ANAESTHESIA MANAGEMENT OF BEATING HEART CORONARY BYPASS SURGERY

Dr. PSN Raju, MD.,
Chief Cardiac Anaesthetist
Apollo Hospitals,Vanagaram,
Chennai.

The treatment of coronary artery disease has evolved rapidly over the last two decades.  The gold standard of surgical revascularization has undergone many changes and in fact made a full circle.  Coronary artery bypass surgery using suture techniques and localized stabilization of the beating heart was introduced by Kolesov in 1967 and continued to be successfully employed by some surgeons over ensuing years. The technological advances of cardiopulmonary bypass (CPB) and cardioplegic arrest around the same time, providing a quiet  and bloodless field, overshadowed the off pump approach to become the dominant modality in coronary surgery. 

Surgery on the beating heart re-emerged with the introduction of so called minimally invasive procedures.  These surgeries, such as minimally invasive direct coronary artery bypass (MIDCAB) are usually performed using limited parasternal incision. Special devices are used to provide surgical exposure and stabilize the epicardium to perform one or two vessel bypass on a beating heart without the use of CPB.  However the use of MIDCAB is limited because it does not readily allow performance of multiple vessel bypass.

The off-pump coronary artery bypass (OPCAB) is a natural extension of the more limited MIDCAB surgery and gained popularity with the development of better devices to stabilize the beating heart.  The key surgical features of OPCAB are the absence of CPB, operation on a beating heart, use of an epicardial stabilizer and intracoronary shunts during micro surgical  anastomoses of distal vessels.

ADVANTAGES OF OPCAB:

The key advantage of OPCAB surgery is avoidance of “the pump”.  In terms of morbidity & mortality the clearest advantage of OPCAB is the neuropsychologic impairment associated with CPB is significantly reduced.  Recent OPCAB studies reported less than 1% incidence of stroke compared to CPB group where it is 2-3% depending on pre-op risk factors.  Other  advantage with  OPCAB is less peri-operative bleeding due to  use of  lower dose of heparin, lack of CPB associated haemodilution and the absence of pump related platelet dysfunction .  While OPCAB involves periods of transient coronary ischaemia, it avoids the potential global myocardial ischaemia that may be associated with CPB.  Many published reports have shown less usage of ionotropes and anti-arrythmic drugs in OPCABs. 

Avoiding CPB confers number of other advantages. Respiratory problems are reduced and renal function is better preserved in OPCABs.  Pharmacokinetics are more predictable, since there is no uptake of drug from the pump and there are fewer disturbances, especially in glucose, potassium and calcium metabolism. Lastly OPCAB avoids complement activation and the systemic inflammatory response associated with CPB, which may facilitate better post-operative analgesia and fluid management.
CPB is associated with usage of costly disposable items like oxygenator, cannulae and other extra corporeal devices, which increase the cost of cardiac surgery. With OPCABs the cost can be brought down to reasonable levels, which is of great help for developing countries like ours.

GOALS OF ANAESTHESIA MANAGEMENT OF OPCAB
Anaesthesia management of OPCAB surgery should focus on the following items:
1) Providing maximum cardiac protection by use of a cardioprotective anaesthetic
Technique
2) Providing safe induction and maintenance of general anaesthesia.
3) Maintaining haemodynamic stability with adequate monitoring and
pharmacological support.
4) Enabling surgical exposure, within the limits of haemodynamic stability.
5) Enabling early emergence and ambulation and providing execellent post-operative
analgesia.
These goals can be achieved by tailored approach to anaesthetic techniques, monitoring, medication and haemodynamic management.

PRE-OPERATIVE MANAGEMENT:
The anaesthesiologist should be very familiar with the coronary anatomy in general and with the patients specific lesions in particular.  It is not enough to know that the patient has triple vessel disease, he must also be aware of the severity of the lesions and their specific locations and also about the LV function.  Cardiac anti-ischaemic drugs like B blockers, calcium antagonists and nitrates should be continued till the day of surgery.  Benzodiazepines like diazepam or midazolam should be given for sedation and anxiolysis.  Some anaesthesiologists prefer to use narcotics like morphine or pethidine as premedication.

MONITORING
Besides the standard monitoring devices like the use of 5 lead ECG with online S.T. segment analysis , pulse oxymetry, capnograpy, intra arterial blood pressure and a central venous pressure, there are additional forms of monitoring which are important in managing OPCABs.  Pulmonary artery catheter with online monitoring of SVO2 and cardiac output is very useful during periods of ischaemia and surgical manipulation of the heart.  P.A. catheter is also useful in fine tuning of haemodynamics peri-operatively.

Transesophageal Echocardiography visualises cardiac function during the different phases of the operation and regional wall motion abnormalities can be observed during the distal anastamosis.  But placement of pads behind the heart and retraction of heart for proper exposure may interfere with the TEE imaging.

