Surgery for Coronary Artery Disease

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Chapter 9 Surgery for Coronary Artery Disease

Coronary artery bypass graft (CABG) surgery is one of the most commonly performed operations in the United States, and it forms the core of most cardiac surgery programs. CABG surgery provides effective symptom control for angina, and in certain patient groups, it improves survival rates.

Although the basic principle of CABG surgery—of bypassing coronary stenoses with native vascular grafts—has remained unchanged for more than 3 decades, important developments in surgical technique have emerged in the past few years. Also, because of advances in percutaneous coronary intervention (PCI) and changing societal demographics, the types of patients presenting for surgical revascularization have changed. Patients are older, sicker, have more extensive coronary disease, and are more likely to have impaired ventricular function than at any time previously.

In this chapter, the indications, techniques, and postoperative care of patients undergoing CABG surgery are reviewed. Surgery for the mechanical complications of myocardial infarction is also discussed. The first section of this chapter deals with preoperative issues, with an emphasis on high-risk and unstable patients.

INDICATIONS FOR CABG SURGERY

The indications for CABG surgery have recently been reviewed by the American College of Cardiology and the American Heart Association (ACC/AHA) (Table 9-1).¹ CABG surgery offers a survival advantage over medical treatment for patients with left main coronary disease (see Fig. 5-4), three-vessel coronary disease, and disease of the proximal left anterior descending (LAD) coronary artery (see Fig. 5-5). This survival advantage is most apparent in patients with impaired left ventricular function. When CABG surgery is compared with PCI, a minimal difference in survival rates is seen²^–⁵—except in diabetic patients, who do better with surgery. Relief from angina and the infrequent need for reintervention are superior with CABG. These trials represent highly selected groups in which patients were considered to be equally treatable by either technique—a feature that may have emphasized the benefits of PCI. The trials were carried out before the introduction of drug-eluting stents, a feature that may have emphasized the benefits of CABG surgery.

Table 9-1 ACC/AHA Indications for CABG Surgery ¹

Clinical Subset	Recommendation
Mild or Asymptomatic Angina
Left main or left main equivalent CAD (class I)	CABG should be performed
3-vessel CAD (class I)	CABG is useful
1-2 vessel CAD involving the proximal LAD (class IIa, but becomes class I if there is extensive documented ischemia and/or EF < 50%)	CABG can be beneficial
1-2 vessel disease not involving the proximal LAD (class IIb, but becomes class I if there is a large area of viable myocardium and high risk criteria are found on noninvasive testing)	CABG may be considered
Stable Angina
Left main, left main equivalent, or 3-vessel CAD (class I)	CABG recommended
2-vessel CAD with proximal LAD stenosis and either EF < 50% or demonstrable ischemia on noninvasive testing (class I)	CABG recommended
1-2 vessel CAD without proximal LAD stenosis, but with a large area of viable myocardium and high risk criteria on noninvasive testing (class I)	CABG is beneficial
Disabling angina in patients on maximal medical therapy when surgery can be performed with acceptable risk (class I)	CABG is beneficial
1-vessel CAD with proximal LAD stenosis (class IIa, but becomes class I if extensive ischemia is documented by noninvasive testing and/or EF is < 50%)	CABG is reasonable
1-2 vessel CAD without proximal LAD stenosis, but with a moderate area of viable myocardium and demonstrable ischemia on noninvasive testing	CABG may be useful
Unstable Angina/NSTEMI^*
Left main, left main equivalent, or 3-vessel CAD (class I)	CABG should be performed
Patients with ongoing ischemia not responsive to maximal non-surgical therapy (class I)	CABG is recommended
1-2 vessel CAD with proximal LAD stenosis (class IIa)	CABG probably indicated
1-2 vessel CAD without proximal LAD stenosis when PCI is not optimal or possible (class IIa, but becomes class I if large area of viable myocardium and high risk criteria met on noninvasive testing)	CABG may be considered
STEMI^*
Indications as for patients with unstable angina/NSTEMI
Emergency CABG in the following circumstances (class I):	CABG recommended
Failed PCI with persistent pain or hemodynamic instability in patients with suitable coronary anatomy
Persistent or recurrent ischemia refractory to medical therapy in patients who have suitable coronary anatomy, who have a significant area of myocardium at risk, and who are not candidates for PCI
At the time of surgical repair of postinfarction myocardial rupture or mitral valve regurgitation
Patients with cardiogenic shock^*
Patients with life-threatening ventricular arrhythmias who have left main or 3-vessel CAD
Poor Left Ventricular Function
Left main, left main equivalent CAD (class I)	CABG should be performed
2-3 vessel CAD with proximal LAD stenosis (class I)	CABG should be performed
Significant viable noncontracting revascularizable myocardium without any of the above anatomic patterns (class IIa)	CABG may be performed
Life-Threatening Ventricular Arrhythmias
Left main, left main equivalent, or 3-vessel CAD (class I)	CABG should be performed
1-2 vessel CAD (class IIa, but this becomes class I if the rhythm is resuscitated sudden cardiac death or sustained ventricular tachycardia)	CABG is reasonable
Following Failed PCI
Ongoing ischemia or threatened occlusion with significant myocardium at risk (class I)	CABG should be performed
Hemodynamic compromise (class I)	CABG should be performed
Removal of foreign body in a crucial anatomic position (class IIa)	CABG is reasonable
Hemodynamic compromise in patients with impaired coagulation and without previous sternotomy (class IIa)	CABG can be beneficial
Hemodynamic compromise in patients with impaired coagulation and with previous sternotomy (class IIb)	CABG can be considered
Patients With Previous CABG
Disabling angina despite optimal nonsurgical therapy (class I)	CABG should be performed
Occluded bypass grafts and class I indications for surgery for native vessel CAD (class I)	CABG should be performed
Bypassable distal vessel(s) with a large area of threatened myocardium by noninvasive studies (class IIa)	CABG is reasonable
Atherosclerotic vein grafts with > 50% stenosis supplying LAD or large areas of myocardium (class IIa)	CABG is reasonable

