Approximately 4.7 million inpatient heart-related procedures including coronary arteriography, angiocardiography, balloon angioplasty, coronary atherectomy, endovascular stent placement, cardiac catheterization, and coronary artery bypass grafting (CABG) are performed in the United States each year.1 Heart disease is known to be the leading cause of death in the United States with coronary artery disease (CAD) being the most common type of heart disease.1 In CAD, coronary artery blood flow is impaired usually due to atherosclerosis, which is a frequent cause of myocardial infarction and sudden cardiac death.2 While there are approximately 395,000 CABG procedures performed annually, this surgery doesn't cure atherosclerosis.1
This article reviews the CABG procedure and select alternate approaches, indications, associated risks, cardiopulmonary bypass (CPB), and postoperative management.
CABG is a surgical revascularization procedure that helps to restore blood flow to an ischemic heart by bypassing the coronary artery stenosis or occlusion. A healthy artery or vein taken from elsewhere in the body, such as the leg, chest, arm and abdomen, is used as the conduit to divert blood around the target artery. Because plaque buildup in the coronary arteries narrows the arterial lumen, the amount of oxygen-rich blood that reaches the myocardium is limited. Therefore, one of the main goals of the surgery is to restore adequate perfusion and oxygenation to the heart. Surgery also relieves signs and symptoms such as chest pain, dyspnea on exertion, and fatigue, improving cardiac function, and the overall quality of the patient's life.
Indications for surgery
The primary indications for CABG include: left main coronary artery stenosis greater than 50%, stenosis of the proximal left anterior descending artery, and proximal circumflex greater than 70%.3 Others include severe or intractable angina, multiple vessel disease, unsuccessful percutaneous coronary intervention or fibrinolytic therapy, and persistent ischemia unresponsive to medical therapy.3
Risks of surgery
Some of the risks associated with surgical revascularization include: bleeding during or after the surgery, incisional/wound infection, myocardial infarction, heart failure, stroke, pneumonia, dysrhythmias, acute kidney injury, and changes in cognition and death.3,4
Most patients undergo a complete cardiac workup prior to surgery, which includes a thorough health history, physical examination, information regarding current medications, allergies, and any comorbidities. Diagnostic studies such as a chest X-ray, ECG, echocardiogram, stress test, and cardiac catheterization are performed. Lab tests include a complete blood cell count, coagulation studies, serum electrolytes, and type and crossmatch. Additional lab studies include serum renal and liver function tests. Patients with chronic lung disease often have pulmonary function tests and arterial blood gas analysis performed. In this evaluation period, one of the goals is to determine if the patient is at a higher risk for developing postoperative complications. Factors that predispose a patient to complications include a history of bleeding disorders, anticoagulation, and comorbidities such as emphysema, chronic kidney disease, diabetes, and peripheral arterial disease.
Traditional surgical approach
CABG has been historically performed through a midline sternotomy to allow exposure of the heart and avoid entering the pleural space. The sternum is cut and the ribs are spread to allow exposure of the anterior mediastinum and pericardium. The pericardium is then opened and the patient is anticoagulated with heparin to prevent thrombosis as blood circulates through the CPB machine. Additional heparin doses may be administered in order to achieve and maintain an activated whole blood clotting time of greater than 480 seconds.5
CPB is initiated by the placement of a cannula through the right atrial appendage to divert venous blood away from the heart and another cannula usually placed in the ascending aorta to return blood from the CPB machine back to the systemic circulation. CPB is necessary to divert blood away from the heart and create a bloodless surgical field, while keeping oxygenated blood circulating throughout the body. The patient's temperature is lowered to approximately 28-32° C, (82.4-89.6° F) to decrease the metabolic rate and provide protection to all major organs from potential ischemic injury.6 The aorta is cross-clamped just distal to the aortic cannula so that a cardioplegic solution can be infused to arrest the heart. It's infused into the aortic root and circulates through the coronary arteries protecting the heart against myocardial ischemia.7 The cardioplegic solution is mainly a hyperkalemic solution with additional additives such as sodium bicarbonate, calcium, magnesium, and sodium chloride.8 During this initial phase of the surgery, the grafts used to bypass the target coronary arteries are harvested.
