Coronary bypass surgery

Author : Dr Kendrick Shunk Interventional Cardiologist University of California San Francisco (UCSF) School of Medicine

2008-07-28
Coronary Bypass Surgery : aka Coronary artery bypass grafting (CABG)

If you think you may be having a heart attack, dial 911 immediately.  Here are warning signs: American Heart Association

Coronary artery bypass grafting (CABG) is the most commonly performed “open heart” surgery in the US and remains a cornerstone of therapy for certain patients with coronary artery disease. Despite a modest decline in the number of CABG (pronounced “cabbage”) surgeries performed in recent years, it is estimated that nearly 500,000 are still performed annually in the US.  The reasons for the recent decline are multi-factorial and do not detract from the value of this procedure for certain patients. The diagnosis of coronary artery disease (CAD) and an overview of CAD treatment options are discussed elsewhere.  This knol focuses on coronary artery bypass surgery including the rationale behind it and how cardiologists go about deciding when to recommend it.  Certain technical details about how it is performed are also discussed with links to other web resources for additional reading.

Introduction and Rationale:

Understanding the rationale behind coronary artery bypass grafting (CABG) requires some level of understanding of coronary artery circulation, atherosclerosis, and medical and catheter-based therapies that are often the foundation of a multifaceted treatment plan for coronary artery.  In brief, although the heart is responsible for, among other things, pumping a supply of oxygenated blood to the entire body, the heart muscle itself still requires its own blood supply, and this supply comes from 3 arteries that course over the surface of the heart muscle (myocardium).  These are known as coronary arteries.  These coronary arteries can become narrowed or blocked (stenosed), most commonly by the effects of cholesterol accumulating in and around the cells lining the artery wall (atherosclerosis).  Depending upon a variety of factors including whether the narrowing occurs suddenly or gradually, the location and severity of the narrowing(s) and whether there are numerous or critically-located narrowings, a strategy of re-routing oxygenated blood around the narrowing may represent the safest and most effective means of managing the problem. 

As a rule, when physicians and patients think about the alternatives for treatment, there should always be consideration of the risk/benefit ratio for each and every alternative as well as a clear understanding that there are only two reasons to proceed with any therapy:
1)    If it can be expected to extend life, and/or
2)    If it can be expected to improve the quality of life.

In the case of coronary artery bypass surgery, there is evidence from clinical trials that in appropriately selected patients, it has the potential to do both.  CABG works primarily by restoring adequate flow of oxygenated blood to areas of the heart that were previously deprived.

“Lifestyle” Diet, Exercise, Tobacco

It should go without saying that CABG as a treatment for coronary artery disease is definitely not an alternative to proper diet, appropriate exercise, quitting smoking, or taking prescribed medications.  Instead it must be viewed as something that is sometimes needed in addition to all of these things. 

The American Heart Association offers numerous recommendations aimed at coronary heart disease patients including recommendations for Diet and lifestyle http://www.americanheart.org/presenter.jhtml?identifier=851 as well as exercise http://circ.ahajournals.org/cgi/reprint/CIRCULATIONAHA.107.185649.  Specific exercise recommendations should be tailored to your specific situation by your physician, but in general, the goal should be aerobic exercise of a moderate intensity for ~30 minutes at least 5 times a week or of high intensity for >20 minutes at least 3 times a week.  Dietary goals should emphasize no more than appropriate total caloric intake with lower saturated fat and dietary cholesterol content, trans-fat intake of <1% of total calories, and for most people an increase in non-processed foods such as fresh fruits and vegetables and whole grains.

Tobacco use is a major contributor to CAD progression, increases the risk of CABG surgery, and if continued, decreases the lifespan of bypass grafts.  Tobacco cessation (http://americanheart.org/presenter.jhtml?identifier=498 ) is of critical importance for anyone undergoing CABG.

Anyone undergoing CABG must have aggressive control of cholesterol levels in the blood.  In addition to diet and exercise, this will almost always include the use of statins with a goal LDL of <70 mg/dl.  A more detailed set of guidelines relating to cholesterol management is available (http://www.nhlbi.nih.gov/guidelines/cholesterol/atp3xsum.pdf) including discussion of alternatives to statins.  

More information about medical therapy for coronary heart disease, which applies to patients who have undergone CABG, can be found here (Link to CAD Tx)

Key Decisions:

Certain features will strongly influence whether a cardiologist will recommend CABG surgery.  In turn, certain features affect both the willingness of a cardiothoracic surgeon to perform the procedure and his/her specific plan for the operation.

After appropriate testing, including cardiac catheterization with coronary angiography, the cardiologist will typically review the entirety of the data for a given patient in order to determine whether CABG is appropriate.  Features likely to influence the decision toward CABG include:

1)    Involvement of a particularly critical location called the left main (a.k.a. the “widowmaker”). 
2)    Involvement of all 3 major coronary arteries.
3)    Presence of impaired heart pumping function (left ventricular ejection fraction <50%).
4)    Presence of diabetes.
5)    Certain situations where a stenting option is likely to be suboptimal or impossible.

