Comparison of transthoracic and transesophageal color Doppler flow imaging in patients with mechanical prostheses in the mitral valve position

Platelet Dysfunction in Cardiopulmonary Bypass: An Experimental Comparative Study between a Centrifugal and a New Pulsatile Pump

The aim of this investigation was to study the effect of a new pulsatile pump for extracorporeal circulation (ECC) on platelet count and platelet function with respect to a Biomedicus centrifugal pump. Thirteen pigs, 8 in the pulsatile group (PG) and 5 in the centrifugal group (CG), underwent a partial extracorporeal circulation lasting 3 h. The animals were sacrificed 3 h post-ECC. The platelet study was both quantitative (platelet count) and qualitative (platelet function analysis) by assessing the closure time (CT) with a PFA-100 system. The decrease in platelet number from basal to 3 h post ECC was only significant in CG (p = 0.009). The platelet function was impaired in both groups, but the value of CT with col/ADP increased significantly only in CG (p < 0.001). The increase of CT with col/EPI was greater in CG (p = 0.07) than in PG (p = 0.2). The results indicated that the new pulsatile pump preserves platelets quantitatively and qualitatively well compared to a Biomedicus pump.

Evaluation of Prosthetic Heart Valves by Transesophageal Echocardiography: Problems, Pitfalls, and Timing of Echocardiography

Transesophageal echocardiography (TEE) is especially suitable for examination of prosthetic valves because of the proximity of the esophagus to the heart and absence of interference with lungs and ribs. This article reviews normal and abnormal morphologic characteristics of prosthetic valves such as spontaneous echocontrast, microbubbles, strands, sutures, vegetations or thrombus. Doppler echocardiographic characteristics of normal and pathologic prosthetic valve function and the management of prosthetic valve pathology is discussed. Physicians taking care of patients with prosthetic valves should be familiar with the characteristics of normal and abnormally functioning prosthetic valves.

Diagnostic evaluation of left-sided prosthetic heart valve dysfunction

Prosthetic heart valve (PHV) dysfunction is a rare, but potentially life-threatening, complication. In clinical practice, PHV dysfunction poses a diagnostic dilemma. Echocardiography and fluoroscopy are the imaging techniques of choice and are routinely used in daily practice. However, these techniques sometimes fail to determine the specific cause of PHV dysfunction, which is crucial to the selection of the appropriate treatment strategy. Multidetector-row CT (MDCT) can be of additional value in diagnosing the specific cause of PHV dysfunction and provides valuable complimentary information for surgical planning in case of reoperation. Cardiac magnetic resonance imaging (CMR) has limited value in the evaluation of biological PHV dysfunction. In this Review, we discuss the use of established imaging modalities for the detection of left-sided mechanical and biological PHV dysfunction and discuss the complementary role of MDCT in this context.

Prosthetic mitral regurgitation can be mimicked by Doppler color flow mapping: Avoiding misdiagnosis

OBJECTIVES

We sought to characterize a region of apparent systolic flow resembling mitral regurgitation (MR) in patients with mechanical disk mitral prostheses as artifact.

BACKGROUND

Diagnosing MR in the presence of mechanical prostheses is challenging. Occasionally, important MR is suggested by a substantial region of systolic Doppler color flow in an acoustically shadowed region of the left atrium when, in fact, only trace MR exists. We hypothesized this pseudo-MR is caused by acoustic mirroring of the left ventricular outflow tract (LVOT) flow by sound reflected off the prosthesis, projecting flow into the left atrium because of longer transit time.

METHODS

We reviewed 19 patients with mechanical mitral valves and trace MR by transesophageal echocardiography who had transthoracic echocardiography studies within 1 week (group A), and prospectively studied 22 consecutive patients by transthoracic echocardiography with subtle transducer angulation variation to detect pseudo-MR and characterize it by pulsed Doppler (group B).

RESULTS

In group A, 12 of 19 patients had evidence of pseudo-MR on review of their transthoracic echocardiograms, absent by transesophageal echocardiography. In group B, this pseudo-MR signal was present in 13 of 22 patients, with velocity and timing by pulsed Doppler comparable with LVOT flow (onset at 125 +/- 27 milliseconds vs 135 +/- 11 milliseconds from QRS, P = not significant). The angle between the mitral plane and the LVOT, which determines whether this mirroring can occur, was smaller for patients with pseudo-MR.

CONCLUSION

Artifactual pseudo-MR is often seen with mechanical mitral prostheses. Its behavior and sensitivity to geometric relationships are consistent with mirroring of LVOT flow. Practically, potential misdiagnosis can be readily avoided by pulsed Doppler sampling, sparing the patient further procedures.

Mitral prosthetic inflow mimicking severe aortic regurgitation

We describe two patients in whom mitral valve prosthetic inflow mimicked severe aortic regurgitation. Repeat transthoracic echocardiography using multiple transducer angulations and a narrow color Doppler sector angle in one patient and a color Doppler M-Mode study in the second patient proved useful in excluding the presence of severe aortic regurgitation.

Use of deep intravenous sedation with propofol and the laryngeal mask airway during transesophageal echocardiography

OBJECTIVE

To describe the use of either deep intravenous sedation with propofol or light sedation with midazolam and topical anesthesia during transesophageal echocardiography (TEE) and to report the incidence of respiratory complications and their management.

DESIGN

Retrospective study from March 2000 through August 2002.

SETTING

Single institution, specialized cancer center.

PARTICIPANTS

All patients undergoing TEE examination in the specified time period (n = 42).

MAIN RESULTS

Eight patients received light sedation and 34 patients received deep intravenous sedation with propofol. An airway event occurred in one patient in the light sedation group and in six patients in the deep sedation group. The patient in the light sedation group was managed with the use of a face-mask and a manual resuscitation bag. All airway events in the deep sedation group were managed successfully using the laryngeal mask airway (LMA).

CONCLUSION

Deep sedation with intravenous propofol can provide both excellent patient comfort and optimal conditions for TEE examination, particularly in patients who may require more lengthy procedures or in whom other techniques have failed. Although the incidence of respiratory depression was higher in patients receiving deep sedation with propofol than in patients who were lightly sedated (17.6% versus 12.5%, respectively), all six patients who had respiratory depression while under deep sedation with propofol were successfully ventilated using the LMA trade mark, without the need to remove the TEE probe and without terminating the examination prematurely. In contrast, in the one patient in the light sedation group who had respiratory depression, the TEE probe had to be removed to ventilate the patient via a face mask, and the procedure was cancelled.

