Coronary Artery Bypass Grafting for Acute Myocardial Infarction with Cardiogenic Shock
Oz M. Shapira, MD and Amit Korach, MD
Department of Cardiothoracic Surgery, Hebrew University, Hadassah Medical Center
Oz M. Shapira, MD and Amit Korach, MD
Department of Cardiothoracic Surgery, Hebrew University, Hadassah Medical Center
Cardiogenic shock occurs in 6 to 7 percent of patient with acute myocardial infarction (MI) (excluding patients with mechanical complications such as mitral insufficiency due to papillary muscle rupture, ventricular septal defect or left ventricular rupture). Cardiogenic shock after MI develops after a loss of at least 40% of contractile mass of the left ventricle. Medical treatment of cardiogenic shock after MI is associated with very high mortality rate – approaching 80%. Observational data suggest that a timely opening of the infarct-related artery translates into improved survival. Most patients with acute MI and cardiogenic shock have significant left main or three-vessel disease. Never-the-less, expeditious primary percutaneous coronary intervention (PCI) of the "culprit" vessel is the revascularization procedure of choice.
Coronary artery bypass grafting (CABG) in patients with acute MI is indicated in only 2 to 5% of patients.
The principal indications for early CABG after acute MI and cardiogenic shock include: a) Complex coronary anatomy, particularly if the "culprit" vessel cannot be identified; b) After unsuccessful or complicated PCI (e.g.; failed stent deployment, extensive coronary dissection, coronary perforation with tamponade and abrupt stent closure). Important factors that should be taken into consideration prior to proceeding with emergency CABG include the amount of viable myocardium at risk, the "quality" of the distal target vessels (i.e. – whether the vessels are technically amenable to bypass), patient co-morbidities and functional status immediately prior to the event, and the ability to achieve revascularization within six hours after onset of symptoms.
Preoperative Diagnostic Studies and Preparations
Patients with acute MI and cardiogenic shock are usually rushed into the cardiac catheterization laboratory to define the coronary anatomy, and determine the feasibility of primary PCI. Patients who are considered for emergency CABG should have a detailed cardiac echocardiography examination to assess ventricular function, valve function and the presence of mechanical complications of MI such as papillary muscle rupture, ventricular septal defect, or free wall rupture. Once a decision was made to proceed with emergency CABG, an expeditious operation to shorten myocardial ischemic time and systemic organ dysfunction due to low cardiac output is a key component of the management of these patients.
Unnecessary testing should be strictly avoided. Patient hemodynamics should be supported en-route to the operating room by liberal use of pharmacologic agents and routine insertion of intra-aortic balloon pump (IABP). Early use of the IABP improves coronary perfusion and decreases myocardial afterload, leading to increased myocardial oxygen supply and decreased demand. This, in turn, decreases myocardial injury, improves hemodynamics, allowing decreased use of inotropic agents.
Operation is performed under general endotracheal anesthesia. Induction and maintenance of anesthesia should be achieved with the goals of minimizing hypotension and tachycardia. To achieve this goals the combination of intravenous fentanyl and ethomidate are used for induction and the combination of intravenous fentanyl and midazolam and inhaled isoflurane are used for maintenance of anesthesia.
Routine monitors and lines include two-channel ECG, a pulse oxymeter, end-tidal CO2, non-invasive blood pressure cuff, body core temperature, urinary bladder catheter to measure hourly urine output, two large-bore peripheral intravenous lines, invasive arterial line and a pulmonary artery (Swan-Ganz) catheter. Trans-esophageal echocardiography is an essential component in all patients. (Figure 1) depicts a typical monitor screen during CABG. TEE is used to systematically assess ventricular function, valve function, shunts and other pathologies after induction of anesthesia, upon weaning off cardiopulmonary bypass and thereafter. It is a very useful adjunct to optimize patient hemodynamics post bypass.
