Coronary Artery Bypass Grafting (CABG) 

Coronary artery bypass grafting involves bypassing native coronary arteries with high-grade stenosis or occlusion not amenable to angioplasty  with stenting. Indications are changing as percutaneous interventions are being increasingly used. 

Traditional CABG Procedure 

The procedure involves thoracotomy via a midline (median) sternotomy. A heart-lung machine is used  to establish cardiopulmonary bypass (CPB), allowing  the heart to be stopped and emptied  of blood to maximize operative exposure and facilitate vessel anastomosis; stopping  the heart also markedly decreases myocardial O2 demand. Before initiation  of CPB, the patient is given very high dose heparin to prevent clotting in  the bypass circuit. Then the aorta  is cross-clamped and the  heart is stopped by injection of  a cardioplegic solution (crystalloid  or more commonly blood-based) that also contains substances that help myocardial cells tolerate ischemia and reperfusion. The cardioplegic solution and the heart are sometimes cooled slightly to enhance tolerance  of ischemia; the patient's body is cooled via the CPB machine for similar reasons. 

The left internal mammary artery is typically used as a pedicled graft  to the left anterior descending  coronary artery. Other grafts consist of segments of saphenous vein removed from the leg. Occasionally, the right internal mammary  artery or radial artery from  the nondominant arm can be used. 

On completion  of the vascular anastomoses,  the aorta is unclamped, allowing  the coronary arteries to be perfused by oxygenated blood, which typically restores cardiac activity. Heparin anticoagulation  is reversed by giving protamine. Despite cardioprotective measures, stopping  the heart is not without consequences. During reperfusion, myocardial dysfunction is  common and  can lead to bradycardia, arrhythmias (eg, ventricular fibrillation), and low cardiac output; these events are  treated by standard measures such as pacing, defibrillation,  and inotropic drugs. 

Typically, hospital stays are 4 to 5 days unless prolonged by complications. 

Complications:  Complications and disadvantages of traditional  CABG involve mainly 

• Sternotomy 
• CPB 

Median sternotomy is surprisingly well tolerated; however, healing takes 4 to 6 wk. Also, wound infections occasionally cause mediastinitis or sternal osteomyelitis, which can  be vexing to treat. 

CPB causes several complications, including 

• Bleeding 
• Organ dysfunction 
• Neuropsychiatric effects 
• Stroke 

Post-CPB bleeding is a common problem caused by various factors, including hemodilution, heparin use, platelet dysfunction due to exposure to  the bypass pump, consumptive coagulopathy, and induced hypothermia. Also, the CPB machine evokes  a systemic inflammatory response (probably due to exposure  of blood components to  the foreign material of  the bypass circuit); this response can cause organ dysfunction in any system (eg, pulmonary, renal, brain, GI). Aortic cannulation, cross-clamping, and release can trigger release of emboli, causing stroke in about 1.5%; microemboli may contribute to post-CPB neuropsychiatric effects, which appear in about 5 to 10%. 

Other common complications of  CABG include focal and global myocardial ischemia and dysrhythmias. Perioperative MI occurs in about 1% of patients. Atrial fibrillation occurs in 15 to 40% of patients, typically 2 to 4 days after surgery. Nonsustained ventricular tachycardia  may occur in up to 50% of patients. 

Mortality depends mainly on patients' underlying health; operator and institutional experience (ie, number of annual procedures)  also is important. In  an experienced program, periprocedural mortality  in otherwise healthy patients is typically < 1 to 3%. 

Alternative CABG Procedures 

• Newer techniques seek to limit  the complications of traditional CABG by 
• Avoiding CPB (off-pump CABG) 
• Avoiding median sternotomy (minimally invasive CABG) 
• Both 

CPB can be avoided in select patients by using new techniques that allow  the surgeon to revascularize the beating heart. Various devices and methods stabilize a portion  of the myocardium, holding the operative site relatively motionless. Off-pump procedures are more commonly done through small parasternal or intercostal incisions (minimally invasive CABG), sometimes with endoscopy or even robotic assistance, but may be done through  a traditional median sternotomy, which provides better operative exposure.
Allowing the  heart to beat means that  the myocardium requires more O2 than when CPB is used. Thus, the  heart is sensitive to the interruption  of blood flow necessitated while the vascular anastomosis is done; this interruption can cause ischemia  or infarction  in the myocardium supplied by the affected vessel. Some surgeons place a temporary coronary artery shunt to provide distal perfusion. 

The minimally invasive technique is somewhat more difficult to do  and may not be suitable when multiple bypass grafts, particularly those involving vessels behind  the heart, are required. Transfusion requirements, length  of stay, and costs  are typically less with off-pump CABG, but in some studies,  the rate of  the more serious complications of death, MI, and stroke  are similar to that  of CABG using CPB. Thus, the theoretic advantages of avoiding CPB do not seem to have been fully realized. 

Minimally invasive CABG is usually done off-pump but may be done using CPB. In such cases, CPB is done endovascularly using special catheters inserted into the arterial  and venous systems; the aorta is occluded by  a balloon at the end  of the aortic catheter rather than an external clamp. Although avoiding median sternotomy complications, this technique otherwise has similar rates of mortality and major perioperative complications as conventional techniques. 

Notes: Dr. M. Kristine Schlossberg

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EditText of this page (last edited February 23, 2010)