Introduction

Since the first implantation of a transcatheter aortic valve in 2002 ¹ this new technology has reached world-wide acceptance as an alternative to surgical aortic valve replacement for patients with severe, symptomatic aortic stenosis who are who are inoperable or at high risk for surgical valve replacement.^2-5 Consensus papers and guidelines suggest that echocardiography is important in the pre-procedural, intra-procedural and post-procedural evaluation of patients undergoing TAVR.^6-9 As experienced centers are moving toward less invasive anesthetic and monitoring techniques,^10-14 greater numbers of commercial sites with lower volumes and less experience have started TAVR programs. For these sites, intra-procedural transesophageal echocardiography (TEE) may be protective, with lower mortality shown in at least one international registry.¹⁵ In addition, the presentation of PARTNER II intermediate risk arm of the third generation balloon-expandable valve data (Kodali et al, ACC 2015) suggests that 30-day mortality of <2% should be expected for this patient population. Arguments against TEE include the risk of general anesthesia and longer hospital stays however not only are the risks associated with general anesthesia¹⁶ and TEE imaging^{17, 18} low, but some sites perform TEE for TAVR under conscious sedation¹⁸. Recent recommendations for intra-procedural imaging¹⁹ should help standardize the approach to TEE imaging for TAVR. Compendia for imaging of TAVR complications also should improve the diagnosis and treatment of intra-procedural emergencies and improve outcomes.^{20, 21} Understanding the utility of real-time TEE imaging for pre-implantation planning, intra-procedural guidance, and post-implantation evaluation is essential for the entire Heart Team; this review will outline the approach to intra-procedural TEE for TAVR.

Intra-procedural guidance

One of the first tasks of intra-procedural TEE is to confirm the valve choice and size. Recent studies have supported the feasibility of 3D TEE annular measurements and THV sizing,^22-28 Using direct planimetry of the annular plane, Tsang et al²⁴ compared cardiac magnetic resonance imaging (CMR), MSCT and 3D TEE measurement of in vitro, ex vivo and in vivo calcified annuli and found that CMR had the highest accuracy and lowest variability. In addition, compared to CMR, MSCT overestimated and 3D TEE underestimated in vivo annular measurements. Direct planimetry of the annulus however has significant limitations and intra-procedural indirect planimetry of the annulus has been validated again MSCT^{29, 30} with high agreement and low inter- and intra-observer variability. Sizing algorithms suggested by manufacturers are available¹⁹ but continue to be refined as new iterations and valve types are implanted.³¹

The significant advantage of real-time TEE imaging is continuous monitoring of all aspects of the procedure. Instead of limiting procedural assessment to burst of fluoroscopic visualization or injection of contrast, TEE allows a safe, continuous imaging of all cardiac structures as well as the aorta. In cases without adequate radiographic imaging, TEE has been successfully used with equivalent short and mid-term outcomes with reduced contrast media use.³² In our experience, discordant TEE and fluoroscopic pre-implant THV positioning may warrant repositioning of the C-arm or TEE probe, to obtain adequate positions for continuous imaging by both imaging modalities.

Wire and cannulation position and complications can easily be evaluated. Ideal pacemaker tip location in the right ventricle or stiff wire position in the left ventricle should be confirmed. Excluding entanglement of wires in the mitral apparatus may prevent acute mitral regurgitation or mal-positioning of the THV. Optimal transapical cannulation site should be routinely assessed in order to avoid right ventricular or interventricular septal perforation.

The diagnostic as well as procedural utility of balloon aortic valvuloplasty (BAV) cannot be underestimated. Not only can BAV be used for final confirmation of THV sizing, but also for prediction of calcium displacement during final THV deployment.^33-35 Complications from BAV occur in up to 16% of patients³⁶ and echocardiographic imaging can help diagnose acute coronary occlusion, severe aortic regurgitation and tamponade.

Positioning the THV requires a thorough understanding of the valve structure, implantation characteristics and ideal location within the aortic landing zone. In addition, the fluoroscopic co-axial imaging plane is typically not the same imaging plane for TEE and realizing that different portions of the THV being imaged is crucial for accurate communication between the interventionalist and imager. For the first and second generation balloon-expandable valve, balloon deployment of the THV is typically performed during rapid pacing and relative hypotension to prevent ejection into the aorta. Dvir et al³⁷ showed that during deployment, device upward movement on fluoroscopy was asymmetrical, occurring more in the lower part of the device than in its upper part (3.2 ± 1.4 mm vs. 0.75 ± 1.5 mm, p < 0.001). In order for the ventricular end of the skirted valve to cover the annulus (typically by at least 1-2 mm) the ideal echocardiographic position of the valve during pacing should be ~ 5-6 mm below the annulus while also ensuring the calcified leaflets are completely covered and the sinotubular junction is not threatened.

