TSRA Content:
Author: Georgia R. Layton, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by Timothy Pirolli, MD, and Sanford Zeigler, MD.
How to Take Down the LIMA
After sternotomy, the internal mammary artery (IMA) is harvested prior to proceeding with coronary artery bypass grafting. The mammary artery is the second branch of the first part of the subclavian artery and descends inferiorly, in a straight plane superficial to the endo-thoracic fascia, approximately 0.5-1cm parallel to the lateral sternal borders. Towards the inferior border of the mammary (which is the sixth intercostal space) it runs deep to the transversus thoracic muscle. Around this point it bifurcates into the superior epigastric and musculo-phrenic arteries. The aim is to harvest the IMA proximally to the first intercostal artery branch and distally just beyond its bifurcation to ensure maximal conduit length is achieved. Both the left and right mammary may be harvested by similar techniques.
The mammary may be harvested as a vascular pedicle (with a surrounding cushion of soft tissue of fat and fascia) or as a skeletonized vessel. Proponents of the pedicle approach cite potential for greater safety and avoidance of iatrogenic injury during harvest while proponents of skeletonization cite that technique’s superior control of side branches, provision of additional length to the vessel and reduction of sternal wound infection by maintaining venous supply to the chest wall. As well as pedicled harvest, the IMA may also be taken as free vessel and used for grafting proximally to the ascending aorta. This is rarely performed for the LIMA, but may be considered if using the RIMA, especially for an-aortic or composite total arterial grafting approaches.
Setting up to harvest the IMA
Technique will vary by surgeon. IMA is harvested after sternotomy and is accessed either through division of the pleura whilst the pericardium remains closed or through division of the pericardium after formation of a pericardial well.
Following sternotomy, the sternum is partially opened using a preferred retractor to elevate the ipsilateral side of harvest. The two most common retractors used are the Rultract Skyhook Retractor (Cleveland, OH, USA) and the Delacroix-Chevalier IMA Couëtil Sternal Retractor (Paris, FR).
Pericardial Approach
The heart is exposed by resecting the covering layers of thymus, mediastinal fat, vessels and pericardium. To open the pericardium, use the bovie to dissect the mediastinal fat. The thymus may be divided by the bovie. Often, there will be multiple medium-size veins (branches of the innominate vein) which need to be controlled with ligaclips or cautery.
Once those layers are divided, the underlying pericardium will be exposed. Sticking within the midline, the pericardium may be elevated off the heart and cautery may be used to open it. Continue elevating the pericardium using pick-ups and protect the heart surface with your finger or the sucker and open it inferiorly down to the diaphragm. Use the same technique to follow the diaphragm to the right and left to open the pericardium in an inverted "T" fashion. Then, elevate the pericardium in the other direction, remaining in the midline, over the medial border of the aorta, up to the level of the pericardial reflection and the innominate vein.
Pleural Approach
Use a small swab to gently retract the mediastinal fat downward, away from the sternal edge. The lung edge is usually quite obvious. Open the pleura with cautery in one small area around the sixth intercostal space approximately 1cm beneath the underside of the chest wall. Breaching the pleura will result in an iatrogenic pneumothorax which will result in the lung collapsing down away from the pleura. Protect the lung by gently retracting it down away from your field and open the pleura in parallel to the sternum along the length. This includes dividing the transversus thoracis muscle and the posterior rectus sheath inferiorly to the xiphisternum. Adequately dividing the muscles inferiorly is essential to facilitate good exposure by retraction. The best exposure requires entrance into the pleural space, and this allows the vessel to fall away from the midline following bypass grafting and therefore allows for safer re-operation.
Rule:
Use caution when dividing the pleura cephalad beyond the fourth rib as the mammary may often be sitting quite superficially just underneath it and may also move medially its course toward the subclavian.
At this point following either technique, the retractor may be opened further to begin to elevate the left hemi-sternum and ribs whilst the operating surgeon sits on a stool to bring their head level with the sternal edge. It may be helpful to rotate the table slightly away from the operating surgeon to give a better view of the underside of the left chest wall. If the pleura is opened, two medium swabs may be temporarily placed in the left hemi-thorax to gently retract the lung down, away from the operating field.
Harvesting the IMA
The IMA is accompanied by two mammary veins; one immediately medial and one lateral to it. The medial is usually the more prominent. When looking at the underside of the chest wall, this vein is often seen very clearly. The location of the mammary artery may be confirmed by observing a pulsating vessel or very gently palpating for pulsation just lateral to the vein with your thumb. Cautery power should be reduced to around 50% of normal to minimize the risk of electrosurgical damage to the vessel. Divide the endo-thoracic fascia in parallel with and approximately half a centimeter medial to the medial mammary vein. It is often easiest to start this in the middle of the mammary around the third rib and extend it inferiorly to the xiphoid process and superiorly. The lateral border of the mammary also has to be divided to complete a pedicled harvest. Some may do this at the beginning, prior to separation of the mammary from the chest wall. Some may choose to do this at the end.
