Abstract OBJECTIVES Favourable outcomes with mitral annuloplasty have been achieved with transapical cardioscopic (TAC) surgery in a survival animal model. In addition, experimental TAC on a non-survival animal model also showed adequate access to remove the native mitral valve and implant a prosthetic valve, but the surgical procedure took a long time and lacked follow-up data. The goal of this study was to develop a clinically translatable TAC mitral valve replacement (MVR) procedure using technical and instrumental refinements to reduce the surgical time and to evaluate functional recovery and short-term durability using a survival porcine model. We hypothesized that MVR could be achieved with subannular implantation of the bioprosthesis via the TAC approach. METHODS TAC MVR using the Hancock II™ (Medtronic)® mitral prosthesis was performed in 6 pigs via an incision over the xiphoid process, under cardiopulmonary bypass and cardiac arrest. COR-KNOT® and minimally invasive cardiac surgery instruments were used. Haemodynamics, echocardiography, cardiac computed tomography, ventriculography and electrocardiography were used to evaluate the function of the mitral prosthesis and left ventricle, coronary system and conduction system in the perioperative period and 4 weeks later. RESULTS A postimplant examination showed that the mitral prosthesis was competent, without a paravalvular leak. The left ventricular ejection fraction was comparable to preoperative values (65.2 ± 4.1 vs 67.2 ± 7.9). The bypass, cross-clamp and implant times were 177.2 ± 44.2 min, 135.3 ± 47.6 min and 94.0 ± 41.2 min, respectively. The prosthesis was in a good position. The apical scar was intact and not aneurysmal 4 weeks after the implant. The valve was properly sutured to the annulus, without a postimplant paravalvular leak. All animals recovered after 1 month of follow-up with preserved ventricular function and normal wall motion. CONCLUSIONS We successfully managed to replace the mitral valve with a biological prosthesis via the apex with encouraging bypass and cross-clamp times. This technique may provide an alternative for a selected group of patients with diseased mitral valves who have indications for MVR and still in a high-risk redo setting with conventional sternotomy or minimally invasive cardiac surgery-MVR. Transapical cardioscopic, Mitral valve replacement, Valvular surgery, Minimally invasive surgery, Valve diseases INTRODUCTION Minimally invasive cardiac surgery (MICS) had been turned in the past decade from an innovative strategy to a mainstream therapy for mitral valve replacement (MVR) and repair . This approach reduced surgical trauma, pain and length of hospital stay and improved cosmesis [2–4]. Recent enthusiasm was directed to transcatheter valve implantations specially for the aortic valve in terms of transcatheter aortic valve implantation (TAVI) , and currently the revolutionary transcatheter mitral valve (MV) procedures  with their wide portfolio of devices, some already approved and others still in progress. Part of the MICS armamentarium is the transapical cardioscopic (TAC) surgical platform, which was heavily investigated by our group in the past couple of years. For the MV procedures, direct visualization of the subvalvular apparatus is considered the main advantage of TAC over the conventional surgical approach, especially after the aortic valve procedure. It serves as a back-up option for frail patients who could not withstand the burden of conventional MVR surgery. Moreover, it targets patients with redo surgeries, particularly with intrathoracic adhesions or chest deformities. Our previous studies emphasized the proof of concept, feasibility and practicality of repairing the MV through TAC. To ensure progress in this area, in our present study, we hypothesized that adequate mitral bioprosthesis function and left ventricular (LV) contractility could be achieved with subannular implantation of a bioprosthesis via the TAC approach. METHODS Use of animals Six female Yorkshire pigs weighing 50–70 kg (Comparative Medicine, National University of Singapore) were used. The study was approved by the Institutional Animal Care and Use Committee of the National University of Singapore. The pigs were monitored for 4 weeks after surgery. Anaesthesia The pigs were sedated with midazolam and anaesthetized with 1.5–2.5% isoflurane and ventilated with 60% oxygen. The tidal volume was maintained