Abstract
Cardiac surgery is a major insult to homeostasis. Attempts have been made to reduce the inflammatory response to cardiac surgery by limiting the stimulus. The two biggest stimuli to this inflammation are thought to be surgical tissue destruction and the interaction with the extracorporeal circuit. Therefore, techniques have been developed to reduce these stimuli – either by minimizing them (minimally invasive surgery) or eliminating them completely (off-pump surgery).
Cardiac surgery is a major insult to homeostasis. Attempts have been made to reduce the inflammatory response to cardiac surgery by limiting the stimulus. The two biggest stimuli to this inflammation are thought to be surgical tissue destruction and the interaction with the extracorporeal circuit. Therefore, techniques have been developed to reduce these stimuli – either by minimizing them (minimally invasive surgery) or eliminating them completely (off-pump surgery).
Off-Pump Surgery
Off-pump coronary artery bypass (OPCAB) surgery describes coronary revascularization without the use of CPB. This should not to be confused with ‘beating heart surgery’, involving the use of CPB without arresting the heart.
History
Coronary artery bypass in humans was developed in the 1950s. Techniques evolved and improved facilitating cardiac surgery on a massive scale in the 1960s onwards with CPB and cardioplegic arrest, transforming both the safety and scope of cardiac surgery. It was rapidly apparent that there were considerable side effects associated with cardiac surgery, many of which were attributed to the use of CPB. The evolution of surgical techniques and the development of cardiac stabilization devices, retractors and intra-coronary shunts led to a resurgence of interest in OPCAB.
OPCAB had been performed for many years, particularly in territories where cost was a limiting factor, enabling surgery without using expensive disposables. The absence of a significant reduction in major morbidity and mortality, coupled with technical challenges, have limited the uptake of OPCAB in the developed world.
Rationale
Resurgence in interest and wider utilization of OPCAB were motivated by the desire to limit morbidity and mortality – much of which was attributed to CPB. Advances in surgical equipment were supported by commercial interest in providing new equipment and devices to facilitate OPCAB. As the profile of the procedure rose, demand from cardiologists and patients rose, although this has now fallen – mainly because the rise in percutaneous intervention has changed the profile of those referred for surgery. Patients are now older, have more complex coronary disease and a greater number of co-morbidities. They also have pathology that is less amenable to OPCAB, including those with deep intramuscular coronary arteries, a need for endarterectomy and poor LV function.
Surgical Approach
The usual approach for multi-vessel revascularization is median sternotomy. The heart will need to be displaced to position it suitably for surgical anastomosis. LAD artery grafts require minimal repositioning of the heart and, if isolated, can be amenable to minimal access surgery via a small left anterior thoracotomy. Bypass grafts to the inferior and right sides of the heart require extensive elevation or rotation of the heart.
Positioning is often achieved using a swab attached to the posterior pericardium or a bespoke suction cup to manipulate the heart. A stabilization device is used to keep the anastomotic site still. Stabilizer devices can be thought of like a ‘sewing-machine foot’, and indeed initial equipment was based on this principle. Equipment has now evolved to include tiny suction cups on the stabilizer and bespoke sternal retractors.
In the initial development of OPCAB, blood flow to the anastomotic site would be interrupted by surgical sutures. Modern practice utilizes flexible olive-ended silicone shunts, which maintain both a relatively bloodless field and arterial flow throughout the process of grafting. Tiny amounts of blood still contaminate the field, which are too small for standard suction equipment. Therefore, CO2 is used to ‘blow’ this away.
The proximal anastomoses on the ascending aorta require the use of a ‘side-biting’ aortic clamp. It is the continued use of an aortic clamp that contributes most to morbidity and it is for this reason that ‘off-pump’ isolated left internal mammary artery (LIMA) to the LAD artery grafting (i.e. the aortic clamp is not needed) has a clear outcome benefit (Table 12.1).