Peak early diastolic velocity rather than pressure half-time is the best index of mechanical prosthetic mitral valve function

Reliable screening of mechanical prosthetic mitral valve (PMV) dysfunction by transthoracic echocardiography (TTE) is mandatory because transesophageal echocardiography (TEE) cannot be routinely used. However, acoustic shadowing seriously hampers detection of PMV dysfunction with TTE, particularly regurgitation. To identify TTE indexes that can detect PMV dysfunction (regurgitation or obstruction), 134 patients (age 60 +/- 12 years, 64 men) with PMV who underwent TTE and TEE within 3 +/- 5 days were assessed. There were 73 normal and 61 dysfunctional valves (40 regurgitant, 21 obstructive). By multivariate analysis, peak E velocity was the best predictor of a dysfunctional valve. Both peak E velocity (E > or =1.9 m/s; sensitivity 92%, specificity 78%) and the ratio of velocity-time integrals of flow through the prosthesis to that of the left ventricular outflow (VTI(pmv/)VTI(lvo) > or =2.2; sensitivity 91%, specificity 74%) were successful in detecting PMV dysfunction. Although pressure half-time (PHT) readily identified PMV obstruction, it did not detect regurgitation. Logistic models including peak E velocity and VTI(pmv)/VTI(lvo) or PHT were equally successful in detecting PMV dysfunction. However, all 3 variables were needed to best distinguish among normal, obstructed, and regurgitant valves. A peak E velocity > or =1.9 m/s and VTI(pmv)/VTI(lvo) ratio > or =2.2 predicted valve regurgitation in 83% of valves when PHT was < 130 ms, and valve stenosis in 95% when PHT was >130 ms. Importantly, a peak E velocity < 1.9 m/s, VTI(pmv)/VTI(lvo) ratio < 2.2, and a PHT < 130 ms had a predictive accuracy for a normal valve of 98%. Thus, TTE Doppler indexes can be used as screening parameters of PMV dysfunction and help select patients for further diagnostic evaluation with TEE.

Transesophageal echocardiographic (TEE) evaluation of prosthetic valves

TEE overcomes many of the imaging constraints associated with transthoracic echocardiography for the assessment of valvular anatomy and function. Additional imaging artifacts and constraints associated with prosthetic valves are minimized or overcome with TEE. As such, TEE allows assessment of prosthetic valve anatomy and function and paraprosthetic anatomy, and serves as the diagnostic imaging modality of choice for patients with suspected prosthesis dysfunction or endocarditis.

Incorrect echocardiographic diagnosis in patients with mechanical prosthetic valve dysfunction: correlation with surgical findings

PURPOSE

To identify the rate of occurrence and type of incorrect echocardiographic diagnoses in patients with mechanical valve prostheses.

PATIENTS AND METHODS

We studied 170 consecutive patients (73 women and 97 men) with a total of 208 prostheses who underwent surgery for mitral (n = 136) or aortic (n = 72) valve dysfunction between January 1991 and December 1997. Preoperative echocardiographic data were compared with surgical findings. Any major discrepancy between the echocardiographic reports and surgery was judged to be unconfirmed when the preoperative echocardiographic diagnosis was not confirmed at surgery, but the prosthesis was found to be dysfunctioning; and was judged to be erroneous when the preoperative echocardiographic diagnosis was not confirmed, and surgical inspection failed to reveal any other prosthetic abnormality.

RESULTS

There were 25 (12%) diagnostic errors. Of the 136 mitral prostheses, there were 9 unconfirmed diagnoses of paravalvular regurgitation (6 had a fibrous tissue overgrowth, 1 had a thrombus with fibrous tissue overgrowth, 1 had endocarditis vegetations, and 1 had a ball variance) and 5 erroneous diagnoses. Eleven diagnostic errors were made in the 72 aortic prostheses: there were 9 unconfirmed diagnoses (paravalvular regurgitation was diagnosed as transvalvular in 7, and transvalvular regurgitation as paravalvular in 2 cases), and 2 erroneous diagnoses.

CONCLUSIONS

Although echocardiography has gained great credibility among clinicians, special care should be taken when assessing patients in whom prosthetic valve dysfunction is suspected.

Usefulness of transthoracic echocardiography in detecting significant prosthetic mitral valve regurgitation

To identify the transthoracic echo-Doppler (TTE) variables most predictive of significant mitral regurgitation (MR) of mechanical prosthetic valves, TTE and trans-esophageal echo (TEE) studies were independently reviewed in 57 patients (mean age [+/-SD] 59+/-12.5 years) undergoing both studies within 2+/-3 days. Several 2-dimensional and Doppler hemodynamic variables from the TTE studies were derived. Prosthetic MR was significant (moderate or severe) by TEE in 20 patients, whereas mild or no MR was seen in 37 patients. The best univariate predictors of significant MR by TTE were peak velocity of mitral inflow, mean gradient, tricuspid regurgitation velocity, isovolumic relaxation time, and ratio of time velocity integral of mitral inflow to time velocity integral in the left ventricular outflow (TVI(MV)/TVI(LVO)). Peak mitral velocity and TVI(MV)/TVI(LVO) were the best predictors of significant MR and performed similarly (area under the receiver-operating characteristic curve: 0.97 for both). A peak velocity of > or =1.9 m/s was 90% sensitive and 89% specific for significant prosthetic MR, whereas a TVI(MV)/TVI(LVO) > or =2.5 had a sensitivity and specificity of 89% and 91%, respectively. A decision tree was constructed to assess the conditional probabilities of having significant MR given all the possible outcomes of the 2 best predictors. None of the patients with peak velocity < 1.9 m/s and TVI(MV)/TVI(LVO) <2.5 by TTE had significant MR. Conversely, all patients with peak velocity > or =1.9 m/s and TVI(MV)/TVI(LVO) > or =2.5 had significant MR. The use of more complex algorithms did not further improve the results. Thus, measurements of hemodynamic Doppler variables on TTE examination can accurately identify a large number of patients without significant prosthetic MR, thereby reducing the need for further investigation with TEE.

