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Current options for treating coronary artery disease include pharmacotherapy, percutaneous coronary angioplasty with or without stent implantation, and coronary artery bypass grafting (CABG). Off-pump coronary artery bypass grafting, which is surgery on a beating heart without the support of cardiopulmonary bypass, is now presenting a challenge to conventional CABG.1 Already about 20% of first-time CABG procedures are performed in this way.2
The original motivation for developing cardiopulmonary bypass more than 45 years ago was to allow the surgical treatment of congenital heart conditions, and subsequently of heart valve disease. It continues to be used for these purposes, but today by far the most frequent application is in CABG surgery, for which every year it is used in hundreds of thousands of patients worldwide. In cardiopulmonary bypass blood is taken from the vena cava or right atrium via gravity, passed to a reservoir and then pumped to a blood oxygenator. Once oxygenated and temperature regulated (warmed or cooled), it is returned through a filter back to the body via the aortic arch or descending aorta. The heart is passive and the lungs are not ventilated during bypass, though oxygen and nutrients are delivered to the tissues throughout the procedure.
Cardiopulmonary bypass allows the cardiothoracic surgeon to perform delicate work in a bloodless and motionless field. The surgeon achieves cardiac arrest by infusing a cold solution (cardioplegia) through the coronary artery circulation, and this solution is used throughout the operation to keep the heart motionless and cool. A cold solution is used because the heart tends to resume electrical activity at a normal temperature, and also because it uses less metabolic energy when cold.3
Against the benefits of allowing a bloodless and motionless operating field, cardiopulmonary bypass has the drawback of activating many cascades, including inflammation and coagulation, with effects resembling an acute, massive, defence reaction.4 Under normal circumstances these cascades offer protection from injury; however, during cardiopulmonary bypass the whole-body response is non-physiological. The components of this systemic inflammatory response syndrome include a consumptive coagulopathy,5 release into the circulation of more than seventy hormones, cytokines, chemokines, vasoactive substances, cytotoxins, reactive oxygen species and proteases of the coagulation and fibrinolytic systems,6 induction of interstitial fluid shifts, generation of microemboli, and temporary dysfunction of nearly every organ.7 These effects arise primarily from the interaction between blood and nonendothelial surfaces; the non-pulsatile nature of blood-flow during bypass and other physiology disturbances contribute less to disruption of the milieu interieur.4
Clearly, since many patients recover well after cardiopulmonary bypass, the body's physiological reserves and inhibitory pathways are capable of preventing wide-spread organ damage after heart operations.8 Today, however, operations are increasingly being done on high-risk patients with limited physiological reserves—for example, those with borderline organ functions and those who will need a long time on cardiopulmonary bypass and will therefore be especially susceptible to the potential adverse effects of the systemic inflammatory response syndrome. An uncontrolled inflammatory response seems to contribute substantially to the acute pulmonary, cardiovascular, neurological, splanchnic, haematological and immunological dysfunction observed after surgery under cardiopulmonary bypass. Moreover there is evidence, less well documented, that it contributes to subacute sequelae such as postoperative fever, postcardiotomy pericarditis and pleural effusions.9
Observational data suggest a relation between poor outcome and the use of (1) aortic cross-clamping and (2) extracorporeal circulation. Avoidance of both might therefore be especially desirable in high-risk patients. Part of the rationale for off-pump coronary artery surgery (OPCAB) is to reduce the inflammatory response.10
The usual approach to multivessel grafting without cardiopulmonary bypass, OPCAB surgery, is via a standard median sternotomy.11 Other incisions are used12 but median sternotomy has the advantages of providing access to most regions to be revascularized and allowing standard techniques for internal mammary artery harvesting and emergency management.13 OPCAB surgery is sometimes employed as a hybrid procedure with percutaneous transluminal coronary angioplasty.13
Little displacement of the heart is needed to expose the left anterior descending artery, its diagonal branches or the proximal right coronary artery. However, exposure of the circumflex artery, its branches, the posterior descending artery and the posterolateral branch of the right coronary artery demands a combination of manoeuvres and techniques including pericardial sutures or a retracting sock.13 To elevate the heart anteriorly, when an anastomosis is being performed on the lateral wall, haemodynamics may be compromised by compression of the heart against the right pleura; this can be lessened by tilting and rotating the operating table and opening the right pleura. In the early days of OPCAB, bradycardia was induced with drugs (beta-blockers, diltiazem) to quieten the site of anastomosis. Today, the site is stabilized by pressure or suction devices.13
The theoretical advantages of off-pump coronary artery surgery might lead one to expect lower mortality and morbidity, less blood transfusion, faster recovery, shorter hospital stays and lower costs.1,14-17 However, the evidence is short of proof. To date, six large randomized trials (≥150 patients) of on-pump versus off-pump surgery have been completed and have shown either no outcome differences or only small differences. Straka et al.,18 in their study of 400 non-selected patients (mean age 63 years), found that OPCAB patients had significantly fewer distal anastomoses (2.3 versus 2.7), less blood loss (560 versus 680 mL), lower postoperative creatine kinase MB (0.15 versus 0.56 microkat/L) and lower total hospital costs €3,451 versus €4,387). They concluded that an off-pump technique is applicable in 85% of non-selected patients and is at least as clinically safe and effective as on-pump surgery. The study by Légaré et al.19 is a trial of 300 selected good-risk patients (out of 933 eligible for randomization). The outcomes were excellent for both groups (1% mortality), and no significant differences in morbid events were noted, including such end-points as transfusion requirement, intubation time, intensive-care time, and hospital stay. Gerola et al.,20 in their multicentre prospective study of 160 patients with lesions in the left descending artery, alone or associated with the right coronary artery, did not find any statistical difference in hospital mortality and morbidity between the two myocardial revascularization techniques.
