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Crystalloid-Based Cardioplegia for Minimally Invasive Cardiac Surgery
TECHNIQUES MY WAY
Crystalloid-Based Cardioplegia for Minimally Invasive Cardiac Surgery Martin Misfeld, MD, PhD, and Piroze Davierwala, MD With the ever-increasing popularity of minimally invasive (MI) cardiac surgery, procedures like aortic valve replacement, with or without concomitant aortic surgery, and mitral and tricuspid valve procedures are now routinely performed through a minimal-access partial upper sternotomy and right anterolateral small thoracotomy, respectively, in our institution. To have optimal visualization through a small incision, it is extremely important to reduce the number of instruments, retractors, and cannulae passing through the incision to a bare minimum and to avoid repeated manipulation of the operative field. Repeated use of blood cardioplegia to maintain myocardial protection can sometimes prevent the surgeon from executing the aforementioned measures. However, if adequate myocardial protection can be achieved and maintained by administering a single dose of crystalloid cardioplegia, it would help expedite the operation with greater ease. At our institution, myocardial protection during aortic valve surgery is achieved using either blood or crystalloid cardioplegia according to surgeon preference. However, crystalloid cardioplegia has become the standard myocardial protection strategy for performing MI mitral valve surgery. Our experience with crystalloid cardioplegia for MI mitral valve surgery is the focus of this article. Semin Thoracic Surg 24:305-307 © 2012 Elsevier Inc. All rights reserved. Keywords: myocardial protection, cardioplegia, minimally invasive cardiac surgery Minimally invasive (MI) techniques have become routine practice in cardiac surgery. Aortic valve procedures, with or without concomitant aortic surgery, performed through a partial upper sternotomy and mitral and tricuspid valve procedures performed through a minimal-access right anterolateral small thoracotomy are the 2 most common examples representing these techniques. Both these approaches have become the standard of care at our institution. Repeated administration of blood cardioplegia to maintain myocardial protection can sometimes be a hindrance to the smooth and efficient performance of an MI procedure. However, if adequate myocardial protection can be achieved and maintained by administering a single dose of crystalloid cardioplegia, it would help expedite the operation with greater ease. At our institution, myocardial protection during aortic valve surgery is achieved using either
Department of Cardiac Surgery, Heart Center, University of Leipzig, Leipzig, Germany. Address reprint requests to Martin Misfeld, MD, PhD, Department of Cardiac Surgery, Heart Center, University of Leipzig, Struempellstrasse 39, 04289 Leipzig, Germany. E-mail: [email protected]
1043-0679/$-see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.semtcvs.2012.06.006
blood or crystalloid cardioplegia according to surgeon preference. The in-hospital mortality rate in 988 patients undergoing MI isolated aortic valve surgery using crystalloid cardioplegia at our institution was 1.3%. Ten patients developed low cardiac output (LCO), 5 patients each requiring intra-aortic balloon pump (IABP) and/or extracorporeal membrane oxygenation (ECMO). For MI mitral valve surgery, crystalloid cardioplegia has become the standard myocardial protection strategy. Our experience with crystalloid cardioplegia for MI mitral valve surgery will be the focus of this article. SCIENTIFIC BACKGROUND Adequate myocardial protection plays a key role in cardiac surgery. Despite this, there still exists a controversy regarding the optimal solution for myocardial protection. It is still not known whether antegrade and/or retrograde, intermittent or continuous, crystalloid or blood cardioplegia with or without additional warm induction and/or “hot shot” administration has any major impact on clinical outcome. A meta-analysis of 34 trials comparing blood versus crystalloid cardioplegia failed to show any statistically significant difference between both 305
CRYSTALLOID-BASED CARDIOPLEGIA groups with regard to myocardial infarction or death.1 In this analysis, blood cardioplegia was associated with less postoperative LCO syndrome in 10 of 34 trials, and the creatine kinase⫺muscle brain (CK-MB) release was higher in the crystalloid group in 7 of 34 trials. However, Jacob et al2 highlighted that the authors could not extract these data from the 2 largest trials (each with ⬎1000 patients) included in this analysis. Another meta-analysis, comparing the effect of warm versus cold cardioplegia,3 showed that the incidence of clinical events (death, myocardial infarction, postoperative use of IABP, stroke, and atrial fibrillation) was not different in both groups. Both meta-analyses1,3 exclusively focused on coronary artery bypass surgery. Whether these results can be extrapolated to other cardiac operations and specifically to MI mitral valve surgery remains unclear. Braathen et al4 recently published a prospective randomized study, comparing single-dose histidinetryptophan-ketoglutarate (HTK) with repetitive administration of cold blood cardioplegia. They concluded that both strategies protect the myocardium equally. To the best of our knowledge, there is only 1 study published in literature that investigated HTK cardioplegia for MI mitral valve surgery. Eight patients studied during this investigation showed neither significant electrocardiographic changes nor raised myocardial cytonecrosis enzymes (creatine kinase and its MB fraction) during the postoperative period.5 However, it can be stated that the choice of myocardial protection mainly relies on the surgeon preference.6 METHODS Custodiol HTK cardioplegia (Bretschneider; Koehler Chemie, Alsbach-Haenlien, Germany) has been used in Germany for many years.7 It is the cardioplegia of choice at our institution, especially for
Figure 1. Cannula used for insertion into the aortic root in minimally invasive mitral valve surgery.
