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by Tomas A. Salerno, MD, Mohan Thanikachalam, MD, Kushagra Katariya, MD, and Anthony L. Panos, MD
Simultanous Antegrade/Retrograde Normothermic Blood Perfusion of the Heart in Valvular and Other Complex Surgery (Beating Heart Valvular Surgery): A New Concept in Myocardial Protection
Abstract
A new concept of myocardial protection during valvular and other complex procedures is presented. The method relies upon continuous, simultaneous, antegrade/retrograde perfusion of the empty-beating heart with normothermic blood. Technical details of the method are described in detail.
Introduction
In the current era of aging population, medical advances, including wide spread use percutaneous interventions, have dramatically changed the spectrum of patients referred for cardiac surgery. Compared to just a decade ago, patients undergoing cardiac surgery are older, present at a later stage of the disease process and often have poor left ventricular (LV) function that entails more complex, multiple valvular surgery and other reparative surgery.
Due to the changing complexity of the disease process, cardiac surgeons are often confronted by new and increasingly multifaceted challenges during the modern era of curative medical action. The primary goal in any cardiac surgical procedure is surgery which is technically adequate and that creates little or no myocardial damage.
The development of extracorporeal circulation in the late 1960’s and the use of cardiopulmonary bypass (CPB) combined with cardioplegia (to achieve electromechanical arrest), became the popular method of reparative cardiac surgery in a bloodless and motionless field. Research initiated by curious and concerned surgeons identified many damaging effects of CPB and cardioplegic arrest. This sparked much interest and directed a great deal of resources toward increasing the safety of CPB and establishing new strategies of myocardial preservation.
Most of the current myocardial protective strategies utilizing cold blood cardioplegia have withstood the test of time, providing safe and effective myocardial protection during various cardiac operations. Nevertheless, all cardioplegic myocardial protective strategies devised to date subject the heart to a period of so called "mandatory ischemia", where the heart is without circulation. This subsequently leads to reperfusion injury when the aorta is declamped [1]. Despite the use of these current myocardial protective strategies with cold blood antegrade and/or retrograde cardioplegia, postcardiotomy cardiogenic shock is still a reality in high risk patients with severely compromised LV function [2].
In the current surgical climate, with increased number of high risk patients, it became obvious that alternative methods to conventional myocardial protective strategies were needed. One response to this challenge involved the resurgence of direct myocardial revascularization without CPB. We believe that off-pump coronary artery bypass (OPCAB) comprises one the most important innovations in cardiac surgery in the last decade. Direct coronary surgery without CPB is currently being performed safely and effectively around the world. The resurgence of OPCAB has led to a paradigm shift in the way in which surgeons approach myocardial protection for coronary surgery.
New Concept in Myocardial protection (Beating Valve Surgery): The Rationale
The theoretical and conceptual basis for this new method of myocardial protection lies in warm heart surgery [3]. The ability of the cardiac muscle to tolerate ischemia is finite. Kirklin, et al., illustrated that mortality and morbidity are directly proportional to the period of aortic clamping, i.e., ischemic time [4]. The combination of hypothermia, introduced by Bigelow [5], and potassium cardioplegic arrest, introduced by Melrose [6], became the most common methods of myocardial protection during the 60’s and 70’s. Later, the addition of blood to cardioplegia was seen as a way of providing oxygen, nutrients and buffer [7]. However, a delicate balance exists between the beneficial and deleterious effects of hypothermia on the myocardium, which is jeopardized at the time of surgery. Hypothermia has been shown to be counterproductive at temperatures <15oC due to the leftward shift of the oxyhemoglobin dissociation curve, leading to decreased oxygen unloading and to impaired utilization of oxygen by the myocardium. Thus, hypothermic blood cardioplegia causes cold ischemia and anaerobic arrest. To prepare the heart for this insult, cold arrest was supplemented with warm induction [8] and terminal hot shot [9]. This method of sandwiched cold cardioplegia proved to be better strategy for myocardial protection, especially in ischemic injured hearts [10].
In light of findings that warm blood cardioplegia added a measure of protection when placed at the beginning and at the end of cross clamp duration, so as to reduce the intervening cold-ischemic-anaerobic arrest, we posed fundamental queries. One of these questions attempted to ascertain why ischemia could not be eliminated altogether by administering continuous warm blood cardioplegia. The notion, which logically followed, given the deleterious effect of hypothermia and the very minimal additive benefit to myocardial protection, was that aggressive cooling of the heart was not warranted.
