This chapter discusses the surgical indications, operative techniques, and early and late follow-up after implantation of mechanical and bioprosthetic mitral valve devices. The valves that are discussed are those that are currently (2015) approved by the Food and Drug Administration (FDA). Figure 42-1 shows the former and current FDA-approved prosthetic mechanical mitral valve devices, including the Starr-Edwards ball-and-cage valve (historical relevance only), the Omnicarbon tilting-disk valve, the Medtronic Hall tilting-disk valve, the St. Jude Medical bileaflet valve, the Carbomedics bileaflet valve, the ATS bileaflet valve, and the On-X bileaflet valve. The FDA-approved bioprosthetic valve devices are shown in Fig. 42-2 and include the Hancock II porcine valve, the Carpentier-Edwards porcine valve, the Carpentier-Edwards pericardial valve, the Mosaic porcine valve, and the Biocor porcine valve.

Heart valve prostheses are continually undergoing iterative advancement by the manufacturers; however, the ideal valve has yet to be developed. This ideal replacement prosthesis would have longevity of a mechanical prosthesis combined with the superior hemodynamic function of the native biologic tissue valve. As a result, this hypothetical ideal replacement device would not require lifetime anticoagulation and carries no risk of either thromboembolic events or valve thrombosis. Achieving this goal will require major advancements to currently available designs.

The indications for mitral valve replacement (MVR) are variable and undergoing evolution. Because of increasing use of reparative techniques, particularly for mitral regurgitation (MR), replacement or repair of a mitral valve often depends on the experience of the operating surgeon. Current indications for valve replacement pertain to the types of valve problems that are unlikely to be repaired by most surgeons or which have been shown to have poor long-term success after reconstruction. Indications are discussed according to (1) pathophysiologic states and (2) type of valve required (ie, mechanical or bioprosthetic).

Mitral stenosis (MS) is almost exclusively caused by rheumatic fever even though a definite clinical history can be obtained in only about 50% of patients. The incidence of MS has decreased substantially in the United States in the last several decades because of effective prophylaxis of rheumatic fever; nevertheless, in certain developing countries MS is still very common. Two-thirds of patients with rheumatic MS are females.

The pathologic changes associated with rheumatic valvulitis are mainly fusion of the valve leaflets at the commissures, shortening and fusion of the cordae tendineae, and thickening of the leaflets owing to fibrosis with subsequent stiffening, contraction, and calcification. Approximately 25% of patients have pure MS, but an additional 40% have combined MS and MR.¹

Stenosis usually develops one or two decades after the acute illness of rheumatic fever with no or slow onset of symptoms until the stenosis becomes more severe. Limitation of exercise tolerance usually is the first symptom, followed by dyspnea that can progress to pulmonary edema. New-onset atrial fibrillation and the risk for thromboembolism, hemoptysis, and pulmonary hypertension are other common symptoms in patients with MS.

Besides echocardiographic imaging, the diagnostic workup of the symptomatic patient with MS should include a complete cardiac catheterization, including coronary angiography in any patient older than 40 years old. In younger patients, echocardiographic assessment of the mitral valve suffices in most symptomatic patients unless there is a history of chest pain or coronary artery disease. Typically echocardiogram establishes the extent of MS by determining valve gradients and valve area, but in complex cases cardiac catheterization with direct measurement might be more beneficial.

Recently published 2014 American Heart Association/American College of Cardiology guideline has a new staging system for MS.² The criteria for severe MS has changed from mitral valve area of 1.0 cm² or less to a mitral valve area of 1.5 cm², the normal native mitral valve area being 4 to 6 cm², and/or a diastolic half pressure time more than 150 ms. Symptomatic severe MS is considered stage D while asymptomatic severe MS is stage C. Finally, a mitral valve area of less than 1.0 cm² or diastolic half pressure time more than 220 ms is considered “very severe MS.”