Evaluation of prosthetic valve function and associated complications

Significant advances in imaging modalities have occurred to evaluate prosthetic valve function and associated complications. These developments involve predominantly the introduction of Doppler technology for the non-invasive determination of gradients and valve areas and TEE for an improved assessment of valve structure, function, and associated complications. The current role of cinefluoroscopy is mostly to complement TEE in the evaluation of motion of mechanical prosthetic valves in the aortic position. Cardiac catheterization is now rarely needed to assess valve function. Diagnosis of prosthetic valve obstruction can be performed in the majority of cases with transthoracic Doppler echocardiography. Differentiation of valve obstruction from normal valve function in small valves with high flow conditions, however, may be difficult. Because of this and the variability in normal valves among different prostheses, knowledge of the type and size of the implanted valve is essential. Patients and ultrasound laboratories are encouraged to seek and provide this information on a routine basis. Although transthoracic echocardiography is the main diagnostic modality for the serial evaluation of prosthetic valve function, it is important to recognize its limitations in assessing prosthetic mitral regurgitation and evaluating structural abnormalities of prosthetic valves. These are the situations in which TEE has the most impact. A summary of general indications of TEE in prosthetic valves is provided in Table 6. Finally, a baseline transthoracic Doppler study is essential in the overall follow-up and serial evaluation of valve function. For future comparisons, the best indices of valve functions are those obtained for patients as their own control, from a baseline Doppler echocardiographic study performed early after the operation.

Evaluation of normal hemodynamic profile of CarboMedics prosthetic valves by Doppler echocardiography

The authors investigated 163 CarboMedics bileaflet prosthetic valves--81 mitral prostheses (MP), and 82 aortic prostheses (AP)--to determine acceptable pressure gradients across normally functioning prostheses and effective mitral valve orifice (MVO) area by Doppler echocardiography. In MP, the mean gradient was 3.6+/-1.7 mm Hg, peak transmitral gradient was 8.7+/-3.7 mm Hg, and mean effective valve area was 2.3+/-0.7 cm2. There was a significant overlap in mean and peak transaortic gradients even with valves of the same size. In AP, the mean gradient was 14.7+/-5.1 mm Hg and peak pressure gradient was 26.1+/-8.2 mm Hg. They observed a weak inverse correlation between valve size and gradients in AP. Mean and peak pressure gradients tended to be higher with smaller valve sizes, but differences were statistically significant (P < 0.5) only when they compared the smallest vs the largest valves. Trivial to mild regurgitation was detected in 28.4% of MP and 54.8% of AP. From the data they conclude that CarboMedics valves offer relatively little resistance to forward flow, both in the mitral and aortic positions, and their hemodynamic profile is comparable to that of the St. Jude bileaflet valves described in published literature.

Improved Assessment of Pathological Regurgitation in Patients with Prosthetic Heart Valves by Multiplane Transesophageal Echocardiography

The aim of this study was to evaluate the diagnostic increment of individually optimized axes in the assessment of pathological prosthetic valve regurgitation. Forty-two patients with pathologically regurgitant prostheses in the aortic (n = 21), mitral (n = 15), and tricuspid (n = 6) positions were examined by multiplane transesophageal echocardiography. The investigation was performed utilizing the transverse axis first, the longitudinal axis second, and the intermediate axes afterwards. The presence of regurgitation, the differentiation between trans- and perivalvular origin, and the localization of perivalvular leakages at the sewing ring were evaluated. Findings in the biplane and intermediate axes were compared to surgery or autopsy in all patients. There was slightly higher detection rate for aortic prosthetic regurgitation using the intermediate axes than the biplane axes. The intermediate axes revealed significantly fewer differences to the morphological control than the biplane axes with regard to the differentiation of peri- and transprosthetic aortic regurgitation and to the localization of a periprosthetic aortic regurgitant origin. The intermediate axes provided significantly better agreement to surgery/autopsy than the biplane axes regarding the localization of the origin of mitral periprosthetic regurgitation. Morphological visualization of the perivalvular gap adds important information on the precise localization of the regurgitant origin. The pathological gap was visualized significantly more often using the intermediate than the biplane axes in all types of prostheses. The data in this study therefore suggest that multiplane transesophageal echocardiography is superior to biplane transesophageal echocardiography in the assessment of pathologic prosthetic regurgitation.

Normal echocardiographic characteristics of the Sorin Bicarbon bileaflet prosthetic heart valve in the mitral and aortic positions

Doppler echocardiographic characteristics of normally functioning Sorin Bicarbon prostheses were prospectively assessed in 226 consecutive patients (135 male and 91 female patients, mean age 61 +/- 10 years) with 233 valves in the mitral (n = 67) and aortic (n = 166) positions whose function was considered normal by clinical and echocardiographic evaluation. Patterns of "normal" transprosthetic leakage were assessed with transthoracic echocardiography in all valves and with transesophageal echocardiography in six selected mitral valve prostheses. For the mitral valve prostheses, we found that peak and mean gradient, as well as pressure half-time, were not significantly different in either the 25 or the 31 mm valves (median values from 15 to 10 mm Hg, from 4 to 4 mm Hg, and from 70 to 83 ms; p = Not significant for all). On transthoracic study, 12 patients (17%) with a Sorin Bicarbon valve in the mitral position showed minimal transprosthetic leakage. On transesophageal study, all patients showed a transprosthetic leakage whose spatial distribution had a complex pattern: in planes orthogonal to the leaflet axis, two to four jets arising from the hinge points and converging toward the center of the valve plane could be visualized; in planes parallel to the leaflet axis, there were three jets, the two lateral ones diverging and the central one perpendicular to the valve plane. For the aortic valve prostheses, there was a significant decrease in transprosthetic gradients and an increase in effective orifice areas as prosthesis size increased. Peak and mean gradients decreased from a median value of 25 and 13 mm Hg in the 19 mm valves to 9 and 5 mm Hg in the 29 mm valves, respectively. Effective prosthetic valve area calculated with the continuity equation increased from a median value of 0.97 cm2 for the 19 mm size valves to 3.45 cm2 for the 29 mm size. With analysis of variance, effective prosthetic aortic valve area differentiated various valve sizes (F = 40.9, p < 0.0001) better than peak (F = 10.3, p < 0.0001) or mean (F = 8.04, p < 0.0001) gradients alone did. Furthermore, effective prosthetic aortic valve area correlated better than peak and mean gradients with prosthetic size (r = 0.76, r = -0.45, and r = -0.39, respectively). On transthoracic study, 109 patients (66%) showed minimal transprosthetic leakage. These normal values, obtained in a large number of patients with normofunctioning mitral and aortic Sorin Bicarbon valves, may help to identify Sorin Bicarbon prosthesis dysfunction.

