Internal Mammary Artery Bypass

Anatomy and Physiology

The internal mammary artery (IMA), also called the internal thoracic artery, supplies the anterior chest wall and the breast. Although the IMA usually has a separate origin from the subclavian artery, it may have a common origin with the thyrocervical trunk. After arising from the subclavian artery, it runs anterior to the pleural cupola and crosses the brachiocephalic vein underneath. It then runs posterior to the sternoclavicular joint and costal cartilages and anterior to the parietal pleura. At the third intercostal space (ICS), it enters the space between the transversus thoracis and the intercostal muscles and gives off branches to intercostal spaces. When it reaches the seventh costal cartilage, IMA divides into the superior epigastric artery and the musculophrenic artery.[8]

Histologically, the IMA is a muscular artery that makes it susceptible to spasm when handled surgically. However, because its media is thinner and less muscular than other arteries and veins, the IMA produces a higher basal and stimulated rate of nitric oxide (vasodilator) and exhibits a lower propensity for spasm. The IMA is also found to be less prone to atherosclerosis as compared to venous grafts. This could be a result of the non-fenestrated internal elastic lamina of the IMA, which is less susceptible to intimal hyperplasia, which contributes to the development of atherosclerosis.

The diameter of the IMA is relatively smaller as compared to venous grafts, and the mean flow velocity in IMA is about three times that in saphenous venous grafts. High flow velocities, low thrombotic threshold velocity, resistance to the development of atherosclerosis, the release of vascular endothelial growth factor, and reactivity to vasodilators are some of the factors responsible for the long term patency of IMA bypass.[9][10][7] That is why the use of IMA to revascularize the left anterior descending artery has a significant impact on repeat revascularization and significantly reduces the chances of re-do coronary artery bypass.[2]

Indications

A review of indications for coronary artery bypass graft (CABG) surgery is beyond the scope of this article. We present guideline recommendations for the use of the internal mammary artery (IMA) as the graft of choice for CABG.

According to the American College of Cardiology/American Heart Association guidelines, indications for CABG using the IMA include the following[11]

• Grafting the left anterior descending artery with left IMA (class 1, level of evidence B)
• Grafting of the left anterior descending artery with right IMA when left IMA is unavailable or unsuitable (class 2a, level of evidence B)
• When anatomically and clinically suitable, use of a second IMA to graft the left circumflex or right coronary artery to improve the likelihood of survival and to decrease reintervention (class 2a, level of evidence B)
• IMA graft to the right coronary artery when more than 90% stenosis (class 2b)

Contraindications

Contraindications for coronary artery bypass graft (CABG) using internal mammary artery (IMA) grafts include the following

• IMA graft not suitable
• Elderly patient (more than 85 years old) 
• Morbidly obese
• Hemodynamic instability 
• Severe pulmonary disease 
• Previous thoracic surgeries (makes it difficult to harvest IMA)
• Advanced malignancy or chronic disease with life expectancy less than six months

Most of these listed contraindications are general contraindications for CABG.

Equipment

Types of equipment for coronary artery bypass graft (CABG) using internal mammary artery (IMA) graft include

• Operating room
• Sterile drapes, gown, gloves
• Electrocautery
• Sternotomy saw
• Coronary scissors
• Needle holders
• Dissecting scissors
• Micro teeth forceps
• Straight and Ring tip forceps
• Clamps
• Suction
• Occlusion tips
• Papaverine hydrochloride
• Vessel dilators and probes
• Graftmarkers
• Silk ligatures
• Harmonic scalpel
• Vessel loops and punches sutures

Personnel

Coronary artery bypass graft (CABG) is a complicated and lengthy procedure. A whole team is required to undergo this surgery. Following personnel form the team who will take care of the patient before during and after the procedure.

• Primary care provider
• Intervention cardiologist
• Structural heart disease specialist
• Cardiothoracic surgeon (specialized in CABG)
• Anesthesiologist 
• Surgical  assistant
• Nurse assistant
• Surgical technician
• Cardiac rehabilitation specialist
• Nutritionist
• Pharmacist

Preparation

Operative risk assessment is an integral part of patient preparation. Commonly used risk models to predict perioperative morbidity and mortality include the Euro score system and the Society of Thoracic Surgeons (STS) Risk Model.[12] Variables commonly required for these models include age, previous myocardial infarction, peripheral vascular disease, renal failure, hemodynamic state, and ejection fraction.

The STS model also requires surgical acuity, re-operative status, shock, and chronic lung disease. Aspirin should be continued, especially in those who present with acute coronary syndrome (ACS). For those patients who take P2Y12 inhibitors (e.g., clopidogrel, prasugrel, ticagrelor) and undergoing elective CABG, these drugs should be stopped for either five days (for clopidogrel) or seven days (for prasugrel) before the procedure. Beta-adrenergic blockers should be continued in the perioperative period unless contraindications exist. Recently, the use of intravenous P2Y12 inhibitor (cangrelor) allowed for earlier CABG operations as cangrelor's antiplatelet effects are very short-acting, and the intravenous infusion can be stopped before CABG.