306
Figure 2. Operative setup in minimally invasive mitral valve surgery. The cardioplegia line is brought out through an upper incision, which is also used for insertion of the mitral valve retractor, and connected to a 3-way cock.
MI mitral valve surgery, as it has been shown to accomplish adequate and safe myocardial protection for ⬎2 hours after its administration. After insertion of a 7-F cardioplegia needle in the ascending aorta (CalMed Technologies, CA, Fig. 1), it is brought out of the operation field either directly through the lateral thoracotomy incision (between the soft-tissue retractor and the thoracic wall for optimal visualization) or through an upper small skin incision through which the mitral valve retractor is inserted (Fig. 2). The cardioplegia needle is then connected to a 3-way cock, the other ends of which are connected to the incoming cardioplegia delivery line from the pump and venting line for the aortic root. After an initial dose of 1800 mL Bretschneider cardioplegia at a temperature of 6-8°C, the aortic root is briefly vented through the aortic root vent. This helps us confirm proper application of the aortic cross-clamp, which, if not the case, will be indicated by suction of blood into the aortic root vent. Because every patient undergoing mitral valve surgery undergoes routine preoperative echocardiography, adequate planning of the operative strategy can be performed before MI surgery. Ideally, aortic regurgitation greater than grade 1 should be excluded preoperatively, and if it does exist, the patient should undergo mitral valve surgery through a conventional median sternotomy. Owing to the inadequate visualization of the left ventricle during administration of cardioplegia through an MI approach, it is extremely important that intraoperative transesophageal echocardiography detects significant aortic regurgitation during cardioplegia delivery. If that is the case, the patient often has to be converted to a median sternotomy to administer selective ostial cardioplegia.
Seminars in Thoracic and Cardiovascular Surgery ● Volume 24, Number 4
CRYSTALLOID-BASED CARDIOPLEGIA Optimal visualization of the mitral valve during mitral valve surgery, especially mitral valve repair, is one of the essential key points. Poor visualization frustrates the surgeon and prevents him from performing reconstructive techniques. In MI mitral valve surgery, visualization of the mitral valve can be accomplished in an ideal way owing to the direct view of the mitral valve through a right anterolateral thoracotomy. However, this presumes that the operating field is optimally set up. If repetitive administration of cardioplegia is necessary, as with the use of blood cardioplegia, the surgeon’s rhythm is repeatedly interrupted. The mitral valve retractor has to be released every time during administration of blood cardioplegia so the aortic valve is not distorted and thus incompetent. In contrast, use of a single shot of antegrade crystalloid cardioplegia gives the surgeon enough time (up to ⬎2 hours) to concentrate on the repair without interrupting his rhythm. The additional use of retrograde cardioplegia during MI mitral valve surgery is limited owing to the difficulty in insertion of the retrograde cardioplegia cannula into the coronary sinus. However, when tricuspid valve surgery is also concomitantly performed, the cannula can directly be inserted into the coronary sinus. There have always been concerns as to whether retrograde cardioplegia alone can sufficiently protect the right heart.8,9 We, therefore, routinely protect the heart during MI mitral valve surgery by a single dose of antegrade crystalloid cardioplegia. RESULTS We could demonstrate that MI mitral valve surgery can be performed with excellent results.10 Be-
1. Guru V, Omura J, Alghamdi AA, et al: Is blood superior to crystalloid cardioplegia? A metaanalysis of randomized clinical trial. Circulation 114(suppl 1):I331-I338, 2006 2. Jacob S, Kallikourdis A, Sellke F, et al: Is blood cardioplegia superior to crystalloid cardioplegia? Interact Cardiovasc Thorac Surg 7:491499, 2008 3. Fan Y, Zhang A-M, Xiao Y-B, et al: Warm versus cold cardioplegia for heart surgery: A meta-analysis. Eur J Cardiothorac Surg 37: 912-919, 2010 4. Braathen B, Jeppsson A, Scherstén H, et al: One single dose of histidine-tryptophane-ketoglutarate solution gives equally good myocardial protection in elective mitral valve surgery as repetitive cold blood cardioplegia: A prospective randomized study. J Thorac Cardiovasc Surg 141:995-1001, 2011