Characterization of normal leakage flow of monostrut tilting disk prosthetic mitral valves by multiplane transesophageal echocardiography

To assess normal prosthetic mitral valve function, multiplane transesophageal Doppler echocardiographic studies were performed on 22 patients with Monostrut tilting disk valves. Mean follow-up after implantation was 63 +/- 12 months. Two holosystolic red low-velocity regurgitant jets were detected in all but one case. The length of these jets ranged from 0.8 to 6.9 cm and the area ranged from 0.5 to 10.1 cm2 in various planes. The origin of the jets was inside the rings of the prosthetic valves and started with a mosaic spot of 2 mm maximum diameter. In this mosaic spot, jet velocity by the high pulse-repetition frequency mode measured greater than 4 m/sec. Thin, high-velocity regurgitant jets originating outside the sewing ring were present in three cases. In one case we found a high-velocity turbulent jet originating outside the ring corresponding to a paravalvular regurgitation of moderate degree. The low-velocity jets were obscured in the patient with moderate paravalvular leak but not in patients with trivial paravalvular leaks.

Significance of "Strands" on Mitral Mechanical Prostheses During Late Follow-Up After Surgery

The presence of "strands" on mitral mechanical prostheses (MMP) has been described, although their significance has yet to be established. The aim of this study was to determine the incidence and clinical implications of "strands" on MMP in late follow-up by a retrospective analysis of 320 consecutive patients who had undergone a transesophageal echocardiographic (TEE) examination under standardized conditions 7 +/- 6 years after mitral surgery. Twenty patients (6.25%) with "strands," defined as highly mobile linear echogenic densities, were identified. This group was compared with a control group of 38 patients, matched for age, sex, and interval between surgery and TEE study, selected at random from this population. Patients with MMP dysfunction were excluded. Type of prosthesis, echographic characteristics (left atrial diameter, spontaneous echo contrast, ejection fraction), and anticoagulant status were not found to bear any relationship to the presence of strands. However, the prevalence of previous embolic events was significantly higher in the "strand" group (10/20 [50%]) than in the control group (7/38 [18.4%]) (P < 0.05). The "strand" group also contained more patients with a prior history of mitral prosthesis thrombosis (7/20 [35%] vs 1/30 [2.6%]; P < 0.05). However, long-term follow-up (1-4 years) was uneventful. These results suggest that: (1) in late follow-up, strands are rare in patients with MMP (6%); and (2) strands were related to previous thromboembolic episodes. (ECHOCARDIOGRAPHY, Volume 13, May 1996)

Echocardiography and Doppler assessment of prosthetic heart valves with transesophageal echocardiography

This article examines the use of transesophageal echocardiography (TEE) in the assessment of prosthetic heart valves. A summary of the commonly used artificial valves and their physiologic regurgitant flow patterns, as identified by color Doppler imaging, is presented. The hemodynamic evaluation of prosthetic valve stenosis using Doppler techniques is reviewed, and the diagnostic utility of TEE in identifying the complications of cardiac prostheses is discussed.

Detection of prosthetic valve strands by transesophageal echocardiography: clinical significance in patients with suspected cardiac source of embolism

OBJECTIVES

The purpose of this study was to determine how frequently prosthetic valve strands are associated with prosthetic mitral and aortic valves, as detected by transesophageal echocardiography, and to assess their significance in relation to clinical cardioembolic events.

BACKGROUND

Strands attached to prosthetic mitral valves are a recently described finding of uncertain clinical significance. There are no reports of strands attached to aortic valve prostheses, and data are limited concerning the relation of valvular strands to cardioembolic events.

METHODS

We identified all transesophageal echocardiographic studies performed during a 5-year period at our institution for evaluation of valve dysfunction or a suspected cardioembolic event in patients with a valve prosthesis. All studies were reviewed. The presence of strands was noted and the prevalence compared between patients evaluated for a suspected cardioembolic event and those evaluated for valve dysfunction. In patients with no strands detected, the presence of other potential cardiac sources of embolism was noted.

RESULTS

Strands were detected in 56 (26%) of 214 studies. There was a significant difference (p = 0.0001) in the prevalence of strands between studies performed for a suspected cardioembolic event (34 [53%] of 64) versus those performed for suspected valve dysfunction (22 [15%] of 150). Strands were more prevalent on mitral than on aortic valves (32% vs. 13%, p = 0.0004) and were more frequently detected on mechanical than on bioprosthetic valves (27% vs. 8%, p = 0.003). Among patients with a suspected cardioembolic event and normal valves, other potential cardiac sources of embolism were detected in 67%.

CONCLUSIONS

Prosthetic valve strands are frequently detected by transesophageal echocardiography. They are more commonly detected in patients being evaluated for a suspected cardioembolic event and thus represent a potential cardiac source of embolism. In patients with apparently normal valves, other potential sources of embolism are frequently detected. Thus, transesophageal echo-cardiography may have a significant impact on the management of these patients.

Echocardiographic assessment of vegetations in patients with infective endocarditis: prognostic implications

Today, echocardiography is the most important technique next to clinical findings and blood cultures in the diagnosis of infective endocarditis. The sensitivity of echocardiography, particularly the transesophageal approach, for detection of vegetations and endocarditis related valvular destructions is high. In addition, echocardiographic findings may have some prognostic implications. The size and mobility of vegetations stratifies endocarditis patients into a high risk group for arterial embolism. In particular, mobile vegetations attached to the mitral valve with a maximal diameter > 10 mm may be prone to embolic events. Furthermore, increase in size of vegetations during antimicrobial treatment may identify patients with no, or at least a prolonged, healing process. Also, a lack of increase in the echo density of vegetations under adequate antibiotic treatment may indicate a poor healing process and may necessitate more aggressive management. The demonstration of paravalvular abscesses by echocardiography, particularly by transesophageal echocardiography, identifies a subgroup of patients who will need urgent cardiac surgery before widespread tissue destruction has occurred.