Based on this premise, our group introduced the concept of warm heart surgery with the use of continuous warm blood cardioplegia as a means of myocardial protection and prevention of ischemia [3]. Since its induction, multiple studies have shown that warm heart surgery is comparable to cold cardioplegia in its safety record. From a metabolic standpoint, it provides superior myocardial protection. As well, high-risk patients who may have metabolically compromised hearts show greater benefit from reduced ischemic damage of the myocardium [11,12].
The initial concept of continuous retrograde warm blood cardioplegia [13] was followed by intermittent antegrade infusions of warm blood cardioplegia, as suggested by the findings of different perfusion beds of the heart with antegrade and retrograde cardioplegia [14]. Subsequently, from the clinical observation that the coronary sinus did not “blow out” when retrograde cardioplegia was being delivered and the aorta was declamped, the concept of simultaneous antegrade/retrograde continuous warm blood cardioplegia was developed and published as an alternative and safe method of myocardial protection [15].
As a natural extension of warm heart surgery, beating heart valve surgery was born. Again fundamental questions were posed: why arrest the heart if technically adequate valve procedures could be accomplished with continuous warm perfusion?
In order to avoid myocardial edema, which is intrinsic to an arrested heart [16] and to avoid ischemia-reperfusion injury [1] our group began performing beating heart valve surgeries about 6 years ago. With the aorta clamped, warm blood is given simultaneously antegrade and retrograde, continuously perfusing the heart. During the aortic clamping period, the electrocardiogram remained normal while the empty heart continued to beat. The technical detail of blood in the operative field was overcome in most of these procedures with small changes in the surgeon’s techniques. Air removal and prevention of air embolization were also overcome utilizing similar techniques as for regular procedures.
Weaning from CPB was noted to be much easier when compared to the cardioplegic method. Furthermore, ventricular fibrillation seldom occurred and patients seemed to do better in general.
Preliminary data from our group, and others, suggest that beating heart valve surgery is safe and that there may be a benefit to high risk patients [17-21]. We believe that combined antegrade and retrograde perfusion provides the best myocardial protection possible for the entire heart although other surgeons, such as Matsmumoto and associates [21], have reported good results with beating heart aortic valve surgery using retrograde perfusion alone. However, animal studies indicate that retrograde perfusion alone may not provide protective flow to all areas of the heart [14]. Furthermore, we currently perform beating heart mitral valve surgery without aortic cross clamp thus eliminating the need for retrograde or antegrade perfusion. Even though theoretically using this technique the risk of air embolism may increase, in our practice, this has been observed. Using a technique for mitral valve surgery similar to ours, Thompson and associates illustrated that the incidence of CVA was 1.6%, none of which were due to air emboli [20].
Surgical Techniques
Aortic Valve Surgery
Aortic cannulation is performed in the standard fashion via the ascending aorta. Venous outflow is achieved by the use of a single, double-stage venous cannula in the right atrium. Prior to initiation of CPB, a catheter is placed on the coronary sinus, and upon initiation of CPB, the catheter is snared by placing a 4-0 prolene suture from the external surface, around the coronary sinus, to prevent it from dislodging during perfusion [22]. As the aorta is being clamped, retroperfusion of the coronary sinus is initiated using blood from the CPB machine, using a separate pump head, at pressures of 55 mmHg mean in the coronary sinus, providing a mean pressure in our system of 260 mmHg back at the pump head. Both of these pressures are important as a computer is programmed not to exceed these pressures to prevent coronary sinus damage. Flows in the coronary sinus system are about 300 mL/min. The aorta is opened in the usual fashion as the heart is beating. The surgeon may vent the LV through the left superior pulmonary vein, which facilitates the procedure, but we usually vent through the aortic valve. A polystan (Polystan, Inc., Denmark), self-inflating, soft catheter is placed in the left ostium and secured in an area distal to the ostium. If possible, a polystan catheter is also placed into the right coronary ostium and secured by suture in the aorta. Both cannulas are connected to the blood perfusion manifold, which also supplies the coronary sinus system of perfusion. Replacement of the aortic valve then proceeds in the usual fashion. De-airing is done in the usual fashion, and the two-ostial cannulae are removed prior to completion of aortotomy closure. Perfusion of the coronary sinus continues with the heart beating until removal of the aortic clamp, at which time flow is discontinued in the perfusion system.
