Abstract: Minimally invasive mitral valve surgery is evolving rapidly since the early 90's and is now increasingly adopted as the standard approach for mitral valve surgery. It has a long and challenging learning curve and there are many considerations regarding technique, planning and patient selection when starting a minimally invasive program. In the current review, we provide an overview of all considerations and the decision-making process during the learning curve.

1. Introduction

Minimally invasive mitral valve surgery (MIMVS) has evolved rapidly since the beginning in the early ’90 (1-3) and is now increasingly adopted as the standard approach for mitral valve surgery by many centers (4-10). The STS database showed an increase from 15,5% (2014) to 24,6% (2018) of all mitral valve procedures in the United States being approached by a less invasive access (11). In experienced centers, rates of minimally invasive access have been reported up to 74% (12) with good short- and long-term results in primary (4-6, 15) and in redo mitral valve surgery (7-10), equal repair rates as in sternotomy approach (13, 14).

2. Learning curve of minimally invasive mitral valve surgery

MIMVS has a long learning curve due to multiple factors complicating the learning process. The decrease of the surgical working space and remotely working with long shafted instruments while the surgical perspective is changed to a videoscopic view, is technically more demanding than working in a wide surgical field with shorter instruments. For the extra-corporeal circulation, the procedure requires peripheral canulation (femoral, axillary or direct aortic) with specific aortic clamp techniques (e.g. Chitwood clamp or endo-aortic balloon). There must be insight into the pitfalls for peripheral cannulation techniques. Adding more complex techniques, such as the endo-aortic balloon will require extra attention from the surgeon while there is already full focus needed for the surgical procedure.

The number of procedures required to complete the learning curve is reported at 75 – 125 cases and at least 2 surgeries per week to maintain optimal surgical outcome (12). To train thoracoscopic skills and surgical sequence of MIMVS, training modules and simulators are available to shorten the learning curve (16, 17).

3. Benefits and limitations of minimally invasive access

3.1. Benefits of minimally invasive access

In literature a range of advantages is described when mitral valve surgery is performed via minimally invasive access. A reduction of blood transfusions, post- operative atrial fibrillation, ventilation time, length of stay in intensive care unit or hospital and risk of deep sternal wound complications are reported for MIMVS (18, 19). In addition, minimally invasive access showed improved quality of life and faster return to daily activity compared to sternotomy in the early postoperative phase (first 3 months) (20, 21).

Figure 1. Marking of the incisions for MIMVS and the incision for conversion to sternotomy

3.2. Limitations of minimally invasive access

MIMVS is by no means a risk-free procedure, it has many advantages for the patients but it carries specific risks: a potentially higher stroke risk, groin complications, aortic dissection, longer cardiopulmonary bypass and aortic occlusion time are described. Concomitant procedures in MIMVS are usually limited to tricuspid valve surgery, closure of defects in the atrial septum and rhythm surgery. Chest deformations (e.g. pectus excavatum) or adhesions in the right thorax (e.g. prior pneumothorax or trauma) can limit the view in MIMVS.

4. Considerations in pre-operative planning and patient selection during the learning curve

4.1. Pre-operative planning and imaging

When starting a MIMVS program, careful patient selection and planning is necessary to establish a safe and successful program. A dedicated multidisciplinary mitral valve heart team can help select the best treatment (MIMVS vs. sternotomy vs. trans-catheter treatment vs. conservative) for each individual patient and is proven to improve long-term survival (22). In addition to the standard preoperative examinations (e.g. Chest X-ray, blood tests, transthoracic and/or trans-esophageal echocardiography and coronary angiogram), a thoracic CT scan and aortic CT from the femoral vessels to the aortic valve provide important information for surgical planning. Based on these images, calcifications in mitral annulus, thoracic deformations, the position of the mitral valve in the thoracic cavity and distance to the sternum is objectified and the optimal position of thoracic incision can be predicted.

4.2. Patient selection

Contraindications for MIMVS are safety related factors that expose unnecessary risks which are not in proportion with the benefits of less invasive access. Relative contraindications reported by the International Society for Minimally Invasive Cardiothoracic Surgery (ISMICS) are shown in table 1 (23).

Table 1. Relative contraindications and concerns for MIMVS reported by the International Society for Minimally Invasive Cardiothoracic Surgery (ISMICS).

Table derived from Ailawadi et al. 2016 (33).

4.3. Considerations of mitral valve pathology during learning curve of MIMVS

Patients with complex repair, such as patients with anterior leaflet or bi-leaflet prolapse, should be avoided early in the learning curve as minimally invasive access should not compromise the outcome valve repair. Experienced centers have reported excellent long-term results of complex repairs (e.g. Barlow’s disease) through MIMVS with freedom from re-operation up to 93.8% in 10-years follow up and freedom from greater than 2+ grade mitral regurgitation up to 88.4% in 10- years follow up (24, 25).

5. Available techniques and safety considerations

5.1. Safety of the minimally invasive techniques

Multiple techniques are available at each step of surgery (e.g. chest access, cardiopulmonary bypass configuration, aortic occlusion). In the early days of MIMVS, there were concerns about the safety of this technique, especially with regard to aortic injury and stroke (19). Since the 1990s, results have improved significantly due to better pre-operative planning, adjustments to surgical technique and as more experienced centers have become familiar with the technique. A recent STS database study from 2021 and recent meta-analysis show similar results for MIMVS versus sternotomy (4-9, 11, 26, 27).

Figure 2. Setup of a MIMVS procedure by right anterolateral mini thoracotomy. Starting from the bottom of the soft tissue retractor in clockwise direction: Carbon dioxide insufflator, Transthoracic Chitwood clamp, video port and left atrial retractor.

