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European Journal of Echocardiography 2008 9(2):273-277; doi:10.1016/j.euje.2007.03.030
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2007. For permissions please email: journals.permissions@oxfordjournals.org

Left internal mammary artery bypass dysfunction after revascularization of moderately narrowed coronary lesions. Colour-duplex ultrasound versus angiography study

Juraj Madaric1,2,*, Augustin Mistrik1, Igor Riecansky1, Ivan Vulev1, Jozef Pacak1, Katia Verhamme3, Bernard De Bruyne2, Viliam Fridrich1 and Jozef Bartunek2

1 National Cardiovascular Institute, Bratislava, Slovak Republic
2 Cardiovascular Centre Aalst, OLV-Clinic, Aalst, Belgium
3 Department of Epidemiology, OLV-Clinic, Aalst, Belgium

Received 18 September 2006; accepted after revision 25 March 2007; online publish-ahead-of-print 22 June 2007.

* Corresponding author. Department of Cardiology, National Cardiovascular Institute, Pod Krasnou horkou 1, 833 48 Bratislava, Slovak Republic. Tel: +421 903 556 831; fax: +421 254 788 736. E-mail address: jurmad{at}hotmail.com (J. Madaric).


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conclusion
 References
 
Background: The left internal mammary artery (LIMA) is the conduit of choice for revascularization of coronary arteries and its popularity further increases in the era of mini-invasive coronary surgery. The aim of this study was first, to assess the accuracy of CDUS in predicting the LIMA graft dysfunction as compared to angiography, and secondly, to correlate the postoperative status of the LIMA graft with preoperative coronary artery stenosis severity of the bridged lesion.

Methods and results: We examined 111 patients (pts) by colour-duplex ultrasound after myocardial revascularization by LIMA bypass (3.8 ± 3.2 years after revascularization). LIMA was detected from the left supraclavicular approach at rest using the 7.5 MHz linear transducer. The ultrasound results were compared to contemporaneous angiography. The LIMA bypass patency was correlated with the preoperative coronary artery stenosis severity.

The LIMA was detected by ultrasound in 92.8% (103) pts. At angiography, LIMA was patent and functional in 85 pts (76.6%, group A); in 25 subjects LIMA was stenosed or dysfunctional (22.5%, group B). In one patient the coronary subclavian steal syndrome was detected (0.9%). Haemodynamically moderate stenosis (50–60% by preoperative quantitative coronary angiography) was grafted in 5 pts of group A (6%), but in 10 pts of group B (40%) (P < 0.0001 vs group A). A peak systolic to peak diastolic velocity ratio (SDVR) of <2.0 yielded optimal accuracy to detect the absence of LIMA bypass dysfunction with a negative predictive value of 95%.

Conclusion: 1. Revascularization of angiographically moderate coronary lesions is associated with a higher risk of postoperative graft dysfunction. 2. Colour-duplex ultrasound is a useful non-invasive tool for the postoperative follow-up of pts with a LIMA graft.

Keywords: Duplex ultrasound; Myocardial revascularization; CABG; LIMA; Internal mammary artery graft


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conclusion
 References
 
The left internal mammary artery (LIMA) is the conduit of choice for revascularization of coronary arteries and is increasingly used in the era of minimally invasive coronary surgery. Long-term postoperative follow-up demonstrated superior patency, reduced incidence of recurrent angina and better survival after using the LIMA graft as compared to saphenous vein grafts.1 Non-cardiac complications associated with the use of LIMA2 are of minor importance compared to advantages of arterial grafts.

