Since the first mitral valve repair procedure for mitral stenosis in 1923 , mitral valve surgery has been evolving rapidly. Prosthetic mitral valves have been developed for mitral valve replacement, which can be performed using transcatheter access. Similarly, mitral valve repair has progressed from closed commissurotomies to open complex mitral valve repairs using artificial chords and rings. Access to the mitral valve is achieved through small incisions with the use of endoscopes and robotic systems, thus avoiding sternotomy.
Minimal access mitral surgery
Endoscopic minimal access mitral surgery
Endoscopic minimal access mitral surgery was introduced in 1996. It incorporates the use of endoscopes and instruments especially designed to minimise surgical trauma caused by the conventional access to the mitral valve through median sternotomy as much as possible.
In a typical setting, the patient is connected to a cardiopulmonary bypass circuit through peripheral vessels. For venous drainage, internal jugular and femoral veins are used, and for arterial access the femoral artery is typically cannulated. The aorta is then clamped with the use of either a cross-clamp or an endoballoon, which acts as an occlusion device. Cardioplegic solution is then instilled into the coronary arteries and the heart is arrested in a similar manner to conventional procedures.
The mitral valve is accessed from a right lateral mini (4-6 cm) thoracotomy or even a right peri-alveolar incision. Visualisation and exposure of the mitral valve is optimised with the use of endoscopes, which can provide two-dimensional (2D) or three-dimensional (3D) images. With the use of thoracoscopic instrumentation, the mitral valve is assessed and repaired with contemporary techniques, which include ring implantation, artificial chordal replacement, leaflet resection, etc. From the same minimal access, the mitral valve can be replaced with a mechanical or a biological prosthesis.
Many concomitant procedures can also be performed using the above set-up, including tricuspid valve surgery, atrial septal defect closure, relief of hypertrophic obstructive cardiomyopathy, excision of masses such as myxomas, atrial fibrillation surgery and closure of the left atrial appendage .
From the description above, one can understand why the phrase “minimal access mitral surgery” is better suited to these techniques, as opposed to the term “minimally invasive mitral surgery”. Despite the fact that access is minimal, the procedure to the heart itself has a similar degree of “invasiveness” to conventional mitral surgery performed through a median sternotomy.
Robotic mitral surgery
Robotic systems mainly consist of a console and a robotic cart. Through small incisions, robotic instruments and endoscopes are introduced to the chest cavity and the operation is performed with the operating surgeon unscrubbed, controlling the robotic instruments from a distance. The main advantages of robotic implementation include superior 3D visualisation, elimination of tremor, seven angles of freedom for the instruments (compared to four in endoscopic access). However, it involves a loss of tactile feedback for the surgeon and higher cost when compared to endoscopic or traditional access mitral surgery .
Establishment of cardiopulmonary bypass and cardiac arrest are similar between endoscopic and robotic mitral surgery, with similar incisions and identical possibilities for concomitant procedures. Therefore, robotic mitral surgery can also be classified as “minimal access”.
The robotic system that has been used most in cardiac surgery worldwide is the da Vinci® family from Intuitive Surgical (Sunnyvale, CA, USA). First applications for mitral operations started in 1998 and since then there has been a gradual adaptation in many centres in Europe and the USA. Currently, over 100 robot-assisted mitral procedures per year are taking place in Europe and over 1,700 in the USA. The main limitation of further expansion is the increased cost and steeper learning curve of robotic training compared to endoscopic .
Contraindications to minimal access mitral surgery
Minimal access mitral surgery, despite being technically more demanding and having a learning curve, offers the complete range of surgical options for the treatment of mitral valve disease. There are, however, a few contraindications and limitations to minimal access approaches. Pleural adhesions and history of extensive radiation to the chest could complicate the entrance to the chest cavity and access to the heart. As single-lung ventilation is required until cardiopulmonary bypass is established, patients with poor lung function or inability to tolerate single-lung ventilation should be excluded from these techniques. Patients with peripheral vessel disease and aortic regurgitation should also be excluded, and chest deformities (such as pectus excavatum) can make access very difficult. Minimal access robotic mitral surgery is also associated with increased operation times compared to traditional sternotomy, leading some surgeons to avoid minimal access surgery in patients with many comorbidities or reduced left ventricular function who may benefit from a quicker operation .
Results of minimal access mitral surgery
Short- and long-term data for traditional mitral valve surgery, and more specifically mitral valve repair through a sternotomy, demonstrate excellent results. The STS database revealed a 1.2% mortality for isolated mitral valve repairs, which is further reduced to 0.6% when the patients are asymptomatic. Reports of long-term outcomes show freedom from reoperation which reaches 95% in 15 years  and survival similar to a control population when the procedure is carried out early and the patient is in NYHA Class I or II .
