Coronary artery bypass surgery

Coronary artery bypass surgery, also coronary artery bypass graft surgery, and colloquially heart bypass or bypass surgery is a surgical procedure performed to relieve angina and reduce the risk of death from coronary artery disease. Arteries or veins from elsewhere in the patient's body are grafted to the coronary arteries to bypass atherosclerotic narrowings and improve the blood supply to the coronary circulation supplying the myocardium (heart muscle). This surgery is usually performed with the heart stopped, necessitating the usage of cardiopulmonary bypass; techniques are available to perform CABG on a beating heart.

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History

The technique was pioneered by Argentine cardiac surgeon René Favaloro at the Cleveland Clinic in the late 1960s.^[1]

Currently, about 500,000 CABGs are performed in the United States each year.

Terminology

There are many variations on terminology, in which one or more of 'artery', 'bypass' or 'graft' is left out. The most frequently used acronym for this type of surgery is CABG (pronounced 'cabbage'),^[2] pluralized as CABGs (pronounced 'cabbages'). More recently the term aortocoronary bypass (ACB) has come into popular use. CAGS (Coronary Artery Graft Surgery, pronounced phonetically) has been used (primarily outside the United States) and should not be confused with Coronary Angiography (CAG).

Number of bypasses

The terms single bypass, double bypass, triple bypass, quadruple bypass and quintuple bypass refer to the number of coronary arteries bypassed in the procedure. In other words, a double bypass means two coronary arteries are bypassed (e.g. the left anterior descending (LAD) coronary artery and right coronary artery (RCA)); a triple bypass means three vessels are bypassed (e.g. LAD, RCA, left circumflex artery (LCX)); a quadruple bypass means four vessels are bypassed (e.g. LAD, RCA, LCX, first diagonal artery of the LAD) while quintuple means five. Less commonly more than four coronary arteries may be bypassed.

A greater number of bypasses does not imply a person is "sicker," nor does a lesser number imply a person is "healthier."^[3] A person with a large amount of coronary artery disease (CAD) may receive fewer bypass grafts due to the lack of suitable "target" vessels. A coronary artery may be unsuitable for bypass grafting it if it is small (< 1 mm or < 1.5 mm depending on surgeon preference), heavily calcified (meaning the artery does not have a section free of CAD) or intramyocardial (the coronary artery is located within the heart muscle rather than on the surface of the heart). Similarly, a person with a single stenosis ("narrowing") of the left main coronary artery requires only two bypasses (to the LAD and the LCX). However, a left main lesion places a person at the highest risk for death from a cardiac cause.^{[citation needed]}

The surgeon reviews the coronary angiogram prior to surgery and identifies the lesions (or "blockages") in the coronary arteries. The surgeon will estimate of the number of bypass grafts prior to surgery, but the final decision is made in the operating room upon examination of the heart.

Indications for CABG

Several alternative treatments for coronary artery disease exist. They include:

• Medical management (statins, antihypertensives, smoking cessation, tight blood sugar control in diabetics)
• Percutaneous coronary intervention (PCI)

Both PCI and CABG are more effective than medical management at relieving symptoms,^[4] (e.g. angina, dyspnea, fatigue), but repeat procedures are required more frequently after PCI.^[4]

CABG is the preferred treatment with:^[4]

• Disease of the left main coronary artery (LMCA). LMCA disease is associated with sudden death; therefore, lesions of the LMCA are sometimes referred to as widow makers.
• Disease of all three coronary vessels (LAD, LCX and RCA).
• Diffuse disease not amendable to treatment with a PCI.

CABG is the likely the preferred treatment with other high-risk patients such as those with severe ventricular dysfunction (i.e. low ejection fraction), or diabetes mellitus.^[4]

Prognosis

Prognosis following CABG depends on a variety of factors, but successful grafts typically last around 10-15 years. In general, CABG improves the chances of survival of patients who are at high risk (meaning those presenting with angina pain shown to be due to ischemic heart disease), but statistically after about 5 years the difference in survival rate between those who have had surgery and those treated by drug therapy diminishes. Age at the time of CABG is critical to the prognosis, younger patients with no complicating diseases have a high probability of greater longevity. The older patient can usually be expected to suffer further blockage of the coronary arteries.

