Copyright © 2004, The European Society of Cardiology
Improvement in diastolic left ventricular function after coronary artery bypass grafting as assessed by recordings of mitral annular velocity using Doppler tissue imaging
Department of Cardiology, Karolinska Institute at Söder Hospital (Södersjukhuset), S-118 83 Stockholm, Sweden
Received 11 March 2004; received in revised form 14 September 2004; accepted after revision 15 September 2004.
* Corresponding author. Tel.: +46 8 6163046; fax: +46 8 6163040. anders.hedman{at}medklin.sos.sll.se
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Abstract |
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Aims The aim of the study was to evaluate the changes in diastolic function after coronary artery bypass grafting (CABG), using pulsed-wave Doppler tissue imaging (DTI).
Methods Fifty-three patients with coronary artery disease were studied before and 3 and 12 months after CABG. Using pulsed-wave DTI, the mitral annular velocities were determined at 4 sites in the left ventricle (LV). Patients were also examined with dobutamine stress echocardiography and myocardial scintigraphy before and 3 months after CABG.
Results The conventional transmitral velocity profiles were unchanged after CABG. DTI showed a marked improvement in diastolic LV function after CABG (early diastolic velocity: 7.5±1.9, 8.2±1.7 and 9.3±2.7cm/s before and 3 and 12 months after CABG, respectively, P<0.01). The improvement in early diastolic velocity was more pronounced in patients showing no sign of residual ischemia in comparison to those with residual ischemia determined by myocardial scintigraphy (7.41±2.04 vs. 9.25±2.61cm/s, P<0.01 in the nonischemic group; 7.29±2.16 vs. 8.41±2.55cm/s, n.s., in the ischemic group). Before CABG, a significant increase in the systolic velocity (6.4±1.3 vs. 8.7±2.5cm/s, P<0.001), but not the early diastolic velocity (7.6±1.9 vs. 8.0±2.2cm/s), was noted during stress echocardiography. Three months after CABG, both the systolic (6.5±1.3 vs. 9.3±2.8cm/s, P<0.001) and the early diastolic velocities (8.1±1.8 vs. 10.3±2.2cm/s, P<0.001) improved during stress echocardiography.
Conclusion The results of the present study show that diastolic function improves at rest and under stress in patients after CABG. The improvement was seen only in patients without postoperative signs of reversible ischemia.
Keywords: Coronary artery bypass grafting; Diastolic function; Doppler tissue imaging
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Introduction |
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In ischemic heart disease, impairment of left ventricular (LV) diastolic function precedes systolic dysfunction. Myocardial ischemia, hypertrophy and aging are associated with a delay in the energy-dependent reuptake of calcium by the sarcoplasmatic reticulum. This phenomenon usually leads to impaired LV relaxation. Diastolic dysfunction is a common finding in patients with coronary artery disease that may lead to cardiac symptom and is of prognostic significance.1–3 Diastolic function may improve following coronary revascularization, e.g. coronary artery bypass grafting (CABG).4–6 However, most papers dealing with diastolic function before and after CABG, compromise a relatively small number of patients with a short follow-up period. Cardiopulmonary bypass surgery by itself causes short term reversible diastolic dysfunction.4,7,8 Cardiac catheterization is the standard technique for direct measurement of filling pressures and the rate of LV relaxation but it is not suitable for widespread application and follow-up. Conventional Doppler echocardiography can assess LV diastolic function by using transmitral flow velocity profiles. These parameters are, however dependent on various factors, e.g. heart rate and preload.3,9 During the last few years, analyses of myocardial velocities using Doppler tissue imaging (DTI) have been made to assess LV function. The myocardial velocities during the different phases of diastole reflect LV relaxation and filling patterns. It has been postulated that DTI is more direct and less preload-dependent in assessing diastolic function.10,11 To our knowledge, the value of DTI for assessing LV diastolic function in patients with coronary artery disease before and after CABG has not been fully elucidated, and especially not in comparison with conventional Doppler. Using the mitral annular velocity as assessed by pulsed-wave DTI, the aim of the present study therefore was to evaluate the effects of CABG on diastolic function at rest and during dobutamine stress echocardiography and compare the results before and after CABG during a follow-up period of 1 year.
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Methods |
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The study was approved by the local ethical committee.