These excellent results serve as a benchmark to which minimal access mitral procedures should be compared. Reports and meta-analysis that compare traditional and minimal access endoscopic techniques demonstrate less pain, improved cosmesis, reduced blood transfusions, reduced wound infections, less incidence of atrial fibrillation, and reduced ventilation time, intensive care length of stay and hospital length of stay for minimal access surgery. In a large series, the mortality rate remained low (1.1%) and in 95% mitral valve repair was feasible with a 94% freedom from reoperation at 15 years . Robotic mitral surgery has demonstrated similar excellent outcomes with mortality rates of <1% despite the fact that cross-clamp and cardiopulmonary bypass times were slightly longer .
In-hospital morbidity for endoscopic minimal access mitral surgery procedures has been low. Conversion to sternotomy has been reported to be as low as 2%, incidence of stroke 0.3%, myocardial infarction 0.6%, new-onset atrial fibrillation 17%, need for permanent pacemaker implantation 2.3%, renal insufficiency 2.6% and wound infection 0% .
The above results show excellent perioperative mortality, morbidity and long-term outcomes of minimal access mitral surgery. However, there are currently no randomised control trials with enough power to demonstrate significant superiority of these techniques when compared to traditional sternotomy mitral surgery.
Other minimally invasive mitral valve therapies
Technological advances in transcatheter aortic valve replacement (TAVR) have also been implemented to the mitral valve. Transcatheter mitral valve replacement procedures have been introduced, which are performed in centres experienced in TAVR. These procedures aim to replace the mitral valve using a catheter delivery system without the need for cardiopulmonary bypass or cardiac arrest. Four different systems have currently been implanted into humans; they are all still under clinical investigation and are not available commercially. These are the CardiAQ (Edwards Lifesciences, Irvine, CA, USA), Tendyne™ (Tendyne Inc. [now Abbott], Roseville, MN, USA), Tiara™ (Neovasc Inc., Richmond, BC, Canada) and the Twelve valve (Medtronic, Minneapolis, MN, USA). All these valves are delivered through the apex of the heart following a small left anterior thoracotomy, with the exception of the CardiAQ valve which is designed also to be delivered transfemorally and transseptally. There are anatomical and morphological limitations which make transcatheter mitral valve replacement more complex compared to TAVR. There is, however, increased interest from the industry as more devices are currently under development .
Minimally invasive chordal replacement techniques have recently become commercially available. The NeoChord (NeoChord, Inc., St. Louis Park, MN, USA) is a device which introduces artificial chords from the cardiac apex and secures them at the edge of the posterior mitral leaflet. The other end is then tied at the epicardial surface of the left ventricle, with chordal length adjustment happening in real time with a beating heart under echocardiographic guidance. Long-term results are awaited from this exciting new technique .
Finally, interventional cardiologists have an armamentarium of innovative therapies for the treatment of mitral regurgitation. These techniques are the MitraClip® (Abbott Vascular, Santa Clara, CA, USA), mitral annular remodelling devices and ventricular remodelling devices.
The MitraClip is by far the one that has been most extensively implanted and investigated. First implantations took place in 2003, and CE mark and FDA approval was granted in 2008 and 2013, respectively. This device, inspired by the surgical edge-to-edge repair initially described by Alfieri, is a clip which is introduced through a femoral vein and advanced to the mitral valve through the atrial septum. Under echocardiographic and fluoroscopic imaging, the clip is deployed to grasp anterior and posterior mitral valve leaflets, which results in increased coaptation and reduced regurgitation. The procedure is performed typically by interventional cardiologists without any use of cardiac arrest or cardiopulmonary bypass and is effective in high- and prohibitive-risk patients suffering from primary and secondary mitral regurgitation. Results from the randomised control trial EVEREST II were promising, despite the fact that a recurrence in mitral regurgitation was observed in 25% of the patients in one year . Furthermore, the randomised COAPT trial recently published results that demonstrate reduced rates of hospitalisations and death, as well as improved quality-of-life and functional capacity for symptomatic patients suffering from secondary mitral regurgitation and heart failure. These patients were receiving maximum tolerated optimal medical therapy and were also treated with MitraClip . As other studies, such as the MITRA-FR, found no benefit for patients suffering from secondary mitral regurgitation treated with MitraClip , we can therefore assume that there is need for further evidence with regards to indications of MitraClip implantation.
The treatment of mitral valve disease is evolving rapidly. The excellent long-term results of conventional mitral surgery can now be achieved through smaller incisions by using endoscopic and robotic techniques. For high-risk and inoperable patients, minimally invasive transcatheter therapies are currently available and many others are under development. These offer the cardiac surgeon and interventional cardiologist a choice of different approaches to suit each patient’s needs.