at 10–12 ml/kg. With the animals in the supine position, we achieved central venous access via the jugular vein and performed invasive arterial pressure monitoring via the femoral artery for continuous monitoring. Arterial blood gas levels, electrolytes and haematocrit were monitored periodically using CG8+ cartridges (Abbott® Point of Care Inc., Princeton, NJ, USA). Instruments A set of endoscopic instruments was used as previously described [7, 8]. We used MICS 350-mm microscissors, non-traumatic microforceps (2.8 × 15 mm—350 mm), a nerve hook (1 × 7 mm—340 mm), a knot pusher (25°—340 mm) and a microneedle holder (3 × 10 mm—340 mm) (Fehling® Instruments GmbH & Co. KG, Germany); a 5-mm EndoEYE™ deflectable-tip endoscope, 3-mm standard endoscopic instruments (Olympus®Inc., Tokyo, Japan); and 5-mm Roticulator™ scissors and a grasper (Covidien®, Medtronic, Minneapolis, MN, USA). In addition, we used a custom-made LV retractor and apex protector to facilitate the apex opening, to prevent the LV chamber from collapsing during the procedure and to facilitate the ergonomic introduction of the instruments. The COR-KNOT® device (LSI Solutions, Victor, NY, USA) was used to secure the sutures (Supplementary Material, Fig. S1). Mitral prosthesis The Hancock II™ (Medtronic)® mitral bioprosthesis was used for this study. Its flexible frame (stent) allowed reshaping of the struts for easier transit through the small apical opening to the mitral annulus (Supplementary Material, Fig. S1). Surgical procedures Cardiopulmonary bypass Cardiopulmonary bypass (CPB) was established via central aortocaval cannulation, via a right minithoracotomy. The access to the apex was achieved via a 10-cm midline incision over the xiphoid area and a 5-cm, Y-shaped partial distal sternotomy (Fig. 1A and B). A pledgeted purse-string suture was inserted to the apex (Fig. 1B). The heart was arrested with intermittent cold blood cardioplegia (St. Thomas’ II cardioplegic solution, 16 mEq/l, 1000 ml for high dose, 10 mEq/l, 500 ml for low dose; Plegisol™Abbott® Laboratories, Abbott Park, IL, USA) delivered via a 7 Fr aortic root cannula. Figure 1: View largeDownload slide Steps for transapical cardioscopic mitral valve replacement. (A) Setup of the operating theatre and the position of the animal, showing the location of the skin incision (arrow); (B) distal sternotomy and incision at the apex; (C, D) positions of the instruments; (E) insertion of the sutures; (F) the bioprosthesis after implantation. Ao: aorta; AML: anterior mitral leaflet; AV: aortic valve; LA: left atrium; LV: left ventricle; PML: posterior mitral leaflet. Figure 1: View largeDownload slide Steps for transapical cardioscopic mitral valve replacement. (A) Setup of the operating theatre and the position of the animal, showing the location of the skin incision (arrow); (B) distal sternotomy and incision at the apex; (C, D) positions of the instruments; (E) insertion of the sutures; (F) the bioprosthesis after implantation. Ao: aorta; AML: anterior mitral leaflet; AV: aortic valve; LA: left atrium; LV: left ventricle; PML: posterior mitral leaflet. The apex was opened with a 2–3 cm cruciate incision (Fig. 1B) and maintained with 4 staying sutures. The LV retractor was positioned between the LV wall and the mitral chordae under video-assisted guidance, using a straight 5-mm/30° scope (Olympus®, Tokyo, Japan), which provided a magnified view and full coverage of the LV cavity. The surgeon could identify any debris that could be removed later either by forceps or suction before closing the heart. With the ergonomically simple LV retractor in place, the LV cavity was opened wide, and the MV, the subvalvular structure, the aortomitral curtain and the aortic valve were clearly visible. This customized LV retractor created sufficient space to remove the native valve and implant the prosthesis without the need for much retraction force. The retraction was relieved during cardioplegia infusion to create less compression on the septal and free wall coronary circulation. An LV vent was inserted through the apex to the LA (Fig. 1C and D). Mitral valve replacement The mitral chordae and leaflets were resected endoscopically, which was followed by insertion of 13 to 15 valve sutures (2-0 PremiCron®, B. Braun, Bethlehem, PA, USA) to the mitral annulus. The needles were mounted in a ‘fish-hook’ fashion so that the suture could be inserted from the atrial to the ventricular side