On-Pump Coronary Artery Bypass Grafting
◆ The named epicardial coronary arteries that serve as the distal anastomotic targets for coronary artery bypass grafting (CABG) are most commonly located just deep to the epicardial fat and superficial to the myocardium. The arteries, usually the left-sided vessels, may be located more deeply within the myocardium (intramyocardial). A straighter course on coronary angiography may suggest an intramyocardial location.
◆The left anterior descending (LAD) coronary artery courses superficially to the interventricular groove, providing diagonal branches to the anterior wall.
◆ The ramus intermedius (RI) artery arises between the LAD and left circumflex artery (LCx) and can often be identified near the base of the left atrial appendage.
◆ The LCx arises from the left main coronary artery as the left main artery bifurcates to give off the LAD in the atrioventricular groove. The LCx provides obtuse marginal branches that supply the lateral and inferolateral myocardium, usually terminating near the lateral margin of the left ventricle.
◆ The right coronary artery (RCA) originates anteriorly from the aortic root and courses in the atrioventricular groove prior to crossing the acute margin of the heart and bifurcating into the posterior descending artery (PDA) and posterolateral ventricular branch (PLVB).
◆ Right or left coronary dominance refers to the artery from the which the PDA originates.
◆ The most commonly used conduits for CABG include the left and right internal mammary arteries (alternatively termed the internal thoracic artery), radial artery, and reversed greater saphenous vein (GSV).
◆ The left internal mammary artery (LIMA) originates from the proximal left subclavian artery opposite the thyrocervical trunk and courses approximately 1.5 cm lateral to the sternocostal junction. Proximally, the LIMA passes inferiorly and medially behind the subclavian vein, where the phrenic nerve usually crosses from lateral to medial as it courses to the pericardium. Care must be taken during proximal harvest of the LIMA to avoid phrenic nerve injury and resultant diaphragmatic dysfunction. The midportion of the LIMA is superficial, lying just deep to the endothoracic fascia, and can be visualized or palpated most easily in this location. Below the sixth rib, the transversus thoracis muscle covers the posterior aspect of the internal mammary artery (IMA). Near the junction of the xiphoid process and body of the sternum, the IMA bifurcates into the musculophrenic and superior epigastric arteries. The IMA is accompanied by paired internal mammary veins that combine to form a single vein proximally.
◆ The radial artery originates from the brachial artery, coursing under the brachioradialis muscle proximally and in the lateral forearm deep to the distal deep fascia. From the antecubital fossa, the artery courses from medial to lateral. Care must be taken during harvest of the distal radial artery to avoid injury to the superficial radial nerve and lateral antebrachial cutaneous nerve.
◆ The GSV is located on the medial side of the lower extremity, coursing superficial to the medial malleolus at the ankle and running deep to the subcutaneous fat as it courses more proximally. At its most proximal portion, the GSV drains into the common femoral vein at the saphenofemoral junction.
Step 2. Preoperative Considerations
◆ The medical history should focus on comorbid conditions that could increase perioperative risk and conduit selection (e.g., history of stroke, gastrointestinal bleeding, liver disease, diabetes mellitus, obesity, chronic obstructive pulmonary disease, renal failure, peripheral arterial disease).
◆ The surgical history should delineate any prior chest surgery and procedures that could affect conduit selection (e.g., radial artery catheterization, varicose vein stripping).
◆ The physical examination should aim to identify any comorbid conditions not obtained during the history and to identify any potential caveats that could alter surgical planning. Bilateral upper extremity blood pressures should be obtained to identify possible subclavian stenosis, which could impair IMA flow. The skin overlying the chest wall and conduit harvest sites should be examined for any evidence of infection, prior irradiation, and scars from prior procedures. Auscultation for a carotid bruit may indicate the presence of stenosis. Radial pulses should be palpated and, in case radial artery harvest is planned, an Allen test should be performed. The presence of an arteriovenous fistula for hemodialysis has been reported to cause steal from the ipsilateral IMA and should be taken into consideration. Femoral, pedal, and posterior tibial pulses should be identified in case an alternative cannulation strategy is needed or for placement of an intraaortic balloon pump. The lower extremities should be inspected for venous stasis changes and large varicosities.