The rate of the mammary artery graft dysfunction is about 10% at 10 to 15 years after revascularization.3 As the atherosclerotic involvement of the mammary artery is very unusual4,5 the causes of mammary graft failure remain unclear. Previous studies suggested the competitive flow throughout the moderately narrowed bypassed coronary artery as the most critical factor affecting bypass dysfunction.6,7

With the extensive use of LIMA for coronary revascularization, a non-invasive diagnostic testing with direct visualization and functional evaluation could be of interest for the clinical follow-up. The value of transthoracic colour-duplex ultrasound (CDUS) imaging for preoperative evaluation of the native LIMA as well as for the postoperative detection of LIMA graft patency was recently suggested.8–15

The objective of our study was twofold. First, we compared the accuracy of CDUS to detect LIMA graft dysfunction as compared to coronary angiogram. Secondly, we analysed the relationship between the preoperative coronary artery stenosis severity of the grafted artery and postoperative LIMA bypass patency.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 Conclusion
 References
 
Patient population
Between 1999 and 2005, we examined 452 LIMA grafts by CDUS in patients (pts) with prior cardiac bypass (CABG) surgery. Among them, 111 pts (mean age 58 ± 8 years) underwent coronary angiography due to chest pain or dyspnoea or evidence of ischaemia on non-invasive stress testing. In all these patients CDUS was performed before angiography within 24 h.

Colour-duplex ultrasound
Ultrasound examination was performed using Hewlett Packard 2500 and 5500 units equipped with a 7.5 MHz linear transducer. Transthoracic visualization of the LIMA was performed in a supine position from the left supraclavicular approach. Using two-dimensional colour flow mapping, the position of the proximal LIMA was identified 1–2 cm after its origin from the left subclavian artery. Intraluminal flow signal was obtained using the pulsed Doppler method with 2–3 mm sampling volume located within the vessel lumen. Angle correction was applied for the velocity measurements. The following parameters were measured: the peak systolic velocity (PSV – cm/s), the peak diastolic velocity (PDV – cm/s), and the end-diastolic velocity (EDV – cm/s). The peak systolic to peak diastolic velocity ratio SDVR (PSV/PDV) and the resistance index: RI = (PSV – EDV)/PSV were calculated. CDUS was always performed before coronary angiography by an experienced observer.

Angiography
Selective left internal mammary artery bypass angiography was performed at the end of routine left heart catheterization. Quantitative analysis (Medis) was performed with a computerised digital system in 2-orthogonal planes using a 6 fr catheter as a calibration device. The LIMA graft was considered being dysfunctional if its occlusion or significant stenosis (>60%) was present, or when slow run-off and persistently small caliber not responding to nitrates were detected.

To analyse the LIMA graft function in relation to the native coronary artery stenosis severity before CABG, the preoperative coronary angiograms were studied retrospectively. On the preoperative angiogram, coronary lesions with stenosis diameter 50–60% were considered as moderate, lesions with stenosis diameter >60% were considered as significant.

Statistical analysis
All data are presented as mean ± SD for continuous data and as a ratio for categorical data. Gaussian distributions of data were tested by means of the Kolmogorov—Smirnov test. Unpaired t-test or non-parametric Mann—Whitney U-test was used to compare the results of the patients with or without patent LIMA. Fisher exact test was used to compare categorical data. Positive and negative predictive values of CDUS and SDVR <2 to predict the LIMA graft dysfunction were determined using angiography as the gold standard. For all analysis, a P-value of >0.05 was considered non-significant. Statistical analysis was performed with the SPSS/PC 11.5 software.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conclusion
 References
 
Baseline characteristics
At control angiogram 25 pts (22.5%) showed a dysfunctional LIMA graft (group B) and 85 pts (76.6%) showed a functional LIMA graft (group A). In 18 pts, angiography showed lesion or occlusion at the distal anastomosis. In 7 pts, the LIMA showed lesion or occlusion at the proximal site or a string sign with a slow run-off. Baseline characteristic of pts with angiographically patent and dysfunctional LIMA grafts are given in Table 1. There were no differences in age, gender, as well as in cardiovascular risk factors between patients with patent and dysfunctional LIMA grafts.


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  		Table 1 Baseline characteristics

 
LIMA graft characteristics
Table 2 shows characteristics of the LIMA grafts in both groups. In all but 6 cases, the LIMA bridged the left anterior descending coronary artery stenosis. In 6 cases, the LIMA grafted the diagonal branch. There was no difference between the age of LIMA grafts in both groups or in the type of surgical procedure (coronary artery bypass grafting – CABG in extra corporal circulation versus off-pump coronary artery bypass – OPCAB). However, the proportion of moderately narrowed grafted lesions, as assessed from the preoperative coronary angiography, was significantly higher in the group of dysfunctional bypasses [10 (40%)] compared to the group of patent grafts [5 (6%), P < 0.0001].


