CABG Case Study Discussion

Coronary artery disease (CAD) results from the development and progression of atherosclerosis which produces obstruction in the coronary arteries. When myocardial blood supply is inadequate to satisfy myocardial metabolic needs, ischemia occurs and can lead to the development of angina pectoris, ventricular arrhythmias, myocardial infarction, congestive heart failure, or sudden death. A Coronary artery by-pass graft (CABG) is performed to relieve ischemia by providing improved myocardial perfusion beyond areas of coronary stenosis. There are several factors that should be evaluated in any patient being considered for surgery. These include an assessment of the degree of anginal symptoms and the presence of associated medical problems, a determination of the physiologic significance of the CAD, documented action of angiography abnormalities of coronary arteries, and an assessment of left ventricular function. To better understand CABG, other key elements also need to be analyzed. These include indications of CABG, risk factors associated with CABG, and the surgical procedure itself.

Indications for CABG Surgery

CABG surgery is an alternative to drug therapy and other less invasive techniques such as percutaneous transluminal coronary angioplasty (PTCA), atherectomay, and laser therapy. CABG therapy is indicated in patients who have left main coronary artery disease or disease affecting three or more blood vessels. Surgery is also indicated when a patient has at least 60% stenosis of the coronary arteries.

Risk Factors

If the patient has multiple medical problems, such as diabetes, recent stroke, and lung or renal disease, or ejection fraction of less than 20%, they are considered a high-risk patient. Surgery may also be more technically difficult if the patient's coronary arteries are small or if they have diffuse coronary artery disease. The typical CABG patient is generally a high-risk patient, as low-risk patients are frequent candidates for percutaneous cardiac procedures such as angioplasty, atherectomy, and stenting.

Surgical Procedure

A midline chest incision and median sternotomy are performed. Retractors elevate either side of the sternum. If the saphenous vein is to be used for the graft, a segment is removed from the leg prior to the opening of the chest. If the internal mammary arteries (IMA) are to be used, they are harvested from their origin at the subclavian artery and infused with papaverine to relax the spasms that occur. If the gastroepiplitic artery is used, the chest incision is extended to the upper abdomen where the artery is taken from the greater curvature of the stomach.

The pericardium of the heart is then cut and retracted. Heparin is used systemically to increase the clotting time. Saphenous vein grafts (SVG) involve the attachment of a removed portion of the saphenous vein proximal to the aorta and distal to the coronary artery below the obstruction. IMA grafts are left attached to the subclavian artery and are connected distal to the coronary artery below the obstruction. Both the right IMA, and left IMA, may be used as conduits. Although the IMA has proven to have superior patency rates to other conduits, during emergent bypass, the SVG is used because of the extra time required to mobilize the IMA.

During the surgery, the patient is placed on cardiopulmonary bypass (CPB) and the heart is cooled to 15 C to decrease its oxygen needs. CPB is a mechanical means of circulating and oxygenating a patient's blood while diverting most of the circulation from the heart and lungs during cardiac surgical procedures. Systemic hypothermia during bypass can reduce tissue oxygen requirements by 50%. This is an additional protection for major organs from ischemic injury. The body temperature is lowered to about 28 C with a heat exchanger. At the end of surgery, the blood is warmed back up to normal body temperature and the bypass is discontinued.

Once the operation is complete, right atrial and right ventricular pacing wires are placed and used as necessary to achieve a supraventricular rhythm at a rate of 80-100 with a sequential AV conduction. Once the body has reached normal temperature systemically, CPB is weaned over the course of about 30 seconds. Pharmacologic therapy is then used as necessary to maintain satisfactory myocardial performance. Protamine is given to counteract the effects of heparin, and chest tubes are placed in the mediastinum or pleural spaces, and the chest is closed.

Intra-aortic balloon pump (IABP)

If a patient is unable to be weaned from CPB, an intra-aortic balloon pump can be used to decrease afterload and increase myocardial blood flow.

 

IABP support is used until the heart function is improved enough to work on its own. The patient is gradually weaned from the IABP by reducing the pumping rate from 1:1 (augmentation with every beat) to 1:2 (augmentation with every other beat) to 1:4 (augmentation with every fourth beat). Cardiac function is assessed at each stage and IABP is removed if heart function is satisfactory.

Increased FIO2's vs. Increased PEEP

If high FIO2's only cause a small increase in PaO2, shunt is present and a person is said to be refractory to oxygen. Normally, if an increase in FIO2 doesn't cause an increase in PaO2, PEEP is initiated or increased. In CABG patients, low PEEP levels are usually indicated even if the PaO2 is low because even a moderate level of PEEP may decrease venous return. A decrease in venous return will further decrease C.O., which is a problem with most CABG patients initially. Low PaO2's are therefore accepted in this patient population. If the lungs are damaged from high FIO2's, the static compliance would decrease.

Evaluation of Pressures

Central venous pressure (CVP) measures right heart filling pressure or preload for the right ventricle. An increase in CVP indicates either increased circulating blood volume, or increased venous tone or right ventricular failure or a combination of these problems. In the patient with normal cardiac function, the CVP is a reliable tool to monitor the patient response to fluid replacement and diuretic therapy. Pulmonary capillary wedge pressure (PCWP) is a measure of the left heart filling pressure or preload for the left ventricle. An increase in PCWP indicates that the patient either has increased circulating blood volume or left ventricular dysfunction or failure or a combination of these problems. The patient with left ventricular failure will have a decrease in ejection fraction (EF) indicating a reduction in systolic function (contractility) and cardiac output. The patient's PCWP will be abnormally high because of incomplete emptying of the left heart during systole resulting in higher blood volume in the left heart. Pulmonary vascular resistance(PVR) measures pulmonary vascular tone. An elevated PVR indicates pulmonary vascular hypertension and an increased afterload for the right ventricle. Systemic vascular resistance (SVR) measures the vascular tone for the systemic circulation. SVR will fluctuate in response to changes in cardiac output and blood pressure. High SVR increases left heart afterload but it may be elevated so the perfusion can be maintained in the presence of heart failure.