Copyright © 2005, The European Society of Cardiology
Evaluation of Tissue Doppler Tei index for global left ventricular function in mice after myocardial infarction: Comparison with Pulsed Doppler Tei index
Department of Cardiology and Angiology, Medical School Hannover, Carl-Neuberg-Str. 1, 30625 Hannover, Germany
Received 18 August 2004; received in revised form 15 January 2005; accepted after revision 21 January 2005.
schaefer.arnd{at}mh-hannover.de
* Corresponding author. Tel.: +49 511 532 3841; fax: +49 511 532 3357.
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Abstract |
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Aim
The Pulsed Doppler Tei index is a parameter to evaluate combined systolic and diastolic function in humans. However, one major limitation is that the parameters of Pulsed Doppler Tei index cannot be measured within one cardiac cycle. Therefore, accuracy of the Pulsed Doppler Tei index may be affected by anesthesia induced heart rate variation in mice echocardiography. Tissue Doppler Imaging (TDI) enables us to measure both relaxation and contraction velocities simultaneously. Thus, the aim of our study was to validate TDI and Pulsed Doppler Tei index and their reproducibility in mice after experimental anterior myocardial infarction (MI).
Methods and Results
Pulsed Doppler Tei index and TDI Tei index were assessed before and 4 weeks after MI. Both parameters increased significantly after MI (Pulsed Doppler: 0.4±0.04 to 0.7±0.03; P < 0.001; TDI: 0.2±0.03 to 0.5±0.04; P < 0.0001). In addition, TDI Tei index showed a good correlation with ejection fraction and fractional shortening, and was indicated by better reproducibility than Pulsed Doppler Tei index.
Conclusion
Tissue Doppler Tei index is appropriate to characterize global left ventricular function in mice after MI.
Keywords: Mouse; Myocardial infarction; Echocardiography; Tissue Doppler; Pulsed Doppler; Tei index
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Introduction |
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Transthoracic echocardiography is the predominant diagnostic tool to evaluate systolic cardiac function noninvasively in mice [1]. Most frequently, echocardiography in mice is performed for evaluation of cardiac dimension, systolic and diastolic function. Systolic function is usually characterized by fractional shortening (FS%), ejection fraction (EF%) and diastolic function can be determined by transmitral Doppler and Tissue Doppler imaging [2,3]. However, it is well known that these parameters are influenced by various anesthetic agents used for sufficient sedation and immobilization of mice during echocardiography [4].
The Tei index is an echocardiographic Doppler index that combines systolic and diastolic function [5]. In humans, the Tei index is simple to calculate, reproducible, independent of heart rate, blood pressure and characterized by a low interobserver and intraobserver variability [6]. However, one major limitation of the Pulsed Doppler derived Tei index is that both relaxation and contraction velocities cannot be measured simultaneously within one cardiac cycle [3]. Therefore, accuracy of the index may be influenced by heart rate (HR) alteration that is common during mice echocardiography due to anesthesia effects on HR [4,7,8]. Tissue Doppler Imaging (TDI) enables us to measure both relaxation and contraction velocities simultaneously.
Therefore, our current study focuses on the reliability, feasibility of TDI Tei index and the comparison with Pulsed Doppler Tei index measurements for evaluation of global left ventricular function in mice before and after experimental anterior myocardial infarction (MI).
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Methods |
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Animals
We used C57BL/6 mice, aged 4–5 months. They were given standard food and water ad libitum. The Hannover Medical School Standing Committee on Animal Research approved the study protocol.
Experimental protocol
Echocardiographic measurements were performed on 12 C57BL/6 mice the day before and 4 weeks after surgery for MI. Animals, which died within the study period were excluded. A total of four mice died, one during surgery and three later in their cage. The data of these mice were excluded from the study.
Myocardial infarction
MI was induced by ligation of the left anterior descending coronary artery (LAD) as previously described in detail [9].
Echocardiography
Echocardiographic studies were performed under light anesthesia and spontaneous respiration with the use of intraperitoneal ketamine (100 mg/kg) and xylazine (1.25 mg/kg). The chest was shaved and warmed acoustic coupling gel was applied. Subsequently, mice were fixed supine in a left lateral position on a heated pad.