The commercially-available self-expanding valve has in the past, been performed under fluoroscopic guidance with little need for TEE assistance; however the second generation, fully repositionable valve may be most efficiently implanted using echocardiographic guidance. Because of the curvature of the aorta, the self-expanding valve will start non-coaxial with the long-axis of the aorta, the tip pointing posteriorly and to the left. Because of the posterolateral orientation of the valve, when initial deployment begins, the posterior edge of the THV is "higher" (more aortic) than the anterior edge. Imaging should confirm that this edge is at least 4 mm (but typically not more than 10 mm) below the annulus for the first generation valve, and 3-5 mm below the annulus for the second generation valve. As the valve is deployed it will typically "pivot" on the posterior end, causing the anterior edge of the THV to move more superiorly; this is the justification for positioning the valve initially based on the location of this posterior THV edge. Because the second generation valve is fully repositionable, a rapid assessment of position and PAR can be made with TEE imaging. Of note, there is significant intra-procedural spontaneous regression of paravalvular regurgitation with the self-expanding valve and a post-dilatation is not frequently required for ≤ mild PAR especially if the regurgitation occurs in regions of good stent-native tissue apposition. If on the other hand, there is significant PAR with under-deployment of the valve, typically with a very elliptical configuration, then post-dilatation should be considered.

Post-implantation assessment

Given the superiority of this imaging modality for the immediate and accurate real-time assessment of cardiac structures, TEE has been shown to add incremental value to intra-procedural guidance particularly for assessment of complications and evaluation of acute hemodynamic changes.^38-42 Complications such as PVR, mal-positioning, and occlusion of the coronaries, may require transcatheter treatment. Other complications such as aortic trauma may require surgical intervention. Intra-procedural TEE permits immediate recognition and treatment of these complications and thus adds to the safety and outcomes of the TAVR procedure.⁴² In the Brazilian registry of 418 TAVR cases from 18 sites, multivariable analysis revealed intra-procedural TEE was associated with lower all cause and late mortality (hazards ratios of 0.57 and 0.47, respectively).

A large meta-analysis of 9,251 patients from 46 studies suggested the rate of bail-out surgery was only 1.1±1.1% (n = 102 pts), although under-reporting was suspected.⁴³ The most frequent reported reason for emergent surgery in this study was embolization/dislocation of the AV prosthesis (41%). Aortic dissection (n=14), coronary obstruction (n=5), severe aortic valve regurgitation (n=10), annular rupture (n=6), aortic injury (n=14), and myocardial injury including tamponade (n=12) were also seen. The majority of all these complications can be imaged with intra-procedural TEE.

Intra-procedurally, interventionalists have relied upon cine-aortography, TEE color Doppler visual assessment^44-47 and hemodynamics.⁴⁸ Intra-procedural TEE has significant advantages over these methods, avoiding large contrast loads and allowing for accurate differentiation between central and paravalvular regurgitation. Grading of PAR continues to be refined. The guidelines for grading prosthetic surgical valve regurgitation⁴⁹ has little data to support the use in TAVR. On the other hand the methodology used by the PARTNER I trial core lab⁵⁰ has been validated with long-term outcomes data^{51, 52} and used a multi-window and multi-parametric approach which relied heavily on color Doppler jet features such as: width of the jet origin, visible jet path around the stent, number of jets, and the circumferential extent of the jet. A recent proposal for a unifying grading scheme for PAR⁵³ adapts criteria from guidelines and trials into a versatile classification system which can be collapsed to correlate with prior grading schemes, but expanded to hopefully improve the accuracy and consistency of grading.⁵⁴ Quantifying regurgitation by volumetric or 3D methods may also be used⁵⁵ however both have limited applicability for rapid intra-procedural assessment.

When assessing for PAR using TEE, color Doppler evaluation should be performed just below the lower border of the transcatheter valve (within the left ventricular outflow tract) and should not be confused with flow within the SOV. The latter flow is frequently between the sinus and the native cusps and does not typically communicate with the ventricle. In addition, some paravalvular leaks fail to reach the left ventricular outflow tract because of the fabric skirt seal at the lower border of the THV. Central regurgitation should be evaluated at the coaptation point of the leaflets.

If PAR is deemed at least mild in severity, treatment may be warranted. This may entail watchful waiting, re-ballooning or post-dilatation (PD).^{44, 46, 47} Daneault et al showed that approximately 30-40% of the PVR jet size may spontaneously regress within minutes following implantation.⁴⁷ This regression as well as the response to PD may be influenced by a number of factors including initial degree of expansion, valve position and severity of outflow tract calcium. When qualitatively > mild PVR is seen on TEE, PD is typically performed unless: maximal expansion of the valve has already been achieved and the risk of central aortic regurgitation outweighs the reduction of paravalvular regurgitation; severe calcification prevents improvement in valve apposition to the annulus, and the risk of PD outweighs the benefit of reduced PVR. Potential risks of PD include: transcatheter heart valve (THV) migration or injury, trauma to the conduction system, rupture of the membranous septum or aorta and cerebrovascular embolism.^{46, 47}.