Echo Doppler evaluation of patients with acute mitral regurgitation: superiority of transesophageal echocardiography with color flow imaging

Acute mitral regurgitation is a medical emergency that requires prompt, accurate diagnosis and urgent therapy. Although the use of echo Doppler imaging has been described in these patients, preliminary observations have suggested that color flow Doppler performed from the standard transthoracic windows may underestimate the severity of mitral insufficiency in this setting. The aim of this study was to compare transesophageal color Doppler quantitation of regurgitation with results obtained from standard transthoracic windows in patients with acute, severe mitral regurgitation. Two-dimensional echocardiography with pulsed, continuous, and color flow Doppler was performed by both transthoracic and transesophageal methods in 16 consecutive patients who were documented to have acute severe mitral insufficiency by catheterization. Transthoracic and transesophageal scans were reviewed by two blinded observers and assessed for the detection of mitral regurgitation by transthoracic pulsed wave (81%), continuous wave (100%), and color flow Doppler (81%) compared with transesophageal color flow imaging (100%; p = NS). Severity of mitral regurgitation was graded as none, mild, moderate, or severe on the basis of existing transthoracic pulsed wave and color flow criteria and compared with transesophageal color flow grading. At first examination patients were critically ill, with elevated pulmonary wedge pressures (mean 27 +/- 7 mm Hg) and V waves (mean 45 +/- 10 mm Hg). Fifteen of the patients underwent emergency surgery, and the overall hospital mortality rate was 12%. Maximal color flow jet areas were significantly greater on transesophageal scanning (mean 10.5 cm2) compared with transthoracic color jets (mean 2.3 cm2).(ABSTRACT TRUNCATED AT 250 WORDS)

Transesophageal echocardiography

Preview Echocardiography is invaluable in assessing patients with known or suspected cardiac disorders. However, there are often impediments to adequate imaging using the standard transthoracic route (eg, mechanical ventilation, bandages). Transesophageal imaging overcomes many of these technical limitations and, as one researcher notes, provides a "new window to the heart." Drs Peterson and Orsinelli review common clinical applications of the procedure and adjunctive uses.

Does multiplane transesophageal echocardiography improve the assessment of prosthetic valve regurgitation?

Assessment of prosthetic valve regurgitation by echocardiography remains difficult. To study the value of the newly introduced multiplane transesophageal technology for this purpose, prosthetic valve regurgitation was examined in 63 consecutive patients with 35 mitral and 33 aortic prostheses (23 bioprostheses and 45 mechanical prostheses). Transvalvular, paravalvular and, in mechanical valves, normal or pathologic transvalvular regurgitation were identified first with 0 degrees (transverse) and 90 degrees (longitudinal) planes combined with flexion of the echoscope tip and then additionally with multiple intermediary planes by transducer rotation. In a subgroup of 20 patients interobserver variability was evaluated. Both methods showed regurgitation in 56 of 68 valves; one additional case of regurgitation was seen by multiplane imaging only. However, 19 cases of regurgitation were not clearly classifiable by biplane transesophageal echocardiography compared with only three with multiplane transesophageal echocardiography. Grading of severity was concordant by both modalities in 66 and discordant in only two cases. Observers disagreed on severity in two of 20 cases based on biplane imaging but in none based on multiplane imaging; classification of regurgitation differed in six of 20 (biplane) and one of 20 (multiplane), respectively. Multiplane transesophageal imaging improves classification of prosthetic regurgitation but has little effect on severity grading.

Transesophageal echocardiography for the diagnosis and management of nonobstructive thrombosis of mechanical mitral valve prosthesis

BACKGROUND

Diagnosis of a mechanical mitral valve prosthesis thrombosis is currently made with transthoracic Doppler echocardiography and occasionally with fluoroscopy. However, identifying a thrombus on a valve prosthesis may be difficult, especially if the thrombus is nonobstructive. To prospectively define the role of transesophageal echocardiography for identification of nonobstructive thrombi, we studied a series of patients in whom the prosthetic valve was considered to function normally on clinical examination and transthoracic echocardiography.

METHODS AND RESULTS

One hundred fourteen consecutive patients with mechanical mitral valve prosthesis were investigated by both transthoracic echocardiography and transesophageal echocardiography. These examinations were performed for recent systemic emboli (15 patients), fever of unknown etiology (11 patients), routine postoperative evaluation (56 patients), and other reasons (32 patients). Based on transthoracic echo diagnosis, all prostheses were considered normal. Yet, in 20 patients transesophageal echocardiography revealed the presence of a 2- to 15-mm-long mobile thrombus localized on the atrial surface of the prosthesis. When compared with the remaining 94 patients with no visible thrombi, there was no significant difference between the two groups in terms of incidence of atrial fibrillation (65% versus 52%), left atrial size (48 +/- 9 versus 51 +/- 13 mm), left ventricular end-diastolic diameter (49 +/- 10 versus 51 +/- 13 mm) and fractional shortening (28 +/- 9% versus 31 +/- 10%), presence of spontaneous contrast to the left atrium (40% versus 41%), transprosthetic mean pressure gradient (4.0 +/- 1.4 versus 3.9 +/- 1.5 mm Hg), or the type of prosthesis used. After we discovered a nonobstructive thrombosis, patients were treated with heparin (n = 9) or oral anticoagulation (n = 11). The presence of a localized thrombus was confirmed in 3 patients who were operated on. In the present study, evolution appeared to depend on thrombus size: of 14 patients exhibiting a small (< 5 mm) thrombus, 10 had an uneventful course, whereas 5 of 6 patients with a large (> or = 5 mm) thrombus developed complications or died.

CONCLUSIONS

Transesophageal echocardiography appears to be a reliable method to diagnose thrombi on a mechanical mitral valve prosthesis, even when transthoracic Doppler echocardiographic parameters appear to be normal. Transesophageal echo assessment of thrombus size may be helpful in deciding whether a patient with mitral prosthesis should be treated by anticoagulation, thrombolysis, or valve rereplacement.

Detection of acute thrombosis of mitral tilting disk prosthesis by transesophageal echocardiography

Acute thrombosis is a very severe complication in the replacement of mechanical prostheses and most often fatal if immediate treatment is not implemented. We describe a case in which an acute thrombosis of a mitral tilting disk prosthesis was adequately diagnosed by transesophageal echocardiography and immediate surgical replacement of the dysfunctioning device was possible without catheterizing the patient.