A commercially available ultrasound system was used for the echocardiographic examinations (ATL 5000 CV system, linear 15 MHz high-frequency transducer). The depth was set to 2 cm and zoomed to 1.2 cm. Wall thickness and LV dimensions were obtained from a short-axis view at the level of the papillary muscles at a frame rate of 260Hz. Fractional shortening was calculated as FS(%)=[(LVEDD–LVESD)/LVEDD)]x100, where LVESD is LV end-systolic dimension and LVEDD is LV end-diastolic dimension. Ejection fraction (EF%) was calculated from the short-axis view using the equitation EF(%)=[(LVDA–LVSA)/LVDA]x100, where LVDA is LV diastolic area and LVSA LV systolic area [10]. As described previously [2], for TDI the smallest sample volume was placed within the LV posterior myocardial wall at the level of the papillary muscles in short-axis view and at the lateral side of the mitral annulus in parasternal long-axis view. Gains were adjusted to eliminate background noise and allow for clear tissue signal. Five to ten cycles were recorded.
Transmitral pulsed Doppler measurements were acquired in a parasternal long-axis view. Transmitral flow velocity recordings were performed by setting the smallest possible pulsed Doppler sample volume in the mitral orifice. For this purpose, the Doppler sample was set parallel to transmitral flow by angle correction to accurately assess flow velocities. For all measurements the maximal available time resolution was used (150 mm/s).
For TDI and Pulsed Doppler flow velocities only full-density velocity signals of at least five cardiac cycles were used and averaged. Freeze-frame images were then downloaded to a magneto-optical disk for off-line analysis.
LV mass was calculated by use of the following formula, assuming a spherical LV geometry. LV mass=1.04x[(LVEDD+LVPWD+LVAWD)3–LVEDD3] [11,12], where 1.04 is the specific gravity of muscle. LVPWD and LVAWD are end-diastolic posterior and anterior wall dimension.
Calculation of Tei index by Pulsed Doppler
Fig. 1 summarizes calculation of Tei index by Pulsed Doppler in detail. The Tei index was calculated as Tei index=a–b/b where a is the sum of IVCT (isovolumic contraction time) and IVRT (isovolumic relaxation time) and b is ET (ejection time). Since E/A and ET can not be measured within the same cardiac cycle, the sum (a) of IVCT and IVRT was determined by measuring the time from the end of atrial filling (end of A-wave) to the onset of atrial filling (onset of E-wave) minus b (ET). ET was determined by measuring the LV-outflow velocity with the Pulsed Doppler in the parasternal long-axis view just below the aortic annulus.
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Calculation of Tei index by Tissue Doppler
For Tissue Doppler all interval measurements were performed within one cardiac cycle as illustrated in Fig. 2. The Tei index was calculated as Tei index=a'–b'/b' where a' is the time interval from the end of Aa-wave to the onset of Ea-wave and b' the time from the onset to the end of the Sa-wave.
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Heart rate variability of Tei index
Heart rate variability was determined from a and b intervals of Pulsed Doppler recordings and from a' and b' intervals of TDI recordings. The difference of both intervals was divided by the a or a' intervals to create a percentage.
Reproducibility
To test reproducibility of Tei index for both approaches, repeated measurements were performed on nine mice without myocardial infarction on 2 days apart by the same observer. In addition, repeated measurements were performed at 5 and 10 min after initiating anesthesia on six mice. These mice were not included in the tests previously.
Statistical analysis
All values are presented as means±SEM. For statistical analysis a paired Student's t-test was used. Differences between the measurements taken by Pulsed Doppler and TDI were analyzed according to the Bland–Altman method [13]. To detect interaction between the different parameters, Pearson's correlation was performed. Cicchetti and Sparrow recommend the following guidelines for intraclass correlation coefficients: 0.75 or better indicates excellent interrater agreement; 0.60 to 0.74 indicates good agreement; 0.40 to 0.59 indicates fair to moderate agreement; and below 0.40 indicates poor agreement [14]. MedCalc. (version 7.5) was used for comparison of the correlation coefficients. For all tests, a P value of less than 0.05 was considered significant.
