Color Doppler diagnosis of mechanical prosthetic mitral regurgitation: usefulness of the flow convergence region proximal to the regurgitant orifice

Directional Doppler in Cardiology: A 50-Year Journey

In 1968, while cardiologists were focused on cardiac structures imaged by ultrasound, Daniel Kalmanson in Paris, France, devised a new ultrasonic modality, directional continuous-wave Doppler, enabling him to record instantaneous cardiovascular blood flow velocities with recognition of their direction (relative to the transducer) in vessels. An innovative presentation of Doppler data also made velocity traces physiologically understandable. Following the noninvasive study of the arterial and venous beds, flow velocity in the right (1969) and left (1970) cardiac chambers was studied by means of a directional Doppler catheter. The curtain was then raised for the renewal of our pathophysiologic understanding of cardiac dynamics and the adoption of a new methodology. Technological evolution paved the way for clever researchers to pioneer important advances, diversifying the technique. Guided by the early principles, which are still valid in 2018, directional Doppler finally gained acceptance from the entire scientific community.

Paravalvular leaks: mechanisms, diagnosis and management

AIMS

To understand the incidence, aetiology and mechanisms of paravalvular aortic and mitral leaks after valvular surgery; reviewing the best methods for diagnosis, procedural guidance and result assessment of these leaks, as well as describing the different approaches to their treatment.

METHODS AND RESULTS

A literature search was undertaken as well as an in-depth analysis of our own experience concerning different imaging modalities and various therapeutic strategies for aortic and mitral paravalvular leaks. The majority of patients were diagnosed using two- or three-dimensional transoesophageal echocardiography, useful in both guiding the procedure as well as assessing the procedural results. Haemoglobin, haematocrit, LDH and haptoglobin values were analysed to assess haemolysis. Procedural success for percutaneous closure of paravalvular aortic leaks are around 90% in the different series, with low complication rates. Mitral leaks have been approached by transfemoral and transapical access; the reported success of this procedure ranges from 75% to more than 90% in different reports. Complication rates at 30 days average 10% and mortality related to the procedure is around 1%. Late follow-up results depend on the initial anatomy, baseline clinical class and procedure results.

CONCLUSIONS

Paravalvular leaks after surgical valve implantation have a multifactorial aetiology, but are mainly related to specific anatomic characteristics of the valvular ring. Mitral leaks are three times more common than aortic leaks and the incidence increases after reoperation. Different percutaneous techniques with several devices have been explored for leak closure, but we are still lacking devices specifically designed to treat this pathology more effectively.

Identification and quantification of prosthetic mitral regurgitation by flow convergence method using transthoracic approach

The present case report illustrates the clinical applicability of the proximal isovelocity surface area (PISA) method in identifying, locating and assessing paravalvular prosthetic mitral regurgitation by transthoracic echocardiography.

Accuracy of the flow convergence method for quantification of aortic regurgitation in patients with central versus eccentric jets

Proximal isovelocity surface area (PISA) has been proposed as a quantitative method to assess the severity of aortic regurgitation (AR). Yet the accuracy of this method in patients with eccentric AR jets is unknown. The aims of this study were to compare the accuracy of the PISA method for the quantification of AR severity in patients with central versus eccentric AR jets and to verify whether imaging from the left parasternal instead of the apical window improves the accuracy of the PISA method in patients with eccentric jets. Fifty patients with AR (21 with central jets and 29 with eccentric jets) underwent PISA and phase-contrast cardiac magnetic resonance (CMR) measurements of AR volume. In patients with eccentric AR jets, PISA measurements obtained from the left parasternal and apical windows were compared. In patients with central AR jets, CMR- and PISA-derived AR volumes were similar (28 +/- 19 vs 30 +/- 20 ml, p = 0.34), were strongly correlated (r = 0.92, p <0.0001), and differed minimally from each other (by 2 +/- 8 ml). In patients with eccentric AR jets, PISA-derived AR volumes underestimated those measured by CMR (38 +/- 22 vs 51 +/- 27 ml, bias -13 +/- 20 ml) and were correlated only fairly (r = 0.69, p <0.001). Imaging from the left parasternal window eliminated the differences between CMR- and PISA-derived AR volumes (51 +/- 27 vs 53 +/- 26 ml, p = 0.24) and improved the correlation between the 2 measures (r = 0.95). In conclusion, in patients with eccentric AR jets imaged from the apical window, the PISA method significantly underestimated AR severity. This was no longer the case when imaging was performed from the left parasternal instead of the apical window.

Percutaneous closure of prosthetic paravalvular leaks: Case series and review

BACKGROUND

Paravalvular leaks (PVLs) are a well-recognized complication of prosthetic valve replacement. Most are asymptomatic and benign, but some may cause symptoms due to a large regurgitant volume or hemolysis. Medical therapy is palliative, while reoperation carries significant morbidity and mortality. Percutaneous transcatheter closure techniques, now routinely applied in the management of pathological cardiac and vascular communications, may be adaptable to PVL closure, potentially offer symptomatic relief.

