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European Journal of Echocardiography Advance Access published online on March 18, 2008
European Journal of Echocardiography, doi:10.1093/ejechocard/jen030
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Doppler echocardiographic assessment of TTK Chitra prosthetic heart valve in the mitral position

Narayanan Namboodiri1,*, Othayoth Shajeem1, Jaganmohan A. Tharakan1, R. Sankarkumar2, Thomas Titus1, Ajitkumar Valaparambil1, Sivasubramonian Sivasankaran1, Kavassery Mahadevan Krishnamoorthy1, Sivadasan Pillai Harikrishnan1 and Santosh Kumar Dora1

1 Department of Cardiology, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum 695 011, Kerala, India
2 Department of Cardiothoracic Surgery, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, Kerala, India

* Corresponding author. Tel: + 91 447838258; fax: +91 471 2446433. E-mail address: kknnamboodiri{at}yahoo.co.in


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusions
 References
 
Aims: TTK Chitra heart valve prosthesis (CHVP), a tilting disc mechanical heart valve of low cost and proven efficacy, has been in use for the past 15 years. Although various studies substantiating its long-term safety and efficacy are available, no study had assessed its echocardiographic characteristics. The purpose of this study was, first, to determine the normal Doppler parameters of CHVP in the mitral position and second, to assess whether derivation of mitral valve area (MVA) using the continuity equation and, more commonly used pressure half-time (PHT) method are comparable in the functional assessment of this tilting disc mitral prosthesis.

Methods and results: Doppler echocardiography was performed in 40 consecutive patients with CHVP in mitral position. All patients were clinically stable, without evidence of prosthetic valve dysfunction such as significant obstruction or regurgitation, endocarditis, left ventricular dysfunction (ejection fraction <40%), or significant aortic regurgitation. Valve sizes studied included 25, 27, and 29 mm. Mitral valve area was derived both by the PHT method and the continuity equation, using stroke volume measured in the ventricular outflow tract divided by the time-velocity integral of CHVP jet. The peak Doppler gradient ranged from 5 to 21 (mean 11.0) mm Hg, and the mean gradient ranged from 1.7 to 9.2 (mean 4.1) mm Hg. Mean gradient negatively correlated with increase in actual orifice area (AOA) derived from the valve orifice diameter given by the manufacturer (r = –0.45, P = 0.004). Mitral valve area calculated by both PHT and continuity equation increased significantly with increase in AOA (r = 0.42, P = 0.007 and r = 0.32, P = 0.046, respectively). Mitral valve area by the continuity equation averaged 1.55 ± 0.36 cm2 (range 0.85 cm2 for a 25 mm valve to 2.41 cm2 for a 29 mm valve), and was smaller than by the PHT (mean 2.04 ± 0.41 cm2, range 1.40–3.14 cm2; P = 0.0001; t-test) irrespective of whether the PHT is less than or more than 110 ms.

Conclusion: The Doppler parameters obtained with CHVP in mitral position are comparable to those obtained with the different prosthetic valves in common use. In selected group of patients with CHVP, assessment of MVA by the PHT method is comparable to that by the continuity equation. Areas by both methods were smaller than the AOA provided by the manufacturer, as seen in other similar design valves.

Keywords: Chitra valve; Prosthetic valve; Doppler echocardiography


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusions
 References
 
TTK Chitra heart valve prosthesis (CHVP) is a tilting disc artificial heart valve designed and developed by Sree Chitra Tirunal Institute for Medical Sciences and Technology (SCTIMST).1 It has an ultra high molecular weight polyethylene disc, Haynes-25 alloy (Haynes International Inc., USA) cage and polyester suture ring. Since its first implant on 6 December 1990, more than 15 000 valves have been implanted in various institutions in India. Because of its low cost and proven efficacy, it has a high potential for more wide spread use in developed countries. Although various studies substantiating its long-term safety and efficacy are available,2–4 no study had assessed its echocardiographic characteristics.

Transthoracic Doppler echocardiography has been the predominant tool for the evaluation of prosthetic valve function since early 1980s.5 As the occluder disc is radiolucent, this assumes particular relevance in the evaluation of prosthetic valve dysfunction of CHVP. Doppler echocardiography can provide the information on the gradients across and the mitral valve area (MVA), which are comparable to those obtained at invasive cardiac catheterization.6,7 Two Doppler methods, the pressure half-time (PHT) method proposed by Hatle et al.8 and the method based on the equation of continuity9,10 were used to estimate the stenotic MVA non-invasively, and its accuracy in varying haemodynamic conditions had been studied previously11–20. In general, although more commonly used in clinical practice, method based on the PHT was considered less reliable than the application of continuity equation in the assessment of MVA in mechanical prosthetic valves.17–20

We used the Doppler echocardiographic parameters of the normally functioning CHVP in the mitral position as the reference parameters so as to assess whether derivation of MVA using the continuity equation and the more commonly used PHT method are comparable in the functional assessment of this tilting disc prosthesis in the mitral position.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusions
 References
 
Study population
The study population consisted of 40 consecutive patients with a normally functioning CHVP at mitral position who were subjected to a routine follow-up echocardiographic examination. Those patients with short-term follow up: <3 months of valve replacement; evidence of prosthetic valve dysfunction, such as significant obstruction or regurgitation; endocarditis; left ventricular dysfunction (ejection fraction <40%); significant aortic regurgitation or unsatisfactory echocardiographic windows were not included in this study. Indications for the MVR were rheumatic heart disease in 95% (38 patients) and mitral valve prolapse in 5% (2 patients).