◆ Routine complete blood counts, coagulation studies, and serum chemistry tests should be performed. A baseline electrocardiogram should be obtained. Preoperative transthoracic echocardiography will provide data regarding ventricular function and any additional valvular pathology that might require concomitant intervention. In addition to providing the coronary anatomy for bypass planning, left heart catheterization can also serve to delineate left ventricular function, aortic valve pathology, and mitral regurgitation. A baseline chest x-ray can identify possible aortic calcification, and a noncontrast computed tomography (CT) scan of the chest may be obtained to evaluate for the presence of a porcelain aorta definitively. Routine carotid duplex ultrasonography is not mandated, but should be considered in patients with symptoms of stroke or a transient ischemic attack, presence of a carotid bruit, or significant left main coronary artery stenosis. If there is concern based on the history or physical examination regarding the radial artery or GSV conduit, duplex ultrasonography can aid in identifying the suitability of these conduits preoperatively.
◆ Patients are placed in the supine position on the operating table. The upper extremities should be secured parallel to the torso and appropriately padded to prevent compressive nerve injury. In the case of planned radial artery harvest, the ipsilateral arm should be abducted 45 to 60 degrees from the patient.
◆ Hemodynamic monitoring should include placement of a noninvasive blood pressure cuff, radial arterial catheter (opposite the site of the planned radial arterial harvest site), and central venous catheter in the internal jugular vein. Placement of a pulmonary artery Swan-Ganz catheter should be based on discussion between the surgeon and anesthesiologist and is generally used for higher-risk patients. A transesophageal echocardiography probe may be passed after induction of general endotracheal anesthesia. Near-infrared spectroscopy (NIRS) monitoring may be performed if there are risk factors or significant cerebrovascular disease. All hair should be removed from incision sites with an electric razor. Electrocardiographic leads, defibrillator pads, and electrocautery grounding pads should be placed away from all potential incision sites. Placement of a roll perpendicular to the spine across the scapulae may facilitate sternotomy and IMA harvest. Bolsters placed under the knees or lower extremities with slight external rotation of the legs may facilitate GSV harvest.
◆ Perioperative antibiotics such as a first-generation cephalosporin, with the addition of vancomycin if methicillin-resistant Staphylococcus aureus (MRSA) colonization is documented, should be administered within 30 minutes prior to incision. Skin preparation with an iodophor solution or chlorhexidine gluconate should be performed from the chin to the toes, with circumferential preparation of the lower extremities.
◆ The following discussion details the procedure of CABG using a pedicled IMA graft, with reversed GSV or free radial artery grafting. Alternative conduits, including the gastroepiploic artery, inferior epigastric artery, and lesser saphenous vein may be used, but are not described here. Alternative strategies and approaches for CABG are described elsewhere in the text.
◆ A standard median sternotomy is performed. To facilitate harvest of the IMA, a variety of self-retaining sternal retractors are available to elevate the ipsilateral sternal edge. Once the sternum has been elevated, the mediastinal pleura is freed from the endothoracic fascia. The pleura may be opened widely into the pleural space to facilitate exposure or may be left intact once it has been freed several centimeters beyond the lateral edge of the IMA. The IMA can be harvested using a pedicled technique in which the IMA is harvested with the endothoracic fascia and paired veins or using a skeletonized technique.1 Using a pedicled technique, the endothoracic fascia is opened laterally to the paired mammary veins, creating a 1.5- to 2-cm pedicle. Dissection can be carried out using electrocautery or with scissors. The IMA and its paired veins should be gently dissected free from the chest wall using hemoclips to ligate intercostal branches. Care should be taken to avoid thermal injury with excess use of elec- trocautery or IMA dissection with excessive manipulation of the artery. Dissection of the proximal 3 cm of the IMA is where phrenic nerve injury is most likely to occur. Both the left IMA and right IMA can be harvested in a similar fashion, but the harvester should be aware that the mammary vein may cross medially earlier on the right side, and intersection of the phrenic nerve and IMA occurs more proximally on the right side. The skeletonized approach is preferred when bilateral IMA harvest is planned to avoid devascularization of the sternum and when IMA length may be an issue. Sharp dissection and avoidance of electrocautery is preferred during skeletonization of the IMA to minimize the risk of thermal injury. When skeletonizing the IMA, the endothoracic fascia is incised sharply, and the IMA is dissected free from the paired veins. The fascia is opened longitudinally under the IMA proximally and distally along the course of the artery. The artery may be bathed in a vasodilator solution such as papaverine until ready for use. The chest wall should then be inspected for hemostasis (Fig. 3.1).