tween 1999 and 2012, 2731 patients underwent MI mitral valve surgery using Custodiol HTK cardioplegia solution at our institution. Forty (1.5%) patients developed LCO postoperatively. Of these, 5 patients were managed conservatively with inotropic support. IABP and ECMO were implanted in 25 and 5 patients, respectively. Four patients with LCO received both IABP and ECMO. In-hospital mortality rate was 1.2%. The rate of conversion to full sternotomy was low (1.1%) and unrelated to the technique of cardioplegia administration. Long-term outcomes and reoperation rates for different pathologies (posterior, anterior, or bileaflet prolapse) were not significantly different.11 An MI approach to surgery could be used in 87.2% of Mitral regurgitation (MR). The cumulative survival rate at 5 years for all patients was 87.3%. The 5-year survival rates were 86.9%, 80.9%, and 90.7% for patients with isolated posterior mitral leaflet prolapse, isolated anterior mitral leaflet prolapse, and bileaflet prolapse, respectively. Similar results could also be achieved in elderly patients.12 CONCLUSIONS As in MI aortic valve surgery with or without replacement of the ascending aorta or arch, different cardioplegic regimes can also be used in MI mitral valve surgery, depending on surgeon preference. However, a single dose of antegrade crystalloid cardioplegia appears to be the most efficient and safe technique of cardioplegia delivery for MI mitral valve surgery. It gives adequate myocardial protection, does not obstruct the operating rhythm of the surgeon, and results in excellent immediate and longterm outcomes in patients undergoing this procedure.
5. Savini C, Camurri N, Castelli A, et al: Myocardial protection using HTK solution in minimally invasive mitral valve surgery. Heart Surg Forum 8:E25-E27, 2005 6. Karthik S, Grayson AD, Oo AY, et al: A survey of current myocardial protection practices during coronary artery bypass grafting. Ann R Coll Surg Engl 86:413-415, 2004 7. Beyersdorf F, Krause E, Sarai K, et al: Clinical evaluation of hypothermic ventricular fibrillation, multi-dose blood cardioplegia, and single-dose Bretschneider cardioplegia in coronary surgery. Thorac Cardiovasc Surg 38:2029, 1990 8. Winkelmann J, Aronson S, Young CJ, et al: Retrograde-delivered cardioplegia is not distributed equally to the right ventricular free wall and septum. J Cardiothorac Vasc Anesth 9:135-139, 1995
Seminars in Thoracic and Cardiovascular Surgery ● Volume 24, Number 4
9. Allen BS, Winkelmann JW, Hanafy H, et al: Retrograde cardioplegia does not adequately perfuse the right ventricle. J Thorac Cardiovasc Surg 109:1116-1124, 1995 10. Seeburger J, Borger MA, Falk V, et al: Minimal invasive mitral valve repair for mitral regurgitation: Results of 1339 consecutive patients. Eur J Cardiothorac Surg 34:760-765, 2008 11. Seeburger J, Borger MA, Doll N, et al: Comparison of outcomes of minimally invasive mitral valve surgery for posterior, anterior and bileaflet prolapse. Eur J Cardiothorac Surg 36: 532-538, 2009 12. Holzhey DM, Shi W, Borger MA, et al: Minimally invasive versus sternotomy approach for mitral valve surgery in patients greater than 70 years old: A propensity-matched comparison. Ann Thorac Surg 91:401-405, 2011
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Crystalloid-Based Cardioplegia for Minimally Invasive Cardiac Surgery Martin Misfeld, MD, PhD, and Piroze Davierwala, MD With the ever-increasing popularity of minimally invasive (MI) cardiac surgery, procedures like aortic valve replacement, with or without concomitant aortic surgery, and mitral and tricuspid valve procedures are now routinely performed through a minimal-access partial upper sternotomy and right anterolateral small thoracotomy, respectively, in our institution. To have optimal visualization through a small incision, it is extremely important to reduce the number of instruments, retractors, and cannulae passing through the incision to a bare minimum and to avoid repeated manipulation of the operative field. Repeated use of blood cardioplegia to maintain myocardial protection can sometimes prevent the surgeon from executing the aforementioned measures. However, if adequate myocardial protection can be achieved and maintained by administering a single dose of crystalloid cardioplegia, it would help expedite the operation with greater ease. At our institution, myocardial protection during aortic valve surgery is achieved using either blood or crystalloid cardioplegia according to surgeon preference. However, crystalloid cardioplegia has become the standard myocardial protection strategy for performing MI mitral valve surgery. Our experience with crystalloid