Each patient should have recommended units of blood, fresh frozen plasma, and platelets cross-matched before the procedure. On the day of the procedure, the patient should have fasting of at least six hours, and intravenous antibiotics should be administered before CABG.

Technique or Treatment

For a standard median sternotomy, the anterior chest is exposed with the patient in a supine position. A roll is placed in the interscapular region to provide better access to the sternum. After general anesthesia induction and intubation, the median sternotomy is performed. The IMA is harvested either directly or as a pedicle. Left IMA is commonly harvested as a pedicle, whereas the right IMA is generally skeletonized because a right IMA pedicle usually interferes with sternal wound healing. To harvest IMA, endothoracic fascia is incised first by using electrocautery. Once the fascia is pulled down with the help of forceps, under-surface of the internal mammary vein is exposed till IMA becomes visible. The cautery tip is used to separate the vein and IMA. Then the trunk of IMA is separated from the chest wall, which allows us to expose and cut the perforating branches. Then IMA is pulled down using curved DeBakey forceps, and surrounding tissue is separated from the IMA. Mediastinal branches are divided, and the skeletonized extra-pleural IMA is then harvested from its origin from the subclavian artery.[13] The fascia is peeled off the ITA while the internal thoracic veins are left in situ to preserve a certain degree of vascularisation of the sternum.

After harvesting the conduit, the patient is put on cardiopulmonary bypass (in on-pump CABG) by cannulating the aorta and right atrium. Aorta is cannulated at the soft and nonatherosclerotic area after giving unfractionated heparin and lowering systolic blood pressure below 100mmHg. Once the aortic cannula is in place, it is made air-free and connected to the arterial pump tubing. Then venous cannula is inserted into the right atrial appendage. Aorta is clamped distal to the cannula, and cardioplegia is infused via the aortic cannula.

After starting the cardiopulmonary bypass, the distal coronary bypass targets on the disease native coronary arteries are identified. The left IMA is usually anastomosed to the left anterior descending artery (ramus intermedius, diagonal branches), and right IMA is anastomosed to either left circumflex or right coronary artery (also ramus intermedius, obtuse marginal, posterior descending and right posterolateral). An incision is given in the distal coronary artery, and then the conduit ostium is sutured around the full circumference of the anastomosis using a 7-0 or 8-0 polypropylene suture. Grafting is done sequentially. Hemostasis is confirmed before the release of the aortic clamp. The chest wound is closed, and a drain is usually placed. The patient is monitored in the immediate postoperative period in the cardiothoracic intensive care unit.

Complications

Complications during and after the coronary artery bypass grafting (CABG) in general include

• Perioperative hemorrhage
• Perioperative myocardial infarction
• Renal dysfunction
• Arrhythmias
• Cerebrovascular accidents (CVA)
• Cardiac tamponade
• Respiratory tract infection
• Sternotomy wound infection
• Graft failure. 

Cerebrovascular accidents are reported in about 1% of patients having IMA bypass graft and mainly determined by underlying risk factors, including diabetes mellitus, advanced age, prior CVA, aortic atherosclerosis, and peripheral arterial disease.[14]

Although the sternal wound infection rate has significantly reduced, however, it is relatively more common in patients with obesity, diabetes mellitus, and advanced age.[15]

The most common perioperative arrhythmia is atrial fibrillation, which can be managed with beta-blockers, amiodarone, and anticoagulation. It is associated with embolic stroke and increases perioperative mortality.

Clinical Significance

Robust data and literature suggest that the internal mammary artery (IMA) undergoes minimal atherosclerotic changes and has excellent long term outcomes and patient survival compared to superior vena cava graft (SVG). Its survival and patency rates are much better as compared to venous and even other arterial grafts. Hence the IMA graft is considered to be the first choice conduit and the gold standard for coronary artery bypass graft.

Enhancing Healthcare Team Outcomes

The coronary artery bypass graft (CABG) is a complicated surgical procedure that requires operative skills and expertise as well as good perioperative care. Therefore it is necessary to have a system to ensure the completion of all aspects of patient care. In pre-operative care, in addition to discussing the risks and benefits of the procedure with the patient and family, the assessment must be done to ensure that patient meets the recommended criteria for the procedure. Proper sterile technique is fundamental to better outcomes.

In the postoperative period, patients need regular monitoring by trained nursing staff for complications. After recovery, every patient should undergo cardiac rehabilitation. It is now recommended to have a multidisciplinary team approach that may include a primary care provider, intervention cardiologist, cardiac surgeon, cardiac rehabilitation specialist, a cardiac nurse, and a cardiac pharmacist to enhance patient care, optimize procedural success and minimize perioperative complications.