Functional mitral regurgitation is a regurgitation that occurs despite a structurally normal mitral valve as a consequence of left ventricular dysfunction. It is a common feature of both dilated and ischaemic cardiomyopathy, and the presence and degree of severity of mitral regurgitation is associated with an increased risk of death and congestive heart failure.1–3 Mechanisms leading to functional mitral regurgitation involve local left ventricular remodelling, mitral valve tenting, annular dilatation and loss of systolic annular contraction.3 There is no specific medical therapy for functional mitral regurgitation (treatment of congestive heart failure). Surgical correction, mainly surgical annuloplasty, is highly debated in patients with severe regurgitation and carries a large risk.4 A safe, non-surgical and efficient procedure is thus desirable. Percutaneous mitral annuloplasty has been introduced as a potential alternative to mitral valve surgery.
The anatomy of the coronary venous system has recently gained interest with the development of cardiac resynchronisation therapy (CRT). It is now well recognised that maximal haemodynamic benefit of CRT may be achieved by optimal placement of the left ventricular lead, highlighting the importance of a careful evaluation of the coronary venous anatomy. The coronary venous system may also be of great importance for percutaneous mitral annuloplasty in patients with functional MR. Several devices have been developed exploiting the anatomical proximity of the coronary sinus and the mitral valve annulus (see Figure 1). These devices are inserted into the coronary sinus in order to shrink the mitral annulus and thus to decrease the degree of MR. In this review, we present anatomical data regarding the anatomy of the coronary venous system, the theoretical principles of actions of the devices currently under evaluation and the information provided by the different imaging modalities.
Coronary Venous Anatomy
The morphological arrangement of the coronary venous system is notoriously variable and the clinician should understand the usual presentation but also the common variants.5,6 The coronary sinus returns blood from most of the left heart. It begins at the termination of the great cardiac vein and runs through the left atrioventricular junction to terminate in the right atrium. Its diameter at its right atrial mouth varies from 5 to 20mm and its length from 2 to 5cm.
Its major tributaries are the great cardiac vein, the middle cardiac vein, the veins draining the inferior diaphragmatic surface of the left ventricle, the left marginal veins, the oblique vein of the left atrium and the small cardiac vein. The anterior interventricular vein originates and runs in the anterior interventricular groove, then courses to the left into the left atrioventricular groove and continues as the great cardiac vein. The anterior interventricular vein terminates in the coronary sinus. The coronary sinus and the great cardiac vein have a variable relationship with the coronary arteries and can course the circumflex artery either deeply or superficially (see Figure 2).
How Do the Different Devices Work?
Three different systems of percutaneous mitral annuloplasty are currently under evaluation: the Edwards Monarc system, the Carillon Mitral Contour System and the PTMA implant system. They are inserted into the coronary sinus and the great cardiac vein and all work on the same principle: they shrink the mitral annulus, increasing leaflet coaptation and thus reducing the regurgitation (see Figure 3).
The feasibility and short-term efficacy of percutaneous mitral annuloplasty have been successfully tested in animal models.7–9 Preliminary human experience has recently been reported, with encouraging results.10–12
What Do We Expect from Imaging?
Appropriate evaluation of the coronary venous system and thus appropriate patient selection are key elements of successful device implantation and for avoiding procedural complications. Three major points need to be addressed carefully before implantation:
- the anatomy of the coronary sinus and the great cardiac vein;
- the relationship between the coronary venous system and the coronary arteries; and
- the relationship between the coronary venous system and the mitral annulus.
The coronary venous system can be evaluated using three imaging modalities: echocardiography, angiography and computed tomography.
Transthoracic and transoesophageal echocardiography can provide some visualisation of the coronary venous system. However, echocardiographic assessment is inconstant and allows visualisation of only a small part of the venous system (the distal part or the coronary sinus). Thus, echocardiography cannot be considered a useful technique for the evaluation of venous system anatomy. The usefulness of 3D echocardiography deserves further evaluation, but its spatial resolution remains low and it seems unlikely that it could provide an accurate assessment of the relationship between the coronary sinus and the coronary arteries.
Catheterisation of the coronary sinus and retrograde venography has been used since the middle of the 20th century. It should be performed in at least two different views to clearly separate the branches and display the course of the main branches. Coronary venous angiography allows direct measurement of the length and diameter of the coronary sinus and the great cardiac vein. Lengths are measured using dedicated measuring catheters inserted into the veins (see Figure 4) that take into account the curvature of the veins. Diameters, either proximal or distal, can be measured all along the veins using quantitative coronary angiography (QCA), as for the quantification of coronary artery stenosis. These dimensions are crucial for the decision on the size of the device that should be implanted. As a standard method, venous angiography requires a central venous access and carries the risks of an invasive investigation. Therefore, it is usually performed just before and during the implantation. Venography is indeed crucial to guide and monitor the procedure. However, venous angiography cannot provide any assessment of the relationship between the venous system and the mitral annulus, and evaluating the relationship between the coronary arteries and the venous system may be challenging since they are difficult to image simultaneously.
