50. Coronary Artery Disease-Review of CT Surgery

Meghan Halub and Hamza Aziz        

This chapter is a revision and update of that included in previous editions of the TSRA Review written by Swaroop Bommareddi (2nd edition) and Kendra J. Grubb (1st edition). 

Introduction

Coronary artery disease (CAD) remains the single largest cause of death in the United States from data collected from the Center for Disease Control, and accounts for over 640,000 deaths per year. Data also shows that the percent of people in the United States with CAD is 12.1% and is increasing with the elderly population. Classic symptoms include angina, substernal chest pain with radiation to the jaw or left arm, and associated diaphoresis and shortness of breath may be present. Other symptoms include dyspnea, dizziness, syncope, pulmonary edema, or valvular dysfunction. Decreased coronary blood flow during conditions of increased demand result in symptoms and ischemia. Coronary angiography remains the gold standard for diagnosis.

Patients with CAD may present with chronic CAD (stable angina) or with acute coronary syndrome (ACS). ACS can present in different ways: ST-elevation MI (STEMI), non-ST-elevation MI (NSTEMI), or unstable angina. A STEMI usually occurs from the rupture of atherosclerotic plaques, which result in mural thrombus formation at the site of rupture with vascular occlusion. There is evidence of myocardial necrosis based on cardiac biomarkers and ST changes in the EKG. Unstable angina or a NSTEMI can occur from plaque rupture and thrombosis (most common), coronary spasm (Prinzmetal’s angina), progressive coronary obstruction, or from a demand ischemia (i.e., tachycardia, anemia, low flow states) in a patient with existing CAD. 

LV dysfunction from CAD can occur via the following mechanisms:

  • Muscle loss due to an MI (irreversible)
  • Hibernating myocardium due to wall motion abnormality at rest from underlying CAD which is reversible with revascularization.
  • Stunned myocardium due to transient wall motion abnormality after reperfusion.

Medical Management

The American College of Cardiology and American Heart Association 2012 Guideline for the Diagnosis and Management of Patients with Stable Ischemic Heart Disease can be found at acc.org/guidelines. Medical management consists of the following:

Risk Factor Modification

  • Lipid Management: Goal total cholesterol <200 mg/dL and LDL cholesterol <100 mg/dL for primary prevention and <70 mg/dL for secondary prevention, and moderate or high dose of a statin therapy should be prescribed if tolerated.
  • Blood Pressure Management: Goal blood pressure <130/80 for all patients.  Antihypertensive drug therapy with ACE inhibitors and/or beta blockers with additional drugs such as thiazide diuretics or calcium channel blockers to achieve goal.
  • Diabetes Management: Goal hemoglobin A1c of <7%.
  • Physical Activity: 30-60 minutes of moderate intensity aerobic activity 5-7 times per week to improve cardiorespiratory fitness with medically supervised programs and physician directed cares.
  • Weight management: Goal BMI to be between 18.5 and 24.9 kg/m2. Encouragement of healthy diet and structured caloric intake.
  • Smoking Cessation: Cessation and avoidance of exposure to tobacco smoke at work and home should be encouraged and supported in all patients with CAD.
  • Management of Psychological Factors: Treatment of depression and other factors that may impede care.

Medical Therapy to Prevent MI and Death Recommendations

  • Antiplatelet Therapy: Aspirin (ASA) 75-162 mg/day in all patients should be continued indefinitely and clopidogrel 75 mg/day if ASA contraindicated. Treatment with both is reasonable in patients who have had ACS or PCI.
  • Beta Blocker Therapy: Should be started and continued in all patients for 3 years with normal LV function after MI or ACS. Beta blockers should also be used in all patients with LV systolic dysfunction (EF <40%) with heart failure or prior MI unless contraindicated.
  • Renin-Angiotensin-Aldosterone Blocker Therapy: ACE inhibitors should be used in all patients who have hypertension, diabetes mellitus, LVEF <40%, or CKD. ARBs are recommended for patients who are intolerant of ACE inhibitors.

Medical Therapy for Relief of Symptoms

  • Beta blockers should be prescribed as initial therapy for relief of symptoms. Calcium channel blockers or long-acting nitrates should be prescribed for relief of symptoms when beta blockers are contraindicated. 
  • Sublingual nitroglycerin or nitroglycerin spray is recommended for immediate relief of angina.

