Coronary Artery Bypass Grafts
Related narrative: Coronary Artery Bypass Grafts
Coronary artery bypass grafting (CABG) with saphenous vein conduits was first used in 1964 as a salvage procedure for patients with severe refractory ischemic cardiac disease. As the benefits became evident, the indications were expanded. The use of the internal mammary artery (IMA) was introduced in 1966 and conferred improved patency (90% v 50% @ 10 years). The more recent introduction and growth of percutaneous coronary angioplasty (PTCA) has provided alternative options for various classes of patients as well as the potential for revision of stenotic grafts. A newer (1995) minimally invasive approach to coronary artery bypass is also evolving.
Ischemic cardiac disease results from atherosclerotic narrowing of the coronary arteries and is most often manifested as anginal pain. A narrowing of a coronary artery by 50% of vessel diameter is considered a hemodynamically significant lesion, and pain is brought on by increased oxygen demand with exercise that cannot be satisfied with the flow limitations of the narrowed feeding vessel. Late high-grade stenosis may be manifested by rest pain. Occlusion of a coronary vessel (acute myocardial infarction) most often results from hemorrhage beneath an atherosclerotic plaque. The Framingham study, done before the use of ASA, beta blockers and risk modification, showed that patients with chronic stable angina had a 4% mortality rate per year.
Symptomatic patients are first evaluated with non-invasive testing. These tests include standard exercise stress testing with EKG monitoring, nuclear medicine (thallium) stress perfusion imaging, and pharmacological (dipyrimadole, adenosine, dobutamine) imaging and echocardiography. Invasive angiographic study is indicated when non-invasive studies identify significant abnormalities that might be benefited by PTCA or CABG, in asymptomatic patients with rest or exercise EKG changes, in survivors of cardiac arrest, history of myocardial infarction (MI), and follow up studies of patients who have had PTCA or CABG.
Both PTCA and CABG relieve anginal symptoms. The VA study showed no overall survival benefit of CABG over medical therapy, but several subsets of patients did have survival benefits. Patients with left main disease greater than 50% or "equivalent" (LAD and L circumflex), three vessel disease and moderate to severe left ventricular dysfunction, three vessel disease with severe proximal LAD narrowing regardless of LVD, LAD with moderate to severe LVD and unrelieved or unstable angina are all high risk and are clearly benefited by surgical intervention. Failure of medical management or inability to follow medical regimens are also indications for intervention. Patients with fewer vessels involved and anatomically accessible lesions who are candidates for intervention, are usually managed with PTCA. The long term results of CABG have been better than with PTCA, but PTCA is evolving dramatically. Life style/occupation are important factors in deciding on intervention. Paradoxically, the patients with the highest surgical risks, may gain the most benefit from surgery.
Patients who are to undergo CABG are prepared with beta blockers, nitrates and calcium channel blockers in the weeks preceding surgery to help protect the myocardium during bypass. A single cross clamp period with intermittent sanguinous cold cardioplegia is the most common technique used. The high potassium cardioplegia solution may be supplemented with oxygen, buffers and free radical scavengers. The addition of retrograde cardioplegia via the coronary sinus gives more uniform distribution of the solution, and the heart is also cooled with topical icy slush. Complications include 2-5% reoperation for bleeding, up to 75% transient impairment of intellectual function, 1-5% stroke rate, 40% early (2-3d) atrial fibrillation and 1% bradyarrhythmia requiring permanent pacemaker. The incidence of sternal wound infections is increased when both internal mammary arteries are used. There is an 8-12% early saphenous vein graft occlusion rate. Reoperations for late restenoses now account for 10% of cases and are increasing. PTCA is also playing an increasing role in late restenoses.
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CORONARY SURGERY Off-Pump and Port Access
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