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  		Table 2 LIMA graft characteristics

 
Colour-duplex ultrasound
In CDUS, 103 out of 111 LIMA grafts (92.8%) were visualized. All 8 grafts not visualized by CDUS were patent on angiography. Figures 1 and 2 show examples of the flow detected in the normal and dysfunctional LIMA grafts. The CDUS parameters of patent and dysfunctional LIMA grafts are given in Table 3. Peak systolic velocity was similar in dysfunctional and patent grafts. In contrast, peak diastolic velocity and end-diastolic velocity were lower in dysfunctional LIMA grafts as compared to patent grafts. In addition, LIMA flow characteristics were affected by the site of the occlusion or stenosis. In patients with ‘distal LIMA dysfunction’ (stenosis at distal anastomosis, 18 pts), the flow pattern was associated with a decrease in the diastolic flow velocities. In patients with proximal LIMA occlusion (7 pts), the Doppler spectrum showed absence of the diastole flow component in the portion proximal to the LIMA occlusion. In none of the patients the sole mid portion of the LIMA was affected.


Figure 1
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  		Figure 1 Example of flow in a patent functional LIMA graft. Normal LIMA graft is characterized by biphasic antegrade flow with dominant diastolic component. In early diastole the blood flow increased rapidly, than gradually decreased and fell rapidly in late diastole; S – systole, D – diastole; PSV = 79.8 cm/s, PDV = 62.4 cm/s, EDV = 30.3 cm/s, SDVR = 1.28, RI = 0.62.

 


Figure 2
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  		Figure 2 Example of flow in a dysfunctional LIMA graft. Dysfunctional LIMA graft is characterized by broader Doppler spectrum with dominant systolic flow caused by significant decrease in diastolic flow velocities; S – systole, D – diastole; PSV = 49.3 cm/s, PDV = 17.3 cm/s, EDV = 11.5 cm/s, SDVR = 2.88, RI = 0.77.

 


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  		Table 3 Color duplex ultrasound – patent vs. dysfunctional LIMA grafts

 
To evaluate the diagnostic value of CDUS and SDVR in assessment of LIMA graft patency discriminant analysis was performed to identify optimal value of SDVR to predict LIMA graft function. The value of SDVR <2 showed the highest negative predictive value for normal LIMA graft (95%). Nevertheless, its positive predictive value was only 52.4%.

Using SDVR <2 as a cut-off value, the CDUS showed agreement with qualitative angiographical evaluation in 81 cases (79%). There were 3 false negative (3%), and 19 false positive CDUS results (18%). Negative predictive value of CDUS was 95%, positive predictive value 52.5%.

In one patient with LIMA 5 years after the surgery, CDUS detected retrograde systolic flow followed by preserved antegrade diastolic component. Aortic arch angiography and selective coronary angiography confirmed the diagnosis of coronary subclavian steal syndrome caused by the significant stenosis of the left subclavian artery (Figure 3).


Figure 3
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  		Figure 3 Coronary subclavian steal syndrome. Retrograde systolic flow followed by preserved antegrade diastolic component; S – systole, D – diastole; PSV = 38.6 cm/s, PDV = 81.2 cm/s.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conclusion
 References
 
The results of our study can be summarized as follows: 1. Coronary artery bypass grafting of moderately narrowed stenosis has a higher risk of postoperative conduit dysfunction compared to grafts bridging severe coronary lesions. 2. Colour-duplex ultrasound is a useful non-invasive technique for the postoperative follow-up of patients with a LIMA graft with a high negative predictive value for LIMA graft dysfunction.