of the mitral annulus, leaving the pledgets on the atrial side. The sutures were passed through the apex, 1 by 1, and affixed to a suture-holding frame to avoid tangling. We always started with 3 to 5 sutures on the anterior leaflet, followed by 10 to 12 sutures on the posterior leaflet, in a counterclockwise order. The sutures were then inserted to the prosthetic valve as usual (Fig. 1E). Cardioplegia was given via the aortic root cannula every 20 min. The annular attachment of the mitral leaflets could be clearly identified. The aortic cusps (left and non-coronary) along with the conduction system were visualized and identified for precise removal of the anterior leaflet and suture insertion and to avoid injuring them. Before sliding up the valve prosthesis, the 3 struts were sutured together to form a dome, thereby converting its rectangular shape (in the lateral view) to an elliptical shape. This technique allows the Hancock II valve (35 mm outer diameter of size 27) to enter easily through the apical opening, with little reversible dilation of the heart muscle. It also helped the valve pass smoothly, without tangling of the suture threads on its way to the mitral annulus. The suture was removed after the valve was secured to the mitral annulus. The prosthetic valve was inserted through the apex to the mitral annulus in a subannular position. Sutures were tied using a COR-KNOT device, starting from the anterior leaflet, then following counterclockwise to complete the annulus (Fig. 1F). As with a conventional MVR, 1 cusp of the prosthesis was positioned facing the left ventricular outflow tract (LVOT) for better haemodynamics. After all the sutures were confirmed to be secure, the retractor was removed, and the left ventricle was filled up and de-aired. The apex was closed and reinforced with 2 3-0 Premilene® sutures (Aesculap,). The LV vent was left inside the LV cavity until the intraoperative echocardiography study confirmed complete withdrawal of air (Fig. 1F). The aortic clamp was removed, and the animal was weaned from CPB as usual. The distal sternotomy was closed with 2 steel wires, and the wound was closed after a 28-Fr chest drain was inserted (Video 1). Video 1 Transapical cardioscopic mitral valve replacement procedure. Video 1 Transapical cardioscopic mitral valve replacement procedure. Close Postimplant assessments Four weeks after the MVR, the animal was anaesthetized, and postimplant assessments were carried out using cardiac computed tomography (CT) scans, ventriculography, echocardiography, invasive haemodynamic monitoring and gross examination of the prosthetic valve. Echocardiography Perioperative transthoracic and intraoperative echocardiography was performed using the GE Vivid S6® system (GE Healthcare, Chicago, IL, USA). Two-dimensional (2D) and Doppler images from the parasternal long- and short-axis view, apical 2- and 4-chamber views were acquired to evaluate cardiac function and MV function. Cardiac computed tomography scan Cardiac CT with contrast was performed 4 weeks after MVR in the Siemens® Somatom Definition Flash (Siemens Healthcare, Erlangen, Germany) system to assess the position of the valve prosthesis and its relation to the surrounding structures. CT coronary angiograms were acquired to evaluate the coronary arteries. Ventriculogram The competency of the mitral prosthesis was evaluated through ventriculography using the Cios Alpha® C-arm machine (Siemens Healthcare) with Omnipaque® 350 contrast. The C-arm data were analysed later using the Syngo® software (Siemens Healthcare). Gross examination of the heart The heart was harvested after acquisition of haemodynamic data on the terminal day. Images were taken from the left ventricle and MV for gross examination. Statistical analysis All data were represented as mean ± the standard deviation. Statistical analyses were performed using the SPSS® software version 22 for Windows® (IBM, Armonk, NY, USA). RESULTS Surgical times The overall bypass and cross-clamp times were 177.2 ± 44.2 min and 135.3 ± 47.6 min, respectively. The insertion of the suture to the annulus consumed approximately 50% of the valve implantation time (48.6 ± 25.3 min and 94.0 ± 41.2 min, respectively). The use of the COR-KNOT® device significantly reduced the time spent on anchoring sutures from a 17–50 min to 5–10 min. The surgical time was significantly reduced along the learning curve: The shortest cross-clamp time was 86 