11.5.3 Echocardiography of prostheses

 

In the aortic position, the continuity equation (SAo = (S_LVOT · VTI_LVOT ) / VTI_Ao ) can be used to estimate the surface area of the prosthesis, taking into account the transvalvular flow, but it has 3 problems: 1) measuring the diameter of the LVOT is difficult because of the brightness and shadows of the prosthesis, 2) the surface area of the valve is not circular, and 3) the velocity through the prosthesis is artificially accelerated, which tends to underestimate its true surface area [3]. The equation is only valid in the absence of aortic and mitral regurgitation. As mentioned above, the ratio of  VTI_LVOT/VTI_Ao avoids these problems; the higher the ratio, the greater the surface area. In the mitral or tricuspid position, the half-pressure time (220/Pt1/2) only applies to valves with a surface area < 1.5 cm² ; outside this range it overestimates the true opening of the prosthesis [4,15]. Unfortunately, it is not valid to use the nominal size of the prosthesis to calculate its opening area, as the value given is based on measurements that vary from manufacturer to manufacturer [15].

 

The normal values for pressure gradient, flow velocity and orifice area of the many prostheses currently on the market vary according to model and size. The reference data are published in the appendix to Reference 15 (see Guidelines Valves prostheses.pdf). The website www.valveguide.ch provides data on all prostheses currently in use [5].

 

 

Prosthetic valves

Prosthetic materials absorb ultrasound (shadow zones) or reflect it (reverberation). Wing movement (mechanical prostheses) - The 2 leaflets must be visible on the same image - Mitral valve: mid-esophageal view 40-80°. - Aortic valve: 0° or 120° transgastric view Cusp motion (biological prostheses) - Mitral valve: 4-cavity 0° view - 2-cavity 90° view - Aortic valve: 40° short axis view Mechanical prostheses show normal autolavage leaks (FR 5%); biological prostheses show optional leaks of variable location   Prostheses are restrictive compared to a normal native valve, in ascending order of pressure gradient (ΔP): autografts < homografts < stentless bioprostheses < mechanical valves < attached bioprostheses (ΔPmax 5 - 40 mmHg). The pressure gradient increases in the following cases: assembled bioprosthesis, small size, aortic position, high cardiac output, hypervolaemia, vasoplegia, IABP, etc.   In the aortic position, ΔP must be calculated using the full Bernoulli equation: ΔP = 4 · (V2VAo - V2LVOT ). After AVR, the ratio VTILVOT/VTIVAo must be > 0.4; a lower value indicates stenosis. Pressure recovery is an important phenomenon in mechanical valves.

 

 

 

 ETO examination after ECC

 

After mitral valve replacement (MVR), aortic valve replacement (AVR) or tricuspid valve replacement (TVR), it is essential to interpret the data according to the patient's haemodynamics. 

 

The examination begins with an assessment of ventricular performance (LV and RV) and blood volume. This is a priority during weaning off bypass. Detailed examination of the prosthesis is only appropriate once the patient's haemodynamics have stabilised. The high echogenicity of the prosthesis material prevents midesophageal views of the ventricles; only transgastric views can provide accurate information on function and filling.

Evaluation of the prosthesis is based on 2D imaging, colour Doppler and gradient calculations. The two-dimensional views highlight the opening/closing of the prosthesis.

• Double winglet mechanical prosthesis: simultaneous movement of the 2 winglets on the same image, in a 30-80° mid-esophageal view (mitral prosthesis) or a 0° and 120° transgastric view (aortic prosthesis). Simultaneous viewing of the 2 winglets is the only two-dimensional proof that the prosthesis is functioning correctly. Seeing only one leaflet in two different planes, even if the orientation of the winglet is different, does not rule out the possibility that the same winglet is actually seen twice and the other remains invisible. This is confirmed by the colour flow (double passage through the two large orifices) and the gradient calculation. In the mitral position, it is always possible to find a plane where the 2 winglets appear simultaneously. In the aortic position, the image depends on the orientation of the prosthesis; it may not be possible to see both winglets.
• Bioprosthesis: Movement of cusps in 0-90° retrocardiac view (mitral bioprosthesis) or 40° short-axis view (aortic bioprosthesis); measurement of orifice area by 40° short-axis planimetry (aortic bioprosthesis).
• It is normal to see a cuff around the aortic root after AVR (haematoma, thrombus, suture).
• Presence of sutures, look for thrombus and fibrin filaments.