◆ GSV harvest should take place concurrently with IMA harvest. Three techniques are commonly used: full open harvest with a longitudinal leg incision, a semiopen (bridged) technique with interrupted sequential leg incisions, and an endoscopic approach. Most GSV harvests performed currently use an endoscopic approach. Once the vein has been isolated, the proximal and distal ends are ligated with silk suture, and the vein is transected and removed from the leg. The distal aspect of the vein is cannulated to allow for gentle pressurization of the vein and branches and areas of leak, or it is clipped or oversewn with fine polypropylene sutures. A longitudinal mark may be applied with a skin marker to prevent unrecognized torsion of the vessel when constructing proximal anastomoses (Fig. 3.2).
◆ The radial artery is usually harvested from the nondominant arm to minimize functional consequences should nerve injury occur.2 The artery may be harvested endoscopically, as with GSV, or an open technique may be used. Here we describe the open technique. A longitudinal incision is created from 2 cm proximal to the styloid process of the radius and extended proximally to 2 cm proximal to the antecubital fossa, extending medially toward the biceps tendon. The artery lies between the flexor carpi radialis muscle and brachioradialis muscle. The subcutaneous tissue is divided with electrocautery, and the deep fascia is incised sharply. The distal end of the radial artery (closest to the wrist) should gently be occluded transiently to ensure good collateral flow to the hand, which can be monitored with a pulse oximetry probe on the finger. The deep fascia should be incised sharply over the artery. Vascular branches are divided in sequence using clips, electrocautery, or a harmonic scalpel. As dissection proceeds proximally toward the brachial artery, the brachioradialis muscle can be retracted laterally. Care must be taken to avoid the lateral antebrachial cutaneous and superficial radial nerves. After systemic heparinization, the proximal and distal ends of the artery are ligated, and the vessel is placed in a heparinized solution with nitroglycerin and/ or a calcium channel blocker to minimize vasospasm (Fig. 3.3).
◆ After harvesting of the conduit, a sternal retractor is placed, and the pericardium is opened in an inverted T fashion. The edges of the pericardium are secured to the retractor or sub- cutaneous tissue to create the pericardial well (Fig. 3.4). The aorta should be palpated for calcium plaques and visually inspected for anticipated cannulation, cross-clamp, and proximal anastomotic site placement. Systemic heparin should be administered. Two concentric purse-string sutures are placed in a diamond shape in the distal ascending aorta, sized to match the appropriately selected arterial cannula. Once appropriate levels of systemic anticoagulation have been achieved, the systemic blood pressure should be lowered to between 100 and 110 mm Hg systolic and the ascending aortic cannula inserted and secured with tourniquets. Venous drainage may be obtained by cannulation of the right atrial appendage after placement of a circumferential purse-string suture and transection of the appendage tip. An antegrade cardioplegia catheter is next placed in the ascending aorta, proximal to the aortic cannula and anticipated location of the aortic cross-clamp. Depending on the surgeon’s preference for cardioprotection, a retrograde cardioplegia catheter may be placed in the coronary sinus via the right atrium. Echocardiographic guidance, manual palpation, return of dark deoxygenated blood, and pressure tracing are used to confirm appropriate retrograde catheter position. Prior to initiation of cardiopulmonary bypass (CPB), all conduits should be inspected for usability (Fig. 3.5).
◆ After confirmation of appropriate systemic anticoagulation and aortic cannula line pressure, CPB is initiated; once venous drainage is sufficient, ventilation is discontinued. Systemic cooling may be initiated at the discretion of the surgeon.
◆ An atraumatic aortic cross-clamp should be placed on the ascending aorta just proximal to the aortic cannula and distal to the antegrade cardioplegia catheter. Prior to clamping the aorta, CPB flow should be lowered to decrease systemic blood pressure and decompress the aorta. On confirming adequate clamp placement, cardioplegia is administered. The aortic root should be palpated to confirm adequate pressure when antegrade cardioplegia is administered. If using retrograde cardioplegia, coronary sinus pressure should not exceed 40 mm Hg. Topical cooling can be performed using iced saline slush, wet laparotomy pads, cooling jackets, or continual cold saline irrigation of the operative field. On completion of the initial dose of antegrade cardioplegia, any vent lines may be opened to suction.