Percutaneous Coronary Intervention (PCI) vs. Coronary Artery Bypass Grafting (CABG)

The goals of coronary revascularization are to improve symptoms, preserve myocardium, and improve survival. Most studies define significant stenosis as 70% diameter narrowing for all vessels except the left main coronary artery, for which 50% or more stenosis is considered significant. One of the initial and most comprehensive trials comparing PCI to CABG for CAD is the SYNTAX trial. This trial compared the two modalities in untreated three vessel or left main CAD or both. It showed that adverse cardiac or cerebrovascular events were higher in the PCI group at 12 months, mostly due to an increased rate of repeat revascularization. The rates of MI and death were similar between both groups, and the rate of stroke was higher in the CABG group. The end conclusion of the trial was that CABG remained the standard of care for patients with three-vessel or left main CAD since the use of CABG resulted in lower rates of the combined end point of major adverse cardiac or cerebrovascular events at one year compared to PCI.

The STITCH trial is another landmark study that compared medical therapy to medical therapy plus CABG in patients with CAD with an EF <35%. The end results showed that medical therapy plus CABG had lower rates of death from cardiovascular causes and death from any cause or revascularization or hospitalization for cardiovascular causes in patients with heart failure.

The EXCEL trial evaluated PCI with contemporary drug-eluting stents as compared with CABG in patient with left main disease. The five-year results found that there was no significant difference between PCI and CABG with respect to the rate of composite outcome of death, stroke, or MI when the left main disease was of low or intermediate anatomical complexity. The most recent landmark study is the ISCHEMIA trial, which looked at patients with stable CAD and moderate or severe ischemia and compared clinical outcomes of those who had an invasive intervention plus medical therapy to medical therapy alone. This study found in this population of patients there was no evidence that an initial invasive strategy (angiography and revascularization when feasible) reduced the risk of ischemic cardiovascular events or death from any cause over an average of 3.2 years when compared to an initial conservative strategy.

Indications for CABG

The most recent clinical practice guidelines for when CABG should be performed is from the 2011 ACCF/AHA Guideline for Coronary Artery Bypass Graft Surgery, but it should be noted more updated guidelines will be available in Spring 2021 and can be found at acc.org/guidelines. These guidelines are based on trials comparing surgical revascularization with medical therapy, and significant stenosis is defined as ≥70% reduction in diameter.

Class I Recommendations for CABG

1. Left main disease ≥50% or left main equivalent of the proximal LAD and one other major coronary artery

2. Significant stenosis in 3 major coronary arteries

3. Disabling angina refractory to medical management

4. Ongoing ischemia in the setting of NSTEMI not responsive to maximal medical therapy

5. Survivors of sudden cardiac death with presumed ischemia mediated ventricular tachycardia caused by significant stenosis in a major coronary artery

6. Failed PCI or unfavorable anatomy for PCI

Class IIa recommendations where CABG is considered reasonable

1. Significant stenosis in 2 major coronary arteries without proximal LAD involvement if there is severe or extensive myocardial ischemia

2. Significant multi-vessel CAD or proximal LAD disease in patients with an EF 35-50%

3. Significant stenosis of the proximal LAD and extensive ischemia and the left internal mammary can be used

4. Complex 3 vessel CAD (i.e., SYNTAX score >22) with or without involvement of the proximal LAD

5. Multivessel CAD in patients with DM

6. Disabling angina in patients with contraindications or intolerance to guideline directed medical therapy

Off-Pump CABG

Off-pump CABG (OPCAB) has gained some popularity in trying to decrease morbidity associated with CPB and can be institution and physician dependent. The procedure utilizes coronary stabilizers and coronary shunts to allow for the creation of the anastomosis. Hemodynamic instability during the procedure can be due to RV compression (due to decreased RV filling and cardiac output) when exposing the lateral wall of the LV, diastolic dysfunction from downward pressure from a coronary stabilizer, or intraoperative ischemia. This can be mitigated by using short-acting alpha-adrenergic agents, positioning the patient in Trendelenburg, creating the internal mammary artery-LAD anastomosis prior to manipulating the heart, and using a variety of exposure techniques (deep pericardial sutures, right pleurotomy, and pericardiotomy to decrease RV compression, right hemi-sternal elevation, and apical suction devices.

Surgical outcomes show that OPCAB need fewer blood transfusions, less early neurocognitive dysfunction, and less renal insufficiency. On-pump CABG is associated with high number of total grafts, improved ability to graft the circumflex targets, and possible better long-term graft patency. 

The ROOBY trial, which was performed by the Department of Veterans Affairs, randomized on-pump and off-pump CABGs and found that at 1 year arterial and saphenous vein graft patency was lower in the off-pump group and there was a higher rate of adverse events at one year. At the 5-year follow up, the ROOBY trial showed that a 28% higher risk of death from any cause was observed after off-pump surgery than after on-pump surgery. In contrast, the GOPCABE trial showed that in patients 75 years of age or older, there was no significant difference between the on-pump and off-pump CABG with regard to composite outcome of death, stroke, MI, repeat revascularization, or new renal replacement within 30 days and 1 year after surgery.

