol infusion rate will be determined by the attending anaesthesiologist according to the weight of the patient, and the target BIS of 50, with the aim to maintain BIS within 40- 60. After loss of consciousness (LOC), patients will receive 0.1mg/kg vecuronium bromide and will be intubated after 4 minutes.

The circle system will be closed after ascertaining the correct position of the endotracheal tube. In the control group, isoflurane infusion will be started into the expiratory limb of the circle at predetermined rates. The isoflurane concentration in the circuit will be built gradually over a period of 10 minutes to attain a specified BIS value. During isoflurane anaesthesia, patients will be ventilated with 100% oxygen. Fresh gas flows will be started at 500ml/min, and the ventilation will be set so as to maintain an end tidal CO2 concentration of 30-35 mmHg. If the fresh gas flows are inadequate as determined by incomplete filling of bellows, the flows will be increased gradually by 100 ml/min till a maximum of 1000 ml/min. If still there is a leak, the study will be abandoned and patient ventilated with higher flows. Thus, patients will be maintained on isoflurane injected into the expiratory limb of the circuit. Analgesia will be maintained with fentanyl 1μg/kg/hr infusion with further boluses of 1μg/kg before skin incision, sternotomy and commencement of cardiopulmonary bypass (CPB). Muscular relaxation will be maintained with a continuous infusion of vecuronium bromide 50μg/kg/hr. After 30 minutes of induction, an arterial sample will be drawn to have baseline values for blood gases, activated clotting time (ACT), and blood glucose. Patients will receive a bolus of 3 mg/kg heparin to raise the ACT above 480s. In cases of lower values, further boluses of heparin will be infused as per bull and colleagues dose response curve for heparin.