To start a pedicled harvest, in the mid-portion of the mammary (approximately 3rd
rib), begin on the underside of a rib where there are no side-branches of the vessels. Retract the free edge of the fascia downwards to expose the vein. Using very gentle blunt dissection, the tip of the cautery may be used to identify branches of the vein in front and mammary behind and to gradually separate the trunk of the mammary vessel from the under-side of the chest wall above it. Once you have traveled along the rib and freed the mammary, continue cephalad or caudal and begin to divide the branches in the respective intercostal spaces. It should be dissected free from the chest wall up to the first intercostal artery, the most cephalad point of dissection.
Exposed branches of both the vein and mammary may be ligated with clips, cautery, a harmonic scalpel or a combination of these. Ligation should always occur away from the mammary itself wherever possible; this may require dissection up into the intercostal spaces to an extent.
As the vein and mammary become free in their mid-portion, the superior and inferior course of the mammary artery becomes clearer. Throughout the dissection, do not grasp the mammary artery directly and similarly, avoid excessive traction which may cause intimal injury, occlusion, or inadvertent transection. The fascia or vein may be directly grasped to aid exposure.
Rule:
Never grab the IMA directly and do not cauterize the vessel; this may cause spasm or haematoma and compromise the quality of flow in the vessel. Always cauterize and clip far away from the vessel. Use caution with electrocautery if using metal clips as heat and current may easily pass through the clips and cause thermal injury to the mammary itself.
At this point, heparin should be given. It is good practice to confirm adequate haemostatic control with your assistants harvesting other conduits (ie. Radial artery or saphenous vein) prior to your requesting of heparin administration. Whilst you are waiting for the heparin to be administered, take the opportunity to inspect the mammary bed and cauterize any bleeding points. After heparin administration, the artery is divided distally at, or just proximal to, its bifurcation. Flow may be confirmed and if appears weak, the vessel may be probed gently to remove any clot or tissue that may be obstructing flow. The pedicle is then treated with papaverine to decrease vasospasm. This may reveal small side branches of the mammary requiring ligation which were not initially noticed.
The vessel should not be under any tension with inflated lungs and a physiologically distended heart. If more length is needed, the pedicle may be dissected away from the fascia at intervals. After resecting to an appropriate length, the surrounding fat is cleared from the orifice of the vessel. An oblique incision is made to prepare a standard hooded cuff with the heel to the fascial side. This incision may be extended with Potts scissors to create a larger sewing orifice. Flow should then temporarily be occluded using a bulldog. The mammary may be gently protected with a swab and placed in the left hemi-thorax until required whilst the procedure continues.
Rule:
A final haemostasis check of the mammary bed should occur at the end of the bypass procedure, following the administration of protamine.
Bilateral Internal Mammary Artery Harvest
In general, three types of conduits are considered: the internal mammary arteries, the radial arteries, and the saphenous vein. The LlMA is the gold standard bypass graft, with a 90% patency at 15 years. The internal mammary arteries show a relative resistance to atherosclerosis and are reliably used for bypass grafts. Bilateral IMA grafts may be used to facilitate total arterial grafting. However, large research studies such as the ART trial have demonstrated an association with increased deep sternal wound infections, particularly in patients with high risk of wound complications already, such as insulin dependent diabetics.
Saphenous Vein Harvest
The greater saphenous vein (GSV or long saphenous vein, LSV) is a widely used conduit for bypass surgery due to its accessibility, length for multiple conduits, and minimal risk of side effects after procurement. The long-term outcomes of graft patency with the saphenous vein are more discouraging than arterial conduits, with graft failure at 10-years reported as high as 80% in some studies. However, the vessel remains a mainstay second conduit of choice after LIMA for bypass surgery. The GSV may be harvested using traditional open, no-touch, bridged or endoscopic techniques. There is increasing evidence that supports improved outcomes following a no-touch technique and better patient reported outcomes, such as pain and cosmesis, with minimally invasive approaches which utilize smaller incisions. Open harvest is discussed here.
The GSV may be identified approximately 0.5-1cm anterior and superior to the medial malleolus of the ankle. A small incision is made, taking caution to only divide skin, and scissors may be used bluntly, opening within the dissection plane, to reveal the GSV which lies very superficially beneath the skin. Once a short segment of vein is exposed, a dissection plane is developed on-top of the vein. To do this, it is often easiest to stand at the patient’s feet, looking cephalad. Scissors may be placed in parallel to the vein, on-top of it and using gentle pressure, dissect the plane on top of the vein. Direct visualization should be used to observe the tunnel being developed for the course of the vein. After every few cm of blunt dissection within the tunnel, the overlying skin may then be divided either with a blade down onto the scissors (in order to protect the vein) or directly with scissors. Once exposed along its length, a cannula should be secured within the vessel at its most distal end. High pressure distention should be avoided but gentle distention may be used to confirm caliber and identify and isolate side branches which may then be ligated with clips. Veins with varicosities should be avoided as the dilated regions may cause slow flow and may form clot; varicosities may be exposed during gentle distention also. A variety of solutions are available for venous distention and include heparinized whole blood and heparinized crystalloid solutions.
