Open Heart Surgery Clinical Trial
Official title:
Comparison of Isoflurane Anaesthesia by Closed Loop Controlled Administration Versus Manually Controlled Administration Using Bispectral Index in Open Heart Surgery
With the advancement in microprocessor technology and better understanding of
pharmacodynamics and pharmacokinetics of anaesthetic agents, computer facilitated closed
loop control of anaesthesia using propofol has been shown to be accurate with better
performance than manual control. Literature on computer controlled administration of
inhalational anaesthetics is few, as it requires the computer to control the dial setting on
the vapouriser. The investigators intend to compare the computer controlled closed loop
administration of isoflurane by infusing it into the anaesthetic circuit with conventional
vaporiser control in elective open heart surgery.
40 patients (ASA (American Society of Anesthesiology) class II-IV; 18- 65 years) undergoing
elective cardiac surgery requiring cardiopulmonary bypass (CPB) will be randomly divided
into manual or closed loop groups. Propofol will be used for induction of anaesthesia in
both groups followed by isoflurane for maintenance. In the manual group, isoflurane will be
administered through the Tech 7 vapouriser during pre and post CPB periods to target
bispectral index (BIS) of 50. In closed loop group, isoflurane will be administered using
infusion of liquid isoflurane into expiratory limb of the closed circuit. This rate of
infusion though a conventional syringe pump will be controlled by algorithm termed
'Improvised Anaesthetic Agent Delivery System' (IAADS) to maintain BIS of 50. Patients in
both groups will receive 500ml of 100 % oxygen as fresh gas flow. The % of time bispectral
index (BIS) is within the 10 of set target BIS of 50 will be the primary outcome measure.
The secondary outcome measures will be median performance error (MDPE)(2), median absolute
performance error (MDAPE)(2), wobble(2), divergence(2), amount of isoflurane used and
hemodynamic parameters will be secondary outcome measures.
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.
In the manual group, after the confirmation of the placement of endotracheal tube, the
circle system will be closed and fresh gas flows will be started at 1000ml/min. Inhalational
anaesthetics will be delivered by Fluotech 7 vapouriser (Datex ohmeda) for isoflurane. The
initial dial setting will be started at 3% and after 5 minutes, flows will be reduced to 500
ml/min and isoflurane concentration will be titrated by the anaesthesiologist to maintain
BIS of 50. The dial setting will be increased or decreased to maintain BIS of 50 and the
number of times the vapouriser dial setting is changed will be noted. Rest of the protocol
will be similar to the computer control group described above.
After the initiation of CPB, arterial blood gases, ACT and blood glucose will be determined
every half an hour. An arterial partial pressure of O2 of 300-400 mmHg, ACT >480s and blood
glucose <180mg/dl will be maintained throughout the bypass period. After successful
completion of cardiopulmonary bypass, heparin will be antagonised with protamine sulphate.
During episodes when the mean arterial pressure exceeds 25% of the baseline or heart rate
exceeds 100 beats/ min or 25% of the baseline, analgesia will be supplemented with fentanyl
1μg/kg. In case of persistent hypertension or tachycardia with BIS < 50, nitroglycerine
infusion will be started and titrated to control blood pressure; Esmolol (upto 0.5mg/kg IV)
will be administered to control the heart rate (HR). Similarly, in case of fall in mean
arterial pressure to less than 25% of the baseline in the presence of normovolemia,
initially phenylephrine will be used in boluses of 0.5μg/kg. For persistent hypotension
after a cumulative Phenylephrine dose of 2μg/kg, dopamine infusion will be started and
titrated to maintain MAP within 25% of the baseline. Likewise, in case of decrease in HR to
<45beats/min., atropine sulphate (10μg/kg IV boluses) will be administered after excluding
other treatable causes.
During CPB, patients will be shifted to propofol infusion, as the equipment for delivering
fresh gases during CPB requires large fresh gas flows and circle system is not possible
presently. MAP will be maintained between 50- 80 mmHg and any deviation from theses limits
shall be treated with Phenylephrine boluses or sodium nitroprusside infusion. Propofol will
be administered as per IAADS in computer control group and manually in the manual group to
maintain BIS of 50.
Five minutes after the end of CPB, patients will be shifted back to isoflurane. In the
computer control group, the concentration in the circuit for the set BIS will be achieved
over 10 minutes. In the manual control group, the dial setting will be started at 3% and
titrated according to the BIS. Fresh gas flows will be maintained at the same rate before
the initiation of CPB.
At the end of skin closure, isoflurane, fentanyl and vecuronium will be stopped and the
study protocol terminated. Patient will be shifted to propofol infusion for post operative
sedation and will be shifted to post anaesthesia care unit without antagonizing muscle
relaxants for elective mechanical ventilation. Patients will be extubated on meeting
standard criteria for extubation. All patients will be continuously monitored for
hemodynamic stability and maintenance of blood gases upto 24 hrs postoperatively. Patients
will be subjected to a structured interview as modified from Brice and colleagues and
described by Nordstrom for conscious awareness; on discharge from post anesthesia care unit,
the day after surgery and approximately a week later.
;
Allocation: Randomized, Endpoint Classification: Safety/Efficacy Study, Intervention Model: Parallel Assignment, Masking: Single Blind (Subject)
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