cardioplegia for MI mitral valve surgery is the focus of this article. Semin Thoracic Surg 24:305-307 © 2012 Elsevier Inc. All rights reserved. Keywords: myocardial protection, cardioplegia, minimally invasive cardiac surgery Minimally invasive (MI) techniques have become routine practice in cardiac surgery. Aortic valve procedures, with or without concomitant aortic surgery, performed through a partial upper sternotomy and mitral and tricuspid valve procedures performed through a minimal-access right anterolateral small thoracotomy are the 2 most common examples representing these techniques. Both these approaches have become the standard of care at our institution. Repeated administration of blood cardioplegia to maintain myocardial protection can sometimes be a hindrance to the smooth and efficient performance of an MI procedure. However, if adequate myocardial protection can be achieved and maintained by administering a single dose of crystalloid cardioplegia, it would help expedite the operation with greater ease. At our institution, myocardial protection during aortic valve surgery is achieved using either
Department of Cardiac Surgery, Heart Center, University of Leipzig, Leipzig, Germany. Address reprint requests to Martin Misfeld, MD, PhD, Department of Cardiac Surgery, Heart Center, University of Leipzig, Struempellstrasse 39, 04289 Leipzig, Germany. E-mail: [email protected]
1043-0679/$-see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1053/j.semtcvs.2012.06.006
blood or crystalloid cardioplegia according to surgeon preference. The in-hospital mortality rate in 988 patients undergoing MI isolated aortic valve surgery using crystalloid cardioplegia at our institution was 1.3%. Ten patients developed low cardiac output (LCO), 5 patients each requiring intra-aortic balloon pump (IABP) and/or extracorporeal membrane oxygenation (ECMO). For MI mitral valve surgery, crystalloid cardioplegia has become the standard myocardial protection strategy. Our experience with crystalloid cardioplegia for MI mitral valve surgery will be the focus of this article. SCIENTIFIC BACKGROUND Adequate myocardial protection plays a key role in cardiac surgery. Despite this, there still exists a controversy regarding the optimal solution for myocardial protection. It is still not known whether antegrade and/or retrograde, intermittent or continuous, crystalloid or blood cardioplegia with or without additional warm induction and/or “hot shot” administration has any major impact on clinical outcome. A meta-analysis of 34 trials comparing blood versus crystalloid cardioplegia failed to show any statistically significant difference between both 305
CRYSTALLOID-BASED CARDIOPLEGIA groups with regard to myocardial infarction or death.1 In this analysis, blood cardioplegia was associated with less postoperative LCO syndrome in 10 of 34 trials, and the creatine kinase⫺muscle brain (CK-MB) release was higher in the crystalloid group in 7 of 34 trials. However, Jacob et al2 highlighted that the authors could not extract these data from the 2 largest trials (each with ⬎1000 patients) included in this analysis. Another meta-analysis, comparing the effect of warm versus cold cardioplegia,3 showed that the incidence of clinical events (death, myocardial infarction, postoperative use of IABP, stroke, and atrial fibrillation) was not different in both groups. Both meta-analyses1,3 exclusively focused on coronary artery bypass surgery. Whether these results can be extrapolated to other cardiac operations and specifically to MI mitral valve surgery remains unclear. Braathen et al4 recently published a prospective randomized study, comparing single-dose histidinetryptophan-ketoglutarate (HTK) with repetitive administration of cold blood cardioplegia. They concluded that both strategies protect the myocardium equally. To the best of our knowledge, there is only 1 study published in literature that investigated HTK cardioplegia for MI mitral valve surgery. Eight patients studied during this investigation showed neither significant electrocardiographic changes nor raised myocardial cytonecrosis enzymes (creatine kinase and its MB fraction) during the postoperative period.5 However, it can be stated that the choice of myocardial protection mainly relies on the surgeon preference.6 METHODS Custodiol HTK cardioplegia (Bretschneider; Koehler Chemie, Alsbach-Haenlien, Germany) has been used in Germany for many years.7 It is the cardioplegia of choice at our institution, especially for
Figure 1. Cannula used for insertion into the aortic root in minimally invasive mitral valve surgery.
