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Myocardial Protection and Perfusion Techniques in Mitral Valve Surgery
LEARNING OBJECTIVES
- 1.
The mitral valve surgeon should become familiarized with all types of myocardial protection, including routes of administration and cardioplegic solutions.
- 2.
The mitral valve surgeon should know how to select patients for beating heart procedures (being fully aware of its advantages and disadvantages).
- 3.
When choosing the route of cannulation for cardiopulmonary bypass, the surgeon should consider both central and peripheral cannulation as options, securing both adequate arterial flow and venous drainage.
- 4.
Minimally invasive procedures should become the standard for mitral valve repair or replacement.
INTRODUCTION
Despite major advances in technologies and strategies aimed at reducing the proinflammatory effects of cardiopulmonary bypass (CPB) on the myocardium, myocardial ischemia and postischemic myocardial dysfunction can still occur. Myocardial damage is an independent predictor of adverse outcomes and reduced long-term survival following heart valve surgery. Myocardial protection and perfusion techniques are key factors to achieve successful outcomes, primarily related to their ability to attenuate ischemia-reperfusion injury on CPB and subsequent tissue damage and organ dysfunction. This chapter is a brief review of myocardial protection and perfusion techniques as currently applied in mitral valve surgery.
MYOCARDIAL PROTECTION AND CARDIOPLEGIA
The two cornerstones of myocardial protection during CPB are hypothermia and electromechanical cardiac arrest. Cardioplegia literally means “heart paralysis”; it relates to the intentional and temporary cessation of cardiac activity during surgery. The main objectives of cardioplegic solutions are the arrest of the heart in diastole and preservation of myocardial function ( Table 14.1 ). Additional protection is provided by the addition of acid–base buffers, energy substrates, additives, or oxygenated blood to the cardioplegia infusion. Electromechanical arrest reduces myocardial metabolism, thereby allowing the patient to tolerate intermittent periods of ischemia. Electromechanical arrest is usually achieved with potassium infusion, which leads to diastolic cardiac arrest. The cardioplegic solutions are dissolved in crystalloid fluids or in the blood of the patient, being delivered intermittently or continuously, using antegrade (aortic root or coronary ostia), retrograde (coronary sinus), or both routes of administration. There are two types of crystalloid cardioplegic solutions: the intracellular type and the extracellular type. The intracellular types are characterized by absent or low concentrations of sodium and calcium. The extracellular types contain relatively higher concentrations of sodium, calcium, and magnesium. Both types avoid concentrations of potassium greater than 40 mmol/L (typical range is 10 to 40 mmol/L), contain bicarbonate for buffering, and are osmotically balanced.
TABLE 14.1
Main Aims of Cardioplegia
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Cardiopulmonary Bypass
CPB is usually instituted by placing two right-angled cannulas into the superior and inferior vena cava or one cannula into the right atrial appendage. A small (22 Fr) plastic or metal cannula is placed directly into the superior vena cava (SVC), above the sinoatrial node. The inferior caval cannula is placed at the entrance of the inferior vena cava, low in the right atrium. Bypass flows above 1.5 L/m 2 /min and mild hypothermia is used with vacuum-assisted suction. In the setting of a minimally invasive procedure, the selection of cannulas and cannulation sites is of paramount importance for a stable CPB and bloodless exposure of the mitral valve. Particularly in this scenario, we use a 22 to 26 Fr percutaneous multi-stage venous cannula placed into the right atrium, at the SVC junction, via the right common femoral vein under transesophageal echocardiographic (TEE) guidance. Because the cannula has multiple holes (multi-stage) and flexibility, reaching the inferior vena cava, the drainage of both vena cava can be easily secured. If a concomitant coronary artery bypass procedure requiring full sternotomy is to be performed, then venous cannulation should be bicaval via the atria. One curved venous cannula should be placed in the SVC above the cavoatrial junction and the other through the lowest part of the right atrium into the mouth of the inferior vena cava. The arterial cannula should be placed directly into the distal ascending aorta. Myocardial protection includes antegrade and retrograde blood cardioplegia and profound myocardial hypothermia. Retrograde cardioplegia is useful for all valvular surgery to protect the ischemic left ventricle and help remove air from the ascending aorta. Antegrade cardioplegia, used as an initial loading dose, is augmented by intermittent retrograde cardioplegia every 20 minutes. This provides safer delivery of cardioplegia because when the atrium is retracted during valve replacement, the aortic valve is distorted, and antegrade cardioplegia tends to fill the ventricle due to aortic regurgitation causing left ventricular distension.