Several complications can be detected immediately by TEE.

• Winglet blockage: lack of movement and flow over half of the prosthesis, excessive gradient; usually caused by the subvalvular apparatus (mitral position) or the LVOT musculature (aortic position). This blockage can be corrected by rotating the prosthesis in its ring, but this requires a return to ECC.
• In the case of a bioprosthesis in the mitral position, the anterior part of the valve may partially obstruct the LVOT; measurement of Vmax in the LVOT (risk of stenosis).
• After MVR:
  • Lateral fixation points of the prosthesis may damage the circumflex artery and lead to lateral akinesia. This is because the CX pathway in the left atrioventricular trine is only a few millimetres from the mitral annulus, and lateral fixation points can easily penetrate and occlude the vessel.
  • Fixation points at the level of the trine can cause restriction of the non-coronary or left coronary cusp of the aortic valve and induce or aggravate insufficiency (AI).
  • The most serious surgical complication that can occur after mitral valve surgery is a tear in the annulus and base of the LV, most commonly during reoperation. Reconstruction is extremely risky and mortality is very high.
• After AVR :
  • Flow control in the common trunk and right coronary artery.
  • Control of the mitral valve: attachment points of the aortic prosthesis at the level of the trine can pull on the anterior winglet and trigger an MI or aggravate a pre-existing leak. Decalcification of the mitral valve angle may result in a tear at the root of the anterior mitral winglet.
  • Septal myectomy in the LVOT, often associated with AVR for aortic stenosis and significant LVH, may be excessive and cross the subaortic septum from side to side, leading to iatrogenic VSD. The coloured flow shows a systolo-diastolic vortex passage between the LV and RV.
  • Complete AV block due to lesion of the His bundle in the part adjacent to the membranous septum below the non-coronary cusp.
• Paravalvular leak, CMO effect, patient-prosthesis mismatch: see below and under prosthesis complications.

 The coloured flow is necessary to guide the positioning of the pulsed/continuous Doppler in the anterograde and retrograde flow. It shows regurgitation.

• Autolavage leaks in mechanical prostheses; their number and size depend on the model; they are intravalvular and small.
• Residual leaks in the bioprosthesis (central and/or at the commissures).
• Paravalvular leaks outside the annulus; these are always pathological.
• Mitral leak due to retraction of the anterior leaflet by the posterior sutures of an aortic prosthesis.
• Aortic leak due to retraction of the non-coronary or left coronary cusp by the sutures used to implant a mitral valve prosthesis.
• Subaortic VSD flow after excessive LVOT myectomy.

The occurrence of MI after AVR may be related to several different phenomena.

• Mesosystolic reopening of the mitral valve in dynamic sub aortic stenosis (SAM); although significant, this MI is brief and does not last throughout systole: it is meso-telesystolic. It is often the first sign to diagnose the HOCM effect (see Complications of heart valve prostheses).
• Retraction of the anterior leaflet by prosthesis anchoring points; the proximity of the aortic and mitral annuli at the trine means that slightly wide points in this area exert traction on the anterior leaflet and may prevent it from closing properly.
• LV dysfunction; MI is a common sequela of acute LV dilatation or ischaemia.
• Persistence of a pre-bypass MI, whether organic (calcific disease, degeneration, age-related changes) or functional (mitral annular calcification, LV dysfunction or dilatation). In most cases, the removal of the obstacle to ejection by the AVR reduces LV wall stress to such an extent that the MI is less significant than preoperatively.

The gradients vary according to the type of prosthesis, but are always greater than those of a normal native valve. They vary with the size of the valve. In simplified terms, the following benchmarks can be used for mean gradients and maximum velocities (which vary with size) (see Table 11.7, Table 11.8 and Table 11.9 for details) [8,9,12,15].

