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J Thorac Cardiovasc Surg 2003;126:1531-1536
© 2003 The American Association for Thoracic Surgery

Cardiopulmonary support and physiology

Changes in left anterior descending coronary artery flow profiles after coronary artery bypass grafting examined by means of transthoracic Doppler echocardiography

^a Division of Cardiovascular Surgery, Sakurabashi-Watanabe Hospital, Osaka, Japan
^b Division of Cardiovascular Surgery, National Kure Medical Center, Hiroshima, Japan
^c Division of Cardiovascular Surgery, Takarazuka City Hospital, Hyogo, Japan

Received for publication September 15, 2002; revisions received October 16, 2002; revisions received October 24, 2002; accepted for publication November 1, 2002.

^* Address for reprints: Masao Yoshitatsu, MD, Division of Cardiovascular Surgery, National Kure Medical Center, 3-1, Aoyama, Kure, Hiroshima 737-0023, Japan
yoshitatsu{at}kure-nh.go.jp

	Abstract

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OBJECTIVE: We sought to investigate the changes of velocity profiles in the left anterior descending coronary artery after coronary artery bypass grafting using transthoracic Doppler echocardiography.

METHODS: Forty-five patients who received a bypass graft to the left anterior descending coronary artery were studied. Before coronary artery bypass grafting, Doppler velocity profiles of the distal left anterior descending coronary artery were recorded with transthoracic Doppler echocardiography. Peak systolic velocity, mean systolic velocity, peak diastolic velocity, mean diastolic velocity, total velocity time integral, systolic velocity time integral, and diastolic velocity time integral were measured. Three weeks after coronary artery bypass grafting, left anterior descending coronary artery antegrade flow in the distal portion of the anastomosis was obtained by using the same method. Coronary angiography was performed before and 3 weeks after coronary artery bypass grafting.

RESULTS: The overall success rate of measuring the left anterior descending coronary artery flow was 60.0% preoperatively and 80.0% postoperatively. In 25 patients, in whom all parameters were obtained both before and after coronary artery bypass grafting, the following increased significantly after coronary artery bypass grafting: peak systolic velocity (14.86 ± 7.50 vs 25.07 ± 17.02 cm/s, P = .0045), mean systolic velocity (9.86 ± 5.42 vs 18.03 ± 12.94 cm/s, P = .0026), peak diastolic velocity (24.26 ± 12.54 vs 48.28 ± 31.66 cm/s, P = .0021), mean diastolic velocity (14.94 ± 6.65 vs 30.36 ± 20.71 cm/s, P = .0022), diastolic velocity time integral (7.22 ± 2.88 vs 15.55 ± 10.39 cm, P = .0009), total velocity time integral (10.50 ± 4.48 vs 19.27 ± 12.63 cm, P = .0034), and diastolic-to-systolic velocity time integral ratio (3.09 ± 1.53 vs 4.97 ± 2.75, P = .0044). Angiography showed graft patency and no significant change in left anterior descending coronary artery stenosis in all patients.

CONCLUSIONS: Transthoracic Doppler echocardiography showed a significant increase in some parameters in left anterior descending coronary artery flow after coronary artery bypass grafting. Measurement of left anterior descending coronary artery flow by means of transthoracic Doppler echocardiography might be a noninvasive method to evaluate the effect of bypass grafting on the left anterior descending coronary artery.

	Methods

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Patients
The study group consisted of 45 consecutive patients who received a bypass graft to the LAD at Sakurabashi-Watanabe Hospital during the period of August 2000 to August 2001. The study population comprised 36 men and 9 women with ages ranging from 46 to 81 years (mean, 65.7 ± 7.3 years) and with ischemic heart disease and significant stenosis in the proximal portion of the LAD or the left main trunk of the coronary artery. Bypass grafting was performed to the midportion of the LAD. The bypass grafts to the LAD consisted of 33 left internal thoracic arteries (ITAs) in situ, 9 right ITAs in situ, 2 radial arteries, and 1 free ITA. Off-pump coronary artery bypass grafting (OPCAB) was performed in 10 patients. Emergency operations were performed in 9 patients. During conventional CABG, myocardial protection was achieved with topical cold saline and intermittent cold blood cardioplegia, followed by terminal warm blood cardioplegia. During OPCAB, a 3-minute period of ischemic preconditioning was performed, and the shunt tube was used in some cases. The characteristics of all patients are shown in Table 1. All patients underwent coronary angiography within 2 months before the operation to evaluate LAD stenosis. Postoperative graft patency and LAD stenosis were evaluated by means of coronary angiography 3 weeks after the operation. Each patient gave informed consent to the study protocol, which was approved by the Committee for the Protection of Human Subjects in Research at Sakurabashi-Watanabe Hospital.