◆ As noted previously, the sequence of distal anastomotic completion is surgeon-dependent. Our standard approach involves creation of right-sided distal anastomoses, followed by the LCx, RI, diagonal, and LAD arteries. On completion of distal anastomoses, proximal anastomoses are completed.
◆ After clearing the epicardial fat from the anterior surface of the target vessel, any vents should be be clamped temporarily to allow for slight distention of the vessel. A no. 15 blade or alternative should be used to incise the vessel, taking care to remain in the midline and avoid the back wall. Pott’s scissors can then be used to extend the arteriotomy proximally and distally. The arteriotomy should be at least 1.5 times the diameter of the target vessel and match the conduit size. A slight bevel of 30 degrees can be created on the conduit to decrease the risk of kinking, and a small longitudinal notch can be created at the heel of the conduit to allow for increased length. Distal anastomoses are generally performed using 7-0 polypropylene running sutures or, in certain cases, 8-0 polypropylene sutures for the IMA anastomosis. On completion of the anastomosis, cardioplegia or heparinized saline may be injected by hand or via a pressurized line to test graft flow and to test for leaks (Fig. 3.6).
◆ When performing an IMA anastomosis with a pedicled graft, proximal inflow may be interrupted with a soft bulldog-style clamp. An incision in the pericardium may be created to allow for the IMA to pass to the distal target without kinking or stretching. Care should be taken to avoid injuring the phrenic nerve when creating the pericardial opening. Once the arteriotomy site for the distal anastomosis has been selected, the graft should be transected after delineating the appropriate length to avoid redundancy. The IMA anastomosis is performed in a similar fashion as described previously. Prior to completing the anastomosis, the IMA clamp should be transiently released to confirm good flow in the conduit. On completion of the anastomosis, release of the IMA clamp may allow for visualization of distal target artery filling, although this is not always visible. If a pedicled graft is used, the lateral edges of the graft may be secured to the epicardial fat with polypropylene sutures to prevent later torsion (Figs. 3.7 and 3.8).
◆ On occasion, when distal targets are small, the conduit is limited, or proximal anastomotic sites are limited, sequential anastomotic techniques should be considered. Sequential grafting uses a single conduit with multiple distal anastomoses (usually two) and one proximal anastomosis—the source of inflow. Although offering the aforementioned advantages, sequential grafting does rely on a single inflow source of blood to supply multiple targets and may result in competitive flow imbalance if the proximal outflow is greater than the distal outflow, and it can be subject to kinking or graft torsion if the targets are not aligned properly.3 Sequential grafting can be performed with a free conduit such as a radial artery graft, GSV graft, or a pedicled IMA. Ideally, the most distal target should be the largest of the targets with the best outflow, and the targets should lie in an anatomic position to the minimize risk of kinking, as can occur with sequential grafting of obtuse marginal targets or diagonal-LAD artery target combinations. When performing sequential grafting using a free graft, we perform the distalmost target anastomosis first, using the technique described previously. The graft and heart are then filled to identify the optimal position of the more proximal distal anastomosis. Both the conduit and target are then incised longitudinally, taking into consideration the size of the prior anastomosis. Depending on the location of the target vessel, an eight-stitch, side-to-side anastomosis can be fashioned with the conduit perpendicular to the target (Fig. 3.9), creating a diamond-shaped anastomosis. Alternatively, a larger side-to-side anastomosis, with the vessels parallel to one another (Fig. 3.10), can be fashioned to avoid a gull wing deformity.
◆ As noted previously, in certain cases for which insufficient conduit is available, and there is concern over conduit to aortic mismatch between the conduit and aorta or there is limited room on the aorta for proximal anastomosis, consideration should be given to Y or T grafting techniques.4 These use an end-to-side anastomosis on a bypass graft for proximal inflow with a standard distal anastomosis for outflow, resulting in a Y- or T-shaped appearance of the proximal anastomosis (Fig. 3.11).
