Cardiology

Valvular Prostheses

I. Prosthetic Valve Malfunction: What every physician needs to know.

Types of malfunction

Structural valve deterioration

The principal complication plaguing cardiac bioprostheses is structural deterioration, often necessitating reoperation and prosthesis replacement. While newer bioprosthetic designs and surface treatments seem to have incrementally increased the lifespan of currently available bioprostheses, all bioprostheses are subject to loss of structural integrity over time. Generally speaking, the younger the patient is when the bioprosthesis is implanted, the faster the bioprosthesis will degenerate.

While many manufacturers suggest that newer manufacturing methods, including low or balanced pressure fixation, new fixation agents, anticalcification treatments, and better tissue mounting within a stent, will probably increase the durability of currently available bioprostheses, there is little peer-reviewed, clinical scientific evidence to support those claims. For several years there was some enthusiasm among cardiac surgeons that bioprosthetic degeneration might be slowed with aggressive treatment of atherosclerotic risk factors, particularly statins to lower cholesterol. Several subsequent studies have failed to demonstrate a statistically significant improvement in bioprosthetic durability with statin therapy.

Of note, the mode of structural failure of porcine and pericardial bioprostheses tends to be different. Porcine bioprostheses usually fail as a result of gradual leaflet wear that results in sudden leaflet tears and clinically obvious regurgitation.

Pericardial valves tend to fail with gradual leaflet calcification that leads to insidious progression of stenosis. Thus, from a clinical patient management point of view, serial echocardiograms may not detect impending porcine bioprosthetic failure, but are indeed necessary to demonstrate progressive pericardial valve stenosis.

Homografts, which for some time have enjoyed a broader clinical application because of the belief that they would not deteriorate as quickly as glutaraldehyde preserved porcine or pericardial tissue, have been shown to have a rate of structural deterioration similar to stented bioprostheses. Currently in most major cardiac surgical centers, homografts are reserved for aortic valve and root replacements for patients with extensive destruction of the valve and proximal aorta secondary to invasive endocarditis.

Historically several early brands of mechanical valves exhibited an increased incidence of fracture or wear of some components of the mechanical prostheses, leading to their malfunction or disruption. These designs have all been removed from the market. Structural deterioration of currently approved mechanical valves is very rare, but not zero. Specifically, implanting cardiac surgeons must be careful not to scratch surfaces coated with pyrolytic carbon, since there is some evidence that this can lead to future fractures of valve components.

Nonstructural dysfunction

Nonstructural dysfunction of a prosthesis is anything other than deterioration of prosthesis components that results in prosthesis malfunction. The two most common clinical causes of nonstructural dysfunction are paravalvular leak and hemolysis.

Paravalvular leak is a complication that occurs with relatively equal frequency between mechanical and bioprosthetic valves. The causes are more related to the patient’s anatomy and pathophysiology, such as excessive annular calcification, or to poorer techniques of surgical implantation.

Hemolysis occurs at a subclinical level in most mechanical valves but uncommonly with bioprostheses. Occasionally a patient may develop a significantly severe hemolysis to warrant prosthesis replacement. Before one assumes that any hemolysis is necessarily due to an implanted mechanical valve, hematologic review of other systemic causes of hemolysis is indicated. Hemolysis may also occur in the setting of a paravalvular leak associated with either mechanical or bioprosthetic valves.

II. Diagnostic Confirmation: Are you sure your patient has Prosthetic Valve Malfunction?

A. History Part I: Pattern Recognition:

Prosthetic valve dysfunction may occur gradually or catastrophically. Catastrophic failure, due either to valve or leaflet dehiscence, results in severe valvular regurgitation, heart failure, and even occasionally shock.

Thrombosis of a mechanical valve produces the clinical syndrome of valve stenosis. Thrombosis of both leaflets of bileaflet mechanical valves is exceedingly rare but thrombosis of a single leaflet usually produces significant hemodynamic embarrassment. Thus the sudden onset of cardiac symptoms in a patient with a prosthetic valve should always arouse a suspicion of valve failure.