In a single-surgeon study, Puskas et al.21 recorded similar (low) mortality and stroke rates for on-pump and off-pump patients, but found the off-pump groups had lower transfusion rates and less enzyme release. A randomized trial from the UK documented lower hospital stay and a decreased risk of atrial fibrillation and blood transfusion for the off-pump group but equivalent mortality and stroke rates.22 Van Dijk et al.23 randomized 281 low-risk patients and found blood utilization slightly higher in the on-pump group, as was enzyme release. Follow-up at 1 year after operation showed no neurocognitive differences.24 The randomized trials that exist, therefore, have involved good-risk or standard-risk patients and have documented low procedure-related risk for both treatments. They have also shown equivalent revascularization rates in terms of mean numbers of grafts performed per patient.
Other studies of on-pump versus off-pump surgery have lacked randomized controls, have included different types of patients and have yielded different conclusions. In some large-database observational studies, attempts were made to adjust for risk, so as to allow for bias in treatment selection. Al-Ruzzeh et al.25 reviewed the UK database for 1997-2001 and found significant differences in mortality and neurological, pulmonary, and renal complications, all in favour of off-pump surgery. A report from the Society of Thoracic Surgeons National Database by Cleveland et al.26 noted a decreased risk of death and neurological, renal, respiratory, and bleeding complications in their off-pump group. Similarly, Mack et al.,27 in their retrospective analysis of all coronary artery bypass grafting in a 3-year period in four centres with off-pump coronary surgery experience, showed that cardiopulmonary bypass was predictive of mortality in high-risk patients including reoperations, female patients, and patients aged ≥75 years. Off-pump coronary artery bypass grafting, on the other hand, was associated with less morbidity, including significantly lower rates of blood transfusion (32.6% versus 40.6%), stroke (1.4% versus 2.1%), renal failure (2.6% versus 5.2%), pulmonary complications (4.1% versus 9.5%), reoperation (1.7% versus 3.2%), atrial fibrillation (21.1% versus 24.99%), and gastrointestinal complications (3.6% versus 4.8%).
From existing evidence, it is conceivable that a more challenging patient category might benefit most from OPCAB surgery. Data from nonrandomized studies show that OPCAB surgery can be safely performed in high-risk patients with multivessel coronary artery disease.28-32 The crucial advantage of off-pump surgery might be a lower risk of perioperative stroke, a major cause of which, in on-pump surgery, is dislodgment of atheromatous plaque by cannulation and clamping of the ascending aorta.33 The presence of aortic atherosclerosis greatly increases the risk of stroke, and aortic atherosclerosis is especially likely in the old, in people with three-vessel disease, and in people with diabetes.33 In CABG patients older than 80 years, the risk of stroke was as high as 9%.35 Since OPCAB does not necessitate manipulation of the aorta36 and avoids the potential risks of cardiopulmonary bypass, we might reasonably expect it to decrease the risk of stroke.36,37
The contraindications to OPCAB surgery are even more uncertain. Selection criteria vary from surgeon to surgeon, and the proportion of patients considered suitable for an OPCAB procedure ranges from 0% to 99%, depending on the skills and opinions of the surgeon and the policy of the institution. To do an OPCAB procedure in a haemodynamically unstable patient is probably unwise but many surgeons no longer consider multivessel disease, diffuse coronary vessel disease, or emergency surgery contraindications. The most important contraindication, therefore, seems to be the situation in which a surgeon feels uncomfortable with an OPCAB procedure in a specific patient with a specific coronary anatomy. Concerns have been raised about incomplete revascularization and suboptimal anastomoses with the potential for early occlusion.14 However, collective data on graft patency after OPCAB are necessary.20,38,39
Occasionally, during a beating heart operation, the patient must be placed on cardiopulmonary bypass in order to safely complete the operation. Common reasons mandating conversion from an off-pump to an on-pump procedure include haemodynamic instability, failure to adequately expose the target vessel, deep intramyocardial course of the target vessel, and global ventricular ischaemia. The reported incidence of conversion to CPB ranges from a low of 1.1% to a high of 16.3%.40 Patients who require conversion may be at greater risk of complications or death, and the procedure will cost more.40
In young, relatively healthy patients OPCAB surgery is safe but has no clear advantage, clinically, over conventional CABG. Older patients with more comorbidity and more advanced atherosclerotic disease are likely to do better with OPCAB surgery but this idea has not yet been tested by randomized controlled trial. Therefore, to return to the original question: will off-pump coronary artery surgery replace conventional coronary artery surgery? The answer at present is ‘no’. Off-pump and on-pump surgery are complementary techniques for achievement of myocardial revascularization, each with its advantages and disadvantages in patients with specific characteristics.