A complete transthoracic echocardiographic examination was performed with Vivid 7 echocardiographic system (GE Vingmed Ultrasound A/S, GE Healthcare, Horten, Norway) in these patients to assess the prosthetic valve function and left ventricular function initially. Two-dimensional and Doppler echocardiographic studies were performed later and parameters were derived. Colour flow Doppler imaging was also done to assess the degree of regurgitation.

Doppler evaluation of mitral prostheses
The following parameters were assessed to evaluate the prosthetic valve in the mitral position; peak velocity, peak gradient, mean gradient, and MVA derived by PHT and the continuity equation.

Flow velocity across the mitral prosthesis was recorded with continuous-wave Doppler guided by colour flow. Measurements were made from the view with the least angulation with flow, most commonly from the apical window. Colour flow Doppler was used in evaluating the direction of flow into the left ventricle and optimizing Doppler recordings of jet velocity. From the tracing of prosthetic inflow velocity, maximal velocity, peak gradient, and mean gradient were measured. Pressure half-time was measured from the tracing of prosthetic inflow velocity as the time required for the peak gradient to be reduced by one-half. Mitral valve area is derived as 220/PHT. Mitral valve area using the continuity equation is derived as stroke volume through the prosthesis divided by the velocity-time integral of the mitral jet velocity. Stroke volume through the mitral valve is substituted for that through the left ventricular outflow. In patients with atrial fibrillation, 10 beats were averaged to obtain the representative measurements. The actual orifice area (AOA) is calculated from the valve orifice diameter (VOD) provided by the manufacturer as {pi}·VOD2/4.

Statistical analysis
All continuous data were reported as mean ± standard deviation. Subgroup analysis was done for each size of the valve implanted. One-way analysis of variance (ANOVA) was used to compare continuous variables between multiple groups. Bivariate correlation analysis was used to assess correlations between each variable and AOA. Discrete variables were compared using the {chi}2 test. A P-value of <0.05 was considered significant. Analysis was performed using SPSS version 14.0 for Windows.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusions
 References
 
Out of the 40 patients studied, 23 had mitral valve replacement alone while the remaining 17 had associated aortic valve replacement also. None had triple valve replacement while 9 had associated tricuspid valve repair. Baseline characteristics of the patients studied are given in Table 1.


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  		Table 1 Baseline characteristics of the patients (total number of patients, n= 40)

 
The sizes of the valves studied were 25, 27, and 29 mm. None of the patients had 23 and 31 mm size valves implanted. An adequate recording of the mitral jet velocity through the prosthetic valve was obtained in all. The normal values for the peak velocity, peak and mean gradients, and MVA by PHT and continuity equation of each valve size are shown in Table 2.


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  		Table 2 Doppler echocardiographic parameters of Chitra valve in mitral position

 
Mean and peak gradients did not show significant correlation with MVA by PHT (r = – 0.22 and r = –0.05, respectively). Similarly no correlation was noted between mean and peak gradients, and MVA by continuity equation (r = –0.23 and r = –0.3, respectively). Peak gradient did not correlate well with AOA (r = –0.26, P = NS). However, mean gradient decreased significantly with increase in AOA (r = –0.45, P = 0.004 and r = –0.39, P = 0.014, respectively). Mitral valve area calculated by both PHT and continuity equation increased significantly with increase in AOA (r = 0.42, P = 0.007 and r = 0.32, P = 0.046, respectively). (Figures 1 and 2).


Figure 1
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  		Figure 1 Scatter diagram showing the correlation between actual orifice area (X-axis) and mitral valve area calculated by pressure half-time method (Y-axis).

 


Figure 2
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  		Figure 2 Scatter diagram showing the correlation between the actual orifice area (X-axis) and mitral valve area calculated by continuity equation (Y-axis).

 
Mitral valve area by PHT showed a significant linear correlation with MVA derived by continuity equation (CE) (r = 0.041, P = 0.009). However, MVA calculated by PHT tended to be higher than that calculated by continuity equation and this difference was statistically significant ( P< 0.001, t-test). This difference was irrespective of whether PHT is less than or more than 110 ms (Table 3). Subgroup analysis between groups with PHT less or more than 110 ms showed no difference in mean or peak mitral gradients. Calculation by CE also showed no difference for calculated MVA between these two groups. The MVA calculated by CE and PHT showed comparable homogeneity in the distribution of MVA (Figures 3 and 4).


Figure 3
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  		Figure 3 Distribution of patients and valve areas according to the continuity equation.