Criteria for surgery are outlined in text.

CABG, coronary artery bypass graft; CAD, coronary artery disease; EF, ejection fraction; LAD, left anterior descending coronary artery; NSTEMI, non S-T elevation myocardial infarction; PCI, percutaneous coronary intervention; STEMI, S-T elevation myocardial infarction.

* Mortality rates increase for 3 to 7 days following NSTEMI and STEMI. Ideally, the patients should be medically stabilized and surgery delayed beyond this period.

Recent advances in PCI technology, particularly the use of coated stents that elute antiproliferative drugs such as sirolimus, have led to an expanded role for PCI and a modest reduction in the number of patients undergoing CABG surgery. CABG surgery remains the treatment of choice for patients with left main or complex multivessel coronary artery disease, particularly for patients with either diabetes or impaired left ventricular function.

CABG SURGERY IN HIGH-RISK PATIENTS

The overall mortality risk for patients undergoing isolated CABG surgery is about 3.5%.⁶ However, operative risk must be stratified according to the presence or absence of various risk factors. In the absence of risk factors, mortality rates are less than 2%, increasing to more than 50% in patients with multiple risk factors. Independent risk factors for adverse outcome following CABG surgery are summarized in Table 9-2.⁷ Patients with cardiogenic shock, New York Heart Association class III or IV symptoms, a history of previous cardiac surgery, or the need for urgent (in-hospital) or emergency (within 24 hours) surgery have the greatest perioperative risk. These and other factors are incorporated into the various cardiac risk scoring systems outlined in Chapter 41.

Table 9-2 Risk factors for Mortality in the Short-Term Following CABG Surgery

Variable	Odds Ratio for Mortality in the Short Term (95% Confidence Interval)
Age (per 10-year increase)	1.44 (1.37-1.53)
Chronic obstructive pulmonary disease	1.22 (1.11-1.35)
Peripheral vascular disease	1.35 (1.23-1.49)
Cerebral vascular disease	1.37 (1.23-1.51)
Serum creatinine ≥1.5 mg/dl (0.13 mmol/l)	1.72 (1.56-1.90)
Prior heart surgery	2.69 (2.40-3.01)
Canadian cardiovascular society anginal class III or IV	1.27 (1.12-1.44)
Prior myocardial infarction	1.28 (1.16-1.42)
Preoperative ST segment depression on ECG	1.37 (1.24-1.51)
Urgent or emergency surgery	1.83 (1.66-2.02)
New York Heart Association class III or IV	1.77 (1.59-1.96)
Left main coronary stenosis ≥ 50%	1.20 (1.09-1.32)
Left ventricular ejection fraction < 45%	1.26 (1.15-1.39)

Adapted from Gardner SC, Grunwald GK, Rumsfeld JS, et al: Comparison of short-term mortality risk factors for valve replacement versus coronary artery bypass graft surgery. Ann Thorac Surg 77:549-556, 2004.