306
Figure 2. Operative setup in minimally invasive mitral valve surgery. The cardioplegia line is brought out through an upper incision, which is also used for insertion of the mitral valve retractor, and connected to a 3-way cock.
MI mitral valve surgery, as it has been shown to accomplish adequate and safe myocardial protection for ⬎2 hours after its administration. After insertion of a 7-F cardioplegia needle in the ascending aorta (CalMed Technologies, CA, Fig. 1), it is brought out of the operation field either directly through the lateral thoracotomy incision (between the soft-tissue retractor and the thoracic wall for optimal visualization) or through an upper small skin incision through which the mitral valve retractor is inserted (Fig. 2). The cardioplegia needle is then connected to a 3-way cock, the other ends of which are connected to the incoming cardioplegia delivery line from the pump and venting line for the aortic root. After an initial dose of 1800 mL Bretschneider cardioplegia at a temperature of 6-8°C, the aortic root is briefly vented through the aortic root vent. This helps us confirm proper application of the aortic cross-clamp, which, if not the case, will be indicated by suction of blood into the aortic root vent. Because every patient undergoing mitral valve surgery undergoes routine preoperative echocardiography, adequate planning of the operative strategy can be performed before MI surgery. Ideally, aortic regurgitation greater than grade 1 should be excluded preoperatively, and if it does exist, the patient should undergo mitral valve surgery through a conventional median sternotomy. Owing to the inadequate visualization of the left ventricle during administration of cardioplegia through an MI approach, it is extremely important that intraoperative transesophageal echocardiography detects significant aortic regurgitation during cardioplegia delivery. If that is the case, the patient often has to be converted to a median sternotomy to administer selective ostial cardioplegia.
Seminars in Thoracic and Cardiovascular Surgery ● Volume 24, Number 4
CRYSTALLOID-BASED CARDIOPLEGIA Optimal visualization of the mitral valve during mitral valve surgery, especially mitral valve repair, is one of the essential key points. Poor visualization frustrates the surgeon and prevents him from performing reconstructive techniques. In MI mitral valve surgery, visualization of the mitral valve can be accomplished in an ideal way owing to the direct view of the mitral valve through a right anterolateral thoracotomy. However, this presumes that the operating field is optimally set up. If repetitive administration of cardioplegia is necessary, as with the use of blood cardioplegia, the surgeon’s rhythm is repeatedly interrupted. The mitral valve retractor has to be released every time during administration of blood cardioplegia so the aortic valve is not distorted and thus incompetent. In contrast, use of a single shot of antegrade crystalloid cardioplegia gives the surgeon enough time (up to ⬎2 hours) to concentrate on the repair without interrupting his rhythm. The additional use of retrograde cardioplegia during MI mitral valve surgery is limited owing to the difficulty in insertion of the retrograde cardioplegia cannula into the coronary sinus. However, when tricuspid valve surgery is also concomitantly performed, the cannula can directly be inserted into the coronary sinus. There have always been concerns as to whether retrograde cardioplegia alone can sufficiently protect the right heart.8,9 We, therefore, routinely protect the heart during MI mitral valve surgery by a single dose of antegrade crystalloid cardioplegia. RESULTS We could demonstrate that MI mitral valve surgery can be performed with excellent results.10 Be-