Mitral Valve Surgery
The aorta is cannulated in the usual fashion. One venous cannula is placed in the superior vena cava and the other cannula is placed in the right atrium, so that blood from the inferior vena cava and coronary sinus are drained; if the other cannula is placed in the inferior vena cava, blood will bypass the right side of the heart and will eventually return to the left atrium, flooding the field. A de-airing catheter is inserted into the aortic root, which is kept at low suction throughout the surgery. CPB is initiated and normothermia is maintained. When using the trans-septal approach to the mitral valve, it is necessary to perform bicaval cannulation and snare the cavae with tapes, thereby completely excluding the venous return from the right atrium. Once the atrium is opened, a flexible cardiotomy cannula is introduced into the coronary sinus to collect the coronary effluent and to improve visibility. To avoid air embolism (1) the patient is maintained in the Trendelenberg position, (2) the aortic root vent is maintained at low suction, (3) the systemic perfusion pressures are kept at 80-90 mm Hg during CPB, and (4) a short period of fibrillatory arrest is used while opening the left atrium if the mitral valve is competent. A vent is introduced into the LV via the mitral valve immediately upon opening the left atrium for further prevention of LV distension and air embolism. Using the above precautions we have not encountered any adverse neurologic outcome attributable to air embolism. Once the LA is entered, a retractor is placed to obtain adequate exposure of the mitral valve. It is important at this time to prevent induction of aortic insufficiency due to retraction. Mitral valve replacement or repair can then be performed. In the end, a soft cardiotomy cannula is left inside the LV across the mitral orifice as the LA is closed. The LV vent is later removed and heart is de-aired completely through the aortic root vent. Surgery is then completed in the usual fashion. The presence of severe aortic regurgitation may compromise coronary perfusion as well as visibility of the mitral valve, making beating heart surgery impossible. In such a situation, we use the same strategy as described below for combined aortic/mitral valve operations on an empty, beating heart is used.
Combined aortic/mitral valve surgery
In patients without aortic insufficiency, the mitral valve is replaced/repaired first as described above. Once this is done, retrograde perfusion is begun through the coronary sinus. Aorta is cross-clamped and the aortic root is opened. Simultaneous antegrade and retrograde perfusion is begun and the aortic operation is completed as described above.
In patients with aortic insufficiency, simultaneous antegrade/retrograde perfusion is established as for an aortic operation. The LA is then opened and the mitral valve surgery is performed. The LA is closed as described for a mitral valve operation. The aortic valve is approached next and the operation is completed as described above for aortic valve operation. If there is the need for tricuspid repair, this is done at the end of the operation with the heart beating and the aorta declamped.
Combined valve surgery/CABG
We routinely perform the CABG on a beating heart without CPB. Next, we place the patient on CPB and perform the valve surgery. In patients requiring AVR/CABG we perform the distal anastomoses first and then use the grafts and the native coronary vessels for antegrade blood perfusion. Once the valve is replaced and the aortic root is closed, the proximal anastomoses of the coronary grafts are constructed. Only rarely, have we observed deterioration of the hemodynamic parameters during performance of off-pump coronary artery bypass, necessitating revascularization on a beating-empty heart after institution of CPB.
Aortic Dissections and other complex procedures
In aortic dissections, after circulatory arrest is initiated, retroperfusion of the brain is begun, and the heart is perfused retrogradely with blood at the same temperature as the reservoir, usually 18oC; if possible, we also perfuse the left and right ostia. For Bentall procedures for ascending aortic aneurysm, the heart will be perfused and beating throughout the procedures by perfusing blood into the coronary sinus and both the left and right ostial buttons [23]. For post-myocardial infarction ventricular septal defect, or LV aneurysm or remodeling procedures, the aorta is left declamped and the heart is allowed to beat throughout the procedure.
Conclusion
The primary aim of the beating heart technique is to avoid ischemic-reperfusion injury in patients with poor ventricular function and little cardiac reserve. Our experience to date and data from other groups suggest that beating heat valve surgery is safe and presents no compromise to other conventional techniques. The data also suggest that there may be a benefit to high risk patients by using the beating valve technique.
Larger randomized studies are needed to study the efficacy of this new modality of myocardial protection aimed at eliminating ischemia-reperfusion injury all together. It appears that the concept of performing reparative valve surgery in a continuously perfused empty beating heart without ischemia-reperfusion injury is valid. As older and sicker patients are referred for cardiac surgery, beating heart techniques may play an important role in expanding the armamentarium of myocardial protective strategies.
References
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