(Radiology. 2000;217:278-283.)
© RSNA, 2000
Coronary Artery Bypass Grafts: Improved Electron-Beam Tomography by Prolonging Breath Holds with Preoxygenation1
Christian N. H. Enzweiler, MD,
Dietmar E. Kivelitz, MD,
Till H. Wiese, MD,
Matthias Taupitz, MD,
Sebastian Höhn, MD,
Adrian C. Borges, MD,
Lars Pietsch, MD,
Pascal Dohmen, MD,
Gert Baumann, MD and
Bernd Hamm, MD
1 From the Departments of Radiology (C.N.H.E., D.E.K., T.H.W., M.T., S.H., B.H.), Internal Medicine I (A.C.B., G.B.), and Cardiovascular Surgery (L.P., P.D.), Charité, Humboldt-Universität Berlin, Schumannstrasse 20/21, 10098 Berlin, Germany. Received July 16, 1999; revision requested September 1; revision received January 7, 2000; accepted February 7. Address correspondence to C.N.H.E. (e-mail: christian.enzweiler@charite.de).
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ABSTRACT
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In 45 patients with coronary bypass grafts, the breath-hold interval with and that without preoxygenation was measured. Its effect on depiction of the distal graft anastomosis at electron-beam tomography was evaluated. Preoxygenation prolonged the breath-hold interval in most patients, thereby allowing greater anatomic coverage including more distal anastomoses. Preoxygenation may improve scanning of coronary bypass grafts and increase detectability of graft stenoses.
Index terms: Computed tomography (CT), electron beam, 54.12112, 54.12116, 54.12117, 54.12118, 54.12119 • Coronary angiography, comparative studies, 54.1244 • Coronary vessels, CT, 54.1211 • Coronary vessels, stenosis or obstruction, 54.754
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INTRODUCTION
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Electron-beam tomography is characterized by high temporal resolution. Electrocardiographically triggered image acquisition allows almost motion-free depiction of the coronary arteries. Nonenhanced studies aim at the detection and quantification of calcium plaques as an indicator of preclinical coronary artery disease (1–5). Contrast material–enhanced studies enable noninvasive depiction of the coronary arteries (3,6–8) and coronary bypass grafts (9–11). Patency of coronary bypass grafts can be reliably assessed with electron-beam tomography (7,9–11). Detection of bypass graft stenosis is poor (10,11), however, which may be attributable to acquisition of contiguous rather than overlapping scans in contrast-enhanced coronary studies (9–12). To depict both proximal and distal anastomoses of bypass grafts, the distance covered in the z axis needs to be greater than that covered in coronary artery scanning. Consequently, a larger number of scans must be acquired.
In electrocardiographically triggered scanning of coronary bypass grafts, overall scanning time depends on the patient’s heart rate. Breathing artifacts on images obtained in patients with low heart rates may compromise studies (9,13,7); therefore, overlapping scanning is usually avoided to reduce breathing artifacts.
Preoxygenation prolongs the breath-hold interval and facilitates intubation of patients undergoing general anesthesia (14–18). After filling of the intrapulmonary oxygen store with 100% oxygen and total nitrogen washout, the functional residual capacity of the lungs may contain as much as 90% oxygen (2,500 mL) (19). During apnea, the pulmonary oxygen reservoir sustains a normal arterial oxygen level for an extended time (apneic oxygenation), which allows a prolonged breath-hold interval and should enable the acquisition of more scans without a parallel increase in breathing artifacts.
The aim of this study was to determine the effect of preoxygenation on the breath-hold interval in patients with coronary artery bypass grafts. Second, the effect of preoxygenation on depiction of distal anastomoses at electron-beam tomography was evaluated with a coronary imaging protocol with overlapping sections.
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Materials and Methods
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Forty-five patients (34 men and 11 women; age range, 39–78 years; mean age, 61 years) with a total of 109 coronary artery bypass grafts (venous, 77; left internal mammary artery, 32) referred to undergo contrast-enhanced electron-beam tomography were included in the study after giving informed consent. This study was approved by a local ethics committee. Twenty-one of 109 (19.3%) bypass grafts were occluded and were excluded from the analysis. All studies were performed at 625 mA and 130 kV (Evolution XP-150; Imatron, San Francisco, Calif). Patients were placed on the scanner couch in a supine position headfirst.