The most common sites of autologous graft retrieval are the saphenous vein, internal mammary artery, and radial artery. The grafts are attached to the ascending aortic wall proximally on one side and then distally below the diseased coronary artery.9
Saphenous vein grafts (SVG), taken from the lower extremities, have been used since the development of CABG. However, research has shown that 50% of these vein grafts are occluded after 10 years.10 Thrombosis, atherosclerosis, and fibrointimal hyperplasia account for the incidence of occlusion. Therefore, surgeons began utilizing other vessels in the body for the grafts.
The internal mammary artery located next to the sternum inside the chest wall is a preferred choice for graft selection because of higher graft patency rates, that is, 90% patency 10 years after surgery and a decrease in the development of atherosclerosis.10 Since the early 1990's, radial artery grafts have been used with growing popularity. Recent clinical trials have shown superior long-term patency over SVG.11
At the completion of the surgery, the blood is rewarmed to 37° C, (98.6° F.).12 Air is removed from the heart chambers and aortic root, then, the aortic cross-clamp is removed.12 This allows blood to reperfuse the coronary arteries. With rewarming and reperfusion, cardiac rhythm may resume. Often times, internal defibrillation may be necessary because of the onset of ventricular fibrillation. Once a perfusing cardiac rhythm is reestablished, the patient is weaned off CPB. Prior to cannulae removal, protamine is given to neutralize the effects of heparin. The cannulae are then removed from the right atrium and aorta when the heart is functioning normally and hemodynamic targets are met.
Prior to sternal closure, epicardial pacing wires are sutured in place along with mediastinal chest tubes. The final step of the surgery is sternal closure.4,9 (See On-pump CABG, Off-pump CABG, and Robotically assisted endoscopic approach.)
Most patients are brought directly to the ICU from the OR and closely monitored for 24 hours.
Postoperatively, a number of complications can occur after CABG and direct the medical management and nursing care of the patient. These complications include hemodynamic instability, dysrhythmias, altered thermoregulation, postoperative bleeding and cardiac tamponade, oxygenation and ventilation abnormalities, neurologic and renal dysfunction, and infection.
Hemodynamic instability. Support of the cardiovascular system involves normalizing and maintaining cardiac output, heart rate, BP, filling pressures, and systemic vascular resistance. Assessing intravascular volume status is a primary concern. Hypovolemia is usually attributed to CPB or diuretic administration at the conclusion of the surgery. With the use of a pulmonary artery catheter, left and right heart filling pressures should be closely monitored.6
To improve preload, volume replacement is achieved by administering a crystalloid solution such as 0.9% sodium chloride solution or colloid such as 5% albumin or hetastarch. If hemoglobin is decreased, then blood products are considered. Heart rate can be regulated through temporary epicardial pacing. Heart rates are usually maintained at 90-100 beats/minute.13 Impaired cardiac contractility can be managed with I.V. inotropes such as dopamine, dobutamine, milrinone, and epinephrine. Hypertension is associated with increased systemic vascular resistance and vasoconstriction due to increased circulating catecholamine and renin levels.6 A variety of pharmacologic agents can help reduce afterload such as nitroglycerin, nitroprusside, labetalol, apresoline, and angiotensin-converting enzyme inhibitors.
Dysrhythmias. Atrial fibrillation is the most common dysrhythmia occurring in up to 40% of patients postoperatively.14 Antiarrhythmic agents such as diltiazem and amiodarone are commonly used to treat atrial and ventricular dysrhythmias. Treating hypokalemia and hypomagnesemia can also decrease ventricular dysrhythmias.
Altered thermoregulation. As previously stated, hypothermia is induced during CABG. Hypothermia can predispose a patient to ventricular dysrhythmias, coagulopathy, increased systemic vascular resistance, and shivering. Temperature regulation is supported through the use of a variety of rewarming devices; however, active rewarming of a patient should occur slowly to decrease the adverse reactions of vasodilation that can occur such as hypotension.6
Postoperative bleeding and cardiac tamponade. Bleeding can occur due to inadequate heparin reversal, disrupted suture lines, hypothermia, and prolonged CPB. Protamine should be administered slowly because it can cause hypotension, bradycardia, and anaphylaxis. The use of blood products such as fresh frozen plasma, platelets, and cryoprecipitate can be administered when bleeding is excessive.15
Oxygenation and ventilation abnormalities. Patients usually remain endotracheally intubated and mechanically ventilated for 2-4 hours after surgery due to advances in cardiac anesthesia techniques. Main criteria for extubation are adequate cardiac index, oxygenation and ventilation without acidosis, minimal chest tube drainage, and the ability to follow commands with adequate strength to cough and deep breath.6
Respiratory complications are due to atelectasis, hemo/pneumothorax, and acute lung injury. Pulmonary care involves early weaning from mechanical ventilation and extubation, institution of coughing and deep-breathing exercises, early mobilization, and aggressive use of the incentive spirometry.