Conduit Options

Internal Mammary (IMA)

When an arterial graft is placed, the left IMA (LIMA) is the conduit of choice and has been associated with excellent long-term patency with over 95% patency at 5 years and right IMA (RIMA) having similar graft patency at 5 years. Ten-year patency is roughly 90%. The use of the IMA has shown early and late survival benefit and superior event-free survival after CABG. IMA grafts tend to be resistant to development of atherosclerosis because the artery has greater resistant to endothelial harvest injury, non-fenestrated internal elastic lamina which may inhibit cellular migration and prevent intimal hyperplasia, and the unique endothelium has higher basal production of vasodilators (nitric oxide and prostacyclin). The graft also shows no vasoconstriction in response to norepinephrine and responds to nitroglycerin. 

Radial Artery

Radial artery is a secondary choice of conduit. The vessel has fenestrated internal elastic lamina and a thicker wall than the IMA. It is more likely to have atherosclerotic changes, spasm, calcification, and is a poor-quality conduit in patients with peripheral vascular disease and DM. Patients should have an Allen Test performed prior to harvest to make sure there is a complete palmer arch prior to harvest, and the artery should not be used if there have been prior interventions in the vessel, although some facilities are willing to use the vessel 3 months after an intervention. Five-year patency is 83-95% and is influenced by competitive coronary flow.

Other Arterial Conduits

Ideally, arterial grafts are the best option for long-term revascularization. Other arterial conduits include the right gastroepiploic artery, inferior epigastric artery, ulnar artery, left gastric artery, splenic artery, thoracodorsal artery, and the lateral femoral circumflex artery.

Greater Saphenous Vein (GSV) Conduits

The saphenous vein is the most common venous graft used for CABG. It is fast and easy to harvest, available in most patients, versatile, and resistant to spasm. There is increased graft failure with GSV as a result of intimal hyperplasia (most common), thrombosis, and graft atherosclerosis. Early occlusions tend to be due to a technical failure, and can be as high as 10% of grafts at 30 days. Ten-year patency is about 50%.

Complications

  • Postoperative Bleeding: This is one of the most common complications after bypass surgery, and both thrombocytopenia and platelet dysfunction are common in up to 60% of patients, and severe bleeding (>10 units transfusion required) occurs in 3-5% of patients. Re-exploration is indicated for bleeding of >400 cc/hr for 1 hour, >300 cc/hr for 2 hours, >200 cc/hr for 4 hours, or if tamponade is suspected. Preoperative use of anticoagulant and antiplatelet medication increases the risk of postoperative bleeding. Medications such as ASA can be continued until the day of surgery. Coumadin should be stopped and transitioned to low molecular weight heparin one week before surgery. Clopidogrel should be stopped 5-7 days before surgery.
  • Perioperative MI/Low Cardiac Output Syndrome: Perioperative MI occurs in 2% of cases and poor cardiac function during the early post operative phase is associated with increased risk of death. This is usually suspected when there is unexplained postoperative hypotension, tachycardia, or pulmonary edema. Low cardiac output syndrome is defined as needing inotropic or mechanical support for >30 minutes after surgery and occurs in 5-10% of patients. Patient who are the most likely to benefit from inotropic support are those with a cardiac index <2.0 L/min/m2 despite optimized heart rate, rhythm, preload, afterload and without evidence of tamponade.
  • Arrhythmias: Atrial fibrillation is one of the most common complications after bypass and occurs in 20-40% of patients. Risk factors include older age, prolonged cross-clamp time, COPD, and beta blockade withdrawal. Prophylactic regimens with beta blockers, amiodarone, or atrial pacing may reduce risk. Ventricular arrhythmias also occur in patients but are only sustained in 1-2% of patients. This is thought to be secondary to reperfusion in patients with non-sustained VT. Bradyarrhythmias are also not uncommon, and temporary electrical pacing or even permanent pacing may be necessary.
  • Pulmonary Dysfunction: Pneumonia occurs in 1-5% of CABG patients and risk factors include COPD, smoking, and older age. Other pulmonary dysfunctions include atelectasis (~70% of patients), pleural effusion, diaphragmatic dysfunction (due to cautery or ice on the phrenic nerve), transfusion related lung injury, or acute respiratory distress syndrome.
  • Neurologic Dysfunction: The incidence of postoperative neurologic sequelae after CABG is approximately 2-6%, with increased frequency among patients >70, history of prior stroke, hypertension, renal failure, DM, smoking, or carotid artery disease. Type 1 deficits (major neurological deficits) occur in 1-3% of patients, while type 2 deficits (global deterioration of memory or function) occur in approximately 3% of patients. 
  • Renal failure: Incidence of postoperative renal failure is high postoperatively, and acute renal injury occurs in approximately 30% of patients with severe enough injury requiring dialysis in 1-5% of patients. Risk factors include age >70, DM, CHF, preoperative renal dysfunction, prior CABG, and prolonged time on cardiopulmonary bypass.
  • Sternal Wound Infection: Mediastinitis and deep sternal wound infection occurs in 1-3% of cases and has a high associated mortality of 15%. Risk factors include cigarette smoking, use of pedicled BIMAs, DM, obesity, advanced age, male gender, and perioperative hyperglycemia.
  • Early Graft Occlusion (<30 days after surgery): This occurs in 5-10% of patients where SVGs are used. This is thought mostly to be thrombotic in origin and mainly occurs at the anastomosis due to technical error or excessive manipulation. Early use of ASA and clopidogrel (also known as dual antiplatelet therapy or DAPT) may decrease the incidence of early vein graft occlusion and is typically prescribed for 12 months after an NSTEMI. Revascularization strategies for early occlusion include balloon angioplasty with or without stent or repeat CABG.
  • Mortality: Overall mortality after CABG is 1-3% on average. Risk factors for surgery include cigarette smoking, urgent status, advanced age, female gender, DM, low EF, CHF, perioperative renal dysfunction, peripheral vascular disease, pulmonary disease, and left main disease. Adjuncts such as the STS Risk Calculator (sts.org/resources/risk-calculator) can help in assessment of patient risk based on clinical characteristics.