Spontaneously appearing microbubbles associated with prosthetic cardiac valves detected by transesophageal echocardiography

During transesophageal echocardiographic studies in patients with prosthetic cardiac valves, we have noticed the presence of bright, highly mobile echoes (termed microbubbles because of their similarity to saline contrast bubbles) that move rapidly away from the valve. These echoes are distinct from spontaneous echo contrast. The purpose of this study was to determine the frequency with which microbubbles are detected in association with prosthetic valves. No microbubbles were detected in association with bioprosthetic valves. Thus analysis was limited to 198 studies performed in 138 patients with left-sided mechanical valves. Microbubbles were detected in 69 (35%) studies. No differences were found between studies with and without microbubbles with regard to patient characteristics or the indication for the study. Among the 173 individual valves studied, microbubbles were more frequently detected in association with mitral valves (34 of 82, 41%) versus aortic valves (14 of 91, 15%; p 0.0001). Microbubbles were more frequently detected with disc-type mitral valves (30 of 42, 17%) in comparison to Starr-Edwards mitral valves (4 of 39, 10%; p 0.0001). Microbubbles were more often detected with abnormal valves (28 of 76, 37%) than normal valves (20 of 97, 21%), p 0.02. We conclude that bright, highly mobile echoes (microbubbles) distinct from spontaneous echo contrast are frequently detected during transesophageal echocardiography in association with mechanical prosthetic valves. Although the cause and clinical significance of these microbubbles are unknown, it is important to distinguish these echoes from valvular masses such as thrombus or vegetations.

Echocardiographic assessment of artificial heart valves: British Society of Echocardiography position paper

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Regurgitant flow of mitral valve prostheses: an intraoperative transesophageal echocardiographic study

To assess the regurgitant characteristics of mitral biologic and mechanical prostheses immediately after implantation, intraoperative transesophageal echocardiography was performed in 27 patients, aged 32 to 69 years, undergoing open-heart surgery for rheumatic heart disease (n = 19), mitral valve prolapse (n = 3), malfunctioning prostheses (n = 3), or periprosthetic leaks (n = 2). The prostheses included 13 biologic (Carpentier-Edwards) and 14 mechanical valves (five Starr-Edwards, five Medtronic-Hall, and four Bjork-Shiley). Physiologic transvalvular regurgitant flow was detected in both biologic and mechanical prostheses. The spatial extent of the regurgitant jets was usually greater in the mechanical than in the biologic valves, and systolic jets, characteristic of each type of valve, were visualized consistently. Trivial periprosthetic jets (PPJs) were observed in many implanted valves (14/27). The median maximal jet area was 0.46 cm2 (range 0.1 to 1.5 cm2). Cardiopulmonary bypass was reinstituted in two patients. In one patient a PPJ was judged extensive enough (area 3.6 cm2) to warrant surgical revision of the implant, but no dehiscence was found. In the other patient a turbulent PPJ (area 5.5 cm2) was associated with a 0.5 cm dehiscence at the surgical inspection. In conclusion, (1) all mitral prostheses exhibit physiologic transvalvular regurgitation, (2) trivial mitral PPJ is a common finding in newly implanted mitral valves and does not require the revision of the implant, and (3) further experience based on larger series of patients is required to determine the maximal acceptable size of a mitral PPJ detected by intraoperative transesophageal echocardiography.

Intraoperative echocardiography: techniques and current applications

Intraoperative echocardiography has opened new dimensions in the evaluation of cardiac surgical patients, and has contributed significantly to the improved surgical results. The use of intraoperative echocardiography has enabled the surgeon to image the working heart, to define the operative anatomy and physiology, and to evaluate the results immediately before the patients leave the operating room. This review summarizes the current status and applications of intraoperative echocardiography in the practice of cardiac surgery.

Usefulness of transesophageal echocardiography in the treatment of critically ill patients

To evaluate the usefulness of transesophageal echocardiography (TEE) in the treatment of critically ill patients, 80 patients (51 male and 29 female; mean age, 53 years) undergoing both transthoracic echocardiography (TTE) and TEE were studied in a 2-year period. Of these, 48 patients were studied in the ICU, while the other 32 patients were directly referred from the emergency departments. Indications for the study included suspected aortic dissection (34 patients), hemodynamic instability (22 patients), suspected cardiac source of embolism (11 patients), evaluation of the severity of mitral regurgitation (7 patients), and suspected infective endocarditis (6 patients). The probe was passed successfully in 78 of 80 attempts (98 percent). No significant complications were recorded during the transesophageal echocardiographic study. Transesophageal echocardiography provided critical information that was not obtained by TTE in 39 of 78 studies (50 percent, p < 0.005). Cardiac surgery was prompted by TEE findings in 14 patients (18 percent) and these findings were all confirmed at operation. Transesophageal echocardiography was a safe, well-tolerated, and valuable diagnostic approach for the rapid detection of specific cardiac abnormalities in patients with critical illness; TEE should be considered in the treatment of critically ill patients especially when TTE provided inadequate information.

Transesophageal echocardiography for prosthetic valve evaluation: is it always necessary?

Transesophageal echocardiography provides excellent images of mitral and aortic prostheses, due to its higher resolution and less interference from other cardiac structures. Since the introduction of transesophageal echocardiography, it has proved to be valuable for the evaluation of prosthetic heart valves. "Classic" indications include the evaluation of prosthetic valve endocarditis, the search for paravalvular abscesses, and the assessment of regurgitation in mechanical, especially mitral, prostheses. New technical developments such as biplane or multiplane transducers enhance the diagnostic capabilities of the technique. Transesophageal echocardiography can also have advantages in recognizing obstructive lesions such as thrombi or pannus, and help understand the complex morphology of prostheses in grafts. Transesophageal echocardiography should be considered a backup technique whenever transthoracic echocardiography, which always remains the primary approach, is inconclusive or negative in the presence of clinically suspected prosthetic valve malfunction.

Outpatient transesophageal echocardiography with intravenous propofol anesthesia in children and adolescents

Outpatient transesophageal echocardiography (TEE) was performed in 10 children and adolescents (aged 3 to 19.5 years, mean 13.5 years; weight 12 to 91 kg, mean 49 kg), including two with Down's syndrome and one with autism, for diagnostic evaluation of issues unresolved by transthoracic echo examination (TTE). Issues for TEE: evaluation for atrial septal defect (two patients); anatomy of left ventricular outflow tract obstruction (one patient); aortic valve anatomy before valvuloplasty for insufficiency (one patient); evaluation for cause of cyanosis after Fontan operation (one patient); determination of source of high-velocity intracardiac turbulence after atrioventricular septal defect repair (one patient); rule out cardiac embolic source in patient with stroke (one patient); evaluate prosthetic valve function and rule out thrombus (one patient); determination of anatomic relationship of mitral valve to a ventricular septal defect before surgery for complex cyanotic heart disease (one patient); and evaluation for aortic dissection in Marfan's syndrome (one patient). Intravenous propofol anesthesia administered without endotracheal intubation by an anesthesiologist allowed successful outpatient TEE in nine patients; midazolam-conscious sedation was used in one. Outpatient TEE resolved diagnostic issues in all patients without complication, thereby avoiding cardiac catheterization in six patients and supplementing catheterization for preoperative planning in four patients. TEE can be performed safely and effectively with propofol anesthesia in the outpatient setting in carefully selected children and adolescents to provide vital diagnostic information. However, given the invasive nature of the procedure and the use of anesthesia, outpatient pediatric TEE should be used judiciously.