Comparison of Breath-Hold Intervals with and Those without Preoxygenation
The efficacy of a commercially available preoxygenation device (NasOral; Klinika, Neufra, Germany) to prolong the breath-hold interval at electron-beam tomography was tested in all patients. The system was connected to a continuous flow of 100% oxygen, and the nose mask was attached to the patient’s head. In addition to the nose mask, the tubes, and the reservoir bag, the preoxygenation device comprises a mouthpiece with an integrated uniflow valve that allows only expiration (20–22). To prevent a feeling of anxiety if a patient needed to breathe during data acquisition, this mouthpiece was not used in our study. On the basis of the principle of unidirectional flow, patients were asked to breathe in through the nose mask of the preoxygenation device and breathe out through the mouth.
The flow rate was adapted to the individual needs of each patient. Patients were instructed to clearly signal the beginning and end of each breath-hold period by raising an arm. After they breathed pure oxygen for 60 seconds, the breath-hold interval was timed with a stopwatch. In addition, the patient was observed and respiratory movements were noted. The analogous procedure was performed without the preoxygenation device with room air conditions. The order of the two breathing tests was randomly selected, and the interval between the tests was 8–10 minutes.
Patients were not trained to breathe with the preoxygenation system prior to the test. The duration of the breath-hold interval with and that without preoxygenation was documented. Results were statistically evaluated with the Wilcoxon matched-pairs signed rank test.
Electron-Beam Tomography of Coronary Bypass Grafts with Preoxygenation
A three-step protocol was used for scanning of bypass grafts. First, an eight-level localization scan was obtained in the "Multislice" mode to choose the starting position (50-msec exposure time). Second, the circulation time in the ascending aorta was determined after injection of 10 mL of iopromide (Ultravist 370 [370 mg of iodine per milliliter]; Schering, Berlin, Germany) at 4 mL/sec via an intravenous catheter inserted in an antecubital vein. Thirty consecutive electrocardiographically triggered scans were acquired on two adjacent levels at the time of injection (baseline) and then at 3-second intervals. Patients breathed during determination of circulation time. In 17 (38%) patients with heart rates below 80 beats per minute, atropine sulfate (0.2–0.4 mg) was administered intravenously to increase heart rate before determination of circulation time. The actual change in heart rate was not documented.
Preoxygenation was performed in all patients immediately before scanning. The preoxygenation device was not removed prior to image acquisition. Instead, the oxygen supply was turned off during the breath-hold interval, and patients were instructed to breathe through the mouth if necessary.
Examinations were performed in a craniocaudal orientation starting at the level of the lower half of the aortic arch. Sixty scans (section thickness, 3 mm; increment, 2 mm) were acquired at end inspiration with an exposure time of 100 msec per image triggered at 80% of the R-R interval (field of view, 18 cm; matrix, 512 x 512). Contrast material (180 mL) was administered intravenously at 3 mL/sec (delay time equals circulation time) during 60 seconds.
Immediately after the bypass graft studies, one radiologist (S.H.) asked the patients about possible adverse reactions to preoxygenation and whether they thought they profited from it.
All studies were reviewed by two radiologists (C.N.H.E., S.H.) in a consensus reading with a commercially available workstation (MagicView; Siemens, Erlangen, Germany). The transverse images were reviewed in two groups: group A, scans 1–40, covering 8 cm in the z axis; group B, scans 1–60, covering 12 cm in the z axis. Depiction of the distal bypass graft anastomosis was evaluated in each image group. The criterion for graft patency was contrast enhancement over the entire length of the bypass graft. The distal graft anastomosis was considered to be included in the data set if the fusion of the bypass graft with the coronary artery was depicted on three or more consecutive scans or over at least 6 mm in the case of an in-plane vessel course.