All patients receive prophylactic antibiotics administered within one hour of skin incision. The agent of choice in most patients is first-generation cephalosporin. Patients with a known allergy to penicillin or cephalosporin and those colonized with methicillin-resistant staphylococci receive Vancomycin alone. A combination of Vancomycin and a third-generation cephalosporin is used in patients who were hospitalized more than 48 hours prior to surgery and patients with active gram-negative infection. Prophylactic antibiotic course is continued for 48 hours only.
The Operative Procedure
Positioning and incision
The patient is placed supine. Lines and monitors are placed as described. The chest, abdomen and lower extremities are prepped with chlorhexidine scrub and alcohol and draped to allow full access to the chest, groins, lower and upper extremities for radial artery harvest in selected cases. The chest is opened via a midline sternotomy incision. The pericardium is opened in the standard inverted T fashion and pericardial stay sutures are placed (2.0 Ethibond Excel, CT-1 pop-off needles, Ethicon, J&J International), creating a pericardial well. The left-sided pericardial sutures are left hanging on a weight and to afford access to the left pleural space (Figure 2).
Choice of conduit
The ultimate goal of the operation is complete revascularization. This is a distinct advantage of CABG compared to PCI. The typical conduit strategy for patients undergoing emergency CABG in the setting of acute MI and cardiogenic shock includes the in-situ left internal mammary artery (LIMA) to graft the left anterior descending coronary artery and saphenous vein grafts for all other targets. This is in distinct contrast to our strategy of multiple arterial grafting in patients undergoing CABG in elective or urgent circumstances. Harvest of a single IMA can be accomplished during a relatively short time and is associated with improved short- and long-term clinical outcomes.
The LIMA and saphenous veins are harvested simultaneously by two team members (Figures 3, 4). Exclusive use of vein grafts should be reserved to the very few, extremely unstable patients. It affords a more expeditious revascularization, an opportunity to directly deliver cardioplegia, to control the reperfusion to the ischemic myocardium and minimize the risk of conduit vasospasm in patients who are typically treated with high doses of inotropes and vasopressors. A second IMA or the radial artery are used only in patients in whom the saphenous vein cannot be utilized.
We prefer to perform CABG in patients with MI and cardiogenic shock using cardiopulmonary bypass (CPB) and cardioplegic arrest. It affords the opportunity to achieve complete revascularization – the ultimate goal of the procedure, in a very precise and expeditious manner. Off-pump grafting of the circumflex and posterior descending coronary arteries can be challenging if not impossible, due to hemodynamic instability that precludes lifting and manipulation of the heart. The aorta is assessed for calcification using finger palpation, TEE and epi-aortic ultrasound in selected cases. The aorta is cannulated via a double purse-string suture of 3-0 Ethibond Excel, RB-1 needle (Ethicon, J&J International) in the distal ascending using a 20 or22Fr right-angled tip cannula (Aortic Perfusion Cannula, Edwards Lifesciences, Irvine, CA, USA). A 40-32Fr two-stage venous cannula (Dual Stage Venous Drainage Cannula, Edwards Lifesciences, Irvine, CA, USA) is placed via a 4-0 Ethibond Excel, SH-1 purse-string suture (Ethicon, J&J International) in the right atrial appendage (Figure 5). The components of our biocompatible perfusion strategy consist of tip-to-tip closed-system (collapsible venous reservoir) heparin-coated CPB circuits (Carmeda®, Medtronic, Minneapolis, MN, USA), centrifugal pump with a membrane oxygenator and lack of a cardiotomy reservoir (Figure 6). A cardiotomy reservoir is added to the circuits requiring left ventricular or pulmonary artery venting. Kinetic venous assist is frequently used. Systemic anticoagulation is achieved by 1 mg/Kg of heparin and is carefully monitored using heparin titration curves (Hepcon®, HMS Plus, Medtronic, Minneapolis, MN, USA) with a target activated clotting time (ACT) of 300 sec. (Figure 7). CPB prime volume is reduced to a minimum using the retrograde autologous priming technique. Systemic body temperature is kept at near-normothermia (34-36ºC) avoiding active cooling. We routinely use a cell saver and anti-fibrinolytics (Tranexamic acid).