Echocardiographic description of the CarboMedics bileaflet prosthetic heart valve

OBJECTIVES

The aim of this study was to describe the echocardiographic appearance of the normal CarboMedics prosthesis in the aortic and mitral positions.

BACKGROUND

Echocardiography is the standard method of assessing prosthetic valves. However, new valve designs may still be marketed without an accompanying echocardiographic description. The CarboMedics prosthesis is in widespread use, but few noninvasive hemodynamic data have been published.

METHODS

Echocardiography was performed in 147 patients with a total of 96 normally functioning CarboMedics prostheses in the aortic position and 75 in the mitral position; in 24 patients, valves were implanted in both positions. The following variables were measured: peak and mean transvalvular velocities, peak and mean instantaneous gradient estimated from the modified Bernoulli equation, aortic acceleration slope, pressure half-time, transvalvular flow and effective orifice area using the continuity equation. Patterns of regurgitation were observed by transthoracic study in all valves and by transesophageal study in selected mitral valve prostheses.

RESULTS

For the aortic valve prostheses, estimated mean gradient ranged between 6 and 19 mm Hg. Effective area differed markedly among the anulus diameters (p < 0.001), with a mean value of 1 cm2 for the 19-mm valve and 2.6 cm2 for the 29-mm valve. For the mitral valve prostheses, mean gradient ranged from 3 to 7 mm Hg. There were a total of four washing leaks, one on either side of each pivotal point, and these lasted throughout systole or diastole. One jet was commonly more prominent than the other three.

CONCLUSIONS

The CarboMedics prosthesis offered relatively little resistance to forward flow except at small anulus diameters. The washing jets were prominent and would be easy to misdiagnose as a sign of paraprosthetic regurgitation.

Value and limitations of transesophageal echocardiography in the evaluation of aortic prostheses

Results of 34 transesophageal (TEE) studies in patients with suspected aortic prosthetic dysfunction were compared with transthoracic echocardiographic (TTE) results and to anatomic findings. Mass lesions noted at surgery (autopsy) were correctly described in 93% by TEE versus 43% by TTE. Abscesses were detected in 88% by TEE versus 18% by TTE. Bioprosthetic degeneration was visualized in 88% versus 38% and prosthetic obstruction correctly identified in 75% versus 50% by TEE and TTE, respectively. Anatomic aortic regurgitant lesions were identified in 96% by TEE versus 77% by TTE, whereas the correct origin was detected in 88% of cases by TEE versus 54% of cases by TTE. TEE provides valuable additional information on morphologic conditions and flow pathology in aortic valve prostheses.

Comparison of transthoracic and transesophageal echocardiography in evaluation of 47 Starr-Edwards prosthetic valves

OBJECTIVES

Our objectives were to characterize by transesophageal echocardiography the normal appearance of the Starr-Edwards prosthetic heart valve and to compare the utility of transesophageal and transthoracic echocardiography in detection of valve abnormality.

BACKGROUND

The Starr-Edwards prosthetic heart valve, the first mechanical valve to be used, has demonstrated excellent durability.

METHODS

Fifty transthoracic and transesophageal echocardiographic studies on 37 patients with 47 Starr-Edwards prosthetic valves were analyzed retrospectively. Six cases of surgically confirmed infective endocarditis were studied.

RESULTS

Vegetation or abscess formation, or both, was identified by transesophageal echocardiography in all six cases of infective endocarditis but was found in only one of these cases by transthoracic echocardiography. Thrombus was detected by transesophageal echocardiography in 9 of 11 patients with transient ischemic attacks or stroke and in 2 patients by transthoracic echocardiography with 3 confirmed at surgery. In 26 of the 30 patients with a mitral Starr-Edwards valve, the valve demonstrated a trivial or mild "closing volume" early systolic or holosystolic leak on transesophageal echocardiography alone. Transthoracic evaluation identified significant mitral regurgitation in six of the eight patients who had this finding on transesophageal echocardiography. Serial studies were performed to assess response to treatment or need for surgical intervention in eight patients. Seventeen valves have been implanted for 12 years; six of these had significant leakage without apparent cause, a finding not observed more recently implanted valves.

CONCLUSIONS

These observations demonstrated the unique utility of transesophageal echocardiography in patients with Starr-Edwards prosthetic valve dysfunction, endocarditis or thrombus formation, and of the clear superiority of transesophageal echocardiography over transthoracic echocardiography in these situations.

Transesophageal color Doppler flow imaging in the evaluation of prosthetic cardiac valves

To determine the value of transesophageal echocardiography in the assessment of prosthetic cardiac valves, 11 patients with clinically suspected cardiac prosthetic valve dysfunction were studied by transesophageal two-dimensional imaging, as well as by color Doppler flow mapping. Among these 11 patients, there were 10 with biological tissue valves and 3 with metallic valves (1 Bjork-Shiley, 2 St. Jude). Nine patients had replacement of mitral valves alone. The remaining two had received both mitral and aortic prostheses. The degree of mitral regurgitation was graded by transesophageal color Doppler according to the area of the regurgitant jet visualized. The degree of aortic regurgitation was graded by the jet height/left ventricular outflow height ratio method. All transesophageal studies were performed without complication and all were well tolerated. The pathological morphology of the cardiac prosthesis was clearly visualized by transesophageal two-dimensional imaging and subsequently proven at surgery. Of those tested, one patient was found to have a torn leaflet, one had a dislodged leaflet, one patient had paravalvular leakage, four had cusp vegetations, and five patients had prosthetic degeneration for other reasons. Mitral regurgitation was graded as absent in one patient, mild in two patients, moderate in two patients, and severe in six patients. Aortic regurgitation was graded as severe in both patients with aortic prostheses. We conclude that in patients with clinically suspected cardiac prosthetic dysfunction, transesophageal two-dimensional imaging combined with color Doppler can provide reliable information that corresponds to surgical findings.