Bypass grafts were divided into three groups according to the bypassed coronary artery. No distinction was made between grafts to the main vessels (left anterior descending coronary artery [LAD], left circumflex coronary artery [LCX], and right coronary artery [RCA]) or to one of their branches. In the case of bypass grafts to the LAD, no distinction was made between venous or left internal mammary artery grafts. Incomplete or complete depiction of the entire course of the bypass grafts including the distal graft anastomosis was documented for both image groups. Results were evaluated with the Wilcoxon matched-pairs signed rank test.
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Results
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Scanning times were 31.0 seconds ± 5.2 (mean ± SD) for group A images and 46.5 seconds ± 6.9 for group B images. Breath-hold intervals were 54.1 seconds ± 32.2 (range, 11–150 seconds) with preoxygenation and 37.2 seconds ± 23.1 (range, 8–129 seconds) without.
The preoxygenation device increased the breath-hold duration in 40 patients. In the remaining five patients, the breath-hold duration with the system was shorter than that with room air conditions. For the 40 patients, the breath-hold intervals were 57.7 seconds ± 32.2 with preoxygenation and 38.3 seconds ± 23.9 without. The maximum increase in breath-hold interval was 313% (95 seconds with preoxygenation versus 23 seconds without). The overall increase in the duration of breath holds after preoxygenation as opposed to room air conditions was significant (P < .001).
The preoxygenation device was tolerated well by all patients. No adverse reactions in terms of ischemic symptoms or anxiety were noted during or after the bypass graft studies. Subjectively, most patients (37 of 45) thought they profited from preoxygenation owing to the achievement of an extended and less exhausting breath-hold interval.
Twenty-one of 109 (19.3%) bypass grafts were occluded (Fig 1). Of the 88 patent bypass grafts, the distal anastomosis of 43 (49%) was not depicted on group A scans and of 17 (19%) on group B scans (P < .001) (Table). Of the 43 bypass grafts not depicted on the group A scans, 13 (30%) were to the LAD (Fig 1), 19 (44%) to the RCA (Fig 2), and 11 (26%) to the LCX (Fig 3). Of the 17 bypass grafts not depicted on the group B scans, one (6%) was to the LAD (P < .01), 13 (76%) to the RCA (P < .05) (Fig 3), and three (18%) to the LCX (P < .05).
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Figure 1a. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 1b. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 1c. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 1d. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 1e. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 1f. Transverse (a-d) CT scans and (e, f) shaded surface display reconstruction images of the heart in a 61-year-old man 3 years after bypass graft surgery. (a) Scan 23 (group A and B) shows two patent venous bypass grafts. One bypass graft (straight arrow) lies lateral to the main pulmonary artery, and the other is seen ventrally (right curved arrow) and at its origin from the ectatic ascending aorta (left curved arrow). (b) Scan 33 (group A and B) depicts the distal anastomosis of one bypass graft with the first diagonal branch of the LAD (closed arrow) and the origin of a third venous bypass graft (open arrow) to the RCA. (c) Scan 43 (group B) shows occlusion of the bypass graft (open arrow) to the RCA. A second patent bypass graft (curved arrow) is seen adjacent to the LAD. (d) Scan 55 (group B) shows the LAD (small arrow) distal to the anastomosis with the graft. Large arrow = RCA. (e) Group A and (f) group B shaded surface display reconstruction images were acquired with a threshold of 80 HU in the left anterior oblique view from the apex of the heart downward. Distal anastomosis (curved arrow) to the LAD is not depicted in e. Small straight arrow in e = proximal LAD. Large straight arrow in e and f = anastomosis of the bypass graft to the first diagonal branch of the LAD.
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Figure 2a. Transverse CT scans in a 67-year-old man with bypass grafts to the second diagonal branch of the LAD and to the posterior interventricular branch of the RCA. (a) Scan 2 (group A and B) shows a patent left internal mammary artery bypass (long arrow) and metal clips adjacent to the graft (bottom short arrow) and at the original site of the left internal mammary artery (top short arrow). (b) Scan 40 (group A and B) shows a patent venous bypass graft (curved arrow) adjacent to the right ventricle. Straight arrow = RCA. (c) Scan 56 (group B), obtained through the base of the heart, shows a patent bypass graft (white curved arrow) and the distal RCA (short straight arrow) with the origin of the posterior interventricular branch of the RCA (long straight arrow). Black curved arrow = posterior interventricular vein draining into the coronary sinus, L = paraesophageal lipoma. (d) Scan 58 (group B) depicts the distal bypass graft anastomosis (straight arrow) to the posterior interventricular branch of the RCA. Curved arrow = posterior interventricular vein.