Growing evidence suggests that CDUS can be a reliable diagnostic tool for the preoperative and postoperative assessment of the internal mammary artery.8–15 Our rates of LIMA graft visualization are within the range of previous studies reporting 81–98% visualization.9,12,14 However, visualization success depends on the imaging approach. The LIMA graft can be imaged either from the left supraclavicular fossa10–14 or from the left parasternal window of the intercostal spaces.9,15 As the native course of the LIMA is often modified by the surgery,we prefer the left supraclavicular approach yielding higher detection rate and reproducible detection at repeated scans. It is interesting to note that we observed LIMA graft lesions exclusively in the proximal segment of the LIMA or at the distal anastomosis and in none of the cases was the mid ‘intrathoracic’ part of the vessel affected.

The site location of the obstruction resulted in 2 distinct patterns of the Doppler spectrum. In the case of proximal occlusion, a LIMA graft functions as a blind-ended tube associated with a monophasic systolic flow and the loss of a diastolic component proximal to the place of occlusion. In the case of distal stenosis/occlusion the Doppler spectrum was characterized by a decrease in the diastolic flow component with the broadening of the Doppler spectrum, but not the complete loss of diastolic flow (Figure 2). Therefore, suspicion of graft dysfunction should be considered if the Doppler spectrum displays no flow or a significant reduction of the diastolic component, and systolic dominance.

Accordingly, these qualitative observations led us to hypothesize that an index incorporating both diastolic and systolic flow components could better reflect the impact of lesion on haemodynamics rather than sole analysis of the diastolic component. This is consistent with previous studies of intracoronary pressure measurements, where consideration of average (mean) pressure differences across the stenoses was more accurate than the diastolic pressure gradient alone.16 Thus, in order to better evaluate haemodynamic importance of the given lesion, both diastolic and systolic parameters should be taken into account.

Our findings of a higher proportion of moderately narrowed lesions (50–60% diameter reduction) in the preoperative coronary angiogram in patients showing the LIMA dysfunction is consistent with the recent report of Berger et al.7 They demonstrated that the diameter stenosis of the native coronary artery is an independent predictor for internal mammary artery graft patency at follow-up. Alternatively, other factors affecting the LIMA flow characteristics include poor distal coronary artery vessel quality, residual LIMA side branches, and the contractility and viability of the perfused myocardium.17 In our patient cohort, LV ejection fraction was similar between both groups.

Limitations
In our study, assessment of the LIMA graft functional status was performed at rest. It is likely that a stress study using administration of vasodilators (adenosine, dipyridamole) with evaluation of flow reserve in the LIMA graft could have increased the sensitivity and accuracy of the CDUS examination. Likewise, quantification of the coronary artery stenosis severity before CABG, as well as LIMA bypass stenosis severity after CABG, was performed without functional assessment of the stenosis severity. In addition, the present study included patients referred to clinical-driven evaluation of graft patency at angiography. Therefore, future prospective studies are needed to define the value of the CDUS for evaluation and clinical decision-making in follow-up of patients with previous LIMA bypass surgery.


   Conclusion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
References
 
Revascularization of angiographically moderate coronary lesions is associated with a higher risk of postoperative LIMA graft dysfunction. Therefore, the use of arterial conduits should be reserved only for bridging of clearly and unequivocally confirmed significant coronary artery stenosis. The colour-duplex ultrasound technique is a useful functional non-invasive tool for the postoperative follow-up of patients with the LIMA bypass, mainly for its appreciatively high negative predictive value of LIMA graft dysfunction. Further prospective studies are needed to establish the value of CDUS in the early diagnosis of coronary bypass failure.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conclusion
 References
 

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  13. Voudris V, Athanassopoulos G, Vassilikos V, Avramides D, Manginas A, Michalis A, et al. Usefulness of flow reserve in the left internal mammary artery to determine graft patency to the left anterior descending coronary artery. Am J Cardiol (1999) 83:1157–63.[CrossRef][Web of Science][Medline]
  14. Chirillo F, Bruni A, Balestra G, Cavallini C, Olivari Z, Thomas JD, et al. Assessment of internal mammary artery and saphenous vein graft patency and flow reserve using transthoracic Doppler echocardiography. Heart (2001) 86:424–31.[Abstract/Free Full Text]
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