METHODS

We reviewed our experience with attempted percutaneous closure of PVLs, using data from medical and procedural records.

RESULTS

Between 2001 and 2004, 14 procedures were performed in 10 patients, all under general anesthesia, with transesophageal and radiographic guidance. Mitral (9) and aortic (1) valve replacements were involved, both mechanical and bioprosthetic. A variety of devices were used, including atrial septal occluders, patent ductus arteriosus occluders, and coils (all of label use). Six had a single procedure, which was technically successful in four: in two, the PVL could not be crossed. Four underwent a second procedure, which was technically successful in three; in one the previously deployed device was dislodged necessitating urgent, but ultimately uneventful, surgical removal and leak repair. One patient had transient severe hemolysis, which resolved after 1 week. At 1-year follow-up (9/10 pts) three had died, five had sustained symptomatic improvement while 1 patient with a residual leak still required regular blood transfusions.

CONCLUSIONS

Percutaneous closure of PVLs is time-consuming but feasible in selected patients, with a reasonable degree of technical and clinical success. A second procedure may be necessary and a variety of complications can occur.

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.

Assessment of severity of mechanical prosthetic mitral regurgitation by transoesophageal echocardiography

OBJECTIVE

To evaluate the ability of colour Doppler transoesophageal echocardiography (TOE) to assess quantitatively prosthetic mitral valve insufficiency.

METHODS

47 patients were studied with multiplane TOE and cardiac catheterisation. Proximal jet diameter was measured as the largest diameter of the vena contracta. Regurgitant area was measured by planimetry of the largest turbulent jet during systole. Flow convergence zone was considered to be present when a localised area of increased systolic velocities was apparent on the left ventricular side of the valve prosthesis. Pulmonary vein flow velocity was measured at peak systole and diastole.

RESULTS

Mean (SD) proximal jet diameter was 0.63 (0.16) cm, with good correlation with angiographic grades (r = 0.83). Mean (SD) maximum colour jet area was 7.9 (2.5) cm2 (r = 0.69) with worse correlation if a single imaging plane was used for measurements (r = 0.62). The ratio of systolic to diastolic peak pulmonary flow velocity averaged 0.7 (1.3) cm (r = -0.66) with better correlation (r = -0.71) if patients with atrial fibrillation were excluded. Mean (SD) regurgitant flow rate was 168 (135) ml/s and regurgitant orifice area was 0.56 (0.43) cm2, with good correlation with angiography (r = 0.77 and r = 0.78, respectively).

CONCLUSIONS

TOE correctly identified angiographically severe prosthetic mitral regurgitation, mainly by the assessment of the flow convergence region and the proximal diameter of the regurgitant jet.

Intraoperative echocardiographic detection of regurgitant jets after valve replacement

BACKGROUND

Paravalvular jets, documented by intraoperative transesophageal echocardiography, have prompted immediate valve explantation by others, yet the significance of these jets is unknown.

METHODS

Twenty-seven patients had intraoperative transesophageal two-dimensional color Doppler echocardiography, performed to assess the number and area of regurgitant jets after valve replacement, before and after protamine. Patients were grouped by first time versus redo operation, valve position and type.

RESULTS

Before protamine, 55 jets were identified (2.04+/-1.4 per patient) versus 29 jets after (1.07+/-1.2 per patient, p = 0.0002). Total jet area improved from 2.0+/-2.2 cm2 to 0.86+/-1.7 cm2 with protamine (p<0.0001). In all patients jet area decreased (average decrease, 70.7%+/-27.0%). First time and redo operations had similar improvements in jet number and area (both p>0.6). Furthermore, mitral and mechanical valves each had more jets and overall greater jet area when compared to aortic and tissue valves, respectively.

CONCLUSIONS

Following valve replacement, multiple jets are detected by intraoperative transesophageal echocardiography. They are more common and larger in the mitral position and with mechanical valves. Improvement occurs with reversal of anticoagulation.

Application of the proximal flow convergence method to calculate the effective regurgitant orifice area in aortic regurgitation

OBJECTIVES

We sought to determine the reliability of the proximal isovelocity surface area (PISA) method for calculation of effective regurgitant orifice (ERO) of aortic regurgitation (AR).

BACKGROUND

The ERO area can be calculated by the PISA method, but this method has not been validated in AR.

METHODS

ERO calculation by the PISA method was undertaken prospectively in 71 consecutive patients with isolated AR and achieved in 64 and compared with two simultaneous reference methods (quantitative Doppler and quantitative two-dimensional echocardiography). In addition, this method was compared with angiography in 12 patients, with surgical assessment in 18 patients and with ventricular volumes in all patients.