• Mitral position:
  • Two-wire mechanical valve      ΔPmoy 3-5 mmHg Vmax 1.2 - 1.6 m/s
  • Attached bioprosthesis            ΔPmoy 6 mmHg Vmax 1.5 - 1.8 m/s
• In aortic position :
  • Bioprosthesis without frame    ΔPmoy 4-8 mmHg Vmax 1.5 - 1.8 m/s
  • Two-wire mechanical valve     ΔPmoy 6-15 mmHg Vmax 2.0 - 2.5 m/s
  • Fixed bioprosthesis                 ΔPmoy 12-20 mmHg Vmax 2.5 - 3.0 m/s

In the aortic position, it is essential to calculate the gradient using Bernoulli's equation, which takes into account the velocity in the outflow chamber (ΔP = 4 · (V²_VAo - V²_LVOT ), as this is often well above 1.5 m/s under the haemodynamic conditions at the pump outlet: concentric LVH, hypovolaemia, catecholaminergic stimulation and reduced afterload due to removal of the stenosis. Dynamic subaortic stenosis is defined by an LVOT Vmax > 2.5 m/s; it alone induces a ΔPmax > 25 mmHg. The ratio between VTI in the LVOT and through the valve (R = VTI_LVOT / VTI_VAo) is useful for assessing the degree of acceleration generated by the prosthesis.

However, several phenomena can cause a falsely high transprosthetic gradient across an aortic prosthesis [10].

• Transient episode of high stroke volume: hypervolaemia, too rapid transfusion from the ECC venous reservoir, haemodilution.
• Pressure recovery: kinetic energy is converted back into pressure beyond the zone of maximum acceleration, where pressure is minimal; as echocardiography measures pressure precisely at maximum velocity, it tends to overestimate the true gradient (see Figure 11.44).
• Dynamic subaortic stenosis (HOCM effect): the removal of the obstacle to ejection may cause a hypertrophied ventricle to collapse in systole, because its cavity is naturally small and the outflow tract is very muscular; the anterior leaflet of the mitral valve is displaced towards the LVOT and is sucked in to the point of partially blocking the flow. This is systolic anterior motion (SAM), a phenomenon similar to obstructive cardiomyopathy (Figure 11.45). This systolic narrowing of the outflow tract creates a dynamic obstruction that can represent a gradient of 25-40 mmHg upstream of the prosthesis (see Complications of Prostheses, Dynamic Subaortic Stenosis).
• Even in the absence of muscular stenosis, LV outflow velocities may exceed 1.5 m/s due to significant sympathetic stimulation and transient hypervolemia. In this case, Bernoulli's equation must be corrected to account for Vmax in the LVOT:  ΔP = 4 (V2² - V1² ), where V2 is Vmax through the prosthesis and V1 is Vmax in the LVOT. The gradient generated in the LVOT is then subtracted from the total gradient to give the prosthesis specific gradient (see Fig. 11.121).
• The presence of intra-aortic balloon pump (IABP) significantly reduces the pressure in the ascending aorta at the start of systole as the balloon deflates; this results in an exaggerated gradient between the LV and the aorta; the gradient should be measured during a brief cessation of IABP. Significant dilatation of the ascending aorta has the same effect.
• Patient-prosthesis mismatch: an orifice area < 0.85 cm /m² creates an excessive gradient (see Complications of prostheses, Patient-prosthesis mismatch).

Figure 11.45: Dynamic obstruction of the LVOT after mitral valve repair. A: A restrictive annulus and/or excessive height of the posterior leaflet (PL) (anterior to posterior leaflet ratio < 1.3) advances the posterior wall and projects the coaptation point towards the LVOT (green arrow). In the protocole, coaptation takes place on the body of the anterior leaflet (AL), the distal part of which is in the LV and not against the PL. B: In mesosystole, intraventricular pressure pushes the AL towards the LVOT; during ejection it is sucked in by the Venturi effect and blocks the LVOT (SAM: systolic anterior motion). The aortic flow drops suddenly and the reopening of the mitral valve causes meso-tesystolic insufficiency (MI). C: When the LV is narrowed, only the posterior wall can move anteriorly (red arrow) because the base of the anterior leaflet is attached to the fibrous trine, which is a fixed point because it is the skeleton and mechanical centre of the heart.