Blood flow velocity was measured by means of pulsed Doppler scanning. The spectral Doppler scan of the LAD flow showed a characteristic biphasic flow pattern with a large diastolic component and a small systolic component (Figure 1). The angle between the blood flow and ultrasonic beam was optimized and adequately corrected if necessary. Velocity measurements were performed by using the internal analysis package on the ultrasound unit. Seven parameters were measured by tracing the contour of the Doppler velocity pattern: (1) peak systolic velocity; (2) peak diastolic velocity; (3) mean systolic velocity; (4) mean diastolic velocity; (5) systolic velocity time integral (VTI); (6) diastolic VTI; and (7) total VTI. The diastolic-to-systolic velocity ratio was computed as the ratio of diastolic-to-systolic mean and peak coronary flow velocities. The diastolic-to-systolic VTI ratio was also computed. Before CABG, measurements were performed in the distal coronary artery. After the operation, recipient LAD flow was measured at a position distal to the anastomotic site of the bypass graft.

View larger version (99K): [in this window] [in a new window]   Figure 1. Spectral Doppler flow before (A) and after (B) CABG in the distal LAD.

Statistical analysis
Statistical analysis was performed with the StatView software package, version 5.0 (Abacus Concepts, Inc). All values are expressed as means ± SD. A paired t test was used to evaluate changes in each parameter after surgical intervention. An unpaired t test was used to evaluate differences in each parameter between 2 groups (conventional CABG vs OPCAB). Comparison between the 2 groups was analyzed by using the ² method or the Fischer exact test for categoric variables.

	Results

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Angiographic assessment
Postoperative angiography showed no stenosis at the anastomotic sites in all patients. In no case was either significant regression or progression of LAD stenosis detected.

Echocardiographic detection of LAD
Doppler velocity profiles and diameters of the LAD were obtained in 27 (60.0%) of 45 patients preoperatively and in 36 (80.0%) of 45 patients postoperatively. All patients were assigned to one of 4 groups to clarify the factors affecting the ability of the LAD to be detected (Table 2). We considered that in group B, in which LAD was not detected preoperatively but rather detected postoperatively, the LAD flow was too small to be detected preoperatively and increased enough to be detected after successful CABG. We divided groups A and B into 2 subgroups: a severe stenosis group consisting of patients with severe stenosis (percent diameter stenosis 99%) in the proximal LAD or the left main trunk and a nonsevere stenosis group of patients without severe stenosis in either the proximal LAD or the left main trunk (Table 3). The rate of severe stenosis in group B was significantly higher than that in group A (P < .005). Before emergency operations, the patient's condition was thought unsuitable for meticulous measurement by means of TTDE. Therefore we analyzed the relationship of the ability of the LAD to be detected preoperatively to the requirement for emergency operation. All groups were divided into 2 subject groups: one group consisting of patients with emergency operations and the other consisting of patients with elective operations (Table 4). There was no significant difference in the ratio of emergency operation between group A plus C in which LAD was detected preoperatively and group B plus D in which the LAD was not detected preoperatively. There were 13 patients with total occlusions of the left main trunk or LAD. In these patients LAD flow was detected in 4 (30.8%) patients, consisting of 3 with antegrade flow and 1 with retrograde flow. In group A all parameters were obtained both preoperatively and postoperatively. The characteristics of group A (25 patients) are shown in Table 5, and the comparison of preoperative and postoperative values is shown in Table 6. Peak systolic velocity, mean systolic velocity, peak diastolic velocity, mean diastolic velocity, diastolic VTI, and total VTI increased significantly after CABG. (In the patient with total occlusion of the left main trunk, preoperative antegrade LAD flow was considered to be 0.) On the other hand, the systolic VTI did not differ after CABG. Neither peak nor mean diastolic-to-systolic velocity ratios differed significantly after CABG, whereas the diastolic-to-systolic VTI ratio increased significantly after CABG.

	Discussion

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Changes in LAD flow profiles after CABG
In our study there were significant changes in diastolic VTI, despite no significant changes in systolic VTI. Thus the diastolic-to-systolic VTI ratio increased significantly after CABG. It is presumed that these data indicate the improvement of myocardial perfusion during diastole by CABG. In normal coronary arteries most blood flow occurs during diastole because myocardial compression during systole increases distal vascular resistance.¹³

There have been few reports about diastolic VTI and systolic VTI of the LAD by TTDE. Crowley and Shapiro¹³ reported that in the normal LAD mean systolic VTI was 3.5 ± 1.9 cm and mean diastolic VTI was 11.7 ± 6.5 cm. It might be thought that these data are as great as our postoperative data. It appears that a graft to the LAD provides enough flow to the ischemic myocardium. On the other hand, our data also showed that the peak and mean diastolic velocities and systolic velocities increased significantly postoperatively. Thus there were no significant changes in the peak diastolic-to-systolic velocity ratio and mean diastolic-to-systolic velocity ratio. It is reported that the diastolic-to-systolic coronary flow velocity ratio in the distal coronary artery provides important information on stenosis in the proximal coronary artery.^7,8 Higashiue and colleagues⁷ reported that both peak and mean diastolic velocities significantly increased after successful coronary angioplasty, and there was no significant change in either peak systolic velocity or mean systolic velocity. Because multiple flow of grafts and native other coronaries would affect LAD flow in patients after CABG, LAD flow characteristics would differ from those in patients after angioplasty only to the LAD. Related to the small sample size of our patient population, further studies involving larger data sets are needed to confirm these results.