For a patient at very high perioperative risk (e.g., Euroscore predicted mortality >10% to 20%), a multidisciplinary approach involving the patient’s surgeon and cardiologist, along with input from intensive care and relevant subspecialty physicians, is appropriate. In this way the risks and benefits of surgery and the potential for preoperative optimization can be carefully considered. Further investigations may be indicated (e.g., a myocardial viability scan in patients with low ejection fraction). Alternatives to CABG, such as medical optimization of angina or heart failure symptoms, PCI, or spinal cord stimulation, should also be considered.

Acute Coronary Syndromes

The effectiveness of CABG in patients with severe or unstable angina has been clearly demonstrated. However, mortality rates are substantially increased if CABG surgery is undertaken within 48 hours of an ST segment elevation myocardial infarction (STEMI).⁸ Increased operative risk is less pronounced following non-ST segment elevation myocardial infarction (NSTEMI).⁸

In the first few days following a myocardial infarction, myocardial stunning occurs and a systemic inflammatory response develops—the latter characterized by fever, leukocytosis, and increases in inflammatory markers. Myocardial stunning results in a reduced ejection fraction, segmental wall motion abnormalities (SWMAs), and increased left ventricular end-diastolic pressure. Patients may develop acute heart failure or even frank cardiogenic shock. Following STEMI or NSTEMI, CABG surgery should be delayed by 3 to 7 days or until such time as the clinical features of congestive cardiac failure have resolved.¹

In patients with critical coronary anatomy, particularly left main or left main equivalent coronary artery disease, the decision regarding the timing of surgery is difficult. Despite a large area of threatened myocardium, it is probably still beneficial to attempt to delay surgery for at least 48 hours following an acute myocardial infarction. Following an anteroseptal STEMI, there may be benefit in waiting several weeks until a scar has developed and then performing CABG surgery in conjunction with a remodeling procedure (see later discussion). Following an inferior myocardial infarction with right ventricular involvement, right ventricular function may take as long as 4 weeks to recover,⁹ and CABG surgery is best delayed beyond this period.

Despite the advantages of delayed surgery following myocardial infarction, emergency CABG surgery is indicated in a few specific situations:

• Cardiogenic shock. (See Chapter 19.)

• Ongoing ischemia. Emergency CABG surgery should be considered in patients with suitable anatomy if there is ongoing ischemia evidenced by postinfarction angina, new or evolving ECG changes, or life-threatening ventricular arrhythmias.

• Mechanical complications. CABG surgery may be performed at the time of repair of a mechanical complication of myocardial infarction (see Surgery for Complications of Myocardial Infarction later in this chapter).

• Failed PCI. About 0.7% of patients require emergency CABG following PCI.¹⁰ The reasons for surgical referral include coronary dissection, stent embolization, coronary rupture (with pericardial tamponade), coronary occlusion, and incomplete revascularization.¹⁰ Approximately one quarter of these patients are extremely unwell, experiencing shock, acute ischemia, or arrhythmias. Stabilization in the intensive care unit (ICU) may not be possible, and immediate transfer to the operating room is required. These patients are also likely to have received recent treatment with heparin, aspirin, clopidogrel, and glycoprotein IIb/IIIa inhibitors.

