BACKGROUND
Coronary artery bypass surgery (CABG) has been proven in multiple clinical trials as a safe and effective treatment for coronary artery disease.
1,
2 and
3 Current American College of Cardiology (ACC) and American Heart Association (AHA) guidelines recommend CABG in specific patients with coronary disease (
Table 8.1).
4 Additional indications exist for those patients who present with acute myocardial infarction (MI) (
Table 8.2).
4 Specific clinical scenarios, such as an acute right coronary artery (RCA) infarction, support early percutaneous revascularization to prevent the severe right ventricular (RV) dysfunction that may occur postoperatively.
4,
5Multiple factors affect surgical outcome (
Table 8.3), and various risk stratification scores have been developed to evaluate their impact.
6,
7 and
8 It is important to determine how comorbidities affect patients to adequately discuss risks and benefits prior to surgery. Optimization of renal function and respiratory status, smoking cessation, and glucose control are the methods by which surgical risk can be improved prior to proceeding with revascularization.
SURGICAL TECHNIQUE
CABG is performed under general anesthesia. All patients receive prophylactic preoperative antibiotics within 30 to 60 minutes of incision and this is redosed if the operation exceeds 3 hours. A cephalosporin, usually cefazolin, is used as the firstline agent, with vancomycin being used for patients who are penicillin and/or cephalosporin allergic, or in areas where methicillin-resistant
Staphylococcus species prevalence is high.
9,
10Intraoperative monitoring includes placement of a radial or femoral arterial catheter, a central venous catheter, a Swan-Ganz pulmonary arterial (PA) catheter (often used but may be omitted in uncomplicated low-risk cases), a Foley catheter, a bladder or rectal temperature probe, and a transesophageal echocardiography (TEE) probe. Intraoperative TEE provides assessment of cardiac anatomy and function, qualifies the presence and severity of aortic plaque, assists in placement of intracardiac catheters and vents, aids in the evaluation of intracardiac air or thrombus, and provides vital data regarding valve and cardiac function.
Cardiopulmonary bypass (CPB) is often used for CABG. This is an extracorporeal circuit that completely replaces the function of the heart and lungs, allowing the surgeon to work with a motionless and bloodless heart. In its simplest form, it consists of a venous cannula that drains blood by gravity from the right atrium or vena cava to a venous reservoir. A pump then pushes the blood through a membrane oxygenator (oxygenates the blood and removes carbon dioxide), a filter (removes particulate matter and prevents embolism), and then back into the patient through an arterial cannula into the aorta (common femoral or axillary artery if the aorta is unsuitable). There are additional catheters that allow for delivery of cardioplegia, venting of the heart, and suctioning blood from the operative field. A heat exchanger that allows systemic cooling and rewarming regulates the blood temperature. The CPB circuit is usually primed with balanced crystalloid solution such as lactated Ringers that is about 30% of the patient’s blood volume.
11Prior to cannulation and initiating CPB, the patient is systemically anticoagulated with intravenous heparin (300 to 400 units per kg) to achieve an activated clotting time (ACT) of >400 to 480 seconds. The ACT is checked every 30 minutes and more heparin given as necessary. After institution of CPB and aortic crossclamping, cold cardioplegia solution is given antegrade through the coronary arteries and sometimes retrograde through the coronary sinus to stop the heart and provide myocardial protection. The patient is then cooled to 28°C or up to 32°C to reduce cellular metabolism and offer some organ protection.
11Conduits used for CABG include the left and right internal mammary arteries (IMA), the greater saphenous vein (GSV), and less frequently, the radial artery. The mammary arteries are frequently left in situ with the proximal end left connected to the subclavian artery and the distal end anastomosed to the target coronary. Segments of GSV or radial artery are used as free grafts and anastomosed in an aorta-to-coronary fashion. Patency rates of IMA grafts are usually >90% at 10 years, whereas that of GSV are 70% to 80% at 5 years and 40% to 60% at 10 years. The GSV grafts have a higher rate of formation of neointimal hyperplasia and atherosclerosis that compromise long-term patency.
4 Radial artery grafts have approximately 84% patency at 5 years.
12At the completion of the procedure, patients are rewarmed and weaned off CPB once they are hemodynamically stable with adequate cardiac output (CO). Acid-base, electrolyte, and oxygenation parameters must also be satisfactory. Protamine, 1 mg for every 100 units of heparin given, is administered to reverse anticoagulation and allow for hemostasis (ACT should normalize; 107 ± 13 seconds). Protamine is given slowly as it can activate the complement cascade and lead to hypotension, anaphylaxis, or right heart failure, particularly in patients who have been presensitized from previous administration of protamine or protamine insulin.
11A number of physiologic changes occur because of CPB. Contact of the patient’s blood with the extracorporeal circuit leads to activation of the coagulation, fibrinolytic, complement, and cytokine cascades. This occasionally results in coagulopathy, a systemic inflammatory response, and varying degrees of multiorgan dysfunction. The CPB circulation is relatively hypotensive (usually mean arterial pressure of 40 to 60 mm Hg) and nonpulsatile, and this stimulates sympathetic activation with resultant increased arrhythmias. It can also lead to end-organ hypoperfusion with resultant prerenal azotemia, stroke, or mesenteric ischemia. Required clamps placed on the aorta may put the patient at risk for embolic stroke, particularly in the setting of an atherosclerotic aorta.
