In the practice guidelines published in 1996 by the American Stroke Association and (SCA), the assessment of valve replacement surgery is a category II indication for transesophageal echocardiography (TEE) (TEE may be useful in improving in improving clinical outcome). Situations in which complicated valve replacement surgery (involving a perivalvular abscess or hemodynamic instability), or valve repair, is contemplated are a category I indication for perioperative TEE (TEE is frequently useful in improving clinical outcome). Since that time, the evidence supporting the use of TEE during routine valve replacement surgery has consolidated. For example, in one study of patients undergoing AV replacement, routine intraoperative TEE changed the surgical plan in 13% of cases (which involved the addition of an unplanned mitral valve (MV) replacement in 1.6%), and the extension of a perioperative TEE service to include all valve replacement surgery resulted in an overall cost saving in another study. More recently, in the 2003 Guideline Update for the Clinical Application of Echocardiography, valve replacement surgery is a class I (evidence, general agreement, or both of usefulness or effectiveness) or IIa (weight of evidence, opinion in favor, or both of usefulness or effectiveness) indication for TEE.
Recently, comprehensive guidelines for the echocardiographic assessment of prosthetic valves have been published. These guidelines have been applied throughout this chapter.
Approach to imaging prosthetic valves
It is important to conduct a systematic TEE examination in all patients presenting for valve replacement surgery and to interpret the findings in light of the patient’s clinical history. In the context of revision surgery, it can be helpful to know the type and size of valve, the year inserted, previously documented pressure gradients, and whether there were any problems at the time of the original surgery.
All prosthetic valves produce a degree of acoustic shadowing (echo dropout) and reverberation artifact due to material in the stents, sewing rings, and discs. This can result in misdiagnosis, first because of overinterpretation of the reverberation artifact and second because pathology or abnormal flow may be hidden by acoustic shadowing and therefore missed. This is a particular problem with mechanical valves, as in addition to the sewing ring and struts, the occluders (leaflets) are strongly echogenic. To avoid missing pathology in the far field, it is important to examine the valve from multiple views so that, as far as possible, the entire structure is visualized. Decreasing the gain setting is helpful in minimizing the intense echo reflections from mechanical surfaces. It is also useful to acquire short digital loops and then toggle the color map on and off, while scrolling through each loop to identify the origins of any jets.
Mitral prosthetic valves
TEE is far superior to transthoracic imaging for assessing prosthetic MV function since the left atrium (where regurgitant jets and other complications such as thrombi and vegetations are most frequently seen) is not in the acoustic shadow of the prosthetic valve. As with native MV dysfunction, the midesophageal ventricular views (four-chamber, commissural, two-chamber, and long axis) are the most useful. Rarely, prosthetic MV pathology occurs on the left ventricular (LV) side of the valve (e.g., thrombus formation), and in these situations transthoracic imaging may be required.
Real-time 3-D TEE imaging can be extremely useful when assessing prosthetic MV function. Three-dimensional (3-D) TEE can be used to display en face views of the MV from both the atrial ( Figure 12-1 ) and the ventricular perspectives. This is useful in assessing leaflet motion and damage in the case of bioprosthetic valves ( Figure 12-2 ). Three-dimensional (3-D) TEE can display the entire sewing ring en face, which is especially helpful in localizing of any areas of dehiscence, and 3-D color flow Doppler imaging can accurately display the location of any paravalvular regurgitant jets. 3-D TEE can also show the location and size of any significant thrombi or vegetations. However, small and highly mobile masses may be missed with 3-D imaging due to the temporal resolution (frame rate) than that of lower 2-D imaging.
Aortic prosthetic valves
In general, prosthetic valves in the aortic position are not as well visualized as those in the mitral position. The midesophageal AV views (short and long axis) are the most useful for detecting regurgitant jets. Unfortunately, with mechanical valves, echo dropout of the anterior elements of the valve (from the posterior sewing ring) makes leaflet motion difficult to visualize at the midesophageal level. Leaflet motion, transvalvular flow patterns, and pressure gradients may be better assessed from the deep transgastric long-axis and transgastric long-axis views. Unfortunately, these views may be difficult to obtain, the AV is in the far field of the image, and poor alignment of the Doppler signal with flow may result in the underestimation of transvalvular gradients. In cases of uncertainty, the transvalvular gradient can be assessed in the operating room by epicardial echocardiogrephy (see chapter 5 ) or by direct needle pressure measurements. Needle pressure measurements may be misleading.
