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J Thorac Cardiovasc Surg 1994;108:549-555
© 1994 Mosby, Inc.
SURGERY FOR ACQUIRED HEART DISEASE |
Duesseldorf, Germany
Received for publication May 11, 1993. Accepted for publication Dec. 14, 1993. Address for reprints: Hermann J. Sons, MD, Department of Thoracic and Cardiovascular Surgery, Herzzentrum Kaiser Wilhelm Krankenhaus, Gerrickstrasse 21, D-47137 Duisburg, Germany.
Abstract
Ultrasonic duplex scanning was used to examine 211 internal thoracic arteries. The investigated vessels were classified as normal, abnormal, and occluded. The results of the duplex examination were compared with angiography as the reference method. The diameter measurements showed virtually no differences between the two methods. Normal internal thoracic arteries showed a continuous decrease of the systolic flow velocities from proximal to distal and a narrow to moderate spectral flow curve, whereas arteries classified as abnormal showed a velocity profile distinct from that—in particular, no decrease of the systolic peak velocities and an increased spectral broadening during systole with peak frequencies greater than 4 kHz at 60 degrees (>1.2 m/sec). In occluded vessels no flow could be detected. The majority of changes were found in the proximal part of the internal thoracic artery. All lesions were detected by duplex sonography. Six normal vessels were misjudged as abnormal by the duplex method. The sensitivity, specificity, and accuracy of duplex sonography compared with angiography as the reference method were 100% (95% CI, 74.4% to 100%), 96.9% (95% CI, 93.2% to 98.8%), and 97.2% (95% CI, 93.6% to 98.8%), respectively. Duplex sonography is a reliable, noninvasive technique for the preoperative assessment of the internal thoracic artery. It allows the detection of potential atherosclerotic changes in the internal thoracic artery and the assessment of adequacy of caliber and flow. (J THORAC CARDIOVASC SURG 1994;108:549-55)
Superior long-term patency and better clinical results 
1,2 have led to the preferential use of the internal thoracic artery for myocardial revascularization. Despite being "protected" from atherosclerosis by specific histologic, 3,4 ultrastructural, 3 and histochemical 5,6 properties, the internal thoracic artery is not totally free from atherosclerosis. 7-9
So far, preoperative investigations have been based on invasive techniques, that is, arteriography or digital subtraction angiography. Apart from our own experiences, 10,11 very little information has been reported about the use of duplex sonography for the preoperative evaluation of the internal thoracic artery without angiographic assessment, as well. 12-14
Before starting this study, we performed a series with 30 healthy volunteers to define the criteria of Doppler signals obtained from the internal thoracic artery from a normal subject. 15 Additional information was collected from existing investigations of the vertebral arteries, 16-18 and parallels were drawn between these two anatomically related vessels.
The aim of this study was to evaluate flow patterns of the internal thoracic arteries and to identify normal, abnormal, and occluded vessels by means of duplex sonography with angiography as the reference method.
PATIENTS AND METHODS
Patient population
The study included 117 unselected patients (mean age 56.8 years, range 22 to 75 years; 84 male, 33 female), all admitted to the department of cardiology for left heart catheterization between January 1988 and August 1989. All patients gave their informed consent to participate in the study.
The ultrasonic examinations were performed before angiography. All patients underwent the duplex examination; five of them had only a unilateral investigation because of problems not related to the ultrasonic procedure. Bilateral thoracic arteriography was performed in all but one patient, who had unilateral angiography. A total of 228 vessels could be evaluated. Seventeen of them were not evaluable either by duplex (n = 10) or by arteriography (n = 6) or by either method (n = 1); thus a final comparison could be made in 211 cases, representing 92.5% of the evaluated internal thoracic arteries.