B. History Part 2: Prevalence:

Incidence. The incidence of mechanical valve failure is on the order of 1% per year. Structural deterioration of bioprosthetic valves is in part determined by the patient’s age at implantation and in general is nonlinear, occurring rarely in the first few years after implantation. For patients aged 45 at implantation, structural deterioration approaches 50% at 10 years while for a 70 year old, the deterioration is about 10% at 10 years.

C. History Part 3: Competing diagnoses that can mimic Prosthetic Valve Malfunction.

D. Physical Examination Findings.

Prosthetic heart valves in the aortic position normally have an accompanying systolic flow murmur. A diastolic murmur is never normal and indicates a paravalvular leak from aortic valves or obstruction of a mitral valve. Mechanical valves should open and close with a typical valve click, the muting or absence of which indicates improper leaflet movement. The presence of any new murmur obviously raises suspicion of valve dysfunction.

E. What diagnostic tests should be performed?

1. What laboratory studies (if any) should be ordered to help establish the diagnosis? How should the results be interpreted?

Occasionally, paravalvular leak may cause severe hemolysis as red cells pass through a small orifice at high velocity. Appearance of anemia or jaundice in a patient with a prosthetic valve, especially in the face of a new murmur should arouse suspicion of valve-induced hemolysis. Standard lab tests, including bilirubin fractionation, reticulocyte count, and LDH, should be performed.

2. What imaging studies (if any) should be ordered to help establish the diagnosis? How should the results be interpreted?

The mainstay of diagnosis is echocardiography, and this technique is especially helpful in diagnosing both the magnitude and location of prosthetic regurgitation. Because most prosthetic valves are inherently stenotic, small transvalvular pressure gradients are normal. Gradients in bioprosthetic valves occur both from abnormally functioning valves (usually from pannus ingrowth) or from inappropriately small valves placed in large patients (patient-prosthetic mismatch).

Evaluation of stenosis in mechanical valves is more difficult. Acoustic shadowing from the moving parts of the valve may interfere with transthoracic images, requiring transesophageal echocardiography. Because bileaflet valves have three ejection orifices, the transvalvular gradient can easily be overestimated. In such cases, cardiac catheterization can be performed using a flow wire to safely obtain a true gradient.

III. Management.

A. Immediate management.

Catastrophic failure. Prosthetic valve patients presenting with shock or extreme cardiac decompensation should be suspected of having valve failure and should undergo emergent echocardiography and heart surgery. For catastrophic regurgitation of a bioprosthesis, it may be possible to implant a new valve percutaneously into housing of the failed valve. Thrombosis of a mechanical valve may be treated with thrombolytic agents recognizing the risk of embolization of a lysed clot, especially in the mitral position where stasis may result in large clots stored in the left atrium. Thrombolysis may be the only option if surgical risk is prohibitive.

Milder valve failure. When a new murmur is heard in a patient with a prosthetic heart valve, echocardiographic evaluation is mandatory. Once bioprostheses begin to deteriorate, further impairment may be rapid. In asymptomatic patients with mild dysfunction, close follow-up with expected eventual re-replacement is reasonable. With more severe disease and if symptoms are already present and depending on the status of the patients general condition, re-replacement should be planned in the near future.

B. Physical Examination Tips to Guide Management.

C. Laboratory Tests to Monitor Response To, and Adjustments in, Management.

D. Long-term management.

E. Common Pitfalls and Side-Effects of Management

Thromboembolism

The principal complication of mechanical cardiac prostheses is thromboembolism and the associated requirement for anticoagulation. The gold standard for anticoagulation is warfarin, which requires frequent blood tests, is subject to wide variability depending on who is managing the anticoagulation, as well as patient diet, and which is associated with a significant risk of increased bleeding. (See Anticoagulant-Related Bleeding below.)