The adequate assessment of an aortic prosthesis (especially in relation to regurgitant jets) is made more difficult by the presence of an adjacent MV prosthesis.
3-D TEE is not particularly useful for prosthetic AV imaging, as the leaflets are in line with the ultrasound beam and the dropout from the sewing ring may obscure the valve leaflets.
Specific points to address during the transesophageal echocardiography examination
The following points should be emphasized during the pre- and post-CPB echocardiographic examinations.
Prior to CPB
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Confirm the indications for intervention by assessing the etiology and severity of the valvular dysfunction.
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Look for associated pathology, which may alter the surgical plan.
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Dilatation of the aortic root may necessitate a combined valve–root replacement.
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Extensive calcification of the valve annulus or aortic root may complicate valve implantation—a particular problem with mitral annular calcification.
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The presence of an ASD or small left atrium may influence the surgical approach (e.g., a trans-septal incision may be used instead of an incision directly into the left atrium for MV surgery).
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Thrombi may be present, particularly in the LA appendage.
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Vegetations, abscesses, or fistulae may be present, particularly in revision valve surgery.
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Subvalvular obstruction may be present rather than aortic stenosis (severe “aortic stenosis” in the absence of valvular calcification raises this possibility).
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Severe LV dysfunction may increase the risk of procedures associated with prolonged CBP.
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Measure dimensions that have implications for specific surgical procedures.
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In patients undergoing AV replacement, measurement of the annular diameter is highly predictive of the required prosthesis size. If a stentless AV prosthesis is planned, it is also important to measure the diameter of the sinotubular junction, as this may identify patients who require sinotubuloplasty. The measurement technique is described in Chapter 10 .
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If the annulus is small relative to the size of the patient and to the anticipated cardiac output, specific valves (e.g., stentless) or surgical techniques (e.g., supravalvular placement or aortic root enlargement procedure) may be required to avoid prosthesis–patient mismatch (as described later).
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Post-CPB
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Assess adequacy of replacement.
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Ensure proper seating of the prosthesis and the absence of paravalvular leaks.
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Check for normal prosthetic valve function, including leaflet motion, Doppler evaluation, and characteristic regurgitant (signature) jets.
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Exclude complications involving other structures (as described later).
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Verify the adequacy of deairing.
Prosthetic valves in current use
More than 100 models of prosthetic valves have been used since 1950. They vary in durability, thrombogenicity, and hemodynamic profile. A basic knowledge of those in common use ( Table 12-1 ), their echocardiographic features, and their transvalvular blood flow patterns is important to differentiate normal from abnormal function.
| Mechanical Valves | Bioprosthetic (Tissue) Valves |
|---|---|
| Bileaflet (e.g., St. Jude Medical, Carbomedics, ATS, Duromedics, On-X) | Stented |
| Porcine AV (e.g., Carpentier-Edwards, Medtronic Mosaic) | |
| Tilting disc (e.g., Bjork-Shiley, Medtronic-Hall) | Bovine pericardial valve (e.g., Carpentier-Edwards Perimount, Ionescu-Shiley) |
| Ball and cage (e.g., Starr-Edwards) | |
| Percutaneous (e.g., Edwards Sapien, Medtronic Core) | |
| Stentless | |
| Porcine (e.g., Toronto SPV, Freestyle) | |
| Homograft (human cadaveric) | |
| Autograft (Ross procedure) |
The main advantage of mechanical valves is durability and therefore longevity. Their main disadvantage is thrombogenicity, necessitating lifelong anticoagulation.
Bioprosthetic valves do not require long-term anticoagulation but are less durable.
Bileaflet valves and tilting disc valves make up the majority of mechanical valves currently in use. In general, most stented bioprosthetic valves and all mechanical valves can be used in the aortic, mitral, tricuspid, and pulmonary positions. The use of stentless bioprosthetic valves is restricted to the aortic and pulmonary positions.
Available prosthetic valve sizes depend on the manufacturer and the specific model. Model-specific sizers are used intraoperatively to determine appropriate valve size. Most adults receive a 19- to 23-mm valve in the aortic position, whereas larger valves are used in the mitral position (usually 27 to 31 mm), but sizes can vary from 19 to 31 mm for aortic prostheses and from 19 to 33 mm for mitral prostheses.