◆ Proximal anastomoses may be fashioned after the completion of each distal anastomosis or after the completion of all distal anastomoses. If done after the completion of all distal anastomoses, they may be performed with the initially placed aortic cross-clamp in place to avoid additional aortic manipulation or with a partially occlusive side-biting aortic clamp to minimize ischemic time. Each graft should be measured to ensure appropriate length to avoid excess stretching when the heart is filled and the lungs are inflated and for appropriate anatomic positioning to avoid kinking with excess length. A no. 11 blade is used to create a small incision in the aorta, taking care to avoid too deep an entry, which could injure the back wall of the decompressed aorta. Some surgeons prefer to use the aortic root vent–antegrade cardioplegia catheter site as a location for a proximal anastomosis. After creating the initial incision, an aortic punch of 4 to 5 mm is used to create a circular aortotomy. Proximal anastomoses can be fashioned with running polypropylene sutures, ranging from 5-0 to 7-0 in size, depending on the thickness of the conduit vessel. The grafts should be inspected for air and a fine 25-G needle can be used for de-airing vein grafts, if necessary (Figs. 3.12 and 3.13).
◆ During the reperfusion period, metabolic parameters and hemodynamics should be optimized. All anastomotic sites and conduits should be inspected for hemostasis and kinking because visualizing the inferior and lateral walls of the heart may be difficult after separation from CPB. Once all parameters are optimized, the electrocardiographic tracing is reviewed, and hemodynamics and echocardiographic imaging are satisfactory, weaning from CPB should commence. During weaning from CPB, the grafts should be monitored closely as anatomic positioning may change with lung insufflation and ventricular filling. After separation from CPB, protamine may be administered and the patient decannulated in standard fashion. The IMA harvest sites should be secondarily inspected for hemostasis with retraction of the sternal edge. Standard sternal closure may then be performed, with particular attention to hemodynam- ics, because alterations in graft positioning may occur with sternal closure.
Step 4. Postoperative Care
· Initial postoperative management in the intensive care unit centers around hemodynamic support. Early neurologic evaluation should be performed to ensure that intraoperative stroke has not occurred. In the hemodynamically stable patient without active bleeding, all attempts should be made for early extubation. Hemodynamic support with goal-directed weaning of inotropic agents and vasopressors should occur early. Chest tube output should be monitored closely for signs of bleeding. Aspirin is given within 6 hours of surgery, and a beta blocker is administered if hemodynamics allow. Glucose control is maintained with an insulin infusion until stabilization. Statins should be initiated early in the postoperative period in the absence of contraindications. Nitrates or calcium channel blockers may be used to decrease the risk of arterial spasm if a radial conduit has been used. Early mobilization should be the goal, with anticipated transfer from the intensive care setting by postoperative day 1 or 2. Chest tubes are generally ready for removal by day 2 or 3, depending on output. Diuresis can be initiated by postoperative day 1 and continued until normal fluid balance has been achieved.
◆ Preoperative evaluation should focus on identifying comorbid conditions, which may affect the operative plan. Specifically, care should be taken to ensure the suitability of distal targets for revascularization, presence of sufficient quality conduit, and absence of severe aortic calcification.
◆ Excess sternal retraction during IMA harvest may result in sternal or rib fracture or brachial plexus injury.
◆ Care should be taken during conduit harvest to avoid excess manipulation of IMA conduits, which may result in dissection, thrombosis, or intimal injury, the latter of which may only become evident with late graft failure.
◆ Excessive pressurization or thermal injury of saphenous vein conduits may result in intimal injury, leading to graft failure.
◆ Early planning for aortic cannula, cross-clamp, and cardioplegia catheter placement should be performed with the heart full to determine placement of proximal anastomoses.
◆ Epicardial targets may not be readily visible on the surface of the heart and require extensive dissection of the epicardial fat or myocardium to be identified. Care should be taken to ensure excellent hemostasis of any dissected fat or myocardium because these may result in significant bleeding after release of the aortic cross-clamp.
◆ If the proximal or midportion of the target vessel is not visible or is difficult to identify, careful passage of a coronary probe retrograde from the distal aspect of the vessel may be useful.
Keyword : Operations for Coronary Artery Disease, On-Pump Coronary Artery Bypass Grafting, coronary artery bypass graft, CABG anastomosis, Operations for Coronary Artery Disease
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