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Figure 2b. Transverse CT scans in a 67-year-old man with bypass grafts to the second diagonal branch of the LAD and to the posterior interventricular branch of the RCA. (a) Scan 2 (group A and B) shows a patent left internal mammary artery bypass (long arrow) and metal clips adjacent to the graft (bottom short arrow) and at the original site of the left internal mammary artery (top short arrow). (b) Scan 40 (group A and B) shows a patent venous bypass graft (curved arrow) adjacent to the right ventricle. Straight arrow = RCA. (c) Scan 56 (group B), obtained through the base of the heart, shows a patent bypass graft (white curved arrow) and the distal RCA (short straight arrow) with the origin of the posterior interventricular branch of the RCA (long straight arrow). Black curved arrow = posterior interventricular vein draining into the coronary sinus, L = paraesophageal lipoma. (d) Scan 58 (group B) depicts the distal bypass graft anastomosis (straight arrow) to the posterior interventricular branch of the RCA. Curved arrow = posterior interventricular vein.
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Figure 2c. Transverse CT scans in a 67-year-old man with bypass grafts to the second diagonal branch of the LAD and to the posterior interventricular branch of the RCA. (a) Scan 2 (group A and B) shows a patent left internal mammary artery bypass (long arrow) and metal clips adjacent to the graft (bottom short arrow) and at the original site of the left internal mammary artery (top short arrow). (b) Scan 40 (group A and B) shows a patent venous bypass graft (curved arrow) adjacent to the right ventricle. Straight arrow = RCA. (c) Scan 56 (group B), obtained through the base of the heart, shows a patent bypass graft (white curved arrow) and the distal RCA (short straight arrow) with the origin of the posterior interventricular branch of the RCA (long straight arrow). Black curved arrow = posterior interventricular vein draining into the coronary sinus, L = paraesophageal lipoma. (d) Scan 58 (group B) depicts the distal bypass graft anastomosis (straight arrow) to the posterior interventricular branch of the RCA. Curved arrow = posterior interventricular vein.
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Figure 2d. Transverse CT scans in a 67-year-old man with bypass grafts to the second diagonal branch of the LAD and to the posterior interventricular branch of the RCA. (a) Scan 2 (group A and B) shows a patent left internal mammary artery bypass (long arrow) and metal clips adjacent to the graft (bottom short arrow) and at the original site of the left internal mammary artery (top short arrow). (b) Scan 40 (group A and B) shows a patent venous bypass graft (curved arrow) adjacent to the right ventricle. Straight arrow = RCA. (c) Scan 56 (group B), obtained through the base of the heart, shows a patent bypass graft (white curved arrow) and the distal RCA (short straight arrow) with the origin of the posterior interventricular branch of the RCA (long straight arrow). Black curved arrow = posterior interventricular vein draining into the coronary sinus, L = paraesophageal lipoma. (d) Scan 58 (group B) depicts the distal bypass graft anastomosis (straight arrow) to the posterior interventricular branch of the RCA. Curved arrow = posterior interventricular vein.
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Figure 3a. Transverse CT images in a 55-year-old man with patent bypass grafts to the LAD, RCA, and LCX. (a) Scan 18 (group A and B) shows a venous graft (straight arrow) and a left internal mammary artery bypass (curved arrow) adjacent to the main pulmonary artery. (b) Scan 40 (group A and B) shows the LAD distal to the anastomosis of the left internal mammary artery bypass with the LAD (open arrow). A third graft (wide straight arrow) is adjacent to the right atrium. Curved arrow = LCX, long straight arrow = bypass graft, short straight arrow = great cardiac vein. (c) Scan 60 (group B) shows the LCX (curved open arrow) and the LAD (straight open arrow) distal to the bypass graft anastomoses. The anastomosis of the venous bypass graft with the RCA (large straight arrow) is not depicted. Small straight arrow = great cardiac vein.