MYOCARDIAL PROTECTION AND HYPOTHERMIA
Hypothermic cardioplegia, introduced in the 1960s, is effective in decreasing myocardial metabolism and reducing myocardial oxygen consumption. Electromechanical arrest leads to a 90% reduction in oxygen consumption. By decreasing the metabolic rate, hypothermia reduces oxygen demand and increases the tolerance to ischemia. Also, hypothermia reduces the amount of potassium required for the induction of cardiac arrest and inhibits the intracellular accumulation of calcium. However, hypothermia has a potential harmful influence on tissues due to oxidative stress (production of free-radicals) as well as inflammation (production of cytokines). Although both solutions can induce electromechanical cardiac arrest, hypothermic blood cardioplegia has been shown to be superior to crystalloid-based solutions in terms of myocardial protection in experimental studies when comparing the release of cardiac enzymes and metabolic response, in part because of the characteristics of its blood-based composition. Noteworthy, the two largest clinical trials comparing blood and crystalloid cardioplegia , showed no statistical differences in terms of postoperative clinical outcomes. Furthermore, the largest meta-analysis (including 36 randomized controlled trials [RCTs]) showed similar results in terms of in-hospital death, perioperative myocardial infarction, and low cardiac output.
Topical Hypothermia
Topical hypothermia with iced slush has been previously used as an adjunct to myocardial protection. Although some studies have demonstrated some efficacy in topical cooling, other studies have shown no additional cardioprotective benefit above systemic hypothermia and cold blood cardioplegia alone. Moreover, several studies have demonstrated that the use of topical hypothermia is associated with increased diaphragmatic paralysis due to phrenic nerve palsy and pulmonary complications. Topical hypothermia became less important in recent years to the point that it was not even a matter of discussion in the section for cardioplegia of the last European Association for Cardio-Thoracic Surgery (EACTS) European Association of Cardiothoracic (EACTA) European Board of Cardiovascular Perfusion (EBCP) guidelines on CPB in adult cardiac surgery.
Normothermia Versus Hypothermia
Normothermic myocardial protection is usually performed by the continuous delivery of hyperkalemic normothermic blood during aortic cross-clamping. Warm blood cardioplegia offers adequate myocardial protection during CPB. The benefits obtained by normothermia include a constant oxygen supply and the preservation of aerobic metabolism, low adrenergic response with a subsequent better cardiac index. Lower incidence of ventricular arrhythmias after cross-clamp release with the use of warm heart protection has been reported. Furthermore, the technique of using normothermic CPB and utilization of warm blood cardioplegia may be particularly helpful to mitigate the potential risk of cold agglutination such as in patients with cryoglobulinemia. Although normothermia protects the heart, it has been associated with increased neurological complications. The safe duration of a cardioplegia administration during normothermia is still under debate, and mild (32°C to 35°C) to moderate (28°C to 32°C) hypothermia cardioplegia constitutes an alternative method. Moderate hypothermia has been shown to be associated with similar myocardial oxygen consumption, but less anaerobic lactate washout than normothermic cardioplegia. A systemic temperature of 32°C to 35°C maintained via CPB, combined with mild hypothermic blood cardioplegia appears to best reduce the risks of neurologic complications, limit myocardial injury, and induce better functional myocardial recovery.
CARDIOPLEGIC SOLUTIONS: ROUTES OF ADMINISTRATION
Cardioplegia is effective only if it is well distributed. Adding retrograde perfusion via transatrial cannulation of the coronary sinus improves subendocardial perfusion, limits the removal of retractors during mitral procedures, avoids ostial cannulation during concomitant aortic valve procedures, and permits flushing of air and atheromas during concomitant coronary reoperations. Experimentally, right ventricular nutritive flow is limited by retrograde perfusion. Antegrade or retrograde cardioplegia alone are each inhomogeneous in their distribution in animals with normal coronary arteries as demonstrated in microsphere studies. Therefore, both are needed to ensure a uniform distribution in the myocardium. Noncoronary collateral flow from mediastinal collaterals displaces cardioplegia with warmer systemic blood. As mentioned previously, topical hypothermia slows rewarming but may cause pulmonary complications (phrenic palsy) without adding cardioprotective effects. Hence, continuous or intermittent cardioplegia is required. Continuous cardioplegic perfusion has been advocated to avoid ischemia, but adequate protection may not be achieved at usual flow rates, and the surgeon’s vision becomes obscured during infusion. Intermittent cardioplegia, delivered at 10- to 20-minute intervals, maintains arrest, slows rewarming, and restores substrates depleted during ischemia. In addition, it flushes accumulated metabolites and counteracts acidosis and edema. A dedicated pump on the CPB machine for cardioplegic perfusion is required for specific cannulas for antegrade and retrograde cardioplegia administration and a monitoring-infusion system ( Fig. 14.1 ).
Antegrade Cardioplegia
An antegrade cardioplegia cannula is placed in the ascending aorta below the site chosen for the aortic cross-clamp. An antegrade cannula is placed high on the ascending aorta and slightly to the right side. This cannula contains a pressure line and a vent port to suction air and blood between infusions. A 4-0 purse-string polypropylene suture is used to secure the cannula. When the CPB is running and the heart is empty, thanks to effective systemic venous drainage, the aorta is clamped, and blood antegrade cardioplegia is delivered for 2 minutes at a rate of 200 mL/min. According to the 2019 EACTS/EACTA/EBCP guidelines, during antegrade infusion, aortic root pressure must be maintained between 60 and 100 mm Hg at a flow rate of 200 mL/min or, in hypertrophied hearts, 250 mL/min. Blood cardioplegia is given over time to ensure maximal oxygen delivery. Failure to arrest means inadequate systemic venous drainage (full heart), incomplete aortic clamping, or aortic insufficiency. Aortic pressure below 30 mm Hg means that antegrade flow is insufficient and only retrograde infusion should be used.