Currently, the first option for MS is percutaneous balloon valvuloplasty and certain anatomic characteristics allow a more successful valvuloplasty (Table 42-1).² The Wilkins score is used to predict the success rates of this procedure by taking into consideration valve thickness, valvular calcification, leaflet mobility, and subvalvular thickening. Each category has a score of 1 to 4 and a total score of 8 or less predicts higher success rate with balloon valvuloplasty. If the score is higher than 8, MVR is recommended.³ Subsequently, the 2014 ACC/AHA guidelines gave class I recommendation for mitral valve surgery in severely symptomatic patients (NYHA class III/IV) with severe MS (stage D) who are not high risk for surgery and who are not candidates for previous balloon valvuloplasty or have failed a previous balloon valvuloplasty and in patients with severe MS (stage C or D) undergoing other cardiac surgery. Class IIa recommendation is given for severely symptomatic patients (NYHA class III/IV) with severe MS (stage D) provided there are other operative indications and Class IIb recommendation for patients with moderate MS (MVA 1.6-2.0 cm²) undergoing other cardiac surgery and patients with severe MS (stages C and D) who have had recurrent embolic events while receiving adequate anticoagulation. We must also mention that the degree of pulmonary artery pressure elevation secondary to MS continues to be an area of concern for the mitral valve surgeon but it has been known for more than 40 years that after MVR for MS, pulmonary artery pressure decreases within hours in most patients and decreases more gradually over weeks to months in others.^4-6

Historically, the success with closed commissurotomy after World War II and the development of the Starr-Edwards valve in the early 1960s led to an enormous increase in operations for rheumatic mitral valve disease. It was until the 1990s that the balloon dilation of fibrotic stenotic mitral valves became increasingly common.^6-8 Open mitral commissurotomy and valvuloplasty also became an alternative for MS patients,^9,10 but some studies have shown better long-term results with MVR using a mechanical valve.¹¹ Many patients with chronic MS now require valve replacement because the valve has developed significant dystrophic changes including marked thickening and shortening of all chordae, obliteration of the subvalvular space, agglutination of the papillary muscles, and calcification in both annular and leaflet tissue. Aggressive decalcification and heroic reconstructive techniques for these extremely advanced pathologic valves generally have produced poor long-term results; nevertheless, some surgeons still advocate aggressive repairs in this subset of patients.¹²

The etiology of MR is very diverse and the decision to operate on patients with MR is more complex than for patients with MS, except in patients with acute ischemic MR and endocarditis in whom indications are more straightforward. The pathologic events that produce MR are related to a number of metabolic, functional, and anatomical abnormalities.¹ These can be categorized into degenerative (eg, mitral prolapse and ruptured/elongated chordae), rheumatic, infectious, and ischemic diseases of the mitral valve. The recent AHA/ACC guideline separates MR into primary MR and secondary MR and is later graded based on patient symptoms and severity of the disease.²

The preferred surgical approach for primary MR is mitral valve repair which is described in a different chapter in this book. Mitral valve surgery has class I recommendation for symptomatic severe MR with left ventricular ejection fraction (LVEF) over 30% or asymptomatic severe MR with decreased LVEF (30-60%). It is important to stress that depressed ejection fraction is a poor indicator of left ventricular function in patients with MR. Ejection fraction can be preserved in patients with irreversible left ventricular failure because of regurgitant flow through the valve.^13,14 Depressed cardiac function (LVEF < 40%) therefore usually indicates severe left ventricular dysfunction, and results of surgery are not as favorable in these patients as they are in those with normal ventricles.^15,16 Measurements of end-systolic volume and diameter have lately surfaced as more reliable noninvasive parameters to evaluate the status of the left ventricle and determine the optimal time for operation.^17,18 Therefore, LV end-systolic diameter over 40 mm is also considered Class I indication in patients with asymptomatic MR.