Mobile strands are frequently attached to the St. Jude Medical mitral valve prosthesis as assessed by two-dimensional transesophageal echocardiography

Two-dimensional transesophageal echocardiographic findings are reported in 13 patients with structurally and functionally normal St. Jude Medical bileaflet mitral valve prostheses. Multiple mobile linear echogenic densities attached to the pivot of the prosthesis were present in 9 of 13 patients. These densities may represent fibrin strands. These mobile strands alternatively resolve and reform over a period of 5 to 14 months after mitral valve replacement. No adverse clinical events were attributable to these prosthetic mitral valve strands. We conclude that mobile stands are frequently attached to the structurally and functionally normal St. Jude Medical mitral valve prosthesis. The clinical significance of these prosthetic mitral valve strands requires clarification.

Single-plane transesophageal echocardiography for assessing function of mechanical or bioprosthetic valves in the aortic valve position

To assess the value and limitations of single-plane transesophageal echocardiography in the evaluation of prosthetic aortic valve function, 89 patients (69 mechanical and 20 bioprosthetic aortic valves) were studied by combined transthoracic and transesophageal 2-dimensional and color flow Doppler echocardiography. In the assessment of aortic regurgitation, the transthoracic and transesophageal echocardiographic findings were concordant in 71 of 89 patients (80%). In 8 patients, the degree of aortic regurgitation was underestimated by the transthoracic approach; in each case the quality of the transthoracic echocardiogram was poor. In 10 patients, transesophageal echocardiography failed to detect trivial aortic regurgitation due to acoustic shadowing of the left ventricular outflow tract from a mechanical valve in the mitral valve position. Transesophageal echocardiography was superior to transthoracic echocardiography in diagnosing perivalvular abscess, subaortic perforation, valvular dehiscence, torn or thickened bioprosthetic aortic valve cusps, and in clearly distinguishing perivalvular from valvular aortic regurgitation. Transesophageal echocardiography correctly diagnosed bioprosthetic valve obstruction in 1 patient, but failed to diagnose mechanical valve obstruction in another. In conclusion, transesophageal echocardiography offers no advantage over the transthoracic approach in the detection and quantification of prosthetic aortic regurgitation unless the transthoracic image quality is poor. Transesophageal echocardiography is limited in detecting mechanical valve obstruction and in detecting aortic regurgitation in the presence of a mechanical prosthesis in the mitral valve position. However, it is superior to transthoracic echocardiography in identifying perivalvular pathology, differentiating perivalvular from valvular regurgitation and in defining the anatomic abnormality responsible for the prosthetic valve dysfunction. Combined transthoracic and transesophageal examination provides complete anatomic and hemodynamic assessment of prosthetic aortic valve function.

Transesophageal echocardiography

Transesophageal echocardiography has provided a new acoustic window to the heart, the great vessels, and the mediastinum. It provides anatomical, functional hemodynamic, and blood flow information. High-quality visualization of left atrial appendage, thoracic aorta, atrial septum, and mitral valvular apparatus can be obtained readily. We discuss historical and technical aspects of transesophageal echocardiography, anatomical views, and major clinical indications for this procedure. These indications include intracardiac masses, thoracic aortic dissection, endocarditis, prosthetic and native cardiac valve function assessment, as well as its value in the detection of intracardiac source of systemic emboli. Furthermore, the role of transesophageal echocardiography in the assessment of coronary artery and congenital heart disease and as an intraoperative diagnostic and monitoring technique is discussed.

Evaluation of the severity of mitral regurgitation by transesophageal Doppler flow echocardiography

The severity of mitral regurgitation (MR) was assessed by transesophageal Doppler flow echocardiography (TEE) using new criteria in 87 patients. The severity of MR assessed by TEE (TEE-MR) was compared with that obtained by left ventriculography (LVG-MR). Although the severity of TEE-MR has been evaluated by MR jet area, it is often difficult because the MR jet extends beyond a single frame image in severe MR. We found that (1) when the MR area was larger than 3 cm2, the severity of MR was more than second-degree by LVG; (2) there was systolic turning flow (STF) of the MR jet within the left atrial cavity in 27 of 30 patients with third- and fourth-degree LVG-MR; and (3) there was late systolic backward flow (SBF) in the pulmonary veins in 9 of 10 patients with fourth-degree LVG-MR. A new grading of the severity of MR by TEE was proposed, which combined the findings of STF, SBF, and the MR area. These new criteria of the severity of TEE-MR excellently correlated with that by LVG (y = 0.94x + 0.08; r = 0.95, p less than 0.01). The criteria for MR by TEE were also useful for assessment of MR in patients with prosthetic mitral valve dysfunction (y = 0.96x + 0.04; r = 0.97, p less than 0.01). We conclude from this study that the severity of MR can be accurately assessed with TEE by measuring the MR area and the specific flow patterns in the left atrium and pulmonary veins.

Quantification of mitral regurgitation: a comparison of transesophageal echocardiography and contrast ventriculography

Transesophageal echocardiography (TEE) allows an unobscured view of the left atrium for the assessment of mitral regurgitation (MR). However, criteria for assessing MR by TEE have not been carefully validated. In order to determine and validate criteria for the assessment of MR severity, 65 clinically stable patients with a TEE color Doppler study and contrast ventriculography within a 2-week period were identified. Maximal or peak mitral regurgitation jet area to left atrial area ratio (MR/LA) derived solely from TEE imaging had the best correlation to MR severity by contrast ventriculography (r = 0.89). Utilizing MR jet area without correction for LA size resulted in r = 0.72 to 0.75. Utilizing LA area data from transthoracic echocardiograms in a subset of 29 patients resulted in r = 0.77. Best sensitivity and specificity for the assessment of MR by TEE were obtained using the following criteria: Peak MR/LA of 0%-9% predicts 0 + MR; 10%-28% 1 + MR; 29%-54% 2 + to 3 + MR; and greater than 55% 4 + MR. Best sensitivity and specificity occurs for assessment of 0 + and 4 + MR. Considerable overlap in data occurs in the 1 + and 3 + MR range utilizing the above stated criteria. Peak MR/LA ratio derived from a single TEE view in which the MR jet is maximally imaged is the best determinant of MR severity.