Coronary Artery Disease

July 10, 2009 | In: CARDIOTHORACIC SURGERY

74 CORONARY ARTERY DISEASE
Joseph C. Cleveland Jr., M.D.

1. What is angina, and what causes it?

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Angina pectoris reflects myocardial ischemia. Patients often describe the sensation as pressure, choking, or tightness. Angina is typically produced by an imbalance between myocardial oxygen supply and myocardial oxygen demand. The classic presentation is a man (male-to-female ratio = 4:1) out shoveling snow on a cold night after a big meal after having a fight with his wife.

2. How is angina treated?

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The treatment options for angina include medical therapy or myocardial revascularization. Medical treatment is directed toward decreasing myocardial oxygen demand. Strategies include nitrates (nitroglycerin, isosorbide), which dilate coronary arteries minimally but also decrease blood pressure (afterload) and therefore myocardial oxygen demand; beta receptor antagonists, which decrease heart rate, contractility, and afterload; and calcium channel antagonists, which decrease afterload and may prevent coronary vasoconstriction. Aspirin (antiplatelet therapy) is also important. Newer antiplatelet agents such as clopidogrel (Plavix) and eptifibatide (Integrilin) are promoted in the management of acute coronary syndromes. Plavix, however, is a very potent, efficacious agent, and operation (i.e., coronary artery bypass grafting [CABG]) within 1 week of Plavix exposure increases the risk of postoperative bleeding by threefold.
If medical therapy is unsuccessful in alleviating angina, myocardial revascularization with either percutaneous transluminal coronary angioplasty (PTCA), with or without placement of a stent, or CABG may be appropriate.

3. What are the indications for CABG?

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1. Left main coronary artery stenosis: Stenosis > 50% involving the left main coronary artery is a robust predictor of poor long-term outcome in medically treated patients. A substantial portion of the myocardium is supplied by this artery; thus, PTCA is too hazardous. Even if the patient is asymptomatic, survival is markedly improved with CABG.
2. Three-vessel coronary artery disease (70% stenosis) with depressed left ventricular (LV) function or two-vessel coronary artery disease (CAD) with proximal left anterior descending (LAD) involvement: In randomized trials, patients with three-vessel disease and depressed LV function showed a survival benefit with CABG compared with medical therapy. CABG also confers survival benefit in patients with two-vessel CAD and ≥ 95% LAD stenosis. An important caveat, however, in managing patients with depressed LV function is that operative mortality increases when the ejection fraction (EF) falls below 30%.
3. Angina despite aggressive medical therapy: Patients who have lifestyle limitations because of CAD are appropriate candidates for CABG. Data from the Coronary Artery Surgery Study (CASS) suggest that patients treated with surgery have less angina, fewer activity limitations, and an objective increase in exercise tolerance compared with medically treated patients.

4. What is done during a “traditional” CABG procedure?

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CABG is an arterial bypass procedure that can be done both on bypass and off bypass. The left internal mammary artery (LIMA) is harvested as a pedicled graft. Cardiopulmonary bypass (CPB) is established by cannulating the ascending aorta and the right atrium, and the heart is arrested with cold blood cardioplegia. Segments of the greater saphenous vein are then reversed and sewn with the proximal (inflow) portion of the bypass graft originating from the ascending aorta and the distal (outflow) portion of the bypass graft anastomosed to the coronary artery distal to the obstructing lesion. The LIMA is typically sewn to the LAD. When the anastomoses are finished, the patient is weaned from CPB, and the chest is closed. Typically, one to six bypass grafts are constructed (hence the terms triple or quadruple bypass).

5. What is an off-pump CABG (OPCAB)?

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CABG can be performed without cardiopulmonary bypass and arrest of the heart. When done with the heart beating through a median sternotomy, CABG is then called an OPCAB. The heart is positioned with commercially available stabilization devices, and the vessel to be bypassed is immobilized and snared to provide temporary occlusion. The venous or arterial conduit is then sewn to the immobilized coronary artery, and the occlusion of the vessel is released.

6. Why would one choose an OPCAB instead of a traditional CABG?

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CABG with cardiopulmonary bypass is the gold standard. However, cardiopulmonary bypass is associated with several adverse clinical consequences such as acute lung dysfunction, stroke, renal failure, liver failure, bleeding, and the promotion of a proinflammatory state. It is thought, although not yet well delineated, that performing CABG without CPB may reduce these complications. Patients with comorbidities of lung disease, cerebrovascular disease, renal disease, or severe peripheral vascular disease may have improved outcomes when CABG is performed without the use of cardiopulmonary bypass.

7. Does CABG improve myocardial function?

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Yes. Hibernating myocardium is improved by CABG. Myocardial hibernation refers to the reversible myocardial contractile function associated with a decrease in coronary flow in the setting of preserved myocardial viability. Some patients with global systolic dysfunction exhibit dramatic improvement in myocardial contractility after CABG.