Mitral valve repair is indicated in patients with primary MR due to a degenerative prolapsing myxomatous valve especially if the prolapse is generalized and local findings that decrease the probability of a successful repair are absent.^19-23 When the probability of successful repair is low as is the case with the encounter of rheumatic MR, calcific deposits throughout the leaflet substance and shortened chordae and papillary muscles, MVR is often the most prudent operation.²⁴ However, good results with reconstructive surgery in this patient group have been reported.²⁵ In patients with endocarditis, MVR may be required because of destruction of the valve leaflets, subvalvular mechanisms and annular abscess formation. Although repair of the valve and avoidance of prosthetic material are very desirable in septic situations, the extent of the destruction may preclude repair. Therefore, MVR is required after careful debridement of the infectious tissue and reconstruction of the valve annulus.^26-28

For secondary MR, severe secondary MR in the setting of cardiac procedure has Class IIa, and symptomatic severe secondary MR has class IIb recommendation to undergo mitral valve surgery. Recently published randomized control study that compared MVR and MV repair in patients with severe ischemic MR showed no difference in mortality and LV remodeling.²⁹ However, MV repair group had 33% recurrence of moderate or severe MR compared to 2% in MVR. Other specific findings that preclude satisfactory repair include restrictive valve motion from shortened, scarred papillary muscles, acutely infarcted papillary muscle, and rupture of chordae associated with extensive calcification of valve leaflets.^30-32

Indications for Mechanical Valve Replacement

For young patients, patients in chronic atrial fibrillation who require long-term anticoagulation, and any patient who wants to minimize the chance of reoperation, a mechanical prosthetic valve should be chosen if valve replacement is required. The St. Jude Medical bileaflet valve is the most widely used prosthetic mitral valve at present because it has good hemodynamic characteristics and is easy to insert. Recent interest in the On-X mechanical valve relates primarily to the possibility for limited anticoagulation, although this remains to be borne out from current clinical trial.³³ Indications to choose one prosthetic or another vary primarily by surgeon preference and occasionally depending on the state of the annulus and whether or not there have been multiple previous operations. The most recent 2014 AHA/ACC guideline has changed its recommendation from class I recommendation of mechanical valve in age 65 years to “shared decision-making process with patients preferences and risks of reoperation.”² There is a class IIa recommendation for mechanical valves in patients under age 60 years and either bioprostheses or mechanical prostheses for age between 60 and 70 years.

We have performed a propensity matching analysis in patients younger than 65 years who underwent MVR. Our results showed higher reoperation rate in bioprostheses group and higher survival in mechanical prostheses group (11 vs 13 years, p = .004; Fig. 42-3). Therefore, we recommend mechanical prosthesis in this patient group.³⁴

Indications for Bioprosthetic Valve Replacement

Patients in any age group in sinus rhythm who wish to avoid anticoagulation may prefer a bioprosthetic valve. Recent 2014 AHA/ACC guideline gave Class I recommendation for bioprostheses in patients at any age in whom anticoagulation therapy is contraindicated, cannot be managed appropriately or is not desired. A bioprosthetic valve is preferred in patients greater than age 65 years and in sinus rhythm because these valves deteriorate more slowly in older patients.³⁵ In addition, some 60-year-old patients may not outlive their prosthetic valves because of coexisting comorbidities.^36,37 Specifically, patients who require combined MVR and coronary bypass grafting for ischemic MR and coronary artery disease have a significantly reduced long-term survival as compared to patients who do not have concomitant coronary artery disease.^38-43 In these individuals with little risk of reoperation, anticoagulation may better be avoided.

As 20-year results have become available for various bioprostheses, it is clear that structural valve degeneration (SVD) is the most prominent complication of these valves.^44-49 The durability of porcine mitral valves is less than that of aortic bioprostheses which is directly proportional to age.⁴⁵ Deterioration occurs within months or years in children and young adults and only gradually over years in septuagenarians and octogenarians.^44,50 Essentially, all valves implanted into patients younger than 60 years of age have to be replaced ultimately, and valve failure is prohibitively rapid in children and adults younger than 35 to 40 years of age; therefore, bioprostheses are not advisable in these age groups.⁵¹ Nevertheless, there are still indications for mitral porcine bioprosthetic valves in young patients. In a woman who desires to become pregnant, a bioprosthesis may be used to avoid warfarin anticoagulation and fetal damage during pregnancy.^52-55

