Evaluation Of Mitral Prostheses By Transesophageal Echocardiography

Assessment of anatomic abnormalities

Because of the vicinity of the oesophagus to the left atrium and mitral valve, high quality images can be obtained of prosthetic valves in mitral position.

Abnormal echoes associated with prosthetic valves are spontaneous echo contrast (SEC), microbubbles or cavitations, strands, sutures, vegetations and thrombus. Ionescu et al. recently have provided definitions for these abnormal echoes23. Spontaneous echo contrast (SEC) is defined as smoke-like echoes with slow swirling motion and is caused by slow flow (for example because of a low cardiac output or severe left atrial dilatation). However, SEC may also indicate slow flow due to pathologic obstruction of a mitral prosthesis. The prevalence of SEC is 7 - 53%. Microbubbles (or cavitations) are characterized by a discontinuous stream of rounded strongly echogenic, fast moving, transient echoes occurring when there is motion of the occluder of the prosthetic valve. The prevalence of microcavitations is approximately 47%. Strands are continuous linear, thin, mildly echogenic, mobile echoes. They are often visible intermittently during the cardiac cycle but are recurring at the same site. Strands are found in 6 to 47% of patients and are probably composed of fibrin. Sutures are defined as linear, thick, bright, multiple, evenly spaced, usually immobile echoes consistently seen at the periphery of the sewing ring of a prosthetic valve; they may be mobile when loose or unusually long. Vegetations and thrombus can not be distinguished by echocardiography alone; the differential diagnosis of these sessile or pedunculated masses depends on the full clinical picture. They may be interpreted as vegetations in a febrile patient and as thrombus in a poorly anticoagulated patient.

Prosthetic valve integrity and motion can be evaluated accurately with TEE. For bioprostheses evidence of leaflet degeneration (leaflet thickening, calcification or tear) can be identified. In mechanical valves abnormal disc excursion or a stuck leaflet can be visualized. Prosthetic valve dehiscence is characterized by a rocking motion of the entire prosthesis. An annular abscess may be recognized as an echo lucent, irregularly shaped area adjacent to the sewing ring of the prosthetic valve. Sometimes an abscess is echo dense.

Prosthetic valve obstruction

All normally functioning mechanical prosthetic valves show some obstruction to forward flow. Obstruction to flow may be determined by TEE by measuring the mean gradient and pressure half time. With the interpretation of the mean gradient one should realize that the mean gradient is not only dependent on the orifice area. Mean gradient also depends on heart rate (the faster the heart rate, the shorter the duration of diastole and the higher the transprosthetic gradient) and on transprosthetic stroke volume (higher in paravalvular leakage). The effective orifice area using the continuity equation can best be determined by transthoracic echocardiography (TTE). Pressure half time (P/ - time) as a measure of obstruction should also be interpreted with great caution. It is not only determined by orifice area but also by the early diastolic transprosthetic pressure gradient, heart rate and compliance of left atrium and left ventricle. In general practice, in a symptomatic patient with a mitral prosthesis and a heart rate of 70 - 100 per minute, pathologic obstruction of the valve prosthesis might be suspected if the mean pressure gradient is > 10 mm Hg and the P/ - time > 160 msec. It is important however, to interpret the aforementioned values of mean gradient and P/ - time in the clinical context of the patient and to look for morphologic abnormalities of the prosthetic valve. Pathologic valve obstruction may be caused by valve thrombosis, tissue in growth and sometimes by a vegetation interfering with normal disc motion.

Acute immobilization of a mechanical prosthesis disc (so called sticking disc) is a rare but life-threatening complication often caused by chord remnants or stitches. This can be easily visualized by TEE.

A high transprosthetic gradient despite a normally functioning prosthetic valve may occur after implantation of a valve prosthesis that is too small for the patient's body surface area (Valve Prosthesis - Patient mismatch). This is discussed in another chapter.

Prosthetic valve leakage

Prosthetic valves can be divided in mechanical and bioprosthetic valves. In vitro studies have demonstrated that mechanical prosthesis have closure backflow (necessary to close the valve) and leakage backflow (starting after valve closure).

The closure and leakage backflow pattern is dependent on the prosthesis design. For example tilting disc valves (like St. Jude Medical and Medtronic Hall valves) do not rest on a ledge of the orifice ring but fit inside the ring with a small space between the disc and ring or disc and pivot. Leakage backflow occurs through these small spaces, and generates specific jet patterns within the left atrium. Ball-in-cage prostheses however consist of a poppet, which rests on the ledge once the valve has been closed leaving no space between ring and ball. Therefore, Starr Edwards valves show only closure backflow and no leakage backflow. See table 3.

Table 3. Normal patterns of back flow in prosthetic valves2

Table 3. Normal patterns of back flow in prosthetic valves2

Figure 2. Reference view displaying the prosthetic mitral valve and its relationship to the aortic root

(Ao) and left atrial appendage (LAA) as seen from the left ventricular apex. The hours of a clock face corresponding to those shown in the surgical perspective, have been overlaid.

(B) Surgical view of prosthetic mitral vah'e and its relationship to the aortic root.

Reprinted with permission from the Society of Thoracic Surgeons. (The Annals of Thoracic Surgery

Figure 2. Reference view displaying the prosthetic mitral valve and its relationship to the aortic root

(Ao) and left atrial appendage (LAA) as seen from the left ventricular apex. The hours of a clock face corresponding to those shown in the surgical perspective, have been overlaid.

(B) Surgical view of prosthetic mitral vah'e and its relationship to the aortic root.

Reprinted with permission from the Society of Thoracic Surgeons. (The Annals of Thoracic Surgery

Figure 3. Normal backflow low-velocity nonaliasing jet encoded in a homogeneous colour and pathologic turbulent crescent shaped jet adhering to the left atrial wall.

(Reprinted with permission from American Journal of Cardiology; 1989; 63:1471-4 van den Brink et a)l.

Figure 3. Normal backflow low-velocity nonaliasing jet encoded in a homogeneous colour and pathologic turbulent crescent shaped jet adhering to the left atrial wall.

(Reprinted with permission from American Journal of Cardiology; 1989; 63:1471-4 van den Brink et a)l.

Pathologic regurgitation is divided in paravalvular and valvular regurgitation. Evaluation of a prosthetic valve for regurgitation is done by centrring the prosthetic valve in the midesophageal four-chamber view. Then the sewing ring is imaged in full by rotation of the imaging plane from 0 ° to 180 keeping the sewing ring in the centre of the image, making small adjustments of the transducer tip. See figure 2. Anatomic landmarks for localization of paravalvular leakage and for communication with the surgeon are the aorta and left atrial appendage. Pathologic regurgitation can be distinguished from normal backflow by the Color Doppler appearance of the jets. See figure 3. Normal closure and leakage backflow jets are low-velocity nonaliasing jets encoded in a homogeneous color (red in mitral valve prostheses). In contrast, pathologic jets are more turbulent and extensive, they are often eccentric (crescent shaped) and adherent to the left atrial wall. Pathologic regurgitation in mechanical valves may be caused by prosthetic valve dehiscence or by interference of structures (f.e. thrombus or vegetations) with disc closure. In bioprosthetic valves pathologic regurgitation may be caused by prosthesis dehiscence or leaflet degeneration (calcification or tear).

Severity of pathologic regurgitation is assessed by measurement ofjet area, assessment of the pulmonary vein flow (looking for systolic flow reversal), and determination of diastolic forward transprosthetic flow (increased mean gradient and short P V time). Jet area measurement in eccentric jets may underestimate the severity of regurgitation because of the Coanda effect (spreading of the jet along the atrial wall).

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