 


Figure 4
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  		Figure 4 Distribution of patients and valve areas according to the pressure half-time method.

 


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  		Table 3 Comparison of groups by mitral valve areas and gradients

 
On colour Doppler imaging, 36 patients (90%) showed minimal to mild intravalvular mitral regurgitation, which was of grade 1 in 26 patients (65%) and grade 2 in 10 patients (25%). They were equally distributed among the various valve sizes.


   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusions
 References
 
This study is unique being the first one to provide the data on Doppler echocardiographic parameters for normally functioning CHVP in the mitral position. CHVP being a low-cost prosthetic valve with proven efficacy, these data are particularly relevant in developing countries. We have obtained data on three sizes of CHVP at mitral position—25, 27, and 29 mm. Although 23 and 31 mm valves are available, they are infrequently implanted. The normal Doppler parameters of CHVP in mitral position as derived in this study are compared to the published data on commonly used prosthetic valves in Table 4.


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  		Table 4 Normal Doppler echocardiographic values for mechanical mitral valve prosthesis21

 
Doppler derived gradients
Data on peak velocity, peak, and mean gradients of various prosthetic valves at the mitral position show a poor correlation with the valve size. Studies have shown a wide range of normal values for the velocities and gradients of different valve types.21 In our study, peak gradients showed no correlation to valve area despite optimizing confounding flow parameters such as mitral and aortic regurgitation, left ventricular dysfunction, etc. The possibility of a correlation may have been reduced by the narrow range of valve sizes and the flow dependency. However, there was a significant relationship between the mean gradient and AOA.

Mitral valve area by continuity equation
Valve areas by the continuity equation are normally smaller than those derived by PHT and relate to valve size. In studies on bioprosthetic and St Jude Medical valves, this index relates well to the area by the hydraulic equation and anatomic valve area.17,18 Our study also showed significant correlation between the valve area derived by continuity equation and AOA, similar to published studies of other valves.22 The valve area calculated by continuity equation tended to be smaller than that calculated by PHT, irrespective of the PHT.

Mitral valve area by pressure half-time
There was a significant correlation in our data between AOA and PHT. The concept of PHT was initially derived from studies on native mitral valve stenosis8 and its use in prosthetic mitral valve is controversial.23 Pressure half-time is known to show poor correlation with valve size, as it is affected by factors affecting net atriovenrricular compliance, peak transmitral gradient, and stroke volume. Most of the studies comparing the methods by PHT and CE in prosthetic valves had patients with these confounding factors in the study population. However, these haemodynamically confounding variables were minimized in our study by excluding immediate postoperative cases, and patients with suspected prosthetic valve dysfunction or significant AR. This might have accounted for the relatively better homogeneity in PHT, and the better correlation of MVA derived the PHT method with AOA in our study.

No Doppler echocardiographic method to assess the functional orifice area in mitral prosthesis is without fallacies. Mitral valve area by the continuity equation is theoretically justified. However, absence of the correlation against manufacturer's area has been reported and it is known to underestimate AOA.24 Valve resistance is useful in native mitral stenosis, but its role in prosthetic mitral valves has not been studied well.24 The proximal isovelocity surface area method based on the continuity equation can only be applied using the transesophageal approach, and is not practical.25,26 Colour flow imaging provides good qualitative or semiquantitative information, and together with continuous wave Doppler act as adjuvant to the clinical suspicion of obstruction. Previous studies have also shown that the valve area calculated in vivo with flow equations are less than AOA. Gorlin formula and the continuity equation are both pressure- and flow-dependent and are primarily related to the effective area occupied by flow rather than to the anatomic area of the valve.27

Limitations
Possible limitation of our study is the absence of cardiac catheterization and derivation of MVA by Gorlin formula as the standard for comparison. Mitral valve area derived from the VOD, the AOA in our study tend to overestimate the physiological MVA of the tilting disc prosthesis. However, in the absence of invasive catheterization, this should be the next best option as the standard for comparison, as used in some previous studies.18 Second, valve sizes of 23 and 31 mm were not included in the study as they were infrequently used.


   Conclusions
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
References
 
Our Doppler echocardiographic study provides the normal values for pressure gradients, and MVA derived by PHT method and continuity equation of all commonly used sizes of CHVP in mitral position. The data collected should act as a guide for assessment of prosthetic valve dysfunction in clinical practice. The Doppler parameters obtained with CHVP in mitral position are comparable to those obtained with the different prosthetic valves in common use. In this selected group of patients with CHVP, assessment of EOA by PHT method is comparable to that by continuity equation. However, areas by both methods were smaller than the AOA provided by the manufacturer, as seen in other similar design valves.

Conflict of interest: none declared.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusions
 References
 

  1. Bhuvaneshwar GS, Muraleedharan CV, Vijayan GA, Kumar RS, Valiathan MS. Development of the Chitra tilting disc heart valve prosthesis. J Heart Valve Dis (1996) 5:448–58.[Web of Science][Medline]
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