min and the shortest bypass time was 115 min (Table 1). Table 1: Surgical times for 6 animals who had a transapical mitral valve replacement Time (min) Animal no. Mean ± SD 1 2 3 4 5 6 Valve removal 17 8 8 9 8 9 9.8 ± 3.5 Valve implantation 122 119 150 59 59 55 94.0 ± 41.2 Suture to annulus 49 78 81 24 29 31 48.6 ± 25.3 Suture tie 52 17 20 10 14 5 19.6 ± 16.6 Bypass time 210 208 224 169 115 137 177.2 ± 44.2 Cross-clamp time 163 178 192 103 90 86 135.3 ± 47.6 Time (min) Animal no. Mean ± SD 1 2 3 4 5 6 Valve removal 17 8 8 9 8 9 9.8 ± 3.5 Valve implantation 122 119 150 59 59 55 94.0 ± 41.2 Suture to annulus 49 78 81 24 29 31 48.6 ± 25.3 Suture tie 52 17 20 10 14 5 19.6 ± 16.6 Bypass time 210 208 224 169 115 137 177.2 ± 44.2 Cross-clamp time 163 178 192 103 90 86 135.3 ± 47.6 SD: standard deviation. Table 1: Surgical times for 6 animals who had a transapical mitral valve replacement Time (min) Animal no. Mean ± SD 1 2 3 4 5 6 Valve removal 17 8 8 9 8 9 9.8 ± 3.5 Valve implantation 122 119 150 59 59 55 94.0 ± 41.2 Suture to annulus 49 78 81 24 29 31 48.6 ± 25.3 Suture tie 52 17 20 10 14 5 19.6 ± 16.6 Bypass time 210 208 224 169 115 137 177.2 ± 44.2 Cross-clamp time 163 178 192 103 90 86 135.3 ± 47.6 Time (min) Animal no. Mean ± SD 1 2 3 4 5 6 Valve removal 17 8 8 9 8 9 9.8 ± 3.5 Valve implantation 122 119 150 59 59 55 94.0 ± 41.2 Suture to annulus 49 78 81 24 29 31 48.6 ± 25.3 Suture tie 52 17 20 10 14 5 19.6 ± 16.6 Bypass time 210 208 224 169 115 137 177.2 ± 44.2 Cross-clamp time 163 178 192 103 90 86 135.3 ± 47.6 SD: standard deviation. Perioperative assessments The details of the TAC MVR procedure are shown in Fig. 2A–L. All 6 animals received a size 27 Hancock II™ prosthesis after a proper sizing manoeuvre using the Hancock II mitral prosthesis sizer set (Medtronic®, USA). Perioperative 2D echocardiography showed that there was no significant difference in the left ventricular ejection fraction (measured by the Teichholz method) between preimplantation (Fig. 3) and the 4-week follow-up examination. There was no paravalvular leak as shown in echocardiographs (Fig. 4A with normal transprosthetic mean pressure gradients (4.6 ± 2.4 mmHg, Fig. 4B), Competent prosthesis in the preterminal ventriculogram (Fig. 4C). 2D echocardiography showed preserved postoperative left ventricular ejection fraction (Fig. 4D). The coronary arteries were intact (Fig. 5A and B). The prosthesis was in a good position inside the LV cavity and did not obstruct the LVOT (Fig. 5C and D). Cardiac CT showed that the prosthesis maintained its original shape and that there was no damage to adjacent tissues (Fig. 5E and F). Figure 2: View largeDownload slide Replacement of the mitral valve. (A) Mitral valve replacement was performed via a 2-cm apical cross-incision (small insert), and the cavity of the LV was opened with the assistance of a custom-made LV retractor (arrow). (B) Full visualization of the mitral valve, papillary muscles and chords was achieved. An LV vent was inserted via the apex and placed in the LA. (C) Mitral valve resection was started at the posterior leaflet. (D) The anterior leaflet was resected. (E) The first valve suture was inserted to the right trigon; the needle was mounted in a fishhook fashion. (F) The first valve suture was completed; the pledget stayed on the atrial side of the annulus. (G) The valve sutures were completed. (H) Intermittent cold blood cardioplegia was given every 20 min via the aortic root to maintain the resting state. Effective cardioplegia delivery could be confirmed by direct visualization of complete closure of the aortic cusps. (I) The sutures were inserted into a Hancock II™ bioprosthetic mitral valve, and the valve was anchored to the mitral annulus via the apical opening. The prosthetic valve was anchored to the underside of the mitral annulus using the COR-KNOT® device, starting at the anterior annulus. The rivet (arrow) was placed around the suture and held tight using a COR-KNOT applicator. (J) The prosthetic valve was anchored to the posterior annulus. The arrow indicates the COR-KNOT rivet after firing. (K) The prosthetic valve implantation procedure was completed once the cusp was facing the LVOT. (L) The apex was closed after de-airing. A vent line was placed inside the LV cavity to evacuate residual air and to aid in emptying the LV cavity during reperfusion. The distal sternum was divided in a Y shape, just sufficient to view the apex. AL: anterior