Ttde as a noninvasive method to assess myocardial perfusion after CABG
In this study the success rate for the detection of LAD flow by means of TTDE was 60.0% preoperatively and 80.0% postoperatively. The preoperative success rate was lower than that reported by other investigators,^1-13 and the postoperative success rate was high enough for clinical applications. As described in the "Results" section, the principal factor contributing to a lower success rate preoperatively was severe stenosis in the native coronary artery. In our study the rate of successful detection of LAD flow in the patients with total occlusion of the LAD or the left main trunk was 30.8% (4/13). The LAD of one patient with total left main trunk occlusion and good collaterals showed retrograde biphasic flow. The preoperative LAD examination of the 3 patients with total LAD occlusion showed antegrade flow. As Watanabe and associates¹² reported previously, it is presumed that in these 3 patients the sample volumes were located distal to the collateral input origin. In group C the factor causing the unsuccessful imaging of the LAD postoperatively is thought to be adhesion induced by surgical intervention. In group D both preoperative and postoperative measurements were unsuccessful, and this was probably due to anatomic factors, such as chest shape, subcutaneous fat, lung adhesion, or emphysema. Our data show that before an emergency operation, this method would be available.

Several studies have reported that in patients who undergo left ITA to LAD bypass, it is useful to evaluate the left ITA graft by means of TTDE for the assessment of graft patency.^13-18

However, this method has some disadvantages, as follows. First, because the ITA is no longer in its original location in the chest wall and there are many variations in position of the grafted ITA after CABG, the ITA graft could not be easily identified. In part, this is due to the fact that ultrasonography does not penetrate bone or air-filled organs (ie, lungs)¹⁵ or to variations in the shape of the chest wall.¹³ Second, previous studies have shown that residual flow in the recipient artery might compete with flow in the patent graft and reduce flow in the graft and that the bypass graft will be occluded or become very thin when a stenotic lesion of the recipient vessel is mild or regressed and when myocardial ischemia in the relevant region is absent.^19-21 These reports suggest that it is more important to evaluate myocardial perfusion in the LAD region rather than to evaluate graft patency. Third, ITA graft flow seems to be influenced by its residual branches. Fourth, only assessment of an ITA to LAD graft can be performed by using this method.

On the other hand, our technique has some advantages, as follows. First, because the distal LAD runs in the interventricular groove near the chest wall, it is more easily accessible to imaging by means of transthoracic ultrasonography, even after CABG, and in many cases the location of the LAD remains in its original location after the operation. Thus our technique would provide more accurate information for comparison of LAD flow before and after CABG. Second, in contrast with the assessment of the ITA graft, in which only the left ITA graft to the LAD is available for imaging, our method provides for the assessment of any graft to the LAD. Third, because our method evaluates the LAD flow directly, this method would be available without consideration of complicated native flow after multirevascularization by means of CABG, consisting of several grafts, or even after hybrid therapy, consisting of CABG and angioplasty.

Subgroup analysis: conventional CABG versus OPCAB
We found a tendency for a greater increase in flow after conventional CABG than after OPCAB in all parameters. OPCAB is associated with temporary myocardial ischemia-related complications.^22,23 Temporary snaring of coronary arteries during heart beating and reperfusion after anastomosis might cause a condition that is similar to the no-reflow phenomenon. It is documented that in a no-reflow zone myocardial blood flow is reduced and coronary vascular resistance is increased.²⁴ These mechanisms are compatible with our data, but further study is necessary to clarify these relationships.

Study limitations
First, the study population consisted of only patients with patent grafts. No patient with anastomotic stenosis was included in this study, and the LAD flow pattern in patients with anastomotic stenosis could not be analyzed. Second, we did not perform any other special examinations for the assessment of myocardial perfusion, such as myocardial scintigraphy or dobutamine stress echocardiography. If we had the data of these examinations, the relationship between the LAD flow pattern and myocardial perfusion would be clearer.

Conclusions
We have described changes in the LAD flow after CABG. Measurement of LAD flow with TTDE could allow noninvasive evaluation of the direct effect of bypass grafting on the coronary circulation.

	Acknowledgments

 
We thank Mr Yuzo Sakagami, Mr Masakazu Ueda, and Mr Yoshiki Jyonishi for excellent technical assistance. We are also very grateful to Dr Hiroshi Ito for his advice.

	References

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