Introduction

There are two mitral valve operations for mitral regurgitation (MR): mitral valve plasty (MVP) and mitral valve replacement (MVR). In MVP, the self-valve is preserved and repaired. The advantage is no reaction to a foreign substance, though it requires skilled techniques. On the other hand, the MVR technique is simple, but anticoagulant therapy is needed during life. As in humans, the first choice is MVP in dogs.

Anatomy of Mitral Valve

Mitral valve complexes are mainly valve ring, valve leaflet, and chorda tendineae, and secondarily papillary muscle, left atrium, and left ventricle. Therefore, MVP involves repair of the valve ring, the valve leaflet, and the chorda tendineae.

Lesion Type of Mitral Regurgitation

There are 4 lesion types of mitral regurgitation in human, according to Carpentier:

 Type I is the enlargement of the valve ring due to dilative cardiomyopathy or ischemic heart disease

 Type II is valve prolapse with elongation or ruptured chorda due to myxomatous degeneration

 Type IIIA is mitral stenosis (MS) and MR with fused and hypertrophied valve leaflet due to rheumatic fever

 Type IIIB is traction of the valve leaflet due to ischemic dysfunction

In the dog, most MR is Type II due to myxomatous degeneration together with Type I accompanied by secondary enlargement of the valve ring. Another characteristic in dog's MR is that the anterior leaflet (AL) is damaged more than the posterior leaflet (PL).

The mitral valve map shows orientation of lesion for MR. Anterior leaflet is separated into A1, A2, and A3 segments. Posterior leaflet is separated into P1, P2, and P3 segments, with clefts between them.

Echocardiography Before Surgery for Mitral Valve Plasty

The best examination before surgery for MVP is echocardiography. The lesion site of MR is presumed at the opposite of mitral regurgitation by color Doppler on the LV long axis sectional view and the mitral valve short axis sectional view.

Recently, we use 3D echocardiography, which provides information for the surgeon's view before the MVP operation.

Mitral Valve Plasty Method

There are mainly three methods in MVP: 1) Annuloplasty for dilated mitral valve ring, 2) valvuloplasty for deformed valve leaflet, and 3) chordal reconstruction for ruptured or prolonged chorda tendineae.

1. Annuloplasty for Dilated Mitral Valve Ring

There are three techniques for annuloplasty:

1.  Commissural annuloplasty and the modified techniques are the oldest technique for MR in humans

2.  Semicircular suture annuloplasty and the modified techniques have been developed

3.  Ring annuloplasty using an artificial ring was recently designed and used in most MVP for human MR

However, we think this ring annuloplasty may be inadequate because the ring disturbs blood inflow due to its large size in small and toy dogs. Therefore, we do not use this method in small dogs. Instead, we always employ the semicircular suture annuloplasty technique only for the posterior valve ring to prevent re-enlargement of the mitral valve ring in our MVP method in small dogs. We sometimes use anterior or posterior commissural annuloplasty as additional techniques, if needed. The advantages of this method are the simple technique, time saving and no blood inflow disturbance in small dogs.

2. Valvuloplasty for Deformed Valve Leaflet

Valvuloplasty is possible for a deformed valve leaflet. Triangle resection and rectangle resection are adapted to resect the redundant leaflet part due to ruptured and/or elongated chordae. This technique is used in most MVP for human MR, especially involving the posterior leaflet. However, we do not use this technique because the resection decreases the valve area and coaptation area, and there is almost never a redundant leaflet part in small dogs. This technique is definitely needed for any annuloplasty and ring annuloplasty commonly used in human MR.

We had a case using this triangle resection of the anterior leaflet with posterior commissural annuloplasty in a small dog in 1990. It was the first successful clinical case of MVP in the dog. We now use the valve cusp direct suture technique on the severely damaged posterior leaflet to remove valve cleft in severe MR cases in small dogs.