Redo CABG

Redo CABG operations are becoming more common and carry increased risk for the patient. Some indications for redo CABG include the following:

  • Significant stenosis on the vein grafts to the LAD or multiple vein grafts, especially in patients with abnormal LV function, positive stress test, and myocardium in jeopardy.
  • A highly symptomatic patient with severe stenosis on grafts supplying viable myocardium.

The use of PCI, CABG, or hybrid approaches should be individualized depending on anatomy, symptoms, comorbidities, and surgeon comfort. Each patient should be evaluated in a multidisciplinary approach and work up should include coronary angiography, angiography of the IMA vessels, review of prior operative report, myocardial viabilities studies, venous and/or arterial conduit evaluations, CT scan to assess anatomy, and echo to evaluate heart function. Manipulation of the grafts should be avoided intraoperatively and a “no touch” technique used to prevent embolization of debris. A patent IMA graft should be clamped during myocardial protection. If an arterial graft will be used to replace a vein graft, the arterial graft should be added and the previous vein graft left intact as long as the stenosis on the previous vein graft is >50% to avoid competitive flow. Distal anastomoses of the new grafts should be performed directly on the native vessel and proximal anastomoses can be performed to the hood of the new or old vein graft. 

Mortality varies based on patient characteristics, but redo CABG mortality risk is about 3-8% with the most common cause of mortality being perioperative MI.

Suggested Readings

  1. Serruys PW, Morice M-C, Kappetein AP, et al.; SYNTAX Investigators. Percutaneous coronary intervention versus coronary-artery bypass grafting for severe coronary artery disease. N Engl J Med. 2009;360(10):961-72.
  2. Velazquez EJ, Lee KL, Deja MA, et al.; STICH Investigators. Coronary-Artery Bypass Surgery in Patients with Left Ventricular Dysfunction. N Engl J Med. 2011;364(17):1607-16.
  3. Stone GW, Kappetein AP, Sabik JF, et al.; EXCEL Trial Investigators. Five-Year Outcomes after PCI or CABG for Left Main Coronary Artery Disease. N Engl J Med. 2019;381(19):1820-1830.
  4. Maron DJ, Hochman JS, Reynolds HR, et al.; ISCHEMIA Research Group. Initial Invasive or Conservative Strategy for Stable Coronary Artery Disease. N Engl J Med. 2020;382(15):1395-1405.  
  5. American College of Cardiology Guidelines can be found at acc.org/guidelines.  
  6. Hattler B, Messenger JC, Shroyer AL, et al.; ROOBY Study Group. Off-Pump coronary artery bypass surgery is associated with worse arterial and saphenous vein graft patency and less effective revascularization: Results from the Veterans Affairs Randomized On/Off Bypass (ROOBY) trial. Circulation 2012;125(23):2827-35.
  7. Diegeler A, Borgermann J, Kappert U, et al.; GOPCABE Study Group. Off-pump versus on-pump coronary-artery bypass grafting in elderly patients. N Engl J Med. 2013;368(13):1189-98.   
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