Coronary artery bypass surgery: Investigations and procedures

Coronary artery bypass surgery

Surgery for coronary artery disease is useful in patients with severe symptoms despite medical treatment. It has also been shown to improve outcome in patients with triple vessel disease or left main stem coronary artery disease.

Cardiopulmonary bypass

In order to operate safely in a bloodless, immobile field whilst maintaining an adequate circulation to the rest of the body cardiopulmonary bypass is most commonly used. A cannula is placed in the right atrium in order to divert blood away from the heart. The blood is then oxygenated by one of two methods:

·        Bubble oxygenators work by bubbling 95% oxygen through a column of blood.

·        Membrane oxygenators work by bringing the blood and oxygen together via a gas permeable membrane.

Bubbles are then removed by passing the blood through a sponge. The blood is then heated or cooled as required. A roller pump compresses the tubing driving the blood back into the systemic side of the circulation at an arterial perfusion pressure of between 50 and 100 mmHg. If the myocardium is to be opened, cross-clamping the aorta gives a bloodless field; the heart is protected from ischaemia by cooling to between 20 and 30ËšC. Systemic cooling also lowers metabolic requirements of other organs during surgery. Beating heart bypass grafting is now possible using a mechanical device to stabilise the target surface area of the heart, but access to the posterior surface of the heart can be difficult.

Coronary artery surgery

The internal mammary artery is the graft of choice as 50% of saphenous grafts become occluded within 10 years. The coronary arteries are opened distal to the obstruction and the grafts are placed. If the saphenous vein is used, its proximal end is sewn to the ascending aorta. The surgery takes approximately 1–2 hours. Once the heart is reperfused, it rapidly regains activity. Ventricular fibrillation is deliberately induced during cardiopulmonary bypass to reduce heart movement and avoid additional ischaemia and internal defibrillating paddles are used to restore sinus rhythm.

Complications

Aspirin is usually continued for the procedure, but other antiplatelet drugs such as clopidogrel are stopped up to 5 days in advance. During the procedure patients are heparinised to prevent thrombosis. Antibiotic cover is provided using a broad spectrum antibiotic to prevent bacteraemia. Operative mortality depends on many factors including age and concomitant disease, it usually varies from 1 to 5%. There is a similar, agerelated risk of stroke.

Prognosis

Approximately 90% of patients have no angina postoperatively, with almost all patients experiencing a significant improvement. Over time symptoms may gradually return due to progression of atheroma in the arteries or occlusion of vein grafts. Less than half are symptomfree at 10 years. Outcome is improved by risk factor modification (stopping smoking, lowering high blood pressure, treating hyperlipidaemia and diabetes effectively, etc).

Angioplasty or redo coronary artery surgery may be possible if medication is insufficient to control symptoms; however, repeat surgery has a higher mortality. Angioplasty using stent implantation is suitable for grafts or native vessels.