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Results |
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Effects of MI on left ventricular dimension, ejection fraction, fractional shortening, heart rate and left ventricular mass
Table 1 shows the effect of MI on LVEDD, LVESD, HR, EF, FS and LVM. There was a significant increase of LVEDD and LVESD and significant reduction of FS and EF. HR and LVM remained unchanged.
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Effects on TDI Tei index
Table 2 and Fig. 3A illustrate the effect of MI on TDI Tei index determined in parasternal short axis. TDI Tei index could be measured in all 12 mice in the short-axis view, but only in 6 mice using the long axis due to insufficient transthoracic visibility of the posterior wall. The increase of TDI Tei index in long-axis view (0.2±0.05 to 0.4±0.06; P = 0.047) was less significant compared to the short-axis view (0.2±0.03 to 0.5±0.04; P < 0.0001). As demonstrated in Table 2, the increase of TDI Tei index was due to a significant prolongation of the sum of IVRT and IVCT for long and short axis, whereas ET remained unchanged.
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Effects on Pulsed Doppler Tei index
As shown in Fig. 3B, MI was associated with a significant increase of Pulsed Doppler Tei index (0.4±0.04 to 0.7±0.03; P < 0.001). In accordance with the TDI Tei index, this increase was caused by a significant prolongation of IVRT and IVCT.
Correlation of EF, FS, HR and Tei index
To evaluate Pearson's r correlation all measurements before and after MI were included. We found a significant negative correlation between TDI Tei index and Pulsed Doppler Tei index and EF and FS (Figs. 4 and 5
). The differences between the correlation coefficients were not significant. There was no correlation of TDI and Pulsed Doppler Tei index and HR.
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Comparison of Pulsed Doppler Tei index and TDI Tei index
As shown in Fig. 6 we found an excellent correlation between the sum of IVRT and IVCT measured by Pulsed Doppler and TDI (r = 0.9; P<0.001). The mean difference between both methods was 3±11 ms (P=not significant; n.s.). There was a moderate correlation between ejection time determined by Pulsed Doppler and duration of Sa wave determined by TDI (r = 0.5; P = 0.01; Fig. 6). The mean difference between both parameters was 12±9 ms (P< 0.05). We found a highly significant positive correlation of TDI Tei index and Pulsed Doppler Tei index (Fig. 7). The mean difference between Pulsed Doppler and TDI Tei index was 0.2±0.1 (P < 0.05). Despite these differences, we found that most of the values remained within the 2 standard deviations of the mean measurements (Fig. 7).
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Reproducibility
Interobserver variability for measurements on two days apart of Pulsed Doppler Tei index and TDI Tei index was small, 12±7% vs. 4±16% (n.s.), respectively.
For repeated measurements at 5 and 10 min the variability was 17±10% vs. 6±13% (n.s.), respectively. There was no significant change of heart rate (288±14 bpm vs. 286±56 bpm), Tei index (Pulsed Doppler, 0.6±0.1 vs. 0.5±0.1; TDI, 0.3±0.1 vs. 0.3±0.04), FS and EF within these two measurements.
If analysis of correlation were restricted to the measurements of the reproducibility study (mice without MI) we found a correlation between TDI index and EF (r=–<0.5; P<0.05) and FS (r=–<0.7; P<0.01). There was no correlation between Pulsed Doppler Tei index and EF (r=–<0.3; P=<0.2) and FS (r=–<0.3; P=<0.3).
Heart rate variability of Tei index
The mean heart rate variability of Pulsed Doppler Tei index was 15±5%. Since measurements of TDI intervals were determined from one cardiac cycle, there was no detectable heart rate variability of TDI Tei index.
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Discussion |
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Our study demonstrates that TDI Tei index and Pulsed Doppler Tei index are feasible and accurate parameters to evaluate global systolic left ventricular function in mice after anterior myocardial infarction and correlate well with left ventricular function characterized by FS and EF.
In our study, we used a murine model of anterior myocardial infarction by ligation of the LAD. It is well known that after 4–6 weeks this method induces profound left ventricular dysfunction [15–18]. To characterize LV-function by Tei index we used two different approaches. First, the Pulsed Doppler Tei index determined by diastolic mitral inflow for IVRT and IVCT and by ET measured just below the aortic annulus. Second, the TDI Tei index was determined from the posterior myocardial wall at a parasternal short and long-axis view. For both measurements procedure duration was prolonged by 10 min including off-line analysis. TDI measurements in the long-axis view were limited by insufficient myocardial border detection after myocardial infarction and this cannot be recommended for routine practice.