3. Chordal Reconstruction for Ruptured or Prolonged Chorda Tendineae

On chordal reconstruction, shortening of prolonged chorda and shifting of approximate chorda technique are very difficult in small dogs. Chordal reconstruction using pericardium, silk, nylon, Teflon, or Dacron sutures was used in the past, but has been discontinued because of the calcification or deterioration. Recently, chordal reconstruction using e-PTFE (Gore-Tex) suture has been designed and used in most MVP for human MR.

We studied this suture experimentally in the dog and presented it at the WSAVA Congress in 1995. Five years later, we presented MVP by chordal reconstruction using e-PTFE suture in a small MR dog (Kanemoto I, et al. presented it in part at the 1999 ACVS Symposium, San Francisco).

Our Mitral Valve Plasty Technique

1.  Basic method: Chordal reconstruction (AL) + Semicircular suture annuloplasty.
The basic method is combined with chordal reconstruction of the anterior leaflet and semicircular suture annuloplasty on the fibrous ring of the posterior leaflet side.

2.  Multiple reconstruction chordal reconstruction (AL + PL) + Semicircular suture annuloplasty + Valvuloplasty + Commissural annuloplasty.
If multiple chordae are ruptured or elongated, we employ multiple reconstruction using a pair of sutures with two sutures at the anterior and/or posterior leaflets. Next, we use semicircular suture annuloplasty on the posterior leaflet side. If necessary, valve cusp direct suture and/or anterior or posterior commissure annuloplasty are additionally combined.

Advantages of Our MVP Technique

 Chordal reconstruction using e-PTFE suture involves no foreign body reaction and does not require anticoagulant therapy.

 Semicircular suture annuloplasty using Prolene suture does not disturb blood inflow and maintains the flexibility of the mitral valve ring.

 Posterior cusp direct suture in the posterior leaflet increases the coaptation area of the posterior leaflet and decreases mitral regurgitation.

 Commissural annuloplasty is a very simple technique that can save time.

Mitral valve plasty seems possible even in severe MR cases which are usually planned by MVR.

Mitral Valve Plasty Challenges in Small and Toy Dogs

Future challenges of MVP in small and toy dogs are stated below.

Cardiopulmonary bypass (CPB) methods will involve:

1.  Ongoing miniaturization of the circuit and oxygenator to prevent excessive hemodilution

2.  Development of low-flow CPB to moderate-flow CPB by increasing venous drain volume

3.  The eventual change from deep HT (20°C) to moderate HT (25–28°C)

4.  Improvement of cardioplegia to provide prolongation of aortic cross-clamp time, and perfect repair of MVP

5.  Improvement of ICU to ensure fewer complications and better results.

New operative methods will include:

1.  Development of MVP in small and toy dogs

2.  Development of bioprosthetic valve (MVR) in small and toy dogs

References

1.  Carpentier A. Cardiac valve surgery - the "French correction." J Thorac Cardiovasc Surg. 1983;86:323–337.

2.  Kanemoto I, Shibata S, Noguchi H, Chimura S, Kobayashi M, Shimizu Y. Successful mitral valvuloplasty for mitral regurgitation in a dog. Jpn J Vet Sci. 1990;52:411–414.

3.  Boggs LS, Dewan SJ, Ballard SE. Mitral valve reconstruction in a toy-breed dog. J Am Vet Med Assoc. 1996;209:1872–1876.

4.  Griffiths LG, Orton EC, Boon JA. Evaluation of techniques and outcomes of mitral valve repair in dogs. J Am Vet Med Assoc. 2004;224:1941–1945.

5.  Kanemoto I, Suzuki H, Taguchi D, Yokoyama S, Mizuno M, Kanamoto T. Successful surgical repair for severe mitral regurgitation in five small-breed dogs. Vet Surg. 2004;33:4355-E24 (Abstract from the 14th annual American College of Veterinary Surgeons symposium, October 7–9, 2004, Denver, CO, USA).

6.  Uechi M. Mitral valve repair in dogs. J Vet Cardiol. 2012;14:185–192.