Surgical Replacement of the Mitral Valve

Mitral Stenosis: Indications for Mitral Valve Repair or Replacement

MS may arise from any number of underlying diseases, ^, but is most often caused by rheumatic fever. ^, Although the prevalence of rheumatic disease has been drastically reduced in developed nations; it continues to impose a substantial burden worldwide. ^, MS may also develop as degenerative calcific disease, usually in elderly populations, and specifically those with cardiovascular comorbidities or renal disease. ^{, ,}

The pathophysiology of varying etiologies of MS often results in distinct valvular lesions with unique patient presentations. ^, For example, rheumatic MS is often characterized as an indolent disease that progresses to severe fibrosis and calcification of the valve apparatus, with fusion of the chordal structures and commissures and gradual onset of symptoms. In contrast, degenerative MS is often characterized by leaflet and mitral annular calcification rather than chordal and commissural fusion. Developing symptoms may be delayed, but when symptoms manifest, deterioration may occur relatively acutely. In this respect, surgeons should be familiar with the American Heart Association/American College of Cardiology (AHA/ACC) guidelines for staging the severity of MS disease and symptoms, as these are often sufficient for determining when surgical intervention is indicated. ^,

To briefly summarize some common recommendations, mitral valve surgery (i.e., MVr/commissurotomy or MVR) is recommended for patients with severe symptoms and severe MS who have either failed percutaneous intervention and/or those who are not candidates for percutaneous intervention. Among other indications, mitral valve surgery is also indicated in asymptomatic patients with severe MS or severely symptomatic patients who are undergoing cardiac surgery for other reasons.

It is important to recognize that the guidelines rarely mention MVR specifically. Rather, they are broadly phrased in terms of “mitral valve surgery,” or more specifically MVr, and it is worth reiterating that whenever possible, MVr is usually preferred to MVR. However, some preoperative and intraoperative findings may indicate that MVR is favored over MVr, including the presence of severe leaflet and/or annular calcification, significant subvalvular apparatus involvement (i.e., chordae tendinae and papillary muscles), and/or coexistent MS with MR, which is often unamenable to MVr.

It is also worth noting that although there are some cases in which asymptomatic and/or less-symptomatic patients may meet criteria for MVr when a surgeon concludes that MVR will likely be necessary for MS, it may be preferential to delay surgical intervention until symptoms become more severe if cardiac function is preserved, due to the inherent risks associated with prosthetic valve implantation in MVR (see “Valve Selection” below).

Mitral Regurgitation: Indications for Mitral Valve Repair or Replacement

Because of the myriad disease states that may lead to MR, deciding when and how to intervene in MR is often more complicated than in stenotic lesions. Nevertheless, MR is most often caused by degenerative valvopathy, with mitral valve prolapse accounting for ~50% of cases in the United States. ^, However, MR can also be caused by rheumatic, ischemic, or infectious etiologies, among others. Proper preoperative evaluation of disease etiology, severity of valvular lesions and/or resulting ventricular dysfunction, and patient symptoms is essential to determining when surgical intervention is indicated. In developing recommendations for surgical intervention, the AHA/ACC guidelines have broadly categorized MR into two classes, either primary or secondary MR. ^,

Other Indications: Mixed Lesions, Concomitant Procedures, and Endocarditis

Although MS and MR are often dichotomized, mitral valve disease etiologies, valvular lesions, and their resulting dysfunction often overlap. Surgeons will certainly encounter patients with mitral valve disease encompassing aspects of both primary and secondary MR and/or MS. ^{, ,} In fact, only about 25% of patients with MS have isolated disease, whereas an additional 40% have some degree of MR. ^, In such cases, one might seriously consider MVR, as successful MVr may be likely to be successful in such complicated lesions with additional concerns for durability of repair depending on disease severity.