Bileaflet mechanical valves
There are many brands and models of bileaflet valves, but all have similar structural and echocardiographic characteristics ( Figure 12-3A ). Each valve consists of two semicircular rigid leaflets attached to a pyrolytic carbon ring. The leaflets pivot at two recessed hinges located slightly lateral to the central axis of the valve. The carbon ring is reinforced and enclosed by a sewing ring. Flow through the valve is symmetrical via two large, lateral, semicircular orifices and a narrow, rectangular, central orifice. Leaflets typically open to about 80 degrees. The result is relatively unobstructed forward flow, a very small area of stagnation on the downstream side of the leaflets (which may reduce the risk of thromboembolism), and a relatively large regurgitant volume (because the leaflets must swing through a large arc to close). After the valve closes, characteristic regurgitant “signature” jets help to “wash” the leaflet surfaces and reduce thrombus formation. Most current models allow the valve mechanism to be rotated within the sewing ring to minimize the risk of leaflets being caught up on perivalvular structures.
The closing angle of most bileaflet valves is approximately 25 degrees with the exception of the On-X. The On-X has an opening angle of 90 degrees and a distinctive closing angle of 43 degrees, which can be misinterpreted as incomplete closure. These valves also have significantly larger washing jets compared to other bileaflet valves.
Bileaflet valves may be placed in an annular or supra-annular position. In the aortic position, supra-annular placement may allow the use of a larger valve than would otherwise be possible; in the mitral position, supra-annular placement may be used to separate the leaflets from the subvalvular apparatus in a chordal-sparing operation. With double valve replacement, supra-annular placement provides a greater separation between the two valves, which facilitates implantation.
Echocardiographic appearances
On 2-D imaging, the echogenic sewing ring is seen at the level of the annulus. When supra-annular implantation is used in the mitral position, the valve is seen sitting in the left atrium more noticeably than when annular placement is used. In the aortic position, it is difficult to differentiate between annular and supra-annular placement.
In the mitral position, the leaflets can be clearly seen with midesophageal imaging. If the image plane is perpendicular to the central line of leaflet coaptation, the two leaflets should be seen opening and closing symmetrically and in synchrony ( Figure 12-4 ). Some imaging planes may give the false impression that the leaflet motion is unequal, so it is important to rotate the transducer through a range of views to obtain one in which the ultrasound beam is aligned perpendicularly to the leaflets. The correct angle varies depending on the orientation of the valve.
Color flow Doppler imaging in the mitral position usually displays two or more characteristic narrow signature jets, usually 2 to 5 cm in length ( Figure 12-5 ). The direction of these jets depends on the angle of the imaging plane relative to the valve. When the image plane is parallel to the central line of leaflet coaptation, there are usually two convergent jets, originating from within the sewing ring at the pivot points. When the image plane is perpendicular to the central line of leaflet coaptation, a central jet (originating from the central coaptation line) and a number of diverging peripheral jets of variable length (originating from the space between the valve leaflets and the ring) may be seen. Although color jets are usually well seen in long-axis views, short-axis imaging often helps to localize the origins of jets and to distinguish transvalvular from paravalvular regurgitation.
In the aortic position, the leaflets and washing jets are usually poorly seen in the midesophageal views because of shadowing from the sewing ring ( Figure 12-6 ). Leaflet motion and washing jets may be better appreciated in the deep transgastric long-axis or transgastric long-axis views ( Figure 12-7 ).
Tilting disc valves
A tilting disc valve consists of a single circular disc suspended from the supporting sewing ring by a single strut (see Figure 12-3B ). The strut attaches to the disc eccentrically so that the back pressure on the larger segment of the disc tends to close the valve. The leaflet opens to 55 to 75 degrees (compared with about 80 degrees for a bileaflet valve). This results in a complex flow pattern (70% of flow passes through the major orifice and 30% through the minor orifice), greater impedance to forward flow, and a relatively large area of stagnation on the downstream surface of the disc, theoretically increasing the likelihood of thromboembolism.
Echocardiographic appearances
The strut can be seen in a central position above the sewing ring ( Figure 12-8 ). Motion of the leaflet is easier to see in the mitral position than the aortic position, and is best visualized by imaging the valve in a scan plane through the hinge point and perpendicular to the axis of disc motion.