Electrocardiogram (ECG)-gated multislice computed tomography has been found to be an alternative.13–16 It provides a superb, detailed and minimally invasive assessment of the coronary venous system closed to anatomical assessment (see Figure 5). It requires injection of 60–100ml of contrast and usually exposes the patient to a radiation dose of approximately 10mSv.17
Computed tomography allows a precise evaluation of the coronary venous system anatomy and of the relationship between this venous system and the coronary arteries and the mitral annulus. It is thus the first-choice method for imaging the venous system before device implantation. Magnetic resonance imaging may be of potential interest, but currently only computed tomography provides enough resolution to evaluate the relationship between the coronary sinus, the coronary arteries and the mitral annulus.
Coronary Sinus Anatomy
The feasibility of assessing the coronary venous anatomy is excellent, reaching almost 100% for the coronary sinus and the great cardiac vein.14,16 Measurement of coronary sinus and great cardiac vein diameters can be performed in the majority of patients with low intraand inter-observer variability. Using multiplanar reconstruction, the coronary veins are manually selected and elongated, and the diameters are automatically measured all along the veins (see Figure 6). It has been shown that the diameter of the coronary sinus is larger in the supero-inferior direction than in the antero-posterior direction, indicating an oval shape. Lengths are also measured using this method. The tortuosity and curvature of the venous system can also be assessed using computed tomography, which helps to anticipate potential technical difficulties for implantation. Figure 7 presents an example of a patient with an abrupt curvature of the distal part of the great cardiac vein who could not be implanted using one of the devices discussed above.
Relationship with the Coronary Arteries
The circumflex artery crosses the venous system at a variable distance and in a variable path. It can run deeply between the mitral annulus and the coronary sinus or more superficially above the coronary sinus. Anatomical studies have shown that the former feature is common. In 61 explanted hearts, a course of the circumflex artery between the coronary sinus and the mitral annulus was noted in 64% of cases by Maselli and colleagues.18 In another anatomical study from our group, the circumflex artery ran between the coronary sinus and the mitral annulus in 45% of patients.19
An appreciation of the relationship between the venous system and the circumflex artery has been defined as a critical factor for the safety of mitral annuloplasty devices. The implantation of devices in patients with a circumflex running between the coronary sinus and the mitral annulus may expose the patient to coronary artery impingement; therefore, the procedure is usually not recommended in such patients. Furthermore, a correlation between the depth of the circumflex artery and its distribution has been reported.18 When the circumflex is deeper than the coronary sinus, the number of marginal branches is higher and their diameter larger, increasing the potentially deleterious effect of circumflex compression.
Unlike venography, computed tomography allows for simultaneous imaging of veins and coronary arteries, and can help identify patients in whom the circumflex artery will or will not be a problem if a coronary sinus annuloplasty is attempted. Several studies have shown that the relationship between the coronary sinus and the circumflex artery can be assessed in a high number of patients using computed tomography.13,16 As in autopsy studies, a crossing between the circumflex artery and the coronary sinus was observed in 60–80% of cases (see Figure 8). It is worth noting that there is significant variability in the distance to the point at which the coronary sinus intersects with the circumflex artery (from 37 to 116mm).
Relationship with the Mitral Annulus
The coronary sinus and the great cardiac vein run around the mitral annulus. However, in a significant number of patients the venous system is not exactly at the level of the mitral annulus. Autopsy studies have shown that the coronary sinus is usually above the mitral annulus on the lateral wall of the left atrium.18 This finding has also been well demonstrated using computed tomography (see Figure 9).13,16 In 27 normal subjects and 14 patients with mitral regurgitation, the authors observed that the plane of the coronary sinus was on the left atrial side of the mitral annulus in all patients.13 In addition, computed tomography clearly shows that the distance between the coronary sinus and the mitral annulus is highly variable between individuals (ranging from 2.2 to 15.3mm at the proximal coronary sinus and from 2.6 to 18.6mm at the distal coronary sinus) and is not uniform (higher posteriorly than anteriorly).13
Furthermore, this distance seems to be greater in patients with a dilated ventricle and severe mitral regurgitation, who are the potential candidates for percutaneous annuloplasty, than in patients with non-severe mitral regurgitation.16 The degree to which the distance between the coronary and the mitral annulus may affect the efficiency of the devices remains unclear, but computed tomography provides a unique way to evaluate the impact of anatomical features on the clinical efficiency of a device.
The high degree of variability in coronary venous anatomy makes a non-invasive pre-procedural assessment crucial, and computed tomography is currently the first-choice imaging modality. Computed tomography plays a key role in patient selection and thus should be systematically performed as a screening test in patients considered for percutaneous mitral annuloplasty. It allows precise assessment of venous system anatomy (especially length, diameters and angulations) and its relationship with the coronary arteries and the mitral annulus. Venous angiography provides useful complementary information, and is performed during the procedure to confirm the findings from computed tomography and provide final measurements based on the size of the device that should be implanted; it is also crucial to guide and monitor the procedure.