Neurologic dysfunction. Major postoperative alterations in neurologic function include stroke, neurocognitive changes, depression, delirium, and encephalopathy. Risk factors are multifactorial and can include general anesthesia, CPB, microembolism, acid-base imbalances, diabetes, and systemic inflammation.6,15 Short-term neurocognitive changes involve memory function and usually are transient but can have long-term effects. The patient will exhibit impaired attention, difficulty concentrating, and memory deficits. By discharge from the hospital, there is an incidence of cognitive decline in up to 52% of patients. Other changes involve language and psychomotor function. Postcardiotomy delirium can occur up to 5 days after surgery and is often manifested as confusion, disorientation, agitation, and hallucinations. Signs and symptoms usually resolve once a patient is transferred out of the ICU but can persist for approximately 2 weeks.16,17
Renal dysfunction. Renal dysfunction can result from hypovolemia, low cardiac output, vasopressor administration, and CPB. These factors can reduce renal blood flow and glomerular filtration rate. Clinical manifestations of acute kidney injury include oliguria and increased serum creatinine and blood urea nitrogen levels. Renal dysfunction can be supported with intravascular volume replacement, diuretics, and dialysis when indicated.15
Infection. Postoperative fever is common after CPB due to systemic inflammatory response syndrome (SIRS) or it can be due to the rewarming process. Rewarming often results in an overshoot in body temperature and has been theorized due to the intraoperative administration of anesthetics and narcotics resulting in resetting the hypothalamic regulatory center.6 As blood is exposed to the surfaces of the CPB circuit, the body's inflammatory response is activated and circulating cytokines are released. Fever that persists for 48 to 72 hours may indicate an infection. The most common source of infection is usually related to the pulmonary system, urinary tract, or sternal/leg wound incisions.18,19 The incidence of sternal wound infection is 1%-5%.18 Risk factors for sternal wound infection include diabetes, hypertension, obesity, chronic obstructive pulmonary disease, and smoking.18
Postoperative prevention of infection includes diligent hand hygiene, frequent wound assessment, blood glucose monitoring and control, and sterile dressing changes.18,19,20
Staphylococcus aureus and coagulase negative staphylococcus are the most common bacteria isolated from deep surgical wound infections and antibiotic therapy is initially targeted to cover these organisms. Prophylactic antibiotics should be administered to prevent surgical site infections. The costs of treating a sternal wound infection can be a devastating financial burden in addition to a significant increase in hospital length of stay, and patient morbidity and mortality.
Pneumonia is reported to be the most common type of infection after cardiac surgery.18 Recent research has shown that most infections will occur approximately in the second week of the postoperative period after discharge from the hospital.18,19,20
Intermediate postsurgical care
Pain management. Pain is subjective and it's multidimensional. Incisional pain must be differentiated from anginal pain as this type of pain may indicate bypass graft occlusion.
Severe pain has been associated with younger age, longer duration of surgery, and surgical incision location. Inadequate pain management is associated with adverse reactions such as atelectasis, hypoxemia, and muscle weakness. In addition, poor pain management can lead to psychological changes that can be manifested as anxiety, feeling of helplessness, insomnia, and poor concentration. Many of these clinical manifestations are signs of depression. Other signs and symptoms of depression include loss of interest in daily activities, persistent sadness, and hypersomnia. It's reported that after CABG approximately 20%-40% of patients exhibit depression.17
Other negative outcomes include a decrease in quality of life as experienced by the patient, and failure to participate in daily activities such as ambulation and pulmonary hygiene measures. Effective pain management relies on frequent, comprehensive pain assessments and interventions to decrease pain. Pharmacologic agents commonly used include opioids such as morphine, hydromorphone, oxycodone, and hydrocodone.21
Pulmonary management. Most postoperative pulmonary complications are related to atelectasis and pneumonia. Other related pulmonary function alterations include hypoxemia, increased work of breathing, and ineffective cough. The incidence of atelectasis has been reported from 16.6%-97.5% after cardiac surgery.22
The mainstay for respiratory care is incentive spirometry, coughing and deep-breathing exercises, and chest incision splinting. The goal of incentive spirometry use is to improve the overall pulmonary function by helping to keep the airways open and the lungs expanded, in addition to assisting in removing secretions.22 Assisting the patient with early and aggressive mobilization and providing adequate pain relief are essential to decrease the incidence of pulmonary complications.23
Patient and family education
After hospital discharge, the patient is often referred to a cardiac rehabilitation program. In general, many cardiac rehabilitation programs involve education on establishing and maintaining heart healthy living, exercise training, and counseling. The emphasis is centered on cardiovascular risk factor identification and modification.