RESULTS

Good correlations between PISA and reference methods were obtained (both r=0.90, both p < 0.0001), but a trend toward underestimation of the ERO by the PISA method was noted (24+/-19 vs. 26+/-22 mm2 and 27+/-23 mm2, respectively, both p=0.04). However, this trend was confined to five patients with an obtuse flow convergence angle (>220 degrees), and on multivariate analysis this variable was the only independent determinant of underestimation of the ERO. In contrast, in 59 patients with a flat flow convergence (< or =220 degrees ), the PISA method, in comparison with reference methods, showed excellent correlations, with a narrow standard error of the estimate (r=0.95, SEE 5.4 mm2, and r=0.95, SEE 5.8 mm2; all p < 0.0001) and no trend toward underestimation (22+/-18 vs. 23+/-16 mm2, p=0.44, and vs. 23+/-18 mm2, p=0.34).

CONCLUSIONS

In patients with AR, the PISA method can be used to measure the ERO with reasonable feasibility. Underestimation of the ERO by PISA may occur in patients with an obtuse flow convergence angle. However, in most patients with appropriate flow convergence, PISA provides reliable measurement of the ERO of AR.

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.

Aliasing-tolerant color Doppler quantification of regurgitant jets

Conservation of momentum transfer in regurgitant cardiac jets can be used to calculate the flow rate from color Doppler velocities. In this study, turbulent jets were simulated by finite elements; pseudocolor Doppler images were interpolated from the computations, with aliasing introduced artificially. Jets were also imaged by color Doppler in an in vitro flow system. To suppress aliasing errors, jet velocities were fitted iteratively to a fluid mechanical model constrained to match the orifice velocity (measured without aliasing by continuous-wave Doppler). At each iteration, the model was used to detect aliased velocities, which were excluded during the next iteration. Iteration continued until the flow rate calculated by the model and number of calculated nonaliased pixels were unchanged. The good correlations between measured and calculated flow rates in the experimental (R2 = 0.933) and computational studies (R2 = 0.990) suggest that this may be a clinically useful approach even in aliased images. Published by Elsevier Science Inc.

Usefulness of color Doppler proximal isovelocity surface area method in quantitating valvular regurgitation

To define the clinical utility of the color Doppler proximal isovelocity surface area (PISA) method for estimating regurgitant stroke volume (SV), 160 regurgitant lesions were evaluated in 104 patients with mitral (MR), aortic (AR), and tricuspid (TR) regurgitation. Regurgitant SV by PISA was calculated as 2 pi R2 x V x (time-velocity integral/peak flow velocity), where R is the radius corresponding to the first blue-red interface velocity of the maximal PISA during the cardiac cycle. The time-velocity integral and peak flow velocity from the continuous-wave Doppler recording of the regurgitant jet were used to correct PISA for phasic variations in regurgitant flow. Fifteen lesions were excluded because of difficulty in tracing the continuous-wave Doppler regurgitant curve. Among 145 remaining regurgitant lesions, PISA was measurable in 50 (78%) of 64 cases of MR and 24 (69%) of 35 cases of TR but in only 12 (26%) of 46 cases of AR (p < 0.001). Regurgitant SV by PISA correlated modestly well with jet area/atrial area in all atrioventricular valve lesions (MR: r = 0.55; TR: r = 0.65; p < 0.001). However, the correlation improved if only central jets were considered (MR: r = 0.70; TR; r = 0.75; p < 0.001). These findings are not unexpected because jet area/atrial area underestimates the true severity of regurgitation in cases of eccentric (wall-impinging) jets. PISA was detected in all severe cases of regurgitation but in only 64% of cases of mild MR, 45% of cases of mild TR, and 6% of cases of mild AR (p < 0.01). The color Doppler PISA method is clinically useful in estimating regurgitant SV in MR and TR, including mild cases, but is less useful in AR.

Determination of regurgitant flow and volume by integrating actual proximal velocities over hemispheres (IPROV) in two orthogonal planes

The proximal acceleration technique is a promising technique for quantification of regurgitant valve flow. Although the shape of the regurgitant proximal isovelocity field has been shown to vary with orifice size, geometry, and driving pressure, normally the centerline velocity alone is used for estimation of flow. In this model study of pulsatile flow, two-dimensional and spectral Doppler data were transferred digitally to a computer in which proximal velocity fields were corrected for time and angle errors. With the purpose of improving accuracy, flow was estimated by integrating proximal velocities over nonisovelocity spheric control surfaces in the best zone of measurement (0.15 to 0.45 m/sec at an angle up to +/- 45 degrees from the center line) in two perpendicular planes. Three regurgitant volumes in the range of 5 to 21 ml were studied for circular (diameters of 4, 6, and 8 mm), crescent, and diagonal orifices. The quotient between effective orifice area, estimated by dividing peak flow with peak velocity in the vena contracta, and true orifice area (Aeff = Q(tm)/Vo(tm)) was 0.66 (range 0.60 to 0.79), 0.50 (0.48 to 0.52), and 0.67 (0.66 to 0.68) for the circular, crescent, and diagonal orifices, respectively. Regurgitant volume estimated by multiplying effective orifice area by the velocity-time integral in the vena contracta (V = Aeff.velocity-time integral) ranged from 92% to 115% of the true volume for the circular, 89% to 92% for the crescent, and 105% to 112% for the diagonal orifices, respectively. It is possible to calculate regurgitant volume correctly with data acquisition from multiple hemispheres and planes and postprocessing of data. This amendment of the proximal acceleration technique has great advantage over the center-line method, especially when the orifice is asymmetric.