Careful myocardial preservation protocol is a key component of the management of these complex patients. Our protocol consists of the following components: Use of 1:4 blood (versus crystalloid) cardioplegia, delivered both antegrade via the aortic root (14G Aortic Root Cannula, Edwards Lifesciences, Irvine, CA, USA) and retrograde via the coronary sinus (14Fr Retrograde Cardioplegia Catheter, Edwards Lifesciences, Irvine, CA, USA), (Figure 8); Induction of myocardial diastolic arrest using warm (37ºC), preferably c-amino-acid enriched blood cardioplegia, followed by intermittent cold (4ºC) blood cardioplegia repeated every 20 minutes during aortic clamping, delivered via the aortic root, the vein grafts and the coronary sinus (Cardioplegia set, Edwards Lifesciences, Irvine, CA, USA); Anastomosis to the infarct-related "culprit" vessel first; Warm reperfusion ("hot shot") using 37ºC c-amino-acid enriched blood cardioplegia and more recently whole warm blood; Topical cooling of the heart using cold (4ºC) saline; Strict prevention of left ventricular distension with liberal placement of left ventricular vent. This strategy has been shown to decrease infarct size and limit reperfusion injury in experimental models, and has been associated with reduced morbidity and mortality in clinical trials.
Grafting Sequence and Technique
In most cases the distal and proximal anastomoses are performed during a single aortic cross-clamping to minimize aortic manipulation and the risk of stroke. The distal anastomoses are completed first, followed by the proximal anastomoses. In patients with very poor LV function the two-clamp technique is preferred to minimize myocardial ischemic time. Using the latter technique the distal anastomoses are performed during aortic cross-clamping while the proximal anastomoses are performed using a partial occlusion ("side-biting") clamp on the ascending aorta. Cardioplegia is delivered through the vein grafts during aortic cross-clamping (Figure. 9). If the two-clamp technique is selected, the vein grafts are perfused with warm blood after unclamping of the aorta, during the construction of the proximal anastomoses.
The distal anastomoses are constructed end-to-side using continuous 7-0 polypropylene, BV175-8 needle (Prolene, Ethicon, J&J International) sutures (Figures 10, 11). Sequential grafting is performed when indicated. The proximal anastomoses are constructed using continuous 6-0 polypropylene (Prolene, Ethicon, J&J International), C1 needle for vein grafts or BV1 needle for radial artery or IMA free grafts (Figure 12).
Separation from CPB and Closure
A pair of atrial and ventricular pacing wires are placed routinely (Temporary Pacemaker Electrodes, FEP 15, Ethicon, J&J International) (Figure 13). Separation from CPB is performed using the IABP and inotropic support as necessary under continuous TEE and Swan-Ganz catheter monitoring. After separation from CPB, protamine is administered according to the heparin-titration curves. Given the initial low systemic anticoagulation, protamine reversal doses are typically very low (0.5 to 1 mg/Kg). Meticulous hemostasis is performed. Strict criteria are used for transfusion of allogeneic blood products. The threshold for red blood cell transfusion is a hematocrit of less than 20% during CPB and 25% after separation form CPB. Fresh frozen plasma and platelets are used to manage coagulopathy due to shock liver or potent dual anti-platelet therapy.
The chest is drained using a single straight 28Fr chest tube in the left pleural space and an angled 28Fr and straight 36Fr chest tubes in the mediastinum. The chest is closed using 5-7 stainless steel wires for the sternum, interrupted figure of eight 0 Ethibond Excel, CT-1 needle sutures (Ethicon, J&J International) for the fascia, two layers of continuous 0 Vicryl Plus, CT-1 needle sutures (Ethicon, J&J International) for the muscle and subcutaneous layers and continuous 4-0 Monocryl Plus, PS-2 needle (Ethicon, J&J International) for the skin.
The principles of postoperative management of these complex patients are no different from those guiding the treatment of other patients undergoing cardiac surgery. Close hemodynamic monitoring is critical to allow careful and safe weaning from inotropic and mechanical circulatory support and weaning from the respirator.