 Paravalvular leak

The paravalvular leak (PVL) is located outside the prosthetic ring. It is always pathological, but its impact varies according to its size.

• Small leaks (grade ≤ I) are not significant; they generally disappear rapidly with protamine or within a few days with fibrin deposition and endothelialisation.
• Small leaks are inevitable when the valve annulus is heavily calcified and congruence with the prosthetic annulus is impossible.
• In the tricuspid region, the surgeon may not fix the prosthesis on the septal side to avoid damaging the His bundle, leaving an extra-annular orifice.
• Moderate to severe leaks (50% of cases) should be corrected whenever possible as they tend to worsen over time and cause significant haemolysis.
• Tolerance of PVL is higher for aortic leaks than for mitral leaks because the velocity is lower and the risk of haemolysis is lower in the aortic position. Tolerance is very high for tricuspid PVL where the velocity is low.

Large paravalvular leaks show characteristic signs (Figure 11.46). 

 Figure 11.46: Paravalvular leak. A and B: Normal autolavage leaks from a bivalvular mechanical prosthesis in the mitral position; fine, intravalvular, parallel or bouquet jets, no zone of acceleration C: Diagram showing that the paravalvular leak is located between the annulus of the prosthesis and the anatomical annulus of the valve (*), whereas the autolavage leaks (ALL) are within the valve. A large AVL has a zone of concentric acceleration (PISA) on the ventricular side. D: Short-axis aortic PVL; two leaks around the prosthetic annulus (arrow) at 3 and 7 o'clock; persistence of flow in diastole through the paravalvular orifices. E: Anterior mitral PVL; a small physiological leak is visible inside the prosthetic annulus (arrow). F: Posterior mitral PVL.

• Free passage localised outside the annulus on 2D imaging (inconstant sign).
• PISA on the anterior surface.
• Passage around the annulus visible on colour Doppler.
• Vena contracta ≥ 0.3 cm.
• Large downstream vortex flow, often eccentric.
• Mitral prosthesis: search for PVL around the prosthesis by placing the colour window at the mitro-aortic angle in 4-cavity view and rotating the sensor angle from 0° to 180° while keeping the window on the left side of the prosthesis on the screen. Then place the window on the other side of the valve and rotate back 180° to 0°, keeping the window on the right edge of the prosthesis on the screen.
• Aortic prosthesis: look for PVL in long-axis mid-oesophagus 120° and transgastric 0° - 120°; locate it in short-axis 40° at the periphery of the annulus (diastolic vortex).
• 3D imaging with reconstruction of an "en-face" view of the valve is used to measure the extent of the paravalvular leak(s). A severe leak represents > 20% of the valve circumference (moderate leak: 10-20%, mild leak: < 10%) (Figure 11.47) [15].

 Video: major posterior paravalvular leak swirling in the LA, together with a small normal intravalvular leak. 

 Figure 11.47: Paravalvular leak (PVL). A: Paravalvular regurgitant orifice outside a Carpentier annulus after mitral valve repair (3D TEE view from the LA). B: Occlusion of a paravalvular leak by a device used for AIC closure. Two devices are in place (occluders), one is still suspended from its guide (3D TEE view from the LA).

 Locating a paravalvular leak on two-dimensional imaging is not always easy, but is facilitated by a very systematic approach.

• After MVR: search around the prosthesis; the 4-cavity  0°, bicommissural 60°, 2-cavity 90° and long-axis 120-140° views define 4 planes visualising 8 quadrants of the annulus (Figure 26.65); the position of the left atrial appendage, the tricuspid valve and the aortic valve are additional landmarks.
• After AVR: The 40° short-axis view of the aortic prosthesis shows where the leak is in diastole around the circumference of the prosthetic annulus (Figure 11.46D).

Figure 26.65: The 4 standard transesophageal planes allow the definition of 8 quadrants of the mitral annulus, ensuring efficient localisation of a paravalvular leak; the left atrial appendage (LAA), the aortic valve and the tricuspid valve are additional landmarks. The orthogonal planes of the body (0° and 90°) are shown in red and the orthogonal planes of the mitral valve are shown in green (60° and 120-140°).