Current status of flow convergence for clinical applications: is it a leaning tower of "PISA"?

Spatial appreciation of flow velocities using Doppler color flow mapping has led to quantitative evaluation of the zone of flow convergence proximal to a regurgitant orifice. Based on the theory of conservation of mass, geometric analysis, assuming a series of hemispheric shells of increasing velocity as flow converges on the orifice--the so-called proximal isovelocity surface area (PISA) effect--has yielded methods promising noninvasive measurement of regurgitant flow rate. When combined with conventional Doppler ultrasound to measure orifice velocity, regurgitant orifice area, the major predictor of regurgitation severity, can also be estimated. The high temporal resolution of color M-mode can be used to evaluate dynamic changes in orifice area, as seen in many pathologic conditions, which enhances our appreciation of the pathophysiology of regurgitation. The PISA methodology is potentially applicable to any restrictive orifice and has gained some credibility in the quantitative evaluation of other valve pathology, particularly mitral and tricuspid regurgitation, and in congenital heart disease. Although the current limitations of PISA estimates of regurgitation have tempered its introduction as a valuable clinical tool, considerable efforts in in vitro and clinical research have improved our understanding of the problems and limitations of the PISA methodology and provided a firm platform for continuing research into the accurate quantitative assessment of valve regurgitation and the expanding clinical role of quantitative Doppler color flow mapping.

Dynamic change in mitral regurgitant orifice area: comparison of color Doppler echocardiographic and electromagnetic flowmeter-based methods in a chronic animal model

OBJECTIVES

The aim of the present study was to investigate dynamic changes in the mitral regurgitant orifice using electromagnetic flow probes and flowmeters and the color Doppler flow convergence method.

BACKGROUND

Methods for determining mitral regurgitant orifice areas have been described using flow convergence imaging with a hemispheric isovelocity surface assumption. However, the shape of flow convergence isovelocity surfaces depends on many factors that change during regurgitation.

METHODS

In seven sheep with surgically created mitral regurgitation, 18 hemodynamic states were studied. The aliasing distances of flow convergence were measured at 10 sequential points using two ranges of aliasing velocities (0.20 to 0.32 and 0.56 to 0.72 m/s), and instantaneous flow rates were calculated using the hemispheric assumption. Instantaneous regurgitant areas were determined from the regurgitant flow rates obtained from both electromagnetic flowmeters and flow convergence divided by the corresponding continuous wave velocities.

RESULTS

The regurgitant orifice sizes obtained using the electromagnetic flow method usually increased to maximal size in early to midsystole and then decreased in late systole. Patterns of dynamic changes in orifice area obtained by flow convergence were not the same as those delineated by the electromagnetic flow method. Time-averaged regurgitant orifice areas obtained by flow convergence using lower aliasing velocities overestimated the areas obtained by the electromagnetic flow method ([mean +/- SD] 0.27 +/- 0.14 vs. 0.12 +/- 0.06 cm2, p < 0.001), whereas flow convergence, using higher aliasing velocities, estimated the reference areas more reliably (0.15 +/- 0.06 cm2).

CONCLUSIONS

The electromagnetic flow method studies uniformly demonstrated dynamic change in mitral regurgitant orifice area and suggested limitations of the flow convergence method.

Effective mitral regurgitant orifice area: clinical use and pitfalls of the proximal isovelocity surface area method

OBJECTIVES

We attempted to determine the accuracy and pitfalls of calculating the mitral regurgitant orifice area with the proximal isovelocity surface area method in a clinical series that included patients with valvular prolapse and eccentric jets.

BACKGROUND

The effective regurgitant orifice area, a measure of lesion severity of mitral regurgitation, can be calculated by the proximal isovelocity surface area method, the accuracy and pitfalls of which have not been established.

METHODS

In 119 consecutive patients with isolated mitral regurgitation, effective regurgitant orifice area was measured by the proximal isovelocity surface area method and compared with measurements simultaneously obtained by quantitative Doppler and quantitative two-dimensional echocardiography.