Our results demonstrate that both indexes are well related to global left ventricular function and reciprocal. However, the absolute difference between both is about 50% which is too high for experimental practice. This difference is in particular caused by minor agreement of the ejection time determined by Pulsed Doppler and the duration of the Sa wave determined by TDI, whereas IVRT and IVCT showed a good agreement. There are two issues that may contribute to these findings. We have to consider that both approaches measure different cardiac parameters. Values derived from Pulsed Doppler measurements are determined by blood flow velocities across mitral and aortic valve whereas values of TDI measurements are determined by myocardial velocities in systole and diastole. Since myocardial contraction begins before the onset of blood flow across aortic valve, it is likely that the duration of the Sa-wave measured by TDI has to be longer than the ejection time measured by Pulsed Doppler as shown in our study.
In addition, although TDI measurements for global systolic and diastolic function in humans are usually assessed in apical long-axis views at the septal or lateral mitral annulus, we and others had to arrange with the current limitation of only parasternal long- and parasternal short-axis views in mouse or rat echocardiography [19]. Therefore, our TDI measurements were restricted to the posterior wall representing rather regional than global myocardial velocities.
Taken together, both indexes can be used for assessment of left ventricular function in mice but a direct comparison (i.e. 1 to 1 fashion) is not appropriate in mouse echocardiography.
To provide sufficient sedation and immobilization for assessment of systolic and diastolic function by echocardiography in mice, various anesthetic agents are used. However, it is well known that these agents have major effects on cardiac function, ejection fraction, fractional shortening and especially on heart rate variation [4,7,8,10,20–23]. Therefore, it is very attractive to use a performance index, which is mainly independent of these conditions. Several studies in humans have shown, that the Pulsed Doppler derived Tei index is a global left and right ventricular performance parameter independent of heart rate, loading conditions, blood pressure, ventricular geometry and in addition is a strong predictor of mortality and prognosis for patients with several cardiac diseases [5,6,20,24–28].
A major limitation of the Pulsed Doppler Tei index measurements in mice is that IVCT, IVRT and ET cannot be measured simultaneously [29]. Therefore, changes of heart rate during inflow (IVRT; IVCT) and outflow (ET) recordings may cause less accuracy for this approach. Heart rate changes of more than 100 bpm over a study period of 20 min have been reported in mice [4,7,8]. In contrast, TDI enables us to measure all parameters within one cardiac cycle and therefore may reduce inaccuracy and has practical advantages over Pulsed Doppler measurements. In our study heart rate variability for Pulsed Doppler Tei index was 15±5%, which is three times higher than previously reported in humans [30]. This may in part explain the lower reproducibility of Pulsed Doppler Tei index compared to TDI Tei index. In addition, it appears that TDI index is able to detect smaller differences in cardiac function than Pulsed Doppler Tei index, as indicated by a significant correlation of TDI Tei index in the reproducibility study whereas Pulsed Doppler Tei index showed no correlation.
There are two limitations which have to be addressed. First, we primarily focused on the feasibility TDI Tei index. Therefore, we used mice with severe reduced LV-function, assumed to have combined systolic and diastolic dysfunction. We did not evaluate mice having predominantly or isolated diastolic dysfunction. However, in one study the Tei index was found to be of no value in identifying patients with diastolic heart failure [31]. This has to be confirmed by further studies in mice.
Second, we did not evaluate the effect of significant heart rate variation on Tei index within repeated measurements. Since heart rate variation due to anesthesia is associated with significant changes of systolic left ventricular function in mice, repeated measurements of both indexes over time would demonstrate temporal changes of systolic function and not reproducibility of Tei index [4]. This would only be possible using inhaled anesthetics or conscious mice with stable HR above 500 bpm. Given the technical settings of currently commercially available ultrasound systems, these heart rates would cause fusion of the Doppler signals [7].
In conclusion, we applied Tissue Doppler Tei index to evaluate global left ventricular function in mice after experimental anterior myocardial infarction. Our study demonstrates that TDI Tei index correlates well with EF and FS.
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