8. Is CABG helpful in patients with congestive heart failure (CHF)?

> Show answer
Possibly. CABG improves CHF symptoms that are related to ischemic myocardial dysfunction. Conversely, if heart failure is secondary to long-standing irreversibly infarcted muscle (i.e., scar), CABG does not prove beneficial. The critical preoperative evaluation must assess the viability of nonfunctional myocardium. A rest-redistribution thallium scan is useful to determine the segments of myocardium that are still viable.

9. Is CABG valuable in preventing ventricular arrhythmias?

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No. Most ventricular arrhythmias in patients with CAD originate from the border of irritable myocardium that surrounds infarcted muscle. Implantation of an automated implantable cardiac defibrillator (AICD) is indicated for patients with life-threatening ventricular tachyarrhythmias.

10. What is the difference between PTCA and CABG?

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Six randomized, controlled clinical trials have compared PTCA with CABG. Although collectively they analyzed data from more than 4700 patients, 75% of patients who originally met inclusion criteria were excluded from participation because they had multivessel CAD, which was not deemed suitable for PTCA.
Several important features emerged from these trials. Overall mortality and myocardial infarction rates were no different for CABG and PTCA in five of the six studies. Only the German Angioplasty Bypass Surgery Investigational Study showed a higher short-term combined incidence of death and myocardial infarction (MI) in the CABG group.
The major difference between the two treatment strategies was freedom from angina and reintervention. Overall, whereas 40% of PTCA-treated patients required repeat PTCA or CABG, roughly 5% of CABG-treated patients required reintervention. The CABG-treated patients also experienced fewer episodes of angina compared with the PTCA-treated patients.
A more recent trial comparing PTCA with stent (percutaneous coronary intervention [PCI]) implantation also showed no difference in the composite endpoint of death or Q-wave MI between the CABG or PCI groups. In this investigation, freedom from reintervention was 80% at 1 year in the PCI group.
The unavoidable conclusion is that the recommendation of PTCA with stenting or CABG should be individualized for each patient. The two therapies should not be viewed as exclusionary or competitive; some patients may benefit from a combination of PTCA and CABG. CABG results in a more durable revascularization, although with the inherent risk of perioperative complications.
KEY POINTS: CORONARY ARTERY DISEASE

1. Hibernating myocardium is improved by coronary artery bypass grafting (CABG).
2. CABG is not helpful in preventing ventricular arrhythmias.
3. The rule of thumb for vessel patency is 90% patency at 10 years for the internal mammary graft, 50% patency at 10 years for saphenous vein grafts, and 80% patency at 1 year for PTCA plus stent of stenotic vessel.

11. What is the rule of thumb for vessel patency?

Internal mammary graft:

90% patency at 10 years

Saphenous vein graft:

50% patency at 10 years

PTCA 1 stent of stenotic vessel:

80% patency at 1 year

12. What operative and technical problems are associated with CABG?

Show answer
The operative complications broadly include technical problems with the bypass graft anastomosis, sternal complications, and incisional complications associated with the saphenous vein harvest incision. Technical problems with the coronary artery anastomosis usually lead to MI. Sternal complications predictably result in sepsis and multiple organ failure. Incisions for saphenous vein harvest also may result in problems with edema, infection, and pain postoperatively.

13. What are the risks of CABG? Which comorbid factors increase the operative risk for CABG?

Show answer
Estimating operative risk is a critical component of counseling patients before surgical revascularization. The Society of Thoracic Surgeons (STS) and the Veterans’ Administration have developed and promoted two large databases. Factors that increase the risk of CABG include depressed left ventricular EF (LVEF), previous cardiac surgery, priority of operation (emergency versus elective), New York Heart Association Classification, age, peripheral vascular disease, chronic obstructive pulmonary disease, and decompensated heart failure at the time of surgery. These comorbidities figure prominently in outcome. Quite simply, raw mortality data for CABG can be misleading. Different surgeons can perform identical operations but have different raw mortality rates if one surgeon operates on young triathletes with CAD and the other surgeon operates on old couch potatoes who smoke two packs of cigarettes per day. Through assessment of these comorbid factors, a fairer representation of predicted to observed outcome can be determined. In this manner, using observed to expected outcomes with risk-adjusted models represents an honest comparison of CABG mortality rates.

14. What steps are taken if a patient cannot be weaned from CPB?

Show answer
The surgeon is in fact treating shock. As in hypovolemic shock (e.g., a bullet transecting the aorta), the basic principles include the following:

* Volume resuscitation until left- and right-sided filling pressures are optimized
* When filling pressures are adequate, initiation of inotropic support
* Push inotropic support to toxicity (usually ventricular tachyarrhythmias) and insert an intraaortic balloon pump (IABP). The ultimate extension of CPB includes the placement of an LV or right ventricular assist device (or both). These devices can support the circulation while allowing for myocardial recovery.

Tags: acute, aortic, Calcium, postoperative, pulmonary

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