Coronary Artery Bypass Surgery

Published on 17/05/2015 by admin

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Chapter 21

Coronary Artery Bypass Surgery

1. What are the indications for coronary artery bypass grafting (CABG)?

    According to the latest published guidelines from the American College of Cardiology Foundation/American Heart Association (ACCF/AHA), class I indications for CABG include significant left main stenosis and three-vessel coronary artery disease (CAD). Class I indications also include two-vessel disease involving the proximal left anterior descending (LAD) artery. CABG should be considered as a reasonable treatment strategy (class IIa) in patients with proximal LAD artery disease, and in 2-vessel disease without proximal LAD artery disease but with extensive ischemia.

    After myocardial infarction (MI), primary surgical revascularization should be considered in patients not suitable for percutaneous coronary intervention (PCI), patients who have failed PCI, or patients with ongoing ischemia or symptoms. After a transmural infarct, mechanical complications such as a postinfarction ventricular septal defect (VSD), acute mitral regurgitation (MR) as a result of papillary rupture, and free-wall pseudoaneurysm should be considered as indications for primary surgical intervention.

2. How does CABG compare with medical management for CAD?

    Three early prospective randomized trials comparing CABG with medical therapy were conducted in the late 1970s and were reported in the early 1980s. The Veterans Affairs (VA) cooperative trial, European Coronary Surgery Study (ECSS), and Coronary Artery Surgery Study (CASS) showed long-term superiority of surgery over medical therapy in patients with left main (LM) artery CAD, significant CAD involving the LAD artery, and multivessel disease.

3. How does CABG compare with stents?

    The Arterial Revascularization Therapies Study (ARTS) was the largest trial comparing CABG with bare metal stents in patients (1) with ejection fraction (EF) greater than 30% and (2) where there was consensus between surgeon and cardiologist that the disease was suitable for both therapies. The study showed improved event-free survival in the CABG group at 1, 2, and 5 years.

4. What about with drug-eluting stents (DES)?

    Drug-eluting stents have reduced the problem of restenosis, but data directly comparing DES with CABG is limited and still relatively short-term. In a registry study from the New York state cardiovascular database, CABG resulted in improved survival at 3 years compared with DES for patients with double- and triple-vessel disease. ARTS II randomized patients to DES or CABG, and showed that the primary composite outcomes at one year were similar. The international SYNTAX trial, involving 85 centers and 1800 patients with multivessel or left main CAD, recently published 3-year follow-up data in 2011. This showed worse outcomes in the PCI group as compared to the CABG group at 3 years, with increased composite major adverse cardiac and cerebrovascular events (MACCE: death, stroke, MI, or repeat revascularization). Although there was no significant difference in all-cause mortality and stroke at 3 years, MI and repeat revascularization were both increased in the PCI group. The study concluded that patients with more complex disease (three-vessel CAD with intermediate-high SYNTAX scores and LM artery CAD with high SYNTAX scores) have an increased risk of MACCE with PCI, and CABG is the preferred treatment option.

5. What is the SYNTAX score?

    The SYNTAX score derives from a scoring system developed by the SYNTAX trial investigators to quantify the extent and complexity of CAD based on findings at cardiac catheterization. Scores are divided into tertiles: low (0-22), intermediate (23-32), and high (≥33), with higher scores representing more extensive and complex CAD. The SYNTAX score was found to correlate with PCI risk and outcome, but not with CABG risk and outcome. Patients with higher SYNTAX scores generally benefited from a revascularization strategy of CABG in preference to PCI. This is reflected in current guidelines, which state that it is reasonable to choose CABG over PCI as a revascularization strategy in patients with complex three-vessel disease and high SYNTAX scores (class IIa recommendation).

6. Which patients benefit most from CABG?

    The decision for surgery is made based on the comprehensive evaluation of the patient. Anatomic considerations that favor recommendation for CABG include presence of significant LM or proximal LAD coronary artery disease, multivessel CAD, and presence of lesions not amenable to stenting. The presence of diabetes also favors surgical revascularization over stenting in operable patients. Depressed ejection fraction has been recognized as an additional indication for CABG.

    Although the coronary anatomy may be suitable for bypass, each patient’s comorbidities should be considered in the overall risk-benefit analysis. Preoperative renal insufficiency, peripheral vascular disease, recent MI, or recent stroke, as well as emergency operation and cardiogenic shock, have been identified as factors that increase mortality.

7. What is the cardiopulmonary bypass (CPB) pump, and how is it used?

    The “pump” involves temporarily placing a patient on a machine to supply circulation and oxygenation during an operation, so that the heart can be stopped to facilitate the procedure. The bypass circuit consists of the tubing, a collection chamber, oxygenator, heater-cooler machines to control temperature, and the pump. An aortic cannula is placed in the distal ascending aorta, and this is connected to tubing that will be used to bring artificially oxygenated blood from the pump back to the patient’s arterial bloodstream. A venous cannula is placed in the right atrium, and advanced down into the inferior vena cava, to collect venous blood and return it towards the pump. Once the venous blood is oxygenated, it is pumped back into the arterial line to the patient’s aorta. A perfusionist runs the pump under the direction of the surgeon.

8. Why is heparin required for CPB?

    The CPB circuit is thrombogenic, and systemic anticoagulation is required to prevent clotting and embolization. The standard anticoagulant is heparin (300 U\kg), which is administered to produce a target activated clotting time (ACT) of greater than 480 seconds. In patients with previously documented heparin-induced thrombocytopenia, direct thrombin inhibitors have been used for anticoagulation. Heparin is used in off-pump CABG at partial dose. After termination of CPB and decannulation, heparin-related anticoagulation is reversed with protamine.

9. How is the heart stopped while on CPB?

    The heart is stopped by placing a completely occluding cross-clamp across the ascending aorta, below the aortic cannula, eliminating arterial blood flow to the coronary arteries. Cardioplegia solution is then used to induce arrest of the heart, and can be given antegrade and/or retrograde. Components of cardioplegia solution are varied in different institutions but include potassium to achieve diastolic arrest. Cardioplegia solution administered into the aortic root, via a small cannula below the cross-clamp, is delivered in an antegrade fashion to the myocardium via the coronary ostia. Retrograde cardioplegia is also used frequently, by infusing cardioplegia solution into the coronary sinus with backward filling of the cardiac veins to reach the myocardium, and is especially important in situations where antegrade may not be as effective, such as with severe CAD and aortic valve insufficiency. Most commonly, cold cardioplegia at 4° C is administered intermittently in 15- to 20-minute intervals. Blood can be mixed to the crystalloid component of cardioplegia in a 4:1 mix to provide oxygenated blood to the myocardium and to buffer the pH of the tissue (Fig. 21-1).

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