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Figure 3b. Transverse CT images in a 55-year-old man with patent bypass grafts to the LAD, RCA, and LCX. (a) Scan 18 (group A and B) shows a venous graft (straight arrow) and a left internal mammary artery bypass (curved arrow) adjacent to the main pulmonary artery. (b) Scan 40 (group A and B) shows the LAD distal to the anastomosis of the left internal mammary artery bypass with the LAD (open arrow). A third graft (wide straight arrow) is adjacent to the right atrium. Curved arrow = LCX, long straight arrow = bypass graft, short straight arrow = great cardiac vein. (c) Scan 60 (group B) shows the LCX (curved open arrow) and the LAD (straight open arrow) distal to the bypass graft anastomoses. The anastomosis of the venous bypass graft with the RCA (large straight arrow) is not depicted. Small straight arrow = great cardiac vein.
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Figure 3c. Transverse CT images in a 55-year-old man with patent bypass grafts to the LAD, RCA, and LCX. (a) Scan 18 (group A and B) shows a venous graft (straight arrow) and a left internal mammary artery bypass (curved arrow) adjacent to the main pulmonary artery. (b) Scan 40 (group A and B) shows the LAD distal to the anastomosis of the left internal mammary artery bypass with the LAD (open arrow). A third graft (wide straight arrow) is adjacent to the right atrium. Curved arrow = LCX, long straight arrow = bypass graft, short straight arrow = great cardiac vein. (c) Scan 60 (group B) shows the LCX (curved open arrow) and the LAD (straight open arrow) distal to the bypass graft anastomoses. The anastomosis of the venous bypass graft with the RCA (large straight arrow) is not depicted. Small straight arrow = great cardiac vein.
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Discussion
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The preoxygenation device tested in this study effectively enables apneic oxygen uptake despite breath holding (20–22). Preoxygenation through a nose mask led to a significant prolongation of the voluntary breath-hold interval in the majority of our patients, and all patients tolerated the preoxygenation system well. Some authors, however, believe coronary spasms might be induced during preoxygenation (23,24), and hyperventilation should be avoided during the procedure (23). Other potential adverse effects of hyperoxia are an increase in peripheral vascular resistance and myocardial ischemia (24). No adverse reactions or anginalike symptoms were noted in our study group, and the majority of patients (37 of 45) subjectively believed they profited from preoxygenation prior to scanning.
In contrast-enhanced electron-beam tomographic studies of coronary arteries, 40 scans are usually obtained with 3-mm section thickness and an increment of 2 mm to cover a distance of 8 cm in the z axis (6–8). This technique has proved to be adequate in that the entire course of the vessels may be seen and three-dimensional, angiographic reconstruction of the coronary arteries is possible. Because of the longer course of bypass grafts in the z axis, the coronary protocol is usually modified by increasing the increment to 3 mm to cover a distance of 12 cm in the z axis despite a potential decrease in quality of three-dimensional reconstruction images (9–11). This may account for the poor detectability of graft stenoses reported by several authors (10,11). In this study, we assessed the patency of bypass grafts and excluded occluded bypass grafts. We did not evaluate graft stenosis because we did not compare a 2-mm increment with the standard 3-mm table feed.
The proximal anastomosis of a venous coronary bypass graft with the ascending aorta is usually placed cranial to the origin of the coronary arteries as distal as the proximal portion of the aortic arch. The distal anastomosis of the graft, particularly with the RCA or one of its branches, may lie on the phrenic side of the heart. The distal anastomosis is crucial in terms of stenosis and occlusion owing to the difficulty of creating an anastomosis between a vein with a relatively large caliber and an artery with a small caliber. Depiction of the distal anastomosis at electron-beam tomography is difficult (10,11). Owing to the small caliber of the distal coronary arteries and the often oblique orientation of the vessels at the site of the distal anastomosis, acquisition of overlapping scans may be advantageous.
For the detection of coronary artery stenosis, shaded