Subjects
Fifty-three patients with a history of coronary artery disease were included prospectively and consecutively. Patients were accepted for CABG because of angina pectoris and significant coronary artery stenosis. Exclusion criteria were a history of recent myocardial infarction (4 weeks before preoperative angiography), atrial fibrillation, significant valvular heart disease or previous CABG. All patients underwent CABG with a cardiopulmonary bypass. During and after the CABG, standard laboratory markers for myocardial infarction were obtained and none of the patients were diagnosed with perioperative myocardial infarction. All patients were followed up for 1 year after the CABG. Two patients developed acute myocardial infarction during the follow-up period. Twenty-four age-matched healthy subjects without a history of cardiac and pulmonary disease or systemic hypertension with normal findings at rest ECG and echocardiography served as controls. All patients underwent echocardiographic examinations at rest: before CABG and 3 and 12 months after CABG. Dobutamine stress echocardiography was performed before and 3 months after CABG. Myocardial scintigraphy was performed simultaneously with the dobutamine stress echocardiography.
Echocardiography
Commercially available echocardiographic equipment was used (Hewlett–Packard Sonos 5500 phased array system equipped with DTI technology, Andover, Mass). Recordings and calculations of different parameters were performed according to the recommendations of the American Society of Echocardiography.12 Transmitral flow was recorded by pulsed-wave Doppler with the sample volume placed between the mitral leaflet tips in an apical 4-chamber view. Myocardial velocities of the LV were recorded at the mitral annulus using pulsed-wave DTI. A best-quality recording was made using a variable frequency phased array transducer (2.0–4.0MHz), a low wall filter setting (50Hz), a small sample volume and an optimal gain. The annular velocities were determined at 4 sites in the LV from the apical 4- and 2-chamber views as described previously (septal, lateral, anterior and inferior walls; and a mean value was obtained from 4 different sites, which was used for the assessment of LV function).13 As shown in Fig. 1, three major velocities were recorded at the annular sites: the peak major positive systolic velocity when the annulus moved towards the apex and 2 major negative velocities when the annulus moved back towards the base (one during the early phase of diastole and another during the late phase of diastole). The velocities were recorded on line. A mean of 3 consecutive cycles was used for the calculations of all echo-Doppler parameters (Fig. 1).
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Dobutamine stress echocardiography
Dobutamine was infused, starting at a dose of 5µg/kg/min, with the dose increasing at 3-min intervals to 10, 20, 30 and 40µg/kg/min. If necessary, e.g. if 85% of the maximum predicted heart rate had not been reached or the test was still negative, up to 1mg atropine was injected intravenously. End-points of the test were the achievement of the peak dose, development of severe angina or the occurrence of severe side-effects. The latter included hypertension (systolic pressure>220mmHg, diastolic pressure>120mmHg), hypotension (>20mmHg fall of systolic pressure), severe dyspnoea, or severe ventricular arrhythmias. When the patients reached the end-points of the test (peak test), the infusion of dobutamine was stopped. The mitral annular velocities were first visualized on the screen at the end-point of the test. If the quality seemed satisfactory, the velocities were recorded in real time at different LV sites, as mentioned before, during rest echocardiography. The recording was started at the septum and then continued to lateral, inferior and anterior sites of the mitral annulus. If the early and late diastolic velocities were totally merged, visualization of the velocities was continued on the screen until separation of the waves was visible or a clear notch of the early velocity on the merged waves was noted (post-peak-stress echocardiography). The recording was then used for calculations.
Myocardial scintigraphy
All the patients underwent single photon emission computed tomography (SPECT) using a 2-day Tc-99m-tetrofosmin acquisition protocol 3 months after CABG. The stress test using dobutamine was carried out on day 1 and the control rest test was performed on day 2. Acquisitions were performed as previously described14 using a 2-detector (Vertex, ADAC) camera to get 64 projections over a 180° non-circular arc extending from right anterior to left posterior oblique. A 20% window around the 140-keV energy peak of Tc-99m was used. All projection images were stored in a 64x64x16 matrix. Projection images were filtered using a 2D Butterworth filter order of 10 and cut-off frequency of 0.50cycles/pixel. Images were constructed into transaxial images using filtered backprojection with a ramp filter. No scatter or attenuation correction was applied. The perfusion SPECT was processed with an automatic reorientation programme and the stress and rest images were interpreted with the consensus of 2 experienced readers using a 16-segment model12 (6 basal, 6 midventricular and 4 apical segments) and a 4-point perfusion score for each segment: (0 point=normal perfusion, 1 point=mildly reduced, 2 points=moderately reduced, 3 points=severely reduced or absent).
Coronary angiography
Selective coronary angiography was performed within a month before CABG. Coronary angiograms were interpreted by experienced physicians. On visual analysis, a 50% or more reduction of the luminal diameter in 2 orthogonal projections of a major coronary artery or one of its major branches or a bypass graft was considered to be significant for coronary artery disease.
Statistics
The results are expressed as the mean and 1 standard deviation. The parameters of patients and healthy subjects were compared using an unpaired t-test. A paired t-test was used to compare results within the same group. A P-value of <0.05 was used as the level of significance.