Preoperative Optimization

Prior to urgent CABG surgery, some patients benefit from a short period of aggressive medical optimization in the ICU. While in the ICU, invasive hemodynamic monitoring can be established and, if not already performed, an echocardiogram can be obtained. A pulmonary artery catheter provides important information but is contraindicated in patients with recurrent arrhythmias. Preoperative treatment with an intraaortic balloon pump (IABP) may improve outcome in high-risk patients undergoing CABG surgery ¹¹ and should be considered in all patients with acute ischemia, arrhythmias, pulmonary edema, or shock. For patients with profound cardiogenic shock secondary to myocardial infarction, primary treatment with a ventricular assist device may be preferable to CABG surgery (see Chapters 19 and Chapter 22).¹²

For ongoing myocardial ischemia, intravenous nitroglycerin and a β blocker should be given. For hypotension or pulmonary edema, diuretic and inotropic drugs may be required.

Tight glycemic control has been shown to improve outcome in diabetic patients undergoing cardiac surgery (see Diabetes in subsequent discussion), and in one randomized trial, maintenance of serum glucose between 80 and 110 mg/dl (4.4 and 6.1 mmol/l) improved survival rates in unselected patients (diabetic and nondiabetic; cardiac surgery and noncardiac surgery) admitted to a surgical ICU. Furthermore, treatment with glucose-insulin infusions (as opposed to simply maintaining glycemic control) may improve outcome after CABG surgery in patients with left ventricular failure ¹³ and acute myocardial ischemia.¹⁴

Chronic Left Ventricular Dysfunction

Patients with depressed left ventricular function who have demonstrable hibernating myocardium derive substantial benefit from CABG surgery in terms of improved ejection fraction, relief of heart failure symptoms, and enhanced survival rates.¹⁵ Hibernating myocardium may be identified by a number of myocardial viability studies, as outlined in Chapter 5. A patient with low ejection fraction (<35%) whose symptoms are predominantly those of heart failure should undergo a myocardial viability study; if hibernating myocardium is demonstrated, and the coronary anatomy is suitable, CABG surgery should be considered. A patient with a low ejection fraction whose symptoms are predominantly those of angina and only minimal heart failure should be offered CABG surgery only if indicated on the basis of their coronary artery disease.¹ Patients with extensive anteroseptal scar formation following a STEMI may be suitable for ventricular remodeling surgery (see Surgery to Remodel the Left Ventricle, in subsequent material).

Preoperative Optimization

Chronic left ventricular dysfunction is not an indication for urgent CABG surgery, and patients should be medically optimized prior to surgery to reduce the likelihood of postoperative problems. Systemic and pulmonary congestion should be treated with diuretics. Renal function, volume status, and electrolytes (sodium, potassium, magnesium) should be monitored. Angiotensin-converting enzyme (ACE) inhibitor and β-blocker therapy can inhibit ventricular remodeling and improve ejection fraction, but they may also produce deleterious effects: β blockers initially worsen heart-failure symptoms and can cause symptomatic bradycardia or heart block; ACE inhibitors may cause hypotension, renal impairment, and hyperkalemia. Thus, in patients with severe left ventricular dysfunction, these drugs should not be introduced nor should their doses be increased immediately prior to planned CABG surgery. Oral protein supplements or nasogastric feeding should be considered in patients who are malnourished.

Valvular Heart Disease

In general, any valve pathology graded as moderate or severe should be considered for repair or replacement at the time of CABG surgery. In deciding whether to perform valve surgery, the additive risk of a combined valve-coronary procedure must be weighed against the risk of leaving the patient with an uncorrected valve lesion.

Aortic Stenosis

A patient with moderate or severe aortic stenosis (mean gradient >25 to 30 mmHg) should undergo aortic valve replacement at the time of CABG surgery. The risks involved in aortic valve replacement in a patient with previous CABG surgery are probably greater than those involved in combined CABG and aortic valve surgery, but given the unpredictable natural history of calcific aortic stenosis, aortic valve replacement for mild disease is not justified.

Mitral Regurgitation

The decision to repair or replace a regurgitant mitral valve is difficult and depends on the mechanism of the regurgitation, the likely ease of valve repair, and the degree to which the regurgitation is due to hibernating myocardium. A preoperative transesophageal echocardiogram can help to define the severity and mechanism of the mitral regurgitation. A patient with moderate or severe ischemic mitral regurgitation who undergoes isolated CABG surgery is at risk for important postoperative mitral regurgitation.¹⁶ In general, a patient with moderate or severe ischemic mitral regurgitation should undergo mitral valve surgery at the time of CABG surgery. However, combined CABG and mitral valve surgery substantially increases the operative risk,⁷ particularly in the presence of impaired ventricular function.

