Early Degeneration of a Bioprosthetic Mitral Valve Complicated by a Large Left Atrial Thrombus

A 71-year-old man with a past medical history significant for a mitral valve replacement, parathyroidectomy, and paroxysmal atrial fibrillation (PAF) was admitted after complaining of shortness of breath. Significant surgical history included a remote parathyroidectomy requiring daily calcium and vitamin D supplementation and, occurring 7 years prior to presentation, a bioprosthetic mitral valve replacement as a result of infective endocarditis. Finally, as a result of PAF that developed in the months prior to this admission, the patient’s medical regimen included warfarin. His international normalized ratio (INR) at presentation was equal to 4.

Evaluation with a transthoracic echocardiogram revealed preserved left ventricular function, heavily thickened mitral leaflets with decreased mobility and severely elevated mitral gradients. The left atrium was mildly enlarged and there was an absence of mitral regurgitation. Subsequent to these findings, the patient was scheduled for a mitral valve replacement.

In the operating suite, 2-dimensional transesophageal echocardiography confirmed a heavily calcified bioprosthetic valve with severe leaflet restriction. Doppler exam revealed a transvalvular mean gradient measuring 16 mm Hg and a pressure half-time of 391 ms coinciding with severe bioprosthetic stenosis. Of note, “pressure half-time seldom exceeds 130 ms across a normally functioning mitral valve prosthesis” and “a markedly prolonged single measurement (>200 ms) may be a clue to the presence of prosthetic valve obstruction.” ¹ Despite the fact that the pressure half-time method may not accurately convey effective valve area in a prosthetic mitral valve, it was calculated to be 0.56 cm² (Figure 1). ² Further notable intraoperative findings consisted of a previously undocumented large left atrial thrombus and an unidentifiable left atrial appendage (LAA).

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Figure 1.

Obtained in the mid-esophageal aortic valve long-axis view, this continuous wave Doppler tracing through the stenotic mitral valve reveals a pressure half-time of 391 ms and a calculated valve area of 0.56 cm². Using this diastolic flow velocity profile, the mean gradient measured 16 mm Hg

After standard cannulation and initiation of cardiopulmonary bypass, the left atrium was opened in the intra-atrial groove. Surgical resection revealed a severely calcified bioprosthetic valve with thrombus extending over the hood of a previously oversewn LAA and onto the roof of the left atrium. Following surgical thrombectomy and the placement of a mechanical valve, the patient easily separated from cardiopulmonary bypass and had an uneventful postoperative course.

Overall, 86% of bioprosthetic valves placed in the mitral position maintain their durability over 10 years. ³ In this case, a possible contributing factor toward early calcification of the bioprosthetic valve was calcium supplementation due to surgical hypoparathyroidism. Structural deterioration, defined as a change in valve function as a result of an intrinsic abnormality, may cause either stenosis or regurgitation. As the primary complication of biologic valves, it is the foremost reason for reoperation and may occur at any time after implantation.^4,5 Although bioprosthetic valves have the advantage of not requiring anticoagulation, they are predisposed to calcification when compared to a mechanical implant. With a history of premature bioprosthetic calcification and concomitant warfarin therapy for PAF, the patient received a mechanical replacement, which relieved the stenotic lesion.

Despite a previously oversewn LAA, a short documented history of paroxysmal atrial fibrillation and a supratherapeutic INR at presentation, intraoperative transesophageal echocardiography revealed a significant left atrial thrombus (Figure 2). In all, 90% of left atrial thrombi are found in the 2 to 4 cm LAA, which has previously been referred to as “our most lethal attachment.” ⁶ In this case, hemodynamically significant mitral stenosis coupled with the lack of mitral regurgitation likely contributed to left atrial stasis leading to thrombus formation. In addition, incomplete exclusion of the LAA may also increase thrombotic risk as it serves to promote stasis in the LAA remnant. While the Left Atrial Appendage Occlusion Study revealed successful exclusion in only 45% of patients with oversewing of the LAA, other authors place the success rate closer to 90% when coupled with mitral valve surgery. ⁶ In our patient, because of thrombotic layering over the hood of the oversewn LAA, it was unclear if the LAA was completely obliterated during the original surgery.

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Figure 2.

A mid-esophageal aortic valve long axis view showing the large thrombus layering the left atrium

With the clinical presentation of symptomatic mitral stenosis, echocardiographic assessment revealed severe bioprosthetic mitral obstruction. Of note, this premature valve degeneration was associated with calcium supplementation in the setting of secondary hypoparathyroidism. This case was further complicated by the presence of a large left atrial thrombus despite a previously oversewn LAA and supratherapeutic INR. Successful placement of a mechanical valve in the mitral position served to resolve the patients symptomatic mitral stenosis, decrease the risk of premature calcification relative to a bioprosthetic implant and, in combination with continued warfarin therapy, decrease the risk of recurrent left atrial thrombosis.