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Results |
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Echocardiographic parameters at rest
The baseline characteristics of the patients are shown in Table 1. Diastolic LV function, as assessed by conventional transmitral flow velocity waves, showed no significant change after CABG (Table 2). Compared to healthy subjects, patients had decreased systolic as well as diastolic mitral annular velocities as determined by DTI (healthy subjects vs. patients: systolic velocity=9.5±1.1 vs. 6.3±1.2cm/s, P<0.001; early diastolic velocity=11.6±2.3 vs. 7.5±1.8cm/s, P<0.001 and late diastolic velocity=12.4±2.3 vs. 8.6±2.4cm/s, P<0.001). Repeated echocardiography using DTI, 3 and 12 months after CABG, showed improvement in both systolic and diastolic LV function in the patients (Table 3 and Fig. 2). The ratio between transmitral early velocity recorded by conventional pulsed Doppler and early diastolic velocity determined by DTI was calculated before and after CABG and the results are shown in Table 3.
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Subdivisions of patients according to myocardial scintigraphy
Patients were divided into 2 subgroups according to the results of myocardial scintigraphy 3 months after CABG: those with and those without residual reversible ischemic LV segments. Patients without signs of reversible ischemia at 3 months after CABG showed a significant improvement in LV diastolic function during the whole follow-up period (Table 4). Improvement in systolic function was also seen after CABG, although it was not noted until after 1 year. On the other hand, patients with signs of residual reversible ischemia after CABG showed no improvement in systolic or diastolic function.
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Stress echocardiography before and after CABG
The increase in the stress-related heart rate was similar before and after CABG. The results of the stress echocardiography could be analyzed in 47 patients (88%) at all the sites. Failure to analyze stress echocardiography in 6 patients during peak or post-peak stress was due to unsatisfactory DTI signals, especially from the anterior wall of the LV. Before CABG, there was a significant increase in the systolic mitral annular velocity during peak stress echocardiography. No improvement in the early diastolic velocity was noted. After CABG, both the systolic and diastolic velocities showed significant improvement during the stress test (Table 5 and Fig. 2).
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Discussion |
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Diastolic function has traditionally been measured by pulsed Doppler echocardiography of the transmitral flow, which has the disadvantage of being preload-dependent.3,9 In recent studies, the mitral annular velocity using DTI has been used to assess LV function. Motion of the mitral annulus represents LV function along its long axis. Several DTI methods are being used to assess LV function, e.g. pulsed-wave Doppler and colour M-mode.10,11 The method of recording the mitral annular velocity at rest using pulsed-wave DTI is easy and feasible in almost all subjects.13,15 Being a different anatomical structure, the mitral annulus is easy to visualize and recording of its velocity is simple because it is devoid of myocardial drop-outs, trabeculae etc. It has been known for many years that LV diastolic function, as assessed by transmitral flow velocity profiles, is altered in patients with coronary artery disease. Using DTI, LV diastolic dysfunction has also been reported in patients with coronary artery disease.15,16 The peak velocity of the mitral annulus during early diastole is usually used as a sensitive marker of diastolic function. As compared to healthy subjects, and similar to the results of a previous study,17 a decreased early diastolic mitral annular velocity in patients with coronary artery disease was noted in the present study. The decreased early diastolic mitral annular velocity is a sign of disturbed LV relaxation, secondary to coronary artery disease.
Patients with angina pectoris usually experience a relief of symptoms after CABG. Using conventional echo-Doppler parameters, several previous studies have shown different results for both systolic and diastolic function after the operation.4,5 These studies had small number of patients and most of them lacked a long term follow-up. The results of the present study show that DTI can be used to assess improvement in diastolic function after CABG although the results of the conventional transmitral Doppler flow velocity profiles remained unchanged. The DTI is probably a more sensitive method for assessing diastolic function. The improvement in LV function, which was noted in patients showing no residual LV ischemia during myocardial scintigraphy after CABG, reflects a relief of ischemia which is one of the most common causes of diastolic dysfunction.
In previous studies it has been postulated that the ratio of transmitral early velocity (E) and early diastolic velocity (e') measured by DTI can predict left ventricular filling pressure.18,19 In the present study we noticed an improvement in E/e', however it did not reach significant level. This is probably due to unchanged early transmitral flow before and after CABG although there was a significant improvement of mitral annular diastolic velocity.
During the preoperative investigation, the early diastolic mitral annular velocity failed to show improvement during the peak dobutamine stress echocardiography although the systolic velocity increased significantly. This might indicate that diastolic function is a more sensitive parameter for ischemia. The improved diastolic velocity during dobutamine stress echocardiography after CABG may be a sign of better LV perfusion after CABG in most of the patients as shown by postoperative myocardial scintigraphy. However, the results of the assessment of mitral annular velocity during dobutamine stress echocardiography in this study reflect the changes in a heterogeneous group of patients in which some patients remained ischemic after CABG. The reason of residual ischemia in some patients may be due to early graft occlusion or incomplete revascularization. There was no intention in the present study to identify individual patients with regional myocardial ischemia. Rather, the purpose of the study was to assess global function using different sites of the mitral annulus.