leaflet; LA: left atrium; LV: left ventricle; LVOT: left ventricular outflow tract; PM: papillary muscle; PL: posterior leaflet. Figure 2: View largeDownload slide Replacement of the mitral valve. (A) Mitral valve replacement was performed via a 2-cm apical cross-incision (small insert), and the cavity of the LV was opened with the assistance of a custom-made LV retractor (arrow). (B) Full visualization of the mitral valve, papillary muscles and chords was achieved. An LV vent was inserted via the apex and placed in the LA. (C) Mitral valve resection was started at the posterior leaflet. (D) The anterior leaflet was resected. (E) The first valve suture was inserted to the right trigon; the needle was mounted in a fishhook fashion. (F) The first valve suture was completed; the pledget stayed on the atrial side of the annulus. (G) The valve sutures were completed. (H) Intermittent cold blood cardioplegia was given every 20 min via the aortic root to maintain the resting state. Effective cardioplegia delivery could be confirmed by direct visualization of complete closure of the aortic cusps. (I) The sutures were inserted into a Hancock II™ bioprosthetic mitral valve, and the valve was anchored to the mitral annulus via the apical opening. The prosthetic valve was anchored to the underside of the mitral annulus using the COR-KNOT® device, starting at the anterior annulus. The rivet (arrow) was placed around the suture and held tight using a COR-KNOT applicator. (J) The prosthetic valve was anchored to the posterior annulus. The arrow indicates the COR-KNOT rivet after firing. (K) The prosthetic valve implantation procedure was completed once the cusp was facing the LVOT. (L) The apex was closed after de-airing. A vent line was placed inside the LV cavity to evacuate residual air and to aid in emptying the LV cavity during reperfusion. The distal sternum was divided in a Y shape, just sufficient to view the apex. AL: anterior leaflet; LA: left atrium; LV: left ventricle; LVOT: left ventricular outflow tract; PM: papillary muscle; PL: posterior leaflet. Figure 3: View largeDownload slide Left ventricular ejection fraction before and 1 month after mitral valve replacement. Figure 3: View largeDownload slide Left ventricular ejection fraction before and 1 month after mitral valve replacement. Figure 4: View largeDownload slide Evaluation of the valve and ventricular function 4 weeks after implantation. (A) Mitral valve Doppler; (B) transmitral mean pressure gradient of the prosthetic valve; (C) ventriculogram showing competent mitral prosthesis and no paravalvular leak; (D) M-mode echocardiography showing good contractility. Figure 4: View largeDownload slide Evaluation of the valve and ventricular function 4 weeks after implantation. (A) Mitral valve Doppler; (B) transmitral mean pressure gradient of the prosthetic valve; (C) ventriculogram showing competent mitral prosthesis and no paravalvular leak; (D) M-mode echocardiography showing good contractility. Figure 5: View largeDownload slide Computed tomography (CT) cardiac and CT coronary angiogram. (A, B) Intact coronary arteries; (C) prosthesis in relation to the left ventricular outflow track; (D) long axis showing the mitral prosthesis aligned with the left ventricular wall; (E, F) long- and short-axis sections at the level of the prosthesis. Figure 5: View largeDownload slide Computed tomography (CT) cardiac and CT coronary angiogram. (A, B) Intact coronary arteries; (C) prosthesis in relation to the left ventricular outflow track; (D) long axis showing the mitral prosthesis aligned with the left ventricular wall; (E, F) long- and short-axis sections at the level of the prosthesis. Complications All 6 animals were alive and well recovered from the CPB except for 1 pig that had a trivial paravalvular leak with no signs of heart failure. All 6 pigs recovered uneventfully and remained uncomplicated until the terminal surgery timing 4 weeks later. Data from echocardiography, ventriculography showed no regurgitation of the aortic and MV. CT-coronary angiography showed intact left circumflex artery (Fig. 5). Gross examination of the valve The fabric ring was covered with endothelial cells. The rivets were stable and firm. The struts were not distorted, and the valve leaflets were intact and completely closed. There was no sign of paravalvular defect (Fig. 6). Figure 6: View largeDownload slide The Hancock II® valve at 1 month post mitral valve