Meanwhile, guidelines exist for evaluating when mitral valve surgery may be considered when other procedures are the primary indication for cardiac surgery and vice versa. For example, in some cases, MVr/MVR may be reasonably considered at the time of coronary artery bypass grafting. On the other hand, when mitral valve surgery is the primary indication for cardiac surgery, prophylactic repair or replacement of other valves may not necessarily be indicated, such as when there is concomitant aortic stenosis that is unlikely to progress. ^, In any case, when multiple procedures are planned, important consideration should be given to preoperative planning that will optimize the order of component procedures, thereby minimizing operative times and reducing the risk of associated complications (see “Concomitant Procedures” below).

Lastly, it should be no surprise that MVr is again preferred over MVR in cases of endocarditis to minimize the amount of foreign material implanted and the subsequent risk for prosthesis infection (see “Prosthetic Valve Endocarditis and Prophylaxis” below). Still, there are circumstances in which MVR may be warranted, particularly when extensive debridement is required without any chance for valve preservation. When MVR is required in endocarditis, normal criteria should be used to decide whether to implant a mechanical or biologic prosthesis.

Age-Related Guidelines for Prosthesis Selection: Considering Valve Durability and Anticoagulation Requirements

After a prosthetic valve is implanted, some degree of tissue overgrowth around the prosthesis is expected as the heart heals itself from the insult of open-heart surgery. In fact, this is important in developing a nonthrombogenic surface at the suture line. However, pathologic overgrowth can occur, particularly in BHVs, often referred to as structural valve degeneration (SVD). Although the precise mechanisms of SVD are unclear, it appears to be an age-related phenomenon that likely results from differences in immunity and calcium metabolism. ^,

Some studies suggest that the risk for SVD at 15-years after BHV implantation exceeds 20% among patients less than 50 years of age, ^{, , , ,} whereas others estimate the incidence to be ~65% in patients less than 40 years of age, compared to just 10% in those 70 years or older. ^, Regardless, SVD remains the most common cause of BHV failure, and generally speaking, it can be expected that BHVs implanted into patients younger than 60 years of age will eventually need to be replaced. ^{, ,} Therefore when determining whether or not to implant a BHV, one must consider the patient’s individual risk for reoperation and the potential morbidity and mortality associated with surgical reintervention.

In contrast, mechanical valves offer excellent durability and have the potential to last the entirety of a patient’s lifetime, regardless of age. Thus, mechanical valves are particularly well-suited for younger patients whose primary concern is the avoidance of reoperation. ^, However, because mechanical valves are especially thrombogenic, all patients who receive a mechanical prosthesis require lifelong anticoagulation (see “Anticoagulation Guidelines” below), which carries additional risks for significant bleeding complications, specifically among the elderly. ^{, , ,} Of course, this means that mechanical valves should be avoided in all patients with any contraindication to anticoagulation and/or those who are unable to adequately monitor their therapeutic levels. Each of these considerations can significantly impact long-term outcomes after MVR, underscoring that prosthetic valve selection must be individualized for each.

Nevertheless, for patients 50 to 69 years of age, it has been repeatedly demonstrated that long-term survival is similar after MVR when implanting either a BHV or mechanical valve. ^{, , ,} On the other hand, increasing evidence suggests that a long-term survival advantage persists up to 70 years of age after MVR with a mechanical prosthesis compared to a BHV. ^{, , ,} Therefore, the recently updated AHA/ACC guidelines have lowered the recommended age for mechanical MVR from 60 to 50 years, noting that it is reasonable to implant either a BHV or mechanical prosthesis in patients 50 to 70 years of age. These guidelines have continued to recommend that BHVs should be implanted in patients over 70 years of age.

It is important to note that evolving technology in MVR is promising but remains uncertain. Specifically, the potential for transcatheter valve-in-valve replacement therapy after surgical MVR may ultimately impact future guidelines such that the minimum age recommended for BHV implantation could be reduced to patients younger than 70 years of age. These existing uncertainties regarding the future of mitral valve surgery underscore the importance of strategic decision-making when determining which type of prosthesis to implant during MVR and highlight that additional factors beyond just the patient’s age should be considered when deciding whether to implant a BHV or mechanical prosthesis. ^,