Table 12.1 Advantages and disadvantages of off-pump surgery
| Advantages | Disadvantages |
|---|---|
| No cardioplegia (K+ load/fluid load) No aortic or atrial cannulation (fewer sites for bleeding/dissection) No AXC (isolated LIMA → LAD grafts) Less inflammatory mediator release Less bleeding/transfusion More time-efficient | Technically more challenging Incomplete revascularization – not all arteries well reached by technique (fewer grafts per patient) Higher rates of early graft failure |
Anaesthetic Management
Off-pump CABG requires a greater degree of communication between anaesthetist and surgeon than ‘on-pump’ surgery. The anaesthetic technique has a much greater impact on the ease of surgery – it is much easier to perform ‘off-pump’ surgery on a ‘soft heart’ with a slow HR.
Manipulation of the heart leads to cardiac chamber compression, valvular distortion, valvular regurgitation (notably the MV), reduced venous return, RV outflow obstruction, myocardial ischaemia and arrhythmias. The resultant decrease in the CO and MAP has consequences for end-organ perfusion. Most haemodynamic disturbances can be resolved by returning the heart to its normal anatomical position or by reducing retraction on the heart. If this does not happen rapidly, myocardial function is likely to be impaired. The rate of return to normal haemodynamics when the heart is returned to its usual position is more predictive of the heart tolerating the surgery than absolute haemodynamic values.
General Considerations
Patients scheduled for OPCAB surgery require the same preoperative assessment and management as those scheduled for conventional surgery. The goals of anaesthetic management are:
Prevention of intraoperative cardiac ischaemia
Tight haemodynamic control
Minimization of cardiovascular depression
Maximization of surgical access – a double-lumen endotracheal tube may be required
Multiple anaesthetic techniques have been utilized; none has been shown to be superior.
Monitoring
Standard cardiac anaesthetic monitoring is required in all cases (Chapter 4). Although CO monitoring (e.g. PA catheter, oesophageal Doppler, pulse contour analysis) is advocated in many centres, it is not routinely required.
TOE is routinely used to assess regional wall motion although cardiac displacement and swabs placed behind the heart may obscure images. Nevertheless, TOE may be used to guide fluid management and exclude other cardiac pathology. Neurological monitoring (e.g. TCD, EEG and near-infrared spectroscopy) have all been used, but with little evidence of alteration in the clinical outcome.
Heat Conservation
Convective, evaporative and radiated heat loss during surgery may cause significant cooling and should be anticipated as rewarming using CPB is not available. Measures to reduce hypothermia include:
Maintaining high ambient temperature (approximately 25 °C) – although this is likely to be resisted by the surgical team
Use of underbody heating, e.g. heated mattresses
Use of a sterile lower-body forced air blanket following saphenous vein harvest
Use of fluid warming devices for all IV fluids
Insulation of the head and neck to decrease cranial heat loss
Haemodynamic Management
Physical
Fluid administration: Avoid hypovolaemia and remember that there is no fluid input from CPB prime. Maintain preload, but do not over fill during grafting as over distension of the heart makes the surgery more difficult.
Maintain the cerebral perfusion pressure: Maintain the MAP and avoid gross/prolonged CVP elevation.
Posture: Use of Trendelenburg position ± lateral tilt ameliorates a decrease in CO from decreased venous return.
Opening the right pleural cavity reduces the impact of cardiac rotation.
IABP placement pre-grafting in high-risk cases has been described.
Pharmacological
HR: Aim for 60–80 bpm. Use IV β-blocker (e.g. esmolol) if required.
Contractility: Avoid inotropes during grafting if possible – tachycardia may cause ischaemia. Low-dose dobutamine or a phosphodiesterase inhibitor may be used if absolutely necessary.
Vasoconstriction: Metaraminol provides vasoconstriction with reflex cardiac slowing. A noradrenaline infusion is generally not useful during grafting as it is not rapidly titratable.
Vasodilatation: GTN, sodium nitroprusside or phentolamine drugs may be useful if there is cardiac distension.
Cardiac rhythm: K+ and Mg2+ supplementation to reduce myocardial irritability.
Electrical
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