Finally, the vessel may be freed from its position in the leg by division of the fascia anchoring it on its underside after division of branches. The vessel may be retracted gently with a vessel loop when needed but direct contact with the vein should be avoided to minimize endothelial damage. Bands of adventitia along the vein surface may be divided with scissors to create an even vessel surface and if any avulsions to the wall occur, it is best to repair them with a 7-0 Prolene suture with longitudinal bites to avoid constriction. Venous valves are generally left in place and the conduit must be reversed for grafting so that flow will not be limited by the valves. The distal end of the reversed graft is prepared with an oblique cut and extended with Potts scissors as with the IMA. The graft is then stored in heparinized solution of choice until it is to be grafted.
Identifying Coronary Vessel Targets
Prior to any cardiac procedure, you will have reviewed and memorized the pre-operative angiogram. For any bypass specifically, you should have an idea of where you would like to place your grafts based on this alone. The angiogram should also be available for viewing in the operating room.
Following successful conduit harvest and confirmation of hemostasis, progress is made by cannulating and going on cardiopulmonary bypass. For off-pump bypasses, the targets will be identified straight away as cannulation is not required.
Prior to aortic cross-clamping and administration of cardioplegia, surgical target vessels should be identified. Doing this prior to cardiac arrest may assist with distinguishing between arteries and veins although this will often be done after cardioplegia administration also.
Exposing the left sided circulation
The LAD, ramus intermedius, and diagonal coronary arteries are easily visualized by placing a cold saline soaked sponge behind the heart to bring the LAD more towards midline and superficial. Stay sutures may also be used to further anchor and expose the chosen vessel portion.
The circumflex system vessels are exposed by retracting the apex towards midline and rotating the ventricles counterclockwise around the base of the heart. Elevating the left side of the table is often beneficial to the operating surgeon. Identify the left atrial appendage and this will be a marker for the course of the circumflex artery and its obtuse marginal vessels.
Exposing the right sided circulation
Exposing the right sided system is assisted by placing the table in Trendelenburg position. The RCA may be exposed by elevating the heart from the acute margin and proceeding with careful dissection through the pericardial fat overlying the atrioventricular groove. The PDA and its branches may then be identified by retracting the apex further towards the patient's right shoulder. All maneuvers must be performed gently to avoid undue traction on the myocardium and possible disruption of the AV groove. Maneuvers will often be adapted for off-pump procedures to ensure maintenance of good systemic blood pressures.
Identifying suitable targets
The primary purpose of coronary artery bypass grafting is to provide flow distal to obstructive disease. Therefore, an ideal target site is distal to any known angiographic disease, of reasonable size caliber, in a position convenient for performing the anastomosis and in a segment of disease-free vessel. Larger disease lesions are often palpable if not visible as a paler irregular area of the coronary wall.
Should the coronary vessels be difficult to visualize on the surface of the heart, you may find it beneficial to start more distally and trace the vessel proximally. If the vessels dive into the myocardium, myocardial bridges may be carefully incised using sharp dissection, and the edges of the myocardium coagulated using very low voltage electrocautery. Fortunately, vessels beneath myocardial bridges typically reward the surgeon with a disease-free segment. Beware of too much intra-myocardial scavenging or you may find yourself inside the ventricular cavity or with very difficult hemostasis.
Once a target site is identified, it may be opened by gentle stroking with a blade on the epicardial fat and its superficial vessel wall. The root vent should be off to ensure no negative pressure within the coronary which could increase the chance of damaging the back-wall of the vessel upon opening. The arteriotomy is performed in two stages. The first is to dissect the epicardium and open the vessel at a single point. The second is to extend this into your arteriotomy proper.
The epicardium over the site of distal anastomosis is first incised with the stroker blade. Visualization is assisted with a self-retaining retractor and often the placement of a silastic tape proximally and distally is helpful. Next, the arteriotomy is performed with a poker blade with a more acutely angled tip. The cutting edge of the blade should remain facing up throughout and a stab incision is made with the blade at 30 degrees.
A short incision is made parallel with the direction of the artery, directly in the midline of the vessel. Care should be taken not to injure the back wall of the vessel. After making the first incision, Potts scissors are used to extend the arteriotomy proximally and distally. The arteriotomy should match the conduit diameter and be approximately 1.5 times the diameter of the distal coronary. In most cases, the vessel should be gently probed proximally and distally to ensure that the correct lesions will be bypassed before the anastomosis is begun.