Procedure (Simplified)

1. The patient is brought to the operating room and moved onto the operating table.
2. An anesthetist places a variety of intravenous lines and injects an induction agent (usually propofol) to render the person unconscious.
3. An endotracheal tube is inserted and secured by the anesthetist or assistant (e.g. respiratory therapist or nurse anesthetist) and mechanical ventilation is started.
4. The chest is opened via a median sternotomy and the heart is examined by the surgeon.
5. The bypass grafts are harvested - frequent conduits are the internal thoracic arteries, radial arteries and saphenous veins. When harvesting is done, the patient is given heparin to prevent the blood from clotting.
6. In the case of "off-pump" surgery, the surgeon places devices to stabilize the heart.
7. If the case is "on-pump", the surgeon sutures cannulae into the heart and instructs the perfusionist to start cardiopulmonary bypass (CPB). Once CPB is established, the surgeon places the aortic cross-clamp across the aorta and instructs the perfusionist to deliver cardioplegia to stop the heart.
8. One end of each graft is sewn onto the coronary arteries beyond the blockages and the other end is attached to the aorta.
9. The heart is restarted; or in "off-pump" surgery, the stabilizing devices are removed. In some cases, the Aorta is partially occluded by a C shaped clamp, the heart is restarted and suturing of the grafts to the aorta is done in this partially occluded section of the aorta while the heart is beating.
10. Protamine is given to reverse the effects of heparin.
11. The sternum is wired together and the incisions are sutured closed.
12. The person is moved to the intensive care unit (ICU) to recover. After awakening and stabilizing in the ICU (approximately 1 day), the person is transferred to the cardiac surgery ward until ready to go home (approximately 4 days).

Minimally Invasive CABG

Alternate methods of minimally invasive coronary artery bypass surgery have been developed in recent times. Off-pump coronary artery bypass surgery (OPCAB) is a technique of performing bypass surgery without the use of cardiopulmonary bypass (the heart-lung machine). Further refinements to OPCAB have resulted in Minimally invasive direct coronary artery bypass surgery (MIDCAB), a technique of performing bypass surgery through a 5 to 10 cm incision.

Conduits used for bypass

The choice of conduits is highly surgeon and institution dependent. Typically, the left internal thoracic artery (LITA) (previously referred to as left internal mammary artery or LIMA) is grafted to the Left Anterior Descending artery and a combination of other arteries and veins is used for other coronary arteries. The right internal thoracic artery (RITA), the great saphenous vein from the leg and the radial artery from the forearm are frequently used. The right gastroepiploic artery from the stomach is infrequently used given the difficult mobilization from the abdomen.

Graft patency

Grafts can become diseased and may occlude in the months to years after bypass surgery is performed. Patency is a term used to describe the chance that a graft remain open. A graft is considered patent if there is flow through the graft without any significant (>70% diameter) stenosis in the graft.

Graft patency is dependent on a number of factors, including the type of graft used (internal thoracic artery, radial artery, or great saphenous vein), the size or the coronary artery that the graft is anastomosed with, and, of course, the skill of the surgeon(s) performing the procedure. Arterial grafts (e.g. LITA, radial) are far more sensitive to rough handling than the saphenous veins and may go into spasm if handled improperly.

Generally the best patency rates are achieved with the in-situ (the proximal end is left connected to the subclavian artery) left internal thoracic artery with the distal end being anastomosed with the coronary artery (typically the left anterior descending artery or a diagonal branch artery). Lesser patency rates can be expected with radial artery grafts and "free" internal thoracic artery grafts (where the proximal end of the thoracic artery is excised from its origin from the subclavian artery and re-anastomosed with the ascending aorta). Saphenous vein grafts have worse patency rates, but are more available, as the patients can have multiple segments of the saphenous vein used to bypass different arteries.

Veins that are used either have their valves removed or are turned around so that the valves in them do not occlude blood flow in the graft. LITA grafts are longer-lasting than vein grafts, both because the artery is more robust than a vein and because, being already connected to the arterial tree, the LITA need only be grafted at one end. The LITA is usually grafted to the left anterior descending coronary artery (LAD) because of its superior long-term patency when compared to saphenous vein grafts.^[5][6]

Complications

People undergoing coronary artery bypass are at risk for the same complications as any surgery, plus some risks more common with or unique to CABG.

CABG associated

• Postperfusion syndrome (pumphead), a transient neurocognitive impairment associated with cardiopulmonary bypass. Some research shows the incidence is initially decreased by Off-pump coronary artery bypass, but with no difference beyond three months after surgery. A neurocognitive decline over time has been demonstrated in people with coronary artery disease regardless of treatment (OPCAB, conventional CABG or medical management).
• Nonunion of the sternum; internal thoracic artery harvesting devascularizes the sternum increasing risk.
• Myocardial infarction due to hypoperfusion, early graft occlusion, or graft failure.
• Late graft stenosis, particularly of saphenous vein grafts due to atherosclerosis causing recurrent angina or myocardial infarction.
• Acute renal failure due to hypoperfusion.
• Stroke, secondary to aortic manipulation or hypoperfusion.

General surgical

• Infection at incision sites or sepsis.
• Deep vein thrombosis (DVT)
• Anesthetic complications such as malignant hyperthermia.
• Keloid scarring
• Chronic pain at incision sites
• Chronic stress related illnesses
• Death

See also

• Angioplasty
• Cardiothoracic surgery
• Dressler's syndrome
• Hybrid bypass

References