Duplex sonography
The duplex unit (ATL Mark 500, Advanced Technology Laboratory, Seattle, Wash.) consisted of a 7.5 MHz annular array short-focus transducer for the two-dimensional images and a 5 MHz short-focus variable single gate pulsed Doppler device with a focal zone of 1 to 4 cm and a lateral resolution of 1.0 mm. The sample volume size was approximately 3 mm3 with a depth of 3 cm. The Doppler signals were processed through fast Fourier transform and displayed so that the frequency was expressed as a function of time and the amplitude was shown in shades of gray. Simultaneous electrocardiographic recordings provided a continuous reference.
The patient lay in a relaxed supine position. The examination was started from the supraclavicular fossa by identifying the subclavian artery. This vessel was followed until the origin of the internal thoracic artery was detected. Measurements were taken on each side and across the thorax along the course of the artery at the intercostal spaces. The measurements were performed with an approximate angle of 60 degrees between the Doppler beam and vessel axis and consisted of the audible interpretation of the velocity signals, registration of spectral curves, and two-dimensional B-mode images on light-sensitive paper. The proximal relative diameter of the internal thoracic artery was measured by means of a caliper, and the true diameter was calculated by comparison with the reference scale on the display. In addition, all data were recorded on videotape for later review. The internal thoracic arteries were classified as normal, abnormal, and occluded according to the observations derived from the Doppler signals and the two-dimensional images.
Arteriography
Semiselective internal thoracic arteriography was performed at the end of routine left heart catheterization. After an initial flush injection into the subclavian artery, the catheter was positioned near the origin of the internal thoracic artery. Posteroanterior views were used routinely, with 20-degree oblique views occasionally taken to visualize the artery away from the paraspinal density. The entire course of the artery was covered from its origin to the distal bifurcation. The arteriograms were recorded on 35 mm movie film and reviewed on a Siemens movie projector (Siemens Corp., New York, N.Y.). Vessels with no evidence of stenosis or wall irregularities were classified as normal, those with discerned luminal narrowing independent of its degree as abnormal, and those with lack of opacification as occluded. The relative diameter of the proximal artery was measured by means of a caliper and correlated to the known diameter of the catheter tip.
Data assessment
The ultrasonic tests were carried out and analyzed by one investigator without knowledge of the angiographic findings, and the angiograms were performed and evaluated by one cardiologist without knowledge of the ultrasonic findings.
Data analysis
The detected lesions were described. The ultrasonic and angiographic diameters were subjected to regression analysis. Comparisons were made by the Student's t test. Sensitivity, specificity, and accuracy were defined as the ability of duplex scanning to identify disease and normality, defining disease as abnormality or occlusion. Predictive values of the method were calculated. The software of the SAS Institute (Cary, N.C.) was used for all statistical analyses.
RESULTS
Diameter
The mean diameter of the proximal right internal thoracic artery by duplex sonography was 2.6 ± 0.6 mm, and that of the proximal left internal thoracic artery was 2.4 ± 0.6 mm. By angiography, the mean diameter of the proximal right internal thoracic artery was 2.8 ± 0.7 mm, and that of the proximal left internal thoracic artery was 2.6 ± 0.7 mm. Both regression equations (Fig. 1) demonstrate virtually no difference between measurements taken by duplex sonography compared with angiography.
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Table I). A narrow to moderate spectral flow curve during systole, systolic peak frequencies of 4 kHz or less at 60 degrees (
1.2 m/sec), antegrade direction of flow, and no visible morphologic changes are characteristic of this classification (Fig. 2). Abnormal classification indicated that internal thoracic arteries showed no decrease of the systolic velocities, the peak systolic velocity exceeding 1.2 m/sec at different sites (Table II). A moderate increase in spectral broadening during systole, systolic peak frequencies greater than 4 kHz at 60 degrees (>1.2 m/sec), antegrade direction of flow, and visible morphologic changes, as shown in Fig. 3, typify this classification. In occluded vessels no flow characteristics could be detected (Fig. 4).
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Table III). All proximal lesions were detected by duplex sonography, one median obstruction was misidentified as proximal, and the only peripheral stenosis was misidentified as median.