replacement. The prosthetic valve was examined from the ventricular side (A) and the atrial side (B). Figure 6: View largeDownload slide The Hancock II® valve at 1 month post mitral valve replacement. The prosthetic valve was examined from the ventricular side (A) and the atrial side (B). Postmortem examination of the endocardial surface showed intact trabeculae and papillary muscles. The apical scar was thick and limited, in continuity with the LV muscle. There was no sign of aneurysmal change in the apex (Supplementary Material, Fig. S2A). The epicardial view of the left ventricle (Supplementary Material, Fig. S2B) showed scar around the apex area, whereas the middle and basal parts were relatively scar- and adhesion-free. DISCUSSION The results of this study, a transapical MVR operation on 6 pigs, show that mitral replacement with a mitral bioprosthesis through the apex is feasible, with satisfactory functional recovery. After 4 weeks of follow-up, the valve was competent, well endothelialized, did not obstruct the LVOT and did not have a significant paravalvular leak. The apical scar remained thick and did not cause any aneurysmal changes. Although 4 weeks is a short follow-up period in which to evaluate the effects of the apical scar on cardiac rhythm, aneurysmal changes and the functioning of the left ventricle, our results consistently showed insignificant changes in cardiac function and arrhythmias, comparable to results from similar studies regarding incidences of ventricular arrhythmias during the TAVI procedure [7–9]. Thus, we remain confident that this procedure poses the same—if not lower—risk level than previously established surgical procedures. We also acknowledge that the apical scar in elderly frail patients may not exhibit the same behaviour. Moreover, TAC MVR guaranteed that the valve prosthesis resided in the subannular plane, which allowed it to be pushed further against the annulus during systole, making that position even more secure than the conventional everted supra-annular position where the valve is ‘suspended’ if viewed from the high-pressure LV chamber during systole. MICS-MVR has become a standard practice in leading cardiac centres all over the world and has been offered to a majority of patients with MV diseases who needed replacement with a prosthetic valve. However, MICS-MVR has been found to be unsuitable for patients with previous right pleural space surgery or for redo cases due to fibrous adhesions and the risk of lung tissue damage. It is also more likely to jeopardize both atria, increasing the risk of postoperative atrial fibrillation [10, 11]. Additionally, it is not ideal for patients with a small or distorted rib cage because of poor alignment of instruments and limited control of prosthesis anchoring. Hence, new alternatives should be established for these selected patients. Procedures performed on the heart valves via the LV apex have been reported in transapical mitral valvotomy , recent TAVI and transapical MV implants . It is also a site for implantation of either a permanent or temporary LV assist device . Except for a LV assist device implant, these procedures are carried out on a beating heart and do not allow removal of the diseased valve. As an alternative, the recently developed transapical approach was adopted to access all internal LV structures. MV-in-valve, MV-in-ring implantation, our TAC technique of mitral annuloplasty  and novel repair techniques using the NeoChord™ system  and dynamic repair with insertion of a Mitra-Spacer™  have been reported using this approach [6, 16, 17]. However, according to recent studies [18, 19], patient selection criteria are strict enough to limit most transcatheter applications with a beating heart [1, 15]. Thus, direct MV examination and radical treatment possibilities are unfeasible. Therefore, surgical intervention is the healthier option in patients who have the indications for MVR. The transapical approach for MVR could be achieved with less manipulation to the heart . Compared to the TAVI procedure [18, 21], our current set of instruments and bioprosthetic valves required a larger apical opening, which nevertheless resulted in an insignificant effect on postoperative cardiac function (Fig. 4). However, the larger opening allowed full assessment, removal of the diseased MV and secure replacement with an accurately sized prosthesis. Taking into account the fact that complete resection of the