Unfortunately, even reasonably high levels of anticoagulation do not completely eliminate the risk of thromboembolism. Therefore, when one is assessing the risk of thromboembolism and the need for anticoagulation, we have suggested looking at the composite thromboembolism and bleeding index, since trying to improve one side of that issue can often lead to an increase in trouble with the other part. For example, trying to lower the thromboembolic rate with an increasing target international normalized ratio (INR) reaches a point where the rate of thromboembolism is not improved but the rate of bleeding rapidly increases. Conversely, trying to decrease the rate of bleeding with a low INR can be associated with increased thromboembolism.

There is some evidence that arteriosclerotic risk factors, such as persistent smoking and hypertension, are associated with an increased incidence of thromboembolism. However there is a background incidence of thromboembolism in the general population without implanted heart valves, and this rate climbs with age and is higher for people with coronary artery disease. The mere occurrence of an embolus does not absolutely mean that it came from an implanted heart valve.

Anticoagulant-related bleeding

The Achilles’ heel of warfarin anticoagulation is an associated risk of bleeding, which is higher with increasing levels of INR. Further very high target levels of INR are also associated with decreased survival.

Several studies have demonstrated a significant advantage to patients who regulate their own warfarin anticoagulation, compared to having it regulated by their physician or even an anticoagulation clinic. Thromboembolism and bleeding often occur in patients on warfarin when their INR is out of the target range—thromboembolism when the INR is too low and bleeding when it is too high.

Patient-managed anticoagulation has been shown to maintain the INR more often within the target range. This more careful control of anticoagulation has been demonstrated to safely allow the lowering of the target INR, with even less bleeding.

The future of anticoagulation may hinge on the increasing use of newer anticoagulants. However, to date none have been proven effective in patients with prosthetic valves.

Prosthetic endocarditis

The development of prosthetic endocarditis can be a devastating complication associated with any implanted cardiac valvular prosthesis. Replacement of the prosthesis is usually indicated and is associated with substantial mortality and morbidity. Prosthetic endocarditis can rarely be completely cured with antibiotics alone.

There does not appear to be any significant difference in the incidence of prosthetic endocarditis with mechanical versus bioprosthetic heart valves.

IV. Patient Safety and Quality Measures

A. Appropriate Prophylaxis and Other Measures to Prevent Readmission.

Valvular prosthesis selection

Then most important step in preventing the malfunction of prosthetic heart valves is proper valve selection prior to implantation.

Historically, the selection of a mechanical or bioprosthetic valve has undergone several paradigm shifts. The earliest prostheses were all mechanical. When the first generation porcine valves became available, surgeons began to convert to bioprostheses to avoid anticoagulation.

When the issue of bioprosthetic structural deterioration and subsequent need for reoperation manifested itself over the next 10 to 15 years, mechanical valves regained favor. Then in the late 1990s and continuing to the current day, the incidence of bioprosthetic implantation began to steadily increase. In the mid-1990s about 60% of aortic valves implanted were mechanical, whereas by 2010, only about 20% of aortic valves were mechanical.

Factors influencing selection

The current tendency to implant more bioprostheses in the aortic position has been based in part on: (1) the gradual aging of the valve replacement population; (2) a growing appreciation that patients who require valve replacement have a diminished life expectancy; (3) the desire to avoid the documented complications of warfarin anticoagulation; (4) relatively low operative mortality risk of bioprosthetic reoperation; and (5) the claimed improved durability of currently available bioprostheses.

The predominant issue influencing prosthesis selection for patients is weighing the risk of structural deterioration of bioprostheses and attendant need for reoperation against the continuing risk of thromboembolism and requirement for anticoagulation of mechanical valves. Several factors enter into evaluating that balance.

Patient age

Many reports suggest that patient age is the primary determinant of prosthetic selection, with younger patients receiving mechanical valves and older patients receiving bioprostheses. For older patients or those with expected lower life expectancy due to other medical causes, the durable bioprostheses may outlast the patient’s projected survival.