Localizing leak is very important, as it can be difficult for the surgeon to identify the orifice in the operating field when the heart is flaccid, and the leak is usually located under the prosthetic annulus. The decision to return to ECC to close the paravalvular orifice is not based on the TEE image alone, but on a series of contingencies: the valve incriminated, the condition of the annulus, the surgical feasibility of the repair, the clinical risk of 2nd ECC, etc... . If surgery is too risky, a paravalvular leak can be subsequently reduced with an occluder device used to close AICs (Amplatzer™ type occluder).

 Examination of an aortic prosthesis

A normal aortic prosthesis gives an image of mild stenosis with a Vmax > 2 m/s and an average pressure gradient of 6 to 20 mmHg depending on the type and size of the prosthesis (see above). The spectral image of the continuous Doppler flow has a triangular shape with an early peak (acceleration time < 80 msec) and an VTI_LVOT/VTI_VAo ratio ≥ 0.4. The opening area calculated by the continuity equation (SAo = (S_LVOT · VTI_LVOT ) / VTI_Ao ) is generally ≥ 1.5 cm² , but must be adapted to the type of valve in question. If the prosthesis is stenotic, Vmax exceeds 3-4 m/s and the mean gradient exceeds 30 mmHg. The flow takes on a rounded shape with a mesosystolic peak velocity (acceleration time > 100 msec) (Figure 11.48). An VTI_LVOT/VTI_VAo < 0.25 and an orifice area < 1 cm² indicate an unacceptably tight stenosis [15]. As these measurements may lead to reintervention with ECC, it is important to consider LV function. Left-sided failure results in a longer acceleration time but a lower Vmax and a reduced gradient, which may indicate that the prosthesis has a sufficient opening area. In contrast, a hyperdynamic LV will result in excess velocity and gradient, but with a short acceleration time and an VTI_LVOT ) / VTI_Ao > 0.4.

 Figure 11.48: Spectral images of continuous Doppler flow in aortic stenosis (transgastric position). A: Mild aortic stenosis flow with short acceleration time (< 30% of ejection time), triangular shape and protosystolic peak. B: Tight aortic stenosis flow with rounded flow, long acceleration time (> 40% of ejection time) and mesosystolic peak.

 The presence of a swirling diastolic jet in the LVOT indicates the likelihood of regurgitation. Quantification of residual regurgitation is not straightforward in the long-axis or short-axis mid-esophageal views because of the shadow cast by the prosthesis frame on the outflow tract and leaflets. Although the valve is further away from the sensor, deep transgastric views at 0° or long-axis views at 120° are more suitable for measuring the diameter of the vena contracta (criteria identical to those for a native valve leak, see Table 11.13). In addition to assessing the size of the insufficiency, the question of its intra- or para-valvular origin is crucial, but not always easy to resolve. In the former, the leak is usually central, whereas in the latter it is more eccentric, but prolapse of a poorly fitting stentless prosthesis can cause an eccentric leak even if it is intravalvular. The presence of a zone of concentric acceleration (PISA) anterior to the prosthesis indicates a significant leak.

 Examination of a mitral prosthesis

A normal mitral prosthesis has a Vmax ≤ 1.9 m/s (mechanical valve) to ≤ 2.4 m/s (bioprosthesis) and a mean gradient ≤ 5 mmHg. The half-pressure time (Pt1/2) is only applicable to stenotic areas (≤ 1.5 cm² ) and does not allow the calculation of the opening area of prostheses. The continuity equation is preferable, using the stroke volume in the LVOT as a reference, provided there is no aortic or mitral regurgitation: MVS = (S_LVOT · VTI_LVOT ) / VTIM ). The ratio between the integral velocities through the mitral prosthesis and the integral velocities in the LVOT (VTI_M / VTI_LVOT ) is the inverse of the ratio used for the aortic valve, but fulfils the same function: to measure mitral stenosis independently of haemodynamic conditions; it is usually < 2.2 (≥ 2.5 in severe stenosis) [4]. All these measurements require a normal haemodynamic status, adequate blood volume and a stable heart rate.