native mitral apparatus had been proven to have adverse effects on LV function postoperatively , in our procedure the preservation of the subvalvular apparatus could cause some difficulties in prosthetic valve insertion such as the increased possibility of prosthetic valve entrapment or obstruction of the LVOT. This approach also provides access to the atrial structures such as the pulmonary veins or the atrial appendage if other procedures, i.e. ablation or atrial appendage exclusion, are required. Furthermore, since the tissues beyond the apical area were not involved during TAC-MVR, the retrosternal space and right pleural cavity could be spared from excessive dissection and associated bleeding or injury in the redo setting, which, in many cases, are predictable events with conventional resternotomy. Another added benefit from this type of procedure is that the orientation of the valve can be fully controlled to avoid LVOT obstruction by prosthetic struts, because the LVOT is fully visible from the apex. The valve can be swiftly anchored to the annulus with similar efficiency using a COR-KNOT device  or with a traditional endoscopic knot-pusher/tying device. Because the mitral prosthesis and instruments determine the size of the apical opening, future smaller or low-profile designs for these items may help to reduce the diameter of the insertion through the apex. Likewise, a stentless mitral prosthesis or a hybrid device combining an expandable valve with a sewing ring may be more suitable for TAC MVR. Because these procedures are still under development, special techniques and instruments ought to be developed and standardized to gain better access and facilitate the procedure during transapical MV implantation (i.e. a special LV retractor and a distinct needle-mounting manoeuvre). Limitations As stated previously, although the aortic cross-clamp time was 59 min shorter than in our previous study (135 vs 194 min) , it was still longer than with the conventional MICS-MVR (135 vs 100 min)  and could be improved with a protracted learning curve. A 4-week follow-up period is not sufficient to evaluate the effects of the apical scar on cardiac rhythm, aneurysmal changes and the function of the left ventricle. Hence, an extended study would be required to understand the chronic changes in the heart. We observed insignificant changes in cardiac function, but this healthy animal model might be not sufficient to validate the effects of this procedure on diseased hearts, especially those with poor ejection fractions. Some instruments used in this study were improvised and specifically designed for the LV inlet. Therefore, further development of a low-profile, more manageable set of instruments would be mandatory to improve the quality of this technique. It is worth mentioning that there are anatomical differences in pigs, i.e. their apex is located directly underneath the lower sternal half, that pose no difficulty when translated to human patients because partial sternotomy could be totally avoided. Instead, the MICS apical approach would be easier from a left anterior minithoracotomy. Ultimately, we agree that the effect of subvalvular preservation on LV function for this TAC approach needs to be studied further. CONCLUSIONS This study presents a safe, reliable and reproducible surgical procedure for inserting a MV bioprosthesis via the apex of the left ventricle. Follow-up examinations showed properly functioning prosthetic valves, with no paravalvular leaks, no significant changes in cardiac function and no arrhythmias. The main difference between this approach and our previous experiences is that the former may provide an alternative for the selected group of patients whose MVs are not suitable for MV repair and who have strong indications for MV replacement and for the high-risk patients in a redo setting with traditional sternotomy or MICS-MVR. SUPPLEMENTARY MATERIAL Supplementary material is available at ICVTS online. ACKNOWLEDGEMENTS We would like to thank the veterinary team from Comparative Medicine, National University of Singapore, the entire perfusion team from the National University Hospital, Singapore and Medtronic Ltd Pte (Singapore) for their support throughout the study. 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Interactive CardioVascular and Thoracic Surgery – Oxford University Press
Published: Mar 26, 2018
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