The robotic assisted approach is the most complex to learn and has a learning curve beyond mini thoracotomy (28). In the robotic assisted technique, a 3D camera is used to visualize the chest and robotic arms are installed through ports and a variety of instruments can be inserted (e.g. scissors, diathermia, forceps).

5.3. Cardiopulmonary bypass: arterial cannulation

Cardiopulmonary bypass can be organized peripherally and centrally with a variety of configurations for many reasons. For peripheral cannulation, arterial access is most frequently obtained through the femoral artery, percutaneously (Seldinger technique) or via a groin incision, as it is a simple and reproducible technique that does not interfere with the surgical field. Whenever the femoral artery is inaccessible (e.g. in case of severe ilio-femoral calcifications, stenosis and tortuosity or thrombus in the thoraco-abdominal aorta), subclavian or central cannulation can be used. (29).

Figure 3. Peripheral arterial and venous cannulation in the right groin. Standard arterial cannula for direct aortic cross clamp technique. When using an endoaortic balloon, there is a sidearm in the arterial cannula to insert the balloon.

5.4. Cardiopulmonary bypass: Venous cannulation

As in arterial cannulation, peripheral venous cannulation is most frequently used in MIMVS. Access is obtained through the femoral vein, percutaneously (Seldinger technique) or through a groin incision, by inserting a two staged cannula positioned with the tip in the superior vena cava under TEE guidance, draining the superior and inferior caval vein. For tricuspid valve surgery or closure of an atrial septum defect, it is preferable to drain both caval veins separately, snaring both caval veins when opening the right atrium to prevent air lock in the extracorporeal circulation. When preferred an additional cannula can be inserted through the interna jugular or subclavian vein next to the femoral cannula (with the tip just below the diafragm) for separate caval venous drainage.

5.4. Cardiopulmonary bypass: Venous cannulation

As in arterial cannulation, peripheral venous cannulation is most frequently used in MIMVS. Access is obtained through the femoral vein, percutaneously (Seldinger technique) or through a groin incision, by inserting a two staged cannula positioned with the tip in the superior vena cava under TEE guidance, draining the superior and inferior caval vein. For tricuspid valve surgery or closure of an atrial septum defect, it is preferable to drain both caval veins separately, snaring both caval veins when opening the right atrium to prevent air lock in the extracorporeal circulation. When preferred an additional cannula can be inserted through the interna jugular or subclavian vein next to the femoral cannula (with the tip just below the diafragm) for separate caval venous drainage.

5.5. Techniques for aortic occlusion and de-airing

In literature, there are two techniques described for aortic cross clamping in MIMVS: the endo-aortic balloon occlusion and the direct transthoracic aortic clamp (e.g. Chitwood clamp).Compared to conventional heart surgery, the surgeon is most accustomed to the direct transthoracic aortic cross-clamp and is therefore most easy to handle (30). For the administration of cardioplegia in this technique, an aortic root vent must be placed. The use of the endo balloon was recently described in detail by van Praet et al (31, 32). In brief, the balloon occludes the aorta and cardioplegia is administered through the balloon. During the aortic occlusion, close monitoring of the pressures in the aortic root, the balloon itself and the radial artery pressure are important. Migration of the balloon is possible whenever there are changes in the pressures, such as during the start and stop of administration of cardioplegia, opening the left atrium and changes in cardiopulmonary bypass flow. In redo-surgery, the balloon can be very useful to occlude the aorta without necessity to perform adhesiolysis.

5.6. Hypothermic fibrillary arrest

In selected patients, such as in redo surgery (especially in patients with patent internal mammary artery grafts after CABG), a calcified ascending aorta without safe clamping site (hostile aorta) or whenever the endo-aortic balloon is contra indicated, hypothermic fibrillary arrest can provide safe myocardial preservation, however in patients with more than mild aortic valve regurgitation the vision can be hampered by retrograde blood.

5.7. Valve repair and replacement techniques

Repair rates of MIMVS have been reported to be comparable to sternotomy approach (13, 14). All techniques for mitral valve repair are feasible through minimally invasive access, although there is a trend towards ‘respect’ rather than ‘resect’ in MIMVS. Recent studies have shown that the neochord technique showed better long-term results than the resection technique in patients with degenerative disease who underwent mitral valve repair through minimally invasive access (33, 34).

Figure 4. View on the mitral valve in MIMVS. A. Prolapse of P2 segment B. Placement of neochords on the prolapsing segment C. Adjusting of the height of the neochords D. Result of mitral valve repair

6. Conversion to sternotomy

Conversion to sternotomy is reported at 1-2% in large series of experienced centers (4-6) and can be divided in an intended and unintended sternotomy. A perioperative unintended conversion is an emergency sternotomy due to a lesion of a major vascular structure or the heart (e.g. aortic dissection, atrioventricular dissociation). An intended conversion is a perioperative anticipated conversion to sternotomy due to inaccessible anatomy for MIMVS (e.g. unexpected peri-operative pulmonary adhesions).

Figure 5. Scar of minimally invasive access for mitral valve surgery

7. Conclusion

MIMVS is increasingly adopted worldwide as the standard approach for mitral valve surgery due to less surgical trauma resulting in faster postoperative recovery and improved functional outcome. MIMVS is a complex technique with a long learning curve. The ideal case to start with is a mitral valve replacement or a straight forward mitral valve repair without pulmonary hypertension in a non-obese patient with good left ventricular function, no mitral calcification, no chest deformity, no previous cardiac surgery and no adhesions in the right chest. The most reproducible and simplest technique is access through right mini thoracotomy (totally thoracoscopic or with video assistance and direct view), cardiopulmonary bypass through peripheral femoral arterial and venous cannulation and aortic occlusion with a direct transthoracic clamp. The number required to complete the learning curve is currently reported at 75 – 125 cases and at least 2 cases per week to maintain optimal surgical outcome.

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