1. Guru V, Omura J, Alghamdi AA, et al: Is blood superior to crystalloid cardioplegia? A metaanalysis of randomized clinical trial. Circulation 114(suppl 1):I331-I338, 2006 2. Jacob S, Kallikourdis A, Sellke F, et al: Is blood cardioplegia superior to crystalloid cardioplegia? Interact Cardiovasc Thorac Surg 7:491499, 2008 3. Fan Y, Zhang A-M, Xiao Y-B, et al: Warm versus cold cardioplegia for heart surgery: A meta-analysis. Eur J Cardiothorac Surg 37: 912-919, 2010 4. Braathen B, Jeppsson A, Scherstén H, et al: One single dose of histidine-tryptophane-ketoglutarate solution gives equally good myocardial protection in elective mitral valve surgery as repetitive cold blood cardioplegia: A prospective randomized study. J Thorac Cardiovasc Surg 141:995-1001, 2011
tween 1999 and 2012, 2731 patients underwent MI mitral valve surgery using Custodiol HTK cardioplegia solution at our institution. Forty (1.5%) patients developed LCO postoperatively. Of these, 5 patients were managed conservatively with inotropic support. IABP and ECMO were implanted in 25 and 5 patients, respectively. Four patients with LCO received both IABP and ECMO. In-hospital mortality rate was 1.2%. The rate of conversion to full sternotomy was low (1.1%) and unrelated to the technique of cardioplegia administration. Long-term outcomes and reoperation rates for different pathologies (posterior, anterior, or bileaflet prolapse) were not significantly different.11 An MI approach to surgery could be used in 87.2% of Mitral regurgitation (MR). The cumulative survival rate at 5 years for all patients was 87.3%. The 5-year survival rates were 86.9%, 80.9%, and 90.7% for patients with isolated posterior mitral leaflet prolapse, isolated anterior mitral leaflet prolapse, and bileaflet prolapse, respectively. Similar results could also be achieved in elderly patients.12 CONCLUSIONS As in MI aortic valve surgery with or without replacement of the ascending aorta or arch, different cardioplegic regimes can also be used in MI mitral valve surgery, depending on surgeon preference. However, a single dose of antegrade crystalloid cardioplegia appears to be the most efficient and safe technique of cardioplegia delivery for MI mitral valve surgery. It gives adequate myocardial protection, does not obstruct the operating rhythm of the surgeon, and results in excellent immediate and longterm outcomes in patients undergoing this procedure.
5. Savini C, Camurri N, Castelli A, et al: Myocardial protection using HTK solution in minimally invasive mitral valve surgery. Heart Surg Forum 8:E25-E27, 2005 6. Karthik S, Grayson AD, Oo AY, et al: A survey of current myocardial protection practices during coronary artery bypass grafting. Ann R Coll Surg Engl 86:413-415, 2004 7. Beyersdorf F, Krause E, Sarai K, et al: Clinical evaluation of hypothermic ventricular fibrillation, multi-dose blood cardioplegia, and single-dose Bretschneider cardioplegia in coronary surgery. Thorac Cardiovasc Surg 38:2029, 1990 8. Winkelmann J, Aronson S, Young CJ, et al: Retrograde-delivered cardioplegia is not distributed equally to the right ventricular free wall and septum. J Cardiothorac Vasc Anesth 9:135-139, 1995
Seminars in Thoracic and Cardiovascular Surgery ● Volume 24, Number 4
9. Allen BS, Winkelmann JW, Hanafy H, et al: Retrograde cardioplegia does not adequately perfuse the right ventricle. J Thorac Cardiovasc Surg 109:1116-1124, 1995 10. Seeburger J, Borger MA, Falk V, et al: Minimal invasive mitral valve repair for mitral regurgitation: Results of 1339 consecutive patients. Eur J Cardiothorac Surg 34:760-765, 2008 11. Seeburger J, Borger MA, Doll N, et al: Comparison of outcomes of minimally invasive mitral valve surgery for posterior, anterior and bileaflet prolapse. Eur J Cardiothorac Surg 36: 532-538, 2009 12. Holzhey DM, Shi W, Borger MA, et al: Minimally invasive versus sternotomy approach for mitral valve surgery in patients greater than 70 years old: A propensity-matched comparison. Ann Thorac Surg 91:401-405, 2011
307