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Duplex sonography is a reliable noninvasive diagnostic tool for the preoperative assessment of the internal thoracic artery. The two-dimensional B-mode image serves as a pathfinder providing the investigator with information about anatomic relations and the structure of the visualized vessel, thus permitting an accurate Doppler measurement. Systolic peak velocity, spectral width, and direction of flow are the Doppler landmarks indicating normality or disease. According to the criteria mentioned earlier, normal, abnormal, and occluded internal thoracic arteries could be identified. The sonographic results were highly consistent with the angiographic results. Identification of disease was feasible in all cases, whereas six normal (in terms of angiography) vessels were misclassified by duplex sonography as abnormal. The sensitivity, specificity, and accuracy obtained at this study are superior to those of comparable investigations of the vertebral arteries. 16-18
All proximal lesions were detected. The differences in varying between median and proximal compared with peripheral and median lesions can be explained by problems with the transthoracic approach at the intercostal spaces.
We did not attempt to assess the degree of the stenoses by duplex sonography. The degree of narrowing is proportional to the increase of the peak velocity, assuming the transducer is correctly placed over the area of stenosis. With regard to the relatively small internal thoracic artery as compared with the size of the sample volume, even in cases with a normal laminar flow profile too much reflection is obtained from the blood cells moving near the vessel wall. Under these circumstances the categorization of stenoses based on the interpretation of the spectral analysis would be very uncertain. Retrospectively, spectral broadening with threshold peak frequencies has led to the misclassification of six normal vessels. Spectral broadening is a sign of minor wall abnormalities if obtained from the carotid arteries. However, in the internal thoracic arteries, as in the vertebral arteries, 16 it might be a normal finding within a certain range, caused by the reflections obtained because of the relatively large-sized sample volume. These pitfalls are reflected by a 100% sensitivity of the duplex method in this study and are also supported by observations 19 that duplex sonography overestimates luminal narrowing within the carotid arteries. In addition, a deviating course or tortuosity of the internal thoracic artery may be responsible for turbulences generating complex Doppler signals. Spectral broadening may also occur in areas with (minor) wall lesions not detected by conventional angiography.
The number of cases that could not be evaluated by duplex sonography (4.8%) was somewhat lower compared with the vertebral arteries. 16-18 Obesity, a short neck, and a pyknic constitution interfered seriously with the examination.
Spierenburg and associates 12 studied both left and right internal thoracic arteries either from the supraclavicular fossa or from the third and fifth intercostal spaces. The mean internal thoracic artery diameter and the mean peak systolic velocity were similar to the values that were obtained from normal internal thoracic arteries in this study.
Canver and colleagues 13 insonated the left internal thoracic artery through the third intercostal space in 59 patients undergoing repeat coronary operations and in 105 patients who were scheduled for first-time revascularization. Mean internal thoracic artery diameter and mean peak systolic velocity were not different in the two groups and were comparable to the values that were measured in normal internal thoracic arteries of our population.
Van Son and coworkers 14 found that the luminal diameter of the internal thoracic artery and its hemodynamics, which were evaluated by duplex sonography at the level of the first intercostal space, did not differ among 15 preoperative patients and young and older control groups.
The relative freedom of the internal thoracic artery from severe atherosclerosis and the lack of angiographic verification might explain the absence of abnormal internal thoracic arteries in the aforementioned studies.
In this study, however, the validity of ultrasonography in detecting abnormal internal thoracic arteries could be established by angiography as the reference method. In this context we have also demonstrated the clinical applicability of duplex scanning in the preoperative detection of potential atherosclerotic changes in the internal thoracic artery and the assessment of the adequacy of caliper and flow. The preoperative assessment is of particular value in patients who underwent previous sternotomy or are at risk 20 for atherosclerotic changes in the internal thoracic artery.
Footnotes
From the Departments of Thoracic and Cardiovascular Surgery (Arbeitsgruppe Biometriea) and Cardiology,b Heinrich-Heine-Universitaet, Duesseldorf, Germany. 
References
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