RESULTS

The effective mitral regurgitant orifice area measured by the proximal isovelocity surface area method tended to be overestimated compared with that measured by quantitative Doppler and quantitative two-dimensional echocardiography (38 +/- 39 vs. 36 +/- 33 mm2 [p = 0.09] and 34 +/- 32 mm2 [p = 0.02], respectively). Overestimation was limited to patients with prolapse (61 +/- 43 vs. 56 +/- 35 mm2 [p = 0.05] and 54 +/- 34 mm2 [p = 0.014]) and was restricted to patients with nonoptimal flow convergence (n = 7; 137 +/- 35 vs. 84 +/- 34 mm2 [p = 0.002] and 79 +/- 33 mm2 [p = 0.002]). In patients with optimal flow convergence (n = 112), excellent correlations with both reference methods were obtained (r = 0.97, SEE 6 mm2 and r = 0.97, SEE 7 mm2, p < 0.0001).

CONCLUSIONS

In calculating the mitral effective regurgitant orifice area with the proximal isovelocity surface area method, the observed pitfall (overestimation due to nonoptimal flow convergence) is rare. Otherwise, the method is reliable and can be used clinically in large numbers of patients.

Regurgitant heart valve flow from 2-D proximal velocity field: continued search for the ideal method

It has been suggested that flow through a leaking heart valve can be determined by studying the proximal velocity field. Normally, only the centre-line velocity is studied as a potential method. The aim of the study is to improve this method by using information from the entire reconstructed proximal velocity field. Four methods are compared: use of the centre-line velocity; use of velocities at three different angles; integration of velocities over a hemisphere; and integration of velocities over an estimated hemi-elliptical isovelocity line. Measurements are performed in a hydraulic model with 4, 6 and 8 mm circular orifices, and these are compared with those from computer simulation. From the results presented in the study, it is suggested that the velocities should be integrated over a hemisphere within a best zone. This zone is dependent on the instrument settings, but in this case it is positioned 1.2-1.4 orifice diameters from the orifice inlet, with an angle of up to +/- 45 degrees from the centre axis, and contains velocities in the range 0.15-0.45 ms-1.

Color flow Doppler determination of transmitral flow and orifice area in mitral stenosis: experimental evaluation of the proximal flow-convergence method

To evaluate the in vivo accuracy of color Doppler flow-convergence methods for determining transmitral flow volumes and effective orifice areas in mitral stenosis, we studied two models for flow-convergence surface geometry, a hemispheric (HS) model and an oblate hemispheroid (OH) model in a chronic animal model with quantifiable mitral flows. Color Doppler flow mapping of the proximal flow-convergence region has been reported to be useful for evaluation of intracardiac flows. Flow-convergence methods in patients with mitral stenosis that use HS assumption for the isovelocity surface have resulted in underestimation of actual flows. Chronic mitral stenosis was created surgically in six sheep with annuloplasty rings (group 1) and 11 sheep with bioprosthetic porcine valves (group 2). Hemodynamic and echocardiographic/Doppler studies (n = 18 in group 1; n = 21 in group 2) were performed 20 to 34 weeks later. Left ventricular inflow obstruction was of varied severity, with mean transmitral valve gradients in group 1 ranging from 1.3 to 18 mm Hg and in group 2 ranging from 6.3 to 25.6 mm Hg. Although transmitral flows derived by both geometric flow convergence models showed significant correlations with reference cardiac outputs, the correlations for the OH model were better than those for the HS model (group 1, r = 0.86 for the OH model vs r = 0.72 for the HS model; group 2; r = 0.84 for the OH model vs r = 0.62 for the HS model). The OH model was also superior to the HS model in determining effective orifice areas compared to reference orifice areas determined by postmortem planimetry of anatomic orifices (group 1 only, r = 0.64 for OH vs 0.58 for HS), by the Gorlin and Gorlin formula (group 1, r = 0.63 for OH vs 0.72 for HS; group 2, r = 0.82 for OH vs 0.76 for HS), and by the Doppler pressure half-time method (group 1, r = 0.76 for OH vs 0.69 for HS; group 2, r = 0.84 for OH vs 0.62 for HS).(ABSTRACT TRUNCATED AT 400 WORDS)

Estimation of regurgitant flow volume based on centerline velocity/distance profiles using digital color M-Q Doppler: application to orifices of different shapes

OBJECTIVES

In this study we investigated the centerline velocity profile method for flow computation as applied to noncircular, as well as circular, orifices using digital color flow data.

BACKGROUND

Recently it has been suggested that flow volume through an orifice can be estimated more accurately by computing the axial "centerline" flow velocity/distance profile proximal to the orifice.

METHODS

A total of seven different orifices were mounted in a constant-flow model: four circular orifices, two rectangular orifices with a major/minor axis ratio of 4:1 and 8:1 and an ovoid orifice having a major/minor axis ratio of 2:1. Three different flow rates were examined (1.68, 3.48 and 6.48 liters/min). Digital measurements of flow velocity at discrete positions along the centerline progressing toward the orifice were analyzed to yield complete flow velocity profiles for each orifice at each flow rate.