Some recently published papers have reported the usefulness of DTI during dobutamine stress for identifying patients with coronary artery disease.20–23 The results of these studies with respect to response during dobutamine stress are, however, inconsistent. Most of the studies used only the systolic velocity, as it is easy to record and analyze during stress. Different studies reported an increase in peak systolic velocity in both healthy and ischemic segments, but the increased velocity was more pronounced in the healthy segments. In the present study of patients with coronary artery disease before CABG, the increase in the systolic mitral annular velocity during dobutamine stress was significant, although the percentage of increase was lower than in previous studies.20,22 This is probably due to the fact that we recorded the velocity during the peak or post-peak phase when the peak heart rate level was lower compared to some of the previous studies. The increase in systolic velocity is partly dependent on the level of the maximum heart rate. In addition, most of our patients were severely ischemic with 3-vessel disease, which might contribute to a modest increase in systolic velocity. After CABG, the systolic velocity during dobutamine stress continued to increase. Compared to the systolic velocity, there is little documentation of diastolic mitral annular or myocardial velocities during peak dobutamine stress.21,22 The response of the early diastolic velocity during stress echocardiography was quite limited and was found to be similar in both healthy subjects and patients in those studies. However, our present study showed a significant increase in the stress-related early diastolic mitral annular velocity after CABG indicating that this parameter can be used to demonstrate functional recovery after revascularization. The feasibility of recording the early diastolic velocity during dobutamine stress echocardiography was greater in the present study than in other ones.24 In our experience heart rates lower than 100 beats per minute after peak stress enabled us to analyze diastolic velocity more easily. Heart rates between 100 and 120 beats per minute might cause merging of the velocities with two clear diastolic velocities. When the heart rate is more than 120 beats per minute, there is a risk of total fusion of early and late diastolic velocities.
The present study showed that an improvement of diastolic function is likely to occur early, at 3 months after CABG, whereas, in some cases, recovery of diastolic function maybe delayed for up to 1 year. This issue was demonstrated in our study by further improvement of diastolic function at late follow-up. It is possible that diastolic function exhibits, like systolic function stunning or hibernation with a variable time course in recovery after revascularization.
Study limitations
This study has some limitations. Diastolic function was assessed using only the mitral annular velocity, which represents only the changes along the longitudinal axis of the left ventricle. No account was taken of the changes in the LV along the short axis. However, the method for assessing the mitral annular velocity is easy and previous studies have demonstrated its usefulness in determining systolic and diastolic LV function.15,25 Hypothetically, using only 4 sites at the mitral annulus might not be sufficient if there is regional ischemic diastolic dysfunction distant from the annulus. However, changes in the mitral annular motion can reflect ischemia, e.g. at the apex, as ischemia at any segment in the same wall on a longitudinal image will decrease the velocity of the corresponding site of the mitral annulus.20,26 The inter- and intra-observer variations were found to be insignificant in previous studies; hence they were not studied in the present study.15 Other diastolic parameters, e.g. pulmonary vein flow was not performed in this study due to unsatisfactory registration in many of the patients. In the majority of the patients recording of velocities were performed during a "submaximum" heart rate response. Thus there is a risk that we did not reach the maximum diastolic response due to a low peak heart rate increase during the stress test. However, the response of the stress-related heart rate was not the only end-point criterion of the stress test. In most of the patients, symptoms were the criterion for ending the stress test. No reference values for stress echocardiography-related changes in mitral annular velocity in healthy subjects have been provided here. The purpose of using stress echocardiography in the present study was to assess the improvement of the mitral annular velocity after CABG with the patient's own resting value being used as a reference. Assessment of LV filling pressure with E/e' ratio could have been interesting during stress echocardiography, however it was not performed in the present study due to lack of transmitral early velocity data during stress echocardiography.
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Conclusion |
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On using myocardial velocities, an improvement of diastolic LV function was noted after CABG in patients with coronary artery disease although conventional Doppler echocardiography remained unchanged. In addition, diastolic function was also improved after CABG during stress echocardiography as assessed by DTI. The improvement was seen only in patients without postoperative signs of reversible ischemia. The results of the study indicate that DTI is a useful method for assessing recovery of diastolic function at rest and under stress after CABG.
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Acknowledgement |
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We thank Johan Wardell and Eva Andersson for their excellent technical support. This study was supported by grants from the Swedish Heart and Lung Foundation and from the Karolinska Institute.
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References |
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