Once the CABG referral is made to a cardiothoracic surgeon, certain additional features will be evaluated to determine the suitability for CABG, and occasionally result in a request to reconsider medical or catheter based therapies or an outright refusal to operate.  To help in this decision, the cardiologist and/or surgeon will use clinical judgment and may take advantage of a risk calculator such as the STS risk calculator http://66.89.112.110/STSWebRiskCalc/ or the logistic Euroscore http://www.euroscore.org/calc.html.  The opinion of many cardiothoracic surgeons is that the STS calculator does not capture all of the features that contribute to risk and may lead to underestimation, while the logistic Euroscore may lead to overestimation of risk of CABG.  In the end, the decision of whether to offer an operation falls to the cardiothoracic surgeon who must rely on experience.  Some of the features which influence the risk scores and the decision to offer an operation include:

1)    Age
2)    Gender
3)    Heart muscle function (left ventricular ejection fraction)
4)    Kidney function
5)    Lung function
6)    Brain function
7)    Need for concomitant procedure such as valve replacement
8)    Prior heart surgery
9)    Smoking status
10)    Emergency status

The risk calculators are by no means perfect and other considerations not included in the calculators often play into the decision.

Once the decision is made to proceed with CABG, the surgeon will make additional decisions regarding the strategy.  These strategic decisions include:

1)    Which branches to bypass and how many bypasses are needed.
2)    What type of bypass(es) to use.
3)    Whether to stop the heart and support the circulatory function with a cardiopulmonary bypass circuit (heart-lung machine) while the bypasses are placed (a.k.a. an “on-pump” surgery) or whether to perform the procedure on a beating heart (“off pump”).  There is some concern that longer times spent “on-pump” may be associated with cognitive decline, memory dysfunction, etc, however this has not been demonstrated to represent a true cause-and-effect relationship.  This decision will depend on the experience of the surgeon, the location and quality of the sewing targets, the pumping function of the heart, and other factors.
4)    Whether to use a standard incision or a minimally invasive incision.

Once a specific CABG plan is created and the patient provides informed consent, the operation can proceed.  Although the final decisions about each of these strategic points is ultimately made “on the table” based on what is found during the operation, it is important to have an up-front plan.

The Surgical Procedure:

The actual CABG procedure is performed by a team headed by the cardiothoracic surgeon.  The Society of Thoracic Surgeons (STS) website is a rich resource for additional details about CABG surgery: http://www.sts.org/sections/patientinformation/adultcardiacsurgery/cabg/index.html.  There are numerous variations on the procedure but in brief, a typical CABG procedure involves the following steps:

1)    The patient is placed under general anesthesia. 
2)    An incision is made in the skin down the center of the chest and the breastbone is divided lengthwise to allow the surgeon to be able to access the heart and surrounding structures.
3)    An incision is made in the leg(s) and a suitable length of vein is removed (typically the saphenous vein) to be used as a bypass conduit.  This is sometimes done by an assistant at the same time that the main surgeon proceeds with the operation in the chest.
4)    If the internal mammary artery is to be used as a bypass conduit, which is common practice and has demonstrated advantages, some time is taken to separate it from its natural location along the inside of the ribcage so that it can reach to the heart.
5)    If the procedure is to be performed “off pump”, a stabilization device is secured to the heart to minimize motion as much as possible while the surgeon prepares to sew the bypass conduit(s) into place.  If the procedure is to be performed “on pump”, then tubes (cannulae) are inserted into various vessels and heart chambers and fed to the cardiopulmonary bypass machine as the heart is cooled with ice and treated with a protective cocktail known as cardioplegia to prevent damage while it is stopped.
6)    The blood is thinned (usually with intravenous heparin) to minimize clots during this portion of the surgery.
7)    The bypasses are sewn into place and carefully evacuated of any air.
8)    The patient is taken “off pump” and the heart rewarmed and re-started if necessary.
9)    The heparin blood thinner is partially or completely neutralized or reversed (with protamine)
10)    Chest tubes are placed which will serve to drain any blood or other fluid that may accumulate around the heart in the early time period after the operation.  There are typically two of these.  They protrude through the skin just below the rib cage through their own small separate incisions.
11)    Temporary pacemaker wires are attached to the surface of the heart and fed through the skin.
12)    The breastbone is sewn back together with strong steel wire and the skin incision is sewn or stapled.

Recovery and rehabilitation after CABG:

The patterns of recovery varies some from person to person.  In some areas, a formal cardiac rehab program may be available (http://www.sts.org/sections/patientinformation/adultcardiacsurgery/heartsurgery/index.html ).  Either way, a fairly typical recovery pattern would include:

1)    Waking up from the surgery in the intensive care unit (ICU), where you would typically spend about 1 day.
2)    Rehab begins right away, with breathing exercises to strengthen the lungs and prevent pneumonia, and physical therapy to begin sitting, standing and walking, usually before even leaving the ICU.
3)    The chest tubes are usually removed at this stage.
4)    A period of several days in a “step-down” unit follow, with ever increasing physical activity.
5)    Prior to discharge home, the temporary pacemaker wires are removed.
6)    Unless there are complications that require a longer stay, it is typical to leave the hospital somewhere between day 4 and 7.
7)    The skin staples are often removed at the first office visit, on about day 10.
8)    Someone will be needed to assist with activities of daily living for the first few weeks.
9)    By approximately 6 weeks, patients often report feeling about as well as before the surgery and should be able to return to most forms of work, part-time at first.
10)    By 3 months, typically patients feel better that before the surgery and are able to resume full vigorous activities.

Additional resources:

Working in conjunction, the American College of Cardiology and the American Heart Association (AHA), issue numerous relevant guidelines that pertain to CAD.  In many cases, other societies have signed on as well. 

The STS is the professional society for cardiothoracic surgeons.  Their website has resources for physicians and patients as well.

www.americanheart.org
www.acc.org
http://www.sts.org
http://www.euroscore.org/