Carotid Disease

Carotid disease is common in patients presenting for cardiac surgery. In two studies, the incidence of severe (≥80%) carotid stenosis in unselected patients undergoing cardiac surgery was 8.5% and 12%, with an incidence of postoperative stroke of 18.2% and 5.3%, respectively.^17,¹⁸ All patients who have carotid bruit or a history of transient ischemic attack, stroke, or previous carotid endarterectomy should undergo ultrasound-flow imaging of the carotid arteries prior to surgery. As age above 65 years, left main coronary disease, smoking, and peripheral vascular disease are all associated with carotid atherosclerosis, routine carotid ultrasonography is justified in these patient groups. Because of the increased risk for postoperative stroke associated with high-grade carotid stenosis in patients undergoing cardiac surgery, carotid endarterectomy is indicated when unilateral or bilateral carotid stenoses are greater than 80%, irrespective of symptoms. The outcomes of staged (with carotid endarterectomy performed first) or combined procedures are similar.¹⁹

Chronic Obstructive Pulmonary Disease

Patients with severe chronic obstructive pulmonary disease (COPD), as evidenced by a forced expiratory volume over 1 second (FEV₁) <1.25 l, have significantly higher mortality rates following CABG surgery—mainly due to cardiac arrhythmias—than do patients without severe COPD.²⁰ In patients with end-stage COPD (FEV₁ <0.5l to 1.0 l), cardiac surgery may be considered to be contraindicated. Patients with moderate to severe COPD are likely to benefit from preoperative optimization, including smoking cessation, incentive spirometry, nutritional support, and effective treatment of the bronchospastic or infective components of their pulmonary diseases.

Diabetes

CABG surgery in patients with diabetes and multivessel coronary disease results in improved long-term survival rates compared to those seen after medical treatment or PCI.^21,²² However, diabetic patients suffer a higher rate of perioperative complications, such as renal failure, stroke, and wound infection, and have increased early mortality rates compared to nondiabetic patients.²³ Patients with diabetes often have silent myocardial ischemia, diffuse coronary disease, renal dysfunction, and peripheral vascular disease.

In diabetic patients undergoing cardiac surgery, including CABG surgery, perioperative control of serum glucose by means of intravenous insulin improves survival rates and reduces the incidence of various complications such as sternal wound infection.²⁴^–²⁶ Management strategies for perioperative glucose control are outlined in Chapter 36.

Renal Dysfunction

A number of factors are associated with developing postoperative renal failure (Chapter 33), the most important of which is preoperative renal dysfunction. A preoperative creatinine level of greater than 2.5 mg/dl (0.22 mmol/l) is strongly associated with the need for long-term dialysis following cardiac surgery.²⁷ Postoperative acute renal failure is associated with a greatly increased mortality rate. In one large series, the mortality rate was 63% in patients who developed dialysis-dependent renal failure following CABG surgery compared to only 0.9% in patients whose renal function remained normal.²⁸

If an elevated creatinine level is discovered prior to cardiac surgery it is essential to know whether it is acute or chronic. An acute deterioration in renal function should be carefully investigated and all potential causes eliminated. In particular, drugs such as spironolactone, nonsteroidal antiinflammatory drugs, and nephrotoxic antibiotics should be discontinued; consideration should also be given to stopping ACE inhibitors and angiotensin receptor blockers. If possible, CABG surgery should be delayed until renal function has returned to baseline following administration of radiocontrast agents.

End-Stage Renal Failure

Patients with dialysis-dependent renal failure have a perioperative mortality rate in excess of 10% following cardiac surgery.²⁹ Such patients frequently have coexisting hypertension, diabetes, anemia, abnormal hemostasis, and electrolyte abnormalities. They are often anuric or oliguric; thus, any fluid that is administered must be removed by dialysis. Immediately following dialysis, patients tend to be hypovolemic and hypokalemic; immediately prior to dialysis, patients tend to be hypervolemic and hyperkalemic. Dialysis should be performed on the day prior to surgery, not on the morning of surgery, to allow postdialysis fluid shifts to equilibrate. Following surgery, dialysis is performed, ideally, on the first postoperative day (see Chapter 33).