Severe central THV regurgitation or severe mal-positioning of the THV may be successfully treated with a valve-in-valve procedure.⁵⁶ Although rarely performed (2.4 % of cases in the PARTNER 1 trial) with the majority (88.5%) of repeat valves were performed immediately. When valve-in-valve is performed in the setting of PVR, the etiology was most commonly mal-positioning. When due to central regurgitation, leaflet malcoaptation was typically the cause possibly related to calcium impingement, leaflet overhang, or a tilted valve. The evaluation of both severity and etiology can be performed with TEE.

Other intra-procedural complications which can be imaged on TEE include are listed in Table 1.¹⁹ Reviews of the imaging for complications of both self-expanding²¹ and balloon-expandable valves⁵⁷ have recently been published.

Table 1: Complications that can be images during intra-procedural TEE for TAVR

Complication	Transesophageal Echo Assessment
Hemodynamic Instability
Severe transvalvular or paravalvular aortic regurgitation	Assess location of regurgitation (central vs paravalvular) Assess position of the transcatheter valve Assess severity of aortic regurgitation
Major bleeding	Assess ventricular size and function (wall collapse due to hypovolemia)
Aortic rupture or dissection	Examine the aortic root / ascending aorta for peri-aortic hematoma, aortic dissection, or rupture
Coronary artery obstruction	Evaluate for regional wall motion abnormalities of the LV or RV Identify the left main ostium; use color flow Doppler to assess blood flow
Severe mitral regurgitation	Evaluate severity of mitral regurgitation and anatomy of the mitral apparatus:  valvular perforation, rupture chordae, tethering of the leaflets
Pericardial effusion	Assess for tamponade physiology and possible etiology (i.e. chamber perforation)
Other Procedural Complications
Balloon aortic valvuloplasty complication	Assess severity of aortic regurgitation Examine the aortic root / ascending aorta for peri-aortic hematoma, aortic dissection, or rupture Identify the left main ostium; use color flow Doppler to assess blood flow
Mal-positioning of the transcatheter heart valve	Too high or too low within the annulus with resulting hemodynamic instability:  rapid deployment of a second valve can be performed. Embolization of the valve (into the left ventricle or into the aorta) may require surgical intervention
Fistula	Ventricular septal defect Aorto-cameral fistula (typically into the RVOT or right atrium)

Conclusion

TAVR has become an acceptable alternative to surgical aortic valve replacement for patients with severe aortic stenosis who are at "high risk" or deemed inoperable. Multi-modality imaging has become an integral part of this procedure. Intra-procedural TEE in particular has become an indispensable tool to confirm patient selection, provide intra-procedural guidance, predict and diagnose complications, and assess post-implant THV function.

References

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Clinical Topics: Cardiac Surgery, Cardiovascular Care Team, Congenital Heart Disease and Pediatric Cardiology, Invasive Cardiovascular Angiography and Intervention, Noninvasive Imaging, Pericardial Disease, Valvular Heart Disease, Vascular Medicine, Aortic Surgery, Cardiac Surgery and CHD and Pediatrics, Cardiac Surgery and VHD, Congenital Heart Disease, CHD and Pediatrics and Imaging, CHD and Pediatrics and Interventions, CHD and Pediatrics and Quality Improvement, Interventions and Imaging, Interventions and Structural Heart Disease, Interventions and Vascular Medicine, Angiography, Echocardiography/Ultrasound, Magnetic Resonance Imaging, Nuclear Imaging, Mitral Regurgitation

Keywords: Algorithms, Anesthesia, General, Anesthetics, Aorta, Aortic Rupture, Aortic Valve, Aortic Valve Stenosis, Aortic Valve Insufficiency, Aortography, Calcium, Catheterization, Conscious Sedation, Consensus, Contrast Media, Coronary Occlusion, Coronary Vessels, Dilatation, Echocardiography, Echocardiography, Transesophageal, Echocardiography, Three-Dimensional, Embolism, Emergencies, Fistula, Fluoroscopy, Heart Atria, Heart Septal Defects, Ventricular, Heart Ventricles, Hematoma, Hemodynamics, Hypotension, Hypovolemia, Magnetic Resonance Imaging, Mitral Valve Insufficiency, Multimodal Imaging, Patient Selection, Pericardial Effusion, Prostheses and Implants, Registries, Stents, Surgical Instruments, Watchful Waiting

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