RESULTS

A clear separation of the flow profiles for the three different flow rates was observed independent of orifice size for all of the circular orifices. The velocity/distance acceleration curves showed highly significant correlations using multiplicative regression fits (y = ax-b, r = 0.94 to 0.99, all p < 0.0001). An equation for quantitatively correlating the a and b coefficients from the multiplicative regression fits with flow rates was derived from stepwise regression analysis: Flow rate = 23a + 3.3b - 1.5 (r = 0.97, p < 0.0001, SEE 0.46 liter/min).

CONCLUSIONS

In view of the various sizes and shapes encountered clinically for regurgitant orifices, the simplicity of this method for the estimation of the severity of regurgitant lesions might be of importance for clinical applications of this method.

Evaluation of mitral regurgitation using a digitally determined color Doppler flow convergence 'centerline' acceleration method. Studies in an animal model with quantified mitral regurgitation

BACKGROUND

The imaging and measurement of the proximal flow convergence region in the left ventricle have been reported to be useful for identifying the site of mitral regurgitation (MR) and for evaluating its severity. However, the application of this method has not gained general acceptance. There have been few in vivo studies with quantified reference standards for determining regurgitant volume, and those that have been reported used spectral Doppler standards and/or nonsimultaneously performed contrast ventriculography. The purpose of the present study was to evaluate the proximal flow convergence centerline velocity-distance profile method applied to chronic MR resulting from flail mitral leaflets in an animal model in which regurgitant flow rates and regurgitant volumes were determined simultaneously with electromagnetic flow probes and flowmeters.

METHODS AND RESULTS

In six sheep, a total of 18 hemodynamically different states were obtained when the animals were restudied 6 months after surgical induction of MR produced by severing chordae tendineae to the anterior (three sheep) or posterior (three sheep) mitral leaflet. Echocardiographic studies with a Vingmed 750 were performed to obtain complete proximal axial flow acceleration velocity-distance profiles for each hemodynamic state. The color Doppler velocity data were directly transferred in digital format from the ultrasound instrumentation to a microcomputer. The severity of MR was assessed by the magnitude of the mitral regurgitant fraction determined using both mitral and aortic electromagnetic flow probes balanced against each other to yield regurgitant volume. MR was classified as grade I when the regurgitant fraction was < 20%, as grade II when it was 20% to 35%, and as grade III to IV when it was > 35%. Thus, of the 18 hemodynamic states, 4 (from two sheep) were grade I, 7 (from five sheep) were grade II, and 7 (from three sheep) were grade III to IV. All of the velocity-distance acceleration curves showed organized acceleration fields with highly significant correlations using multiplicative regression fits (y = a.x-b, r = .90 to .99, all P < .01). Grade III to IV MR resulted in rightward and upward shifts of the velocity-distance profile curves compared with those produced by grade II and grade I MR. All of the centerline velocity-distance profiles for grade III or IV regurgitation resided in a domain encompassed by velocities > 0.5 m/s at distances from the orifice > 0.6 cm; the profiles for grade I regurgitation resided in a domain encompassed by velocities < 0.3 m/s at distances from the orifice of < 0.45 cm. The profiles for grade II regurgitations resided in a domain between them. Regression analysis for the distance at which a velocity of 0.5 m/s was first reached bore a close relation to regurgitant fraction (r = .92, P < .0001) and peak regurgitant flow rate (r = .89, P < .0001). In addition, an equation for quantitatively correlating both a and b (coefficients from the multiplicative regression fits) with the peak regurgitant flow rate (Qpeak in L/min) was derived from stepwise regression analysis: Qpeak = 12a + 2.7b-2.4 (r = .96, P < .0001, SEE = .45 L/min).

CONCLUSIONS

In this study, using quantified MR volume, we demonstrate that the proximal flow convergence axial centerline velocity-distance profile method can be used for evaluating the severity of MR without any assumption about isovelocity surface shape geometry.

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

Images

Determination of the most appropriate velocity threshold for applying hemispheric flow convergence equations to calculate flow rate: selected according to the transorifice pressure gradient. Digital computer analysis of the Doppler color flow convergence region

BACKGROUND

While flow convergence methods have been promising for calculating volume flows from color Doppler images, it appears that the velocity threshold used and the transorifice pressure gradient dramatically influence the accuracy of application of the simple hemispheric flow convergence equation for calculation of flow rate. The present in vitro study was performed to determine whether the value of velocity threshold at which the shape of proximal isovelocity surface best fits given shape assumptions with different orifice sizes and flow rates is predictable as a function independent of orifice size from clinically measurable peak velocity or transorifice pressure gradient information.