11In some cases, CABG can be performed without the use of CPB, termed “off-pump” CABG. Mechanical cardiac stabilizers along with various anesthetic and surgical techniques allow the surgeon to operate on coronary arteries while the heart is beating. Off-pump CABG aims to avoid the possible complications of CABG that are directly because of CPB. Benefits are seen in terms of decreased hospital cost and length of stay. Decreased operative mortality and postoperative morbidity in terms of renal failure, myocardial dysfunction, atrial fibrillation, gastrointestinal complications, pulmonary dysfunction, blood loss, and transfusion requirements have been reported.
13,
14 and
15 Significant benefits for female patients in particular have been documented.
16 Multiple factors are considered in the recommendation of off-pump surgery including the skill and experience of the surgeon, stability of the patient, presence of LV dysfunction, target vessels, and concomitant valve disease.
INITIAL CRITICAL CARE MANAGEMENT
The first phase of critical care management begins at the completion of surgery with the safe and expedient transfer of the patient from the operating room to the cardiac care unit (CCU) (
Table 8.4). The patient should be completely stable and transported with continuous EKG, pulse oximetry, and blood pressure tracings on a portable monitor. All necessary drug infusions to maintain hemodynamic stability are continued through fully charged battery-powered infusions pumps. Additional medications that may be necessary in case of an emergency should also be available. The patient should be adequately hand-ventilated with 100% oxygen.
On arrival to the CCU, the patient is reconnected to the ventilator at appropriate settings. A standard ventilator setting is the use of a volume-cycled mode in which the ventilator is programmed to deliver a set tidal volume and fixed minute ventilation. This can be in the form of assist-control ventilation (A/C) in which every breath, whether ventilator- or patientinitiated, is fully ventilator-delivered. Another common mode is synchronous intermittent mandatory ventilation (SIMV) in
which a preset number of breaths are fully ventilator-delivered, but the patient is also allowed additional spontaneous breaths that can be supported to varying degrees. Ventilator settings should be tailored to suit the patient (
Table 8.5). A pressurecycled mode in which a peak inspiratory pressure is set can be used to avoid barotrauma in a chronically ventilated patient, but this mode can result in varying tidal volumes and minute ventilation based on pulmonary compliance and is not commonly used immediately after cardiac surgery.
17The patient is connected to CCU monitors that follow continuous EKG, pulse oximetry, arterial blood pressure, and central venous pressure (CVP) readings. A PA catheter, if present, can be used to monitor core temperature, PA pressures, left heart filling pressures measurements, mixed-venous oxygen saturation, and CO. Other continuous CO monitors are also available. Hypothermic patients should be actively rewarmed to 37°C using forced-air warming devices. The CCU clinician then checks to ensure that all intravenous lines and tubing are in place (not occluded or kinked), and that all medications are correctly labeled and being infused at the appropriate dosage.
At this point, a full “sign-out” between the surgeon, the anesthesiologist, and the CCU clinician should take place. This should include the patient’s past medical history, intraoperative course, details of the bypass surgery, CPB and aortic crossclamp times, and blood loss and products transfused, and pre- and postoperative cardiac function. A complete physical examination should then be performed. Note should be taken of the adequacy of the CO and end-organ perfusion as evidenced clinically by the following parameters:
Mental status. Although the effects of residual anesthesia are present immediately postoperatively, the patient’s mental status should be assessed as soon as they become conscious. Specific neurologic changes such as lack of spontaneous movement or lack of movement to command should be documented.
Urine output (UOP) measured via an indwelling Foley catheter. At least 0.5 cc/kg/hour indicates satisfactory renal perfusion in the adult patient. Hypothermia or diuretics such as furosemide or mannitol received during CPB can cause the UOP to be relatively high initially.
Skin perfusion and peripheral pulses. Many CABG patients also have concurrent peripheral arterial disease in the lower extremities. Warm skin with good capillary refill and palpable pulses indicate good peripheral perfusion.
All patients receive a postoperative chest X-ray to evaluate for any intrathoracic pathology such as hemothorax, pneumothorax, pulmonary edema, atelectasis, widened mediastinum, or enlarged cardiac silhouette. The X-ray also confirms the correct position of all catheters and tubes including the endotracheal tube (2 to 4 cm above the carina), the central venous catheter (at the junction between the superior vena cava (SVC) and the right atrium), the PA catheter (no more than 2 cm from the hilum), the intraaortic balloon pump (IABP; tip in descending aorta just distal to the left subclavian artery), and the pleural and mediastinal tubes.
18Immediate laboratory studies on admission to the CCU include arterial blood gas, hemoglobin (Hb) and hematocrit (Hct) levels, prothrombin time and international normalized ratio (PT/INR), activated partial thromboplastin time (aPTT), basic metabolic panel, magnesium, and blood glucose. Necessary labs are then repeated every 4 to 6 hours depending on the clinical circumstances. All abnormalities should be corrected with the necessary ventilator adjustment, blood product transfusion, and/or electrolyte repletion. Blood glucose should be strictly controlled within a range of 80 to 110 mg per dl within 24 hours of CCU admission using adequate insulin coverage, as tight glucose control has been associated with decreased rates of sepsis, renal failure, and death.
19A 12-lead EKG is performed on admission to the CCU and each day thereafter to assess for myocardial ischemia and arrhythmia. Continuous telemetric EKG monitoring is mandatory to monitor for the development of arrhythmias. Patients with epicardial pacing wires should have these checked to ensure adequate capture.
Pain control and adequate sedation with intravenous narcotics, benzodiazepines, and/or propofol should be titrated to individual patient needs. Pain and anxiety can stimulate catecholamine release and lead to tachycardia, arrhythmia, hypertension, elevated systemic vascular resistance (SVR), and increased myocardial oxygen consumption.