INDUCTION AND MAINTENANCE OF ANAESTHESIA
Induction of anaesthesia is determined by the patients status  and the aim is to extubate as early as possible.  Opiod based balanced anaesthesia technique using moderate doses of Fentanyl (10-20 mcg/kg is ideal in most cases. Small doses of benzodiazepines like midazolam can be supplemented for providing hypnosis, retrograde amnesia and to prevent awareness.  Midazolam also reduces requirement of opiods. For induction small dose of thiopentone or propofol can be used though the later agent may cause hypotension.  Anaesthesia is maintained using either volatile agents like Isoflurane , sevoflurane or propofol infusion.  Any of the intermediate acting neuromuscular blockers like pancuronium, vecuronium  or rocuronium can be used for providing muscle relaxation. Whatever technique is used Anaesthesiologist must make sure that myocardial oxygen supply demand ratio is maintained favourably.

Heparin dose of 1.5 to 2 mg/kg body weight is given before commencing distal anastomosis aiming to keep activated clotting time (ACT) greater than 250 seconds.Usually half the dose of Protamine is given at the end of the anastomosis to reverse anticoagulation
.
Regional Anaesthesia and Analgesia:
Thoracic epidural anaesthesia supplemented with general anaesthesia gives good analgesia during operation and reduces the requirement of opiods.  TEA also causes thoracic sympathectomy, stress response attenuation and intense post-operative analgesia.

HAEMODYNAMIC MANAGEMENT OF OPCABs:
In contrast to CABG procedures that use CPB, beating heart CABG requires the anaesthesiologist to proactively maintain stable haemodynamics and rhythm in an environment that changes rapidly because  of regional ischaemic and cardiac manipulation.  The following haemodynamic abnormalities occur commonly and they need prompt treatment.
Hypotension
Hypotension is quite common during the distal anastomosis especially on the obtuse marginal branches of circumflex artery.  Apart from infusion of fluids to improve the preload it is also useful to put   the patient in approximately 20 degree Trendelenburg position.  If the hypotension is still persistant vasopressor like phenyleprine (100-200 mcg/bolus) or infusion of noradrenaline can be used.  Sometimes the stabilizer has to be adjusted and the heart should be repositioned to maintain the blood pressure.

Low Cardiac Output:
Cardiac output may fall during OPCAB procedure due to compression, decreased contractility from ischaemia and heart displacement.  The fall in cardiac output is more prominent in patients with pre-operative LV dysfunction.  The consequences can be very severe for the brain, intestines, kidney and other organs that require high blood flow.  Infusion of small dose of inotropes like dobutamine or dopamine along with improving the preload may help in maintaining cardiac output.  Maintenance of cardiac output appears to be more important than maintaining systemic blood pressure.

Dysrhythmias:
Arrhythmias during OPCAB surgery may be due to surgical manipulation, ischaemia or reperfusion.  It is better to prevent them by keeping electrolytes at near normal levels.  Infusion of magnesium sulphate or lignocaine before manipulating the heart will be useful in preventing arrhythmias
.
Controlling the heart rate:
With use of modern day stabilizers strict control of heart rate is not needed during the distal anastomosis.  However it is ideal to keep the heart rate around 70-80 beats per minute.
Drugs like Esmolol, Metaprolol can be used for this purpose
.
Hypothermia:
Maintaining normothermia is highly desirable in OPCAB surgery to maintain the cardiac output and for early extubation.  Rewarming of IV fluids, a humidified airway, a warm operating room and positioning of patient on a water heating mattress, keeps the patient normothermic during the surgery.

 

COMMUNICATION BETWEEN THE SURGEON AND THE ANAESTHESIOLOGIST :

The Anaesthesiologist is integral to the success of beating heart CABG.  In contrast to CABG procedures that use  CPB, beating heart CABG requires the anaesthesiologist to proactively maintain stable hemodynamics and rhythm in an environment that changes rapidly because of regional ischaemia and cardiac manipulation.  The anaesthesiologist's role during beating heart surgery requires a new level of communication with the surgeon.  The surgeon must communicate with the anaesthetist whether the heart is being displaced, when a coronary artery is occluded and when a shunt has been inserted or removed.  Likewise the anaesthesiologist must keep the surgeon informed about the use of inotropes or vasopressors, ST segment or rhythm disturbances and the patients general condition.  In no other cardiac procedure has it been more important for the anaesthesiologist to continually observe and treat haemodynamic and rhythm responses to cardiac manipulation and regional ischaemia.

POST OPERATIVE MANAGEMENT:
One goal of anaesthetic management is focused on early awakening.  Patients with thoracic epidural anaesthesia are extubated in the operating room or within first two hours.  If  the patient was anaesthetised  with moderate doses of opiods  extubation can be done in 5-6 hours post-operatively.  Pain management is not only important to prevent post-operative complications but also helps in providing patient comfort and satisfaction.  Small doses of non steroidal agents like Ketorolac in conjunction with low dose opiates effectively alleviates musculoskeletal pain and avoids the sedation produced by opiods. 

Coronary artery bypass grafting on the beating heart is a safe surgical procedure provided anaesthetic management is focused on  physiologic aspects of this operation.