Over the last decade several publications, mainly from European centers, reported on the use of unstented cryopreserved homografts^56-59 and stentless heterografts^60-63 for MVRs, particularly in patients with endocarditis. The prosthetic valve is transplanted so that donor papillary muscles are reattached to recipient papillary muscles and the annulus is sutured circumferentially. This technique has been shown to be safe and reproducible, but it does not always provide durable results and therefore should not be used in young patients.⁶⁰ Other reports suggest that these operations may be a feasible alternative to stented valve replacement in patients with endocarditis. Pulmonary autografts also have been used for replacing the mitral valve (Ross II procedure), but these series are small, and follow-up is relatively short.^64-66

Mitral valve surgery has been in a state of constant evolution since its inception. Data regarding all cardiac surgical procedures reported to the Society of Thoracic Surgeons Adult Cardiac Surgery Database (STS ACSD) demonstrate this dynamic state as more surgeons begin to repair rather than replace mitral valves. Gammie et al evaluated trends in mitral valve surgery in the United States using the STS ACSD evaluating the years between 2000 and 2007.⁶⁷ In this time period, 210,529 mitral valve procedures were performed in all settings. From the study population, 58,370 patients undergoing primary mitral valve operations were identified. Over this 7-year study time line, a 50% increase in repair rates was documented. When considering the valve replacement, a 100% increase in use of bioprosthetic devices coincided with a decline in the use of mechanical valves.

Gammie and coworkers identified clear trends with respect to patient selection as well.⁶⁷ Compared with patients undergoing mitral valve repair, those undergoing mitral replacement tended to be older, females, more likely to have multiple comorbidities (eg, diabetes mellitus, hypertension, chronic lung disease, stroke), concomitant tricuspid valve disease, MS, and were less likely to be asymptomatic. With respect to survival, overall risk adjusted mortality was lower for mitral valve repair versus replacement (OR 0.52, 95% CI: 0.45 to 0.59, p < .0001). The outcome with respect to choice of valve type and survival was not addressed but was felt to be confounded by patient factors that led to the initial device selection.

Mechanical Prostheses

The designs of mechanical and bioprosthetic heart valves have evolved over the last five decades in an effort to develop the ideal replacement device for the pathologic mitral valve. The optimal heart valves exert minimal resistance to forward blood flow and allow only trivial regurgitant backflow as the occluder closes. The design must cause minimal turbulence and stasis in vivo under physiologic flow conditions. The valve must be durable enough to last a lifetime and must be constructed of biomaterials that are nonantigenic, nontoxic, nonimmunogenic, nondegradable, and noncarcinogenic. The valve also must have a low associated incidence of thromboembolism.

The opening resistance to blood flow is determined by the orifice diameter; the size, shape, and weight of the occluder; the opening angle; and the orientation of leaflet or disk occluders with respect to the plane of the mitral annular orifice for any given annular size. Least resistance to transvalvular blood flow during diastole for valves in the mitral position is provided by a large ratio of orifice to total annular area. A wide opening angle also improves the effective orifice area and results in decreased diastolic pressure gradients. Table 42-2 shows hemodynamic assessments of each of the FDA-approved mitral valve prostheses for the most commonly used mitral valve sizes.^68-71 The results of in vivo assessments at rest by invasive (catheterization) or noninvasive (Doppler echocardiography) techniques are tabulated.

Blood turbulence flowing across mitral valve devices results from impedance to forward or reverse flow. This impedance can be minimized by occluder design and orientation, central flow through the orifice and limited struts or pivots extending into flow areas (Fig. 42-4). Hemolysis is the product of red blood cell destruction that is caused by cavitation and shearing stresses of turbulence, high-velocity flow, regurgitation, and mechanical damage during valve closure.⁷² Areas of perivalvular blood stagnation and turbulence increase platelet aggregation, activation of the coagulation proteins, and thrombus formation.

Dynamic regurgitation is a feature of all prosthetic valves and is the sum of the closing volume during occluder closure and the leakage volume that passes through the valve while it is closed. The closing volume is a function of the effective orifice area and the time needed for closure. Leakage volume is inherent to the design of the valve and depends on the amount of time the valve remains in the closed position.⁷³ A small amount of regurgitant volume can be beneficial by minimizing stasis and reducing platelet aggregation; this decreases the incidence of valve thrombosis and valve-related thromboembolism.