While CABG improves coronary artery blood flow, atherosclerosis occurs in bypass grafts just as in native coronary arteries. Therefore, risk factor modification is vital. Many risk factors are modifiable and form the basis for the healthy lifestyle recommendations. These include smoking cessation, glycemic control, weight loss, physical activity, healthy eating, dyslipidemia treatment, and stress reduction.3,20
CABG continues to be the mainstay for surgical revascularization. However, as technology advances, newer and less-invasive techniques continually evolve. Nursing care plays a pivotal role in the preoperative, intraoperative, and postoperative phases. Lifestyle modification after surgery plays a major role in slowing CAD progression and its effects on the coronary bypass grafts. Long-term outcomes are dependent on healthy lifestyle recommendations.
CPB is the mechanical means to oxygenate and circulate blood and it assumes the function of the heart and lungs and provides a bloodless operative field. Once cannulae are placed, blood is routed through the CPB machine that has many functions. Deoxygenated venous blood is first emptied into the venous reservoir where filters will remove particulate or air emboli, platelet aggregates, or plasma proteins. Blood pumps, whether roller or centrifugal-type, control the blood flow through the circuit during CPB. Blood then goes through a bubble or membrane oxygenator that allows for blood to be oxygenated and carbon dioxide to be removed. A heat exchanger allows blood to be cooled initially and later rewarmed at the end of surgery. Sensors and filters in the machine detect air bubbles, oxygen saturation, and the levels of blood in the collection chambers. Oxygenated blood then moves through an arterial filter to be returned to the ascending aorta.3,6
The main physiologic variables during CPB include:
- Hemodilution to decrease the patient's hematocrit to approximately 20%-25% with the infusion of an isotonic crystalloid solution and to decrease the blood viscosity and tendency of the blood to hemolyze or form microemboli
- Hypothermia that's induced by cooling the patient's blood and body surfaces to decrease myocardial oxygen consumption and protect vital organs from ischemic injury
- Hemodynamic targets aimed at maintaining a mean arterial BP of 70 mm Hg and a cardiac index of 2.1 liters/minute in order to maintain blood flow rates through the CPB machine.3,6,7
A less-invasive technique such as off-pump CABG doesn't employ the use of CPB. This technique, often referred to as “beating heart surgery,” has the main goal of avoiding the morbidity associated with CPB. CPB has been associated with complications such as coagulopathy, blood cell injury, increased capillary permeability, and microembolism.
Patients may also experience a decreased hospital length of stay, quicker recovery time, less renal complications, and improved cardiac function with this technique. In addition, by not cross-clamping the aorta, certain risks such as systemic embolization and stroke are reduced. Patients who are considered high-risk for CABG or who may have contraindications to conventional on-pump CPB, may be candidates for this technique.24
Robotically assisted endoscopic approach
A totally endoscopic coronary artery bypass (TECAB) procedure using robotic assistance was first performed in 1998.6 This procedure has been performed on both the beating heart and arrested heart.25
TECAB is a minimally invasive technique that uses a closed-chest approach to surgery through tiny port incisions. Main advantages associated with this procedure include less surgical trauma as a result of an intact sternum, elimination of CPB, reduced blood loss, and a quicker return to normal daily activities.7,26
TECAB isn't an option for several patients including those who have had previous thoracic surgeries or a recent stroke, are in cardiogenic shock or multisystem organ failure, or have severe pulmonary dysfunction.