Retrograde Cardioplegia
Retrograde cardioplegia delivery is performed through the coronary sinus via the right atrium or direct ostial cannulation. The retrograde cannula has a self-inflating or manually inflated balloon at its tip; it also contains a line for monitoring-infusion pressure. It is placed low in the right atrium, anterior to the venous cannula, and just above the caval junction. It is directed into the coronary sinus by aiming at the patient’s left shoulder. The cannula is advanced until it meets resistance within the coronary sinus, usually adjacent to the left atrial appendage. Coronary sinus cannulation can be performed before venous cannulation to prevent the venous cannula from being an obstacle to the insertion of the retrograde cannula. Otherwise, coronary sinus cannulation can be done on partial bypass with the right atrium slightly distended, with the aim of keeping the sinus ostium open. TEE-guided techniques or surgeon palpation are effective methods to guide the retrograde cannula into its correct position. Commercially available retrograde cannulas usually have a malleable stylet and inflatable balloon cannula. The site of introduction on the atrial wall is secured with a 4-0 purse-string polypropylene suture around the cannula. The coronary sinus can be injured by forceful cannulation or continued administration of cardioplegia when coronary sinus pressure exceeds 50 mm Hg. The perfusionist notes high pressure, then low pressure as a consequence of acute perforation, or the surgeon sees red blood accumulate in the pericardial well. Perforation can be repaired directly with 5-0 polypropylene sutures or pericardial pledgets if the tear site is not distinct. Hematomas are often self-contained after heparin reversal. During retrograde infusion, coronary sinus pressure is maintained between 30 and 50 mm Hg at a flow of 200 to 400 mL/min. A coronary sinus pressure greater than 50 mm Hg means improper positioning or cardiac retraction. This is corrected by reducing the flow rate immediately, repositioning the catheter, and then resuming the flow. If severe aortic valve insufficiency is present, direct coronary ostial administration or retrograde cardioplegia can be delivered. If there is concomitant coronary artery disease, myocardial protection in this circumstance should be antegrade and retrograde to ensure adequate distribution of the cardioplegia, , considering that the coronary artery bypass graft/mitral operation presents one of the highest-risk operative settings in cardiac surgery.
BASIC CONCEPTS IN CARDIOPLEGIA-STANDARD VS MICROPLEGIA
Cardioplegic solutions are classified as crystalloid or blood-based solutions delivered either warm or cold. Cold blood cardioplegia is the most commonly used cardioplegic technique. The rationale for using blood for hypothermic potassium-induced cardiac arrest is that (a) it provides an oxygenated environment and a method for intermittent reoxygenation of the heart during arrest; (b) limits hemodilution when large volumes of cardioplegic solution are used; (c) affords an excellent buffering capacity and osmotic properties; (d) allows for electrolyte composition and pH that are physiologic; (e) offers a number of endogenous antioxidants and free-radical scavengers; and (f) is less complex to prepare than other solutions. The early development of cardioplegia used crystalloid-containing solutions only, which permits rapid cardiac arrest via the effects of high potassium, cold temperature, and hypoxia induced by the absence of oxygen. Generally, crystalloid solutions cause myocardial arrest induced by depolarization. Solutions that depolarize contain high extracellular potassium, which may lead to myocardial dysfunction during the post-CPB period due to accumulation of intracellular calcium and sodium during the arrest period. Recently, several crystalloid solutions have been reintroduced. The two most common solutions used are the del Nido solution and Custodiol-HTK solution. Both arrest the heart in diastole although by different mechanisms. In the del Nido solution, the high potassium concentration prevents repolarization, so the cell stays depolarized. When the resting potential approaches −50 mV, sodium channels are inactivated resulting in a diastolic arrest of cardiac activity. In Custodiol-HTK solution, the low extracellular sodium reduces the transmembrane sodium gradient so that not enough sodium can enter the myocyte during phase 0 of the cardiac action potential to cause depolarization, the myocyte membrane remains hyperpolarized, and is arrested in diastole. These solutions are thought to provide superior myocardial protection and increase the duration of time before repeat cardioplegia is required, allowing prolonged uninterrupted surgery. , This may reduce the aortic cross-clamp and CPB times and potentially the need for a retrograde catheter, as suggested by LeBon et al. Limitations of a purely crystalloid approach are that high volumes are administered and lack of oxygen is delivered to the myocardium. Blood cardioplegia adds blood to the crystalloid solution. These typically consist of 4:1 ratios of blood to cardioplegia, but this may be greatly reduced in the microplegia systems available today. The dose of additional crystalloid used in microplegia is small, thus allowing large volumes of cardioplegia to be delivered without causing hemodilution or myocardial edema.
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