The age at which clinicians judge mortality to precede bioprosthetic degeneration has been changing with time, lowering in recent years as the actual incidence of loss of structural integrity of bioprostheses has been reported to be improved with current bioprostheses compared to first generation porcine valves. Many clinicians currently suggest bioprostheses in any patient over age 60, for some clinicians even age 55 has become the trigger point.

Several studies have documented that the projected life expectancy of patients who require valve replacement is lower than the average population. As noted above, this is one factor that has driven the age at which bioprostheses are inserted lower.

Compounding this fact is the additional observation that patients who require concomitant coronary artery bypass grafting with their valve replacement have an even lower life expectancy. Poorer life expectancy after aortic valve replacement has also been reported for patients in atrial fibrillation, patients with both aortic regurgitation and aortic stenosis, and patients who have more advanced congestive heart failure at the time of operation.

Survival

Every randomized trial that has compared mechanical valve recipients to bioprosthetic valve recipients has documented longer survival time, not always statistically significant, however, for mechanical valve patients. Despite these data, bioprostheses have enjoyed increased rates of insertion.

One of the issues related to comparing survival for mechanical and bioprosthetic recipients has been the inability of many trials to account for surgeon bias, not all elements of which can be measured or counted. Thus, when the end-point of comparison is all-cause mortality, the fact that surgeons can often predict who is expected to have better survival preoperatively is not adequately factored into prosthesis selection. If surgeons implant mechanical valves in patients they believe to have a longer life expectancy and the surgeon's choice is verified by the results, then one is really seeing a self-fulfilling prophecy.

Risk of reoperation

The risk of reoperation for currently available bioprostheses is probably better than that for first generation valves, but the breakpoint in the curves of freedom from reoperation has merely been moved out several years while the subsequent down slope of the curves has not been appreciably altered. As noted above, contrary to some popular belief, the rate of degeneration necessitating reoperation for pericardial valves is not substantially different than that of porcine bioprostheses.

One thing that has improved with time has been the operative risk of replacement of a failed bioprosthesis. Current mortality rates for isolated, first-time replacement of a failed bioprosthesis are only a few percentage points higher than mortality rates for initial valve implantation.

Another feature impacting the selection of bioprostheses for younger patients has been the belief that in the near future replacement of a failed bioprosthesis may be able to be accomplished with transcatheter techniques, thus avoiding the standard reoperative strategy with cardiopulmonary bypass. Transcatheter implantation of aortic valves has been approved in Europe for several years and more recently in the United States for sicker patients at high risk for standard operative valve implantation.

One caution must be emphasized: if a physician is to suggest this option, then the bioprosthesis that is implanted must be of a fairly large size to allow future valve-in-valve implantation by transcatheter techniques of an adequate sized prosthesis.

Risks of anticoagulation

There is no doubt that warfarin anticoagulation is associated with an increased risk of bleeding. Comparative trials of mechanical and bioprosthetic valves all demonstrate higher rates of anticoagulant-related bleeding for mechanical valve recipients. The higher the target INR, the greater is the incidence of bleeding.

Special issues

Valve replacement of patients on renal dialysis

Although historically there has been a tendency to recommend bioprostheses for patients on renal dialysis because of the limited life expectancy of dialysis patients and the desire to avoid warfarin anticoagulation, several studies have demonstrated no difference in survival for dialysis patients who receive either mechanical valves or bioprostheses.

Valve replacement for patients who desire to become pregnant

For patients who desire to become pregnant, the choice of a mechanical or bioprosthetic valve has become strongly tilted toward bioprostheses, despite the fact that such patients are younger and would otherwise be more often considered for mechanical valve replacement. The principal issue driving this choice is the desire to avoid warfarin anticoagulation, since warfarin is known to be potentially teratogenic.

Mechanical valve implantation during pregnancy leads to high rates of fetal and maternal complications and death. One study has demonstrated that the structural deterioration of bioprostheses is not accelerated by pregnancy. Obviously, the choice of a bioprosthesis in this younger population almost certainly commits the patient to future reoperation.

B. What's the Evidence for specific management and treatment recommendations?

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