BALL AND CAGE MECHANICAL VALVES

Starr–Edwards

Introduced in 1965, the Starr-Edwards Model 6120 was the only ball-and-cage mitral valve prosthesis approved by the FDA; however, the production was discontinued in 2007 (see Fig. 42-1A). The occluder is a barium-impregnated Silastic ball in a Stellite alloy cage that projects into the left ventricle. This valve has a large Teflon/polypropylene sewing ring that produces a relatively smaller effective orifice and larger diastolic pressure gradients than other prosthetic valves of similar annular sizes. Leakage volumes are not inherent in the ball-and-cage design, and in contrast with other mechanical valves, the presence of regurgitation may indicate a pathologic process. The central ball occluder causes lateralization of forward flow and results in turbulence and cavitation that increase the risk of hemolysis and thromboembolic complications (see Fig. 42-4A).^74-76

MONOLEAFLET MECHANICAL VALVES

Medtronic-Hall

Tilting-disk mitral valve prostheses had better hemodynamic characteristics than ball-and-cage valves (see Fig. 42-4B) but with the development of new valve designs and technology they were taken off the market. The Medtronic Hall central pivoting-disk valve was available between 1977 and 2009 and it was based on engineering design modifications of the earlier Hall-Kaster valve⁷⁷ (see Fig. 42-1B). The opening angle of 70° produced regurgitation volumes of less than 5% of left ventricular stroke volume without significantly compromising forward flow. The disk occluder was allowed to slide out of the housing at the end of the closing cycle to provide a gap through which blood could flow to minimize stasis at the contact surfaces.⁷⁸ The large opening angle and slim disk occluder, along with a thinner sewing ring, provide improved hemodynamics with comparably larger effective orifice areas and lower mean diastolic pressure gradients for each valve size.

Omniscience-Omni Carbon

The Omniscience tilting-disk valve was first introduced in 1978 as a second-generation device derived from improvements to the design of the Lillehei-Kaster pivoting-disk valve.⁷⁹ It had a larger orifice to annular ratio, a larger opening angle of 80° and better hemodynamics compared to its predecessor. A special disk design reduced the regurgitant volume, the turbulence and the areas of stasis and shear stress. Clinical studies, however, opened a huge debate concerning this valve’s hemodynamics since there was reports of a postoperative mean opening angle between 44.8⁶⁹ and 75.9°.⁸⁰ Implicated factors causing this variation included valve sizing, orientation during implantation, and anticoagulation status.^80,81 A subsequent generation of the Omniscience valve is the all-carbon Omnicarbon monoleaflet valve that was released in 2001 in the United States but has been in clinical use in Europe since 1984 (see Fig. 42-1C). The housing material is made of pyrolytic carbon instead of titanium. As a result of this change, the incidence of thromboembolism, valvular thrombosis, and reparations was decreased significantly compared with the Omniscience valve prostheses.⁸² Since 2005, however, the production of the Omnicarbon valve ended as the company decided to stop this production line.

Currently the only valve design available for implantation in the United States is the bileaflet design; it is being provided by many manufacturers.

BILEAFLET MECHANICAL VALVES

St. Jude Mechanical Valve

The unique design of the bileaflet St. Jude Medical valve was introduced in 1977, and it is currently the prosthesis used most commonly worldwide (see Fig. 42-1D). Two separate pyrolytic carbon semidisks in a pyrolytic carbon housing are attached to a Dacron sewing ring. The housing has two pivot guards that project into the left atrium. The bileaflet design produces three different flow areas through the valve orifice that provide overall a more uniform, central, and laminar flow than in the caged ball and monoleaflet tilting-disk designs. The improved flow results in less turbulence and decreased transmitral diastolic pressure gradients^73,83 (see Fig. 42-4C) at any annulus diameter size and cardiac output compared with the caged ball and single-leaflet tilting valves.⁸⁴ The favorable hemodynamics in smaller sizes makes it especially useful in children. The central opening angle is 85°, with a closing angle of 30 to 35°, which along with a thin sewing ring, provides a large effective orifice area for each valve size at the expense of greater regurgitant volumes, especially at low heart rates. Asynchronous closure of the valve leaflets in vivo also contributes to the regurgitant volume.⁸⁵ The design of this prosthesis provides excellent hemodynamic function even in small sizes in any rotational plane.⁸⁶ The antianatomical plane, however, with the central slit between the leaflets oriented perpendicular to the opening axis of the native valve leaflets decreases the potential risk of leaflet impingement by the posterior left ventricular wall.⁸⁷ Rotatable cuff designs are available on newer generation models.

Carbomedics Valve

The Carbomedics bileaflet valve was approved by the FDA in 1986 (see Fig. 42-1E). This low-profile device is constructed of pyrolytic carbon and has no pivot guards, struts, or orifice projections to decrease blood flow impedance and turbulence through the valve.⁷³ It has a rotatable sewing cuff design and is available with a more generous and flexible sewing cuff (the OptiForm variant) that conforms more easily to different patient anatomies and allows subannular, intraannular, or supraannular suture placement. The leaflet opening angle is 78°, which with the bileaflet design provides a relatively large effective orifice area and transvalvular diastolic pressure differences only slightly greater than the St. Jude Medical bileaflet valve. Because of its narrow closing angle and large leakage volume, the Carbomedics valve does not reduce the relatively large regurgitant volume associated with the bileaflet design. Although this valve has good hemodynamic function overall, in the mitral position, the 25-mm Carbomedics valve has a relatively high diastolic pressure gradient and large regurgitant energy loss across the valve, especially at high flows. Hemodynamic studies suggest that the Carbomedics valve should be avoided in patients with a small mitral valve orifice.⁷³

Advancing the Standard (ATS) Valves

The ATS Open Pivot® bileaflet mechanical prosthesis has been in clinical use in the United States since 2000. Similar to the Carbomedics valve, the ATS valve is a low-profile bileaflet prosthesis with a pyrolytic housing and pyrolytic carbon leaflets containing graphite substrate (see Fig. 42-1F). The pivot areas are located entirely within the orifice ring, and the valve leaflets hinge on convex pivot guides on the carbon orifice ring. This design minimizes the overall height of the valve and provides a wider orifice area, and the absence of cavities in the valve ring theoretically reduces stasis or eddy currents that may develop. Valve noise, a bothersome problem for some patients, also is reduced by this design.⁸⁷ The opening angle is up to 85°, and the sewing cuff is constructed of double velour polyester fabric that is mounted to a titanium stiffening ring, which enables the surgeon to rotate the valve orifice during and after implantation.

On-X Prosthesis Valves

The On-X prosthesis was approved by the FDA in 2002. It has a bileaflet design similar to the St. Jude Medical, Carbomedics, and ATS prostheses with comparable hemodynamic performance, that is, a relatively large orifice diameter and a wide opening angle (90°) (see Fig. 42-1G). Instead of silicon-alloyed pyrolytic carbon, as used in the other mechanical prostheses, the On-X valve is made of pure pyrolytic carbon. This material is stronger and tougher than silicon-alloyed carbon and allows incorporation of hydrodynamically efficient features into the valve orifice, such as increased orifice length and a flared inlet that reduces transvalvular gradient. Early clinical results are promising and the valve produces very little hemolysis with postoperative levels of serum lactate dehydrogenase in the normal range.^88,89

Anticoagulation for Mechanical Valves

Warfarin remains to be the mainstay of anticoagulation after mechanical valve replacement in both aortic and mitral positions.⁹⁰ The need for anticoagulation with mechanical valves has been the leading cause of people shying away from this type of valves because of the life style changes this treatment instills. In fact, this has also been a major driving force in the recently observed shift in use of the bioprosthetic valves in younger patients despite its decreased lifespan compared to mechanical valves.^90,91 Given the time needed to heparin-warfarin bridging as well as the frequent need of monitoring during Warfarin treatment, efforts are being made to find an alternative. The new oral anticoagulants (dabigatran, edoxaban, rivaroxaban …) are promising substitutes to the warfarin therapy due to their shorter half life and the lack of need to constant monitoring.⁹⁰ Those drugs have been FDA approved for use in nonvalvular atrial fibrillation and in treatment or prevention of deep vein thrombosis/pulmonary embolism but not for anticoagulation in patients with mechanical valves. dabigatran was the only drug to be compared to warfarin in patients with mechanical valves during a phase 2 trial, the RE-ALIGN trial that was initiated in 2012.⁹² Unfortunately, this trial was terminated prematurely owing to an increase in stroke, myocardial infarction and major bleeding in the dabigatran group. The FDA declared, after this study, that dabigatran is contraindicated in this patient population. In the absence of other randomized controlled studies comparing a new oral anticoagulant to warfarin in patients with mechanical valves, warfarin still holds the crown of oral anticoagulation in this patient population.

Bioprostheses

PORCINE VALVES

The porcine bioprosthetic mitral valves are designed to mimic the flow characteristics of the in situ aortic valve. The Hancock I mitral valve bioprosthesis was introduced in 1970. It has three glutaraldehyde-preserved porcine aortic valve leaflets on a polypropylene stent attached to a Dacron-covered silicone sewing ring. The design allows for central laminar flow through the valve, which tends to decrease diastolic pressure gradients and minimize turbulence.⁸³ The stent, however, impedes forward flow and results in relatively large diastolic pressure gradients across the bioprosthesis. The stent and the large sewing ring contribute to effective orifice areas that are smaller than those of size-matched mechanical valves (see Table 42-2).

The Hancock II porcine bioprosthesis (see Fig. 42-2A) is the more modern version of the Hancock I prosthesis. The stent is made of Delrin with a scalloped sewing ring and reduced stent profile. The leaflets are fixed in glutaraldehyde at low pressure then at high pressure for a prolonged period. To delay calcification, the leaflets are treated with sodium dodecyl sulfate.

The Carpentier-Edwards porcine valve uses a flexible stent to decrease the stress of leaflet flexion while maintaining its overall configuration (see Fig. 42-2B). The effective orifice-to-total-annulus-area ratio for the Carpentier-Edwards valve is relatively small, but exercise studies show that the effective orifice area increases significantly with increased blood flow across the valve; diastolic gradients also increase but to a lesser degree.^70,71,93 Porcine bioprostheses in the mitral position should be avoided in patients with small left ventricles because of the possibility of ventricular rupture or left ventricular outflow obstruction caused by the large struts.⁹⁴

The Mosaic porcine bioprosthesis is a third-generation bioprosthesis using the Hancock II stent (see Fig. 42-2C). It was introduced in the United States in 2000 and has a Delrin stent, scalloped sewing ring, and reduced stent profile. The valve tissue is pressure-free fixed with glutaraldehyde, and the prosthesis is treated with alpha-oleic acid to retard calcification.

In 2005, the FDA approved the Biocor porcine bioprosthesis (St. Jude Medical) (see Fig. 42-2E); however, it has been used and investigated for almost two decades in Europe. It belongs to the third generation of bioprostheses, and the valve tissue is pretreated in glutaraldehyde at very low pressure (<1 mm Hg), making the valve cusps less stiff with less tendency to tissue fatigue. A newer generation of this valve, the St. Jude Medical Epic valve, is identical to its precursor except that the later is treated with Linx AC ethanol-based calcium mitigation therapy.

PERICARDIAL VALVES

Previous studies indicated poor durability of pericardial valves, namely the Ionescu-Shiley valve, caused by leaflet tearing. This led to significant changes in design, including mounting of the pericardium completely within the stent, causing less leaflet abrasion and increased durability. The Carpentier-Edwards pericardial valve uses bovine pericardium as material to fabricate a trileaflet valve and that is cut, fitted, and sewn onto a flexible Elgiloy wire frame for stress reduction (see Fig. 42-2D). The tissue is preserved with glutaraldehyde with no applied pressure and the leaflets are treated with the calcium mitigation agent XenoLogiX. Compared to the Carpentier-Edwards porcine bioprosthesis, the stent profile is reduced. Long-term durability for the Carpentier-Edwards pericardial valve is strong and compared to third-generation porcine valves, valve-related complications are similar (see Discussion later in this chapter).

Hemodynamically, pericardial valves provide the best solution to flow problems. The design maximizes use of the flow area, which results in minimal flow resistance. Figure 42-5A shows how the cone shape of the open valve and circular valve orifice minimizes flow disturbance compared with the more irregular cone shape of the porcine valves that allow for central unimpeded flow (see Fig. 42-5B).

Structural valve deterioration is seen after long-term follow-up of patients with both porcine and pericardial bioprostheses and results in MS or MR or both. Hemodynamic studies early after operation and at 5 years reveal higher average diastolic pressure gradients and smaller effective orifice areas when compared in the same patients at the follow-up study. In some patients, these changes are sufficiently severe to require reoperation as soon as 4 to 5 years postoperatively and by 10 years the rate of primary tissue failure averages 30%. It then accelerates, and by 15 years postoperatively, the actuarial freedom from bioprosthetic primary tissue failure has ranged from 35 to 71%^{44,46,48,49,68} (Table 42-3). Most of these patients show hemodynamic evidence of valvular deterioration before any clinical signs or symptoms.⁷¹ Bioprosthetic valves have the advantage of low thrombogenicity, which must be weighed against poor long-term durability and subsequent hemodynamic deterioration and the risk of reoperation.

Preoperative Management and Anesthetic Preparation

Congestive heart failure (CHF) secondary to MS usually can be treated with aggressive diuretic therapy and sodium restriction preoperatively. If the patient is in rapid atrial fibrillation digoxin, beta blockers, and calcium channel antagonists can be used to slow down the ventricular rate. Patients with acute MR often are in cardiogenic shock, and they can be stabilized preoperatively with inotropes and arterial vasodilators to reduce systemic afterload. Intraaortic balloon counterpulsation also can be used for this purpose. Symptoms of CHF in patients with chronic MR are treated with diuretics and oral vasodilators. The vasodilators lower the peripheral vascular resistance, forward cardiac flow is hence increased and the regurgitant volume into the left atrium reduced.

Preferred anesthesia for MVR typically involves a combination of narcotic and inhalational agents. Ultimately, anesthetic management is dictated by the wide range of functional disabilities and hemodynamic abnormalities of patients who present for MVR.⁹⁴

Preoperative intravenous prophylactic antibiotics are administered to all patients. Monitoring should include arterial and venous lines, a urinary catheter and a pulmonary artery catheter placed before bypass to measure pulmonary pressures and cardiac output. Transesophageal echocardiography is also critical throughout the entire operation. Last but not least, temporary ventricular pacing wires are placed postoperatively, and in many instances temporary atrial pacing wires are placed for possible pacing or diagnosis of various atrial arrhythmias.

Cardiopulmonary Bypass for Mitral Valve Replacement

Cardiopulmonary bypass is instituted by placing two right-angled cannulas into the superior and inferior venae cavae. A small (22 French) plastic or metal cannula is placed directly into the superior vena cava, above the sinoatrial node. The inferior caval cannula is placed at the entrance of the inferior vena cava, low in the right atrium. These insertion sites keep the caval catheters out of the operative field and yet maintain excellent bicaval drainage. An arterial cannula is placed in the distal ascending aorta. Bypass flows are approximately 1.5 L/m² per minute, and mild hypothermia is used with vacuum-assisted suction. Myocardial protection includes antegrade and retrograde blood cardioplegia and profound myocardial hypothermia.⁹⁵ Retrograde cardioplegia is useful for all valve surgery to protect the ischemic left ventricle and help remove ascending aorta bubbles. Antegrade cardioplegia, used as an initial loading dose, is augmented by intermittent retrograde cardioplegia every 20 minutes. This provides safer delivery of cardioplegia because when the atrium is retracted during valve replacement, the aortic valve is distorted, and antegrade cardioplegia tends to fill the ventricle due to aortic regurgitation.