METHODS AND RESULTS

In an in vitro model built to facilitate ultrasound imaging, steady flow was driven through circular discrete orifices with diameters of 3.8, 5.5, and 10 mm. Flow rates ranged from 2.88 to 8.28 L/min with corresponding driving pressure gradients from 14 to 263 mm Hg. At each flow rate, Doppler color-encoded M-mode images through the center of the flow convergence region were obtained and transferred into the microcomputer (Macintosh IIci) in their original digital format. Then, the continuous wave Doppler traces of maximal velocity through the orifice were derived for the calculation of driving pressure gradient. Direct numerical spatial velocity measurements were obtained from the digital color encoded M-mode velocities with computer software. For each flow rate, we could calculate flow volume from any number of velocity distance combinations with a number of assumptions and use the results to assess expected flow convergence shape based on a priori knowledge of the progression from oblate hemispheroid to hemisphere to prolate hemispheroid changes observed previously. Our results showed that for a given ratio of calculated flow rate to actual flow rate (0.7 and 1), the velocity threshold that could be used for the calculation of flow rate with a hemispheric flow convergence equation correlated well with the pressure gradient for a given orifice size, and the differences in velocity threshold that could be used this way among different orifice sizes once they were adjusted for the covariate pressure gradients were not statistically significant (P = .79 for ratio = 0.7, and P = .81 for ratio = 1).

CONCLUSIONS

Our present study provides an orifice size-independent quantitative method that can be used to select the most suitable velocity threshold for applying a simple hemispheric flow convergence equation based on clinically predictable pressure gradients ranging from 40 to 200 mm Hg, and it offers a correction factor that can be applied to the hemispheric flow convergence equation when the pressure gradient is less than 40 mm Hg.

Determination of most appropriate Nyquist velocity for applying hemispherical flow convergence equation to calculate flow rate using the transorifice pressure gradient: digital computer analysis of Doppler color flow convergence region

Nyquist velocity and transorifice pressure gradient dramatically influence color aliasing shape and accuracy of simple hemispherical flow convergence equation for calculation of flow rate. The present in vitro study was performed to determine whether the value of Nyquist velocity, at which the shape of proximal isovelocity surface is best fit for a given shape assumption in different orifice size, and the flow rate may be a determinable and orifice size independent function of clinically measurable peak velocity or transorifice pressure gradient. Steady flow was driven through circular discrete orifices with diameter of 3.8 mm, 5.5 mm and 10 mm and flow rate ranging from 2.88 L/min to 8.28 L/min. For every flow rate, Doppler color encoded M-mode images through the center of flow convergence region were transferred into the microcomputer in their original digital format. The continuous wave Doppler traces of maximal velocity through the orifice were performed for the calculation of pressure gradient. Direct numerical spatial velocity measure using color pixel intensity was obtained from the transferred color encoded M-mode images with computer software. The shape of isovelocity surface was determined by the ratio of calculated flow rate with hemispherical flow convergence equation to the actual flow rate. Both the flow rate and orifice size influence the position of the velocity profile curve. The shape of isovelocity surface is not constant and changes with the velocities used for the calculation of flow rates for a given flow rate and orifice size or pressure gradient and also changes with the flow rate or transorifice pressure gradients for a constant Nyquist velocity and orifice size.(ABSTRACT TRUNCATED AT 250 WORDS)

The shape of the proximal isovelocity surface area varies with regurgitant orifice size and distance from orifice: computer simulation and model experiments with color M-mode technique

The hemispheric proximal isovelocity surface area method for quantification of mitral regurgitant flow (i.e., Qc = 2 pi r2v), where 2 pi r2 is the surface area and v is the velocity at radius r, was investigated as distance from the orifice was increased. Computer simulations and steady flow model experiments were performed for orifices of 4, 6, and 8 mm. Flow rates derived from the centerline velocity and hemispheric assumption were compared with true flow rates. Proximal isovelocity surface area shape varied as distance from each orifice was increased and could only be approximated from the hemispheric equation when a certain distance was exceeded: > 7, > 10, and > 12 mm for the 4, 6, and 8 mm orifices, respectively. Prediction of relative error showed that the best radial zone at which to make measurements was 5 to 9, 6 to 14 and 7 to 17 mm for the 4, 6, and 8 mm orifices, respectively. Although effects of a nonhemispheric shape could be compensated for by use of a correction factor, a radius of 8 to 9 mm can be recommended without the use of a correction factor over all orifices studied if a deviation in calculated as compared with true flow of 15% is considered acceptable. These measurements therefore have implications for the technique in clinical practice.

Accuracy of flow convergence estimates of mitral regurgitant flow rates obtained by use of multiple color flow Doppler M-mode aliasing boundaries: an experimental animal study

The proximal flow convergence method of multiplying color Doppler aliasing velocity by flow convergence surface area has yielded a new means of quantifying flow rate by noninvasively derived measurements. Unlike previous methods of visualizing the turbulent jet of mitral regurgitation on color flow Doppler mapping, flow convergence methods are less influenced by machine factors because of the systematic structure of the laminar flow convergence region. However, recent studies have demonstrated that the flow rate calculated from the first aliasing boundary of color flow Doppler imaging is dependent on orifice size, flow rate, aliasing velocity and therefore on the distance from the orifice chosen for measurement. In this study we calculated the regurgitant flow rates acquired by use of multiple proximal aliasing boundaries on color Doppler M-mode traces and assessed the effect of distances of measurement and aliasing velocities on the calculated regurgitant flow rate. Six sheep with surgically induced mitral regurgitation were studied. The distances from the mitral valve leaflet M-mode line to the first, second, and third sequential aliasing boundaries on color Doppler M-mode traces were measured and converted to the regurgitant flow rates calculated by applying the hemispheric flow equation and averaging instantaneous flow rates throughout systole. The flow rates that were calculated from the first, second, and third aliasing boundaries correlated well with the actual regurgitant flow rates (r = 0.91 to 0.96). The mean percentage error from the actual flow rates were 151% for the first aliasing boundary, 7% for the second aliasing boundary, and -43% for the third aliasing boundary; and the association between aliasing velocities and calculated flow rates indicates an inverse relationship, which suggests that in this model, there were limited velocity-distance combinations that fit with a hemispheric assumption for flow convergence geometry. The second aliasing boundary with an aliasing velocity, of 102 cm/sec, (which was achieved by use of a 4 kHz pulse repetition frequency, a 3.75 MHz transducer, and no color baseline shift), provided the closest fit to the actual regurgitant flow rates (r = 0.99; y = 0.95x + 0.07). The averaged calculated flow rates from all aliasing velocities also resulted in excellent correlation (r = 0.97; y = 0.99x + 0.5). A hemispheric flow convergence method that is based on color Doppler M-mode echocardiography is a feasible and automatable method for quantifying mitral regurgitant rate.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of flow convergence with other transthoracic echocardiographic indexes of prosthetic mitral regurgitation

Prosthetic shadowing of the left atrium may prevent detection of mitral regurgitation during transthoracic echocardiography. In 60 patients with mitral valves, Carpentier-Edwards (n = 20), St. Jude (n = 22), and cage-ball (n = 18), we blindly evaluated the accuracy of three transthoracic Doppler signs of significant (> 2+) mitral regurgitation: (1) color Doppler flow convergence, (2) a color Doppler jet of significant regurgitation in the left atrium, and (3) an intense continuous wave Doppler signal. All 60 patients had transesophageal echocardiography, 26 had cardiac catheterization, and 28 had surgery. The sensitivity and specificity of flow convergence for significant regurgitation by transesophageal echocardiography was 73% and 70%, respectively, compared with 33% and 93% for left atrial color Doppler, and 15% and 97% for continuous wave Doppler. The sensitivity of flow convergence in Carpentier-Edwards, St. Jude, and cage-ball valves was 80%, 73%, and 67%, respectively; whereas the sensitivity of left atrial color Doppler was 70%, 27%, and 0%, and the sensitivity of continuous wave Doppler was 33%, 0%, and 13%. Flow convergence was the only sign of significant regurgitation in 12 of 30 patients (40%); 10 of these patients had mechanical valves. We conclude flow convergence is a more sensitive, though less specific, predictor of significant mitral regurgitation than color Doppler, spatial mapping of the left atrium, and continuous wave Doppler, especially when a mechanical valve is present.

Value of acceleration flow signals proximal to the leaking orifice in assessing the severity of prosthetic mitral valve regurgitation

To test the value of acceleration flow signals proximal to the leaking orifice in assessing the severity of prosthetic mitral valve regurgitation, 39 consecutive patients undergoing left ventriculography were examined by Doppler color flow imaging. Acceleration flow signals proximal to the regurgitant orifice were detected in 27 of the 31 patients who had prosthetic mitral regurgitation by left ventriculography (sensitivity 87%). All four patients without acceleration flow signals had mild prosthetic mitral regurgitation by angiography. No acceleration flow signals were detected in any patient without prosthetic regurgitation by left ventriculography (specificity 100%). Individual values of the maximal area of acceleration flow signals obtained from three orthogonal planes in seven patients with mild prosthetic mitral regurgitation by angiography ranged from 0 to 17 mm2 (mean 4 +/- 6). In 8 patients with moderate prosthetic mitral regurgitation by angiography, the maximal area of acceleration flow signals ranged from 21 to 58 mm2 (mean 33 +/- 15), whereas the maximal area of acceleration flow signals in 16 patients with severe prosthetic regurgitation ranged from 20 to 173 mm2 (mean 102 +/- 41). The maximal area of the acceleration flow signals from three planes correlated well with the angiographic grade of prosthetic mitral regurgitation. There was a significant difference in the maximal area of acceleration flow signals between mild and moderate (p less than 0.001), moderate and severe (p less than 0.001) and mild and severe (p less than 0.001) prosthetic mitral regurgitation. Thus, measurement of acceleration flow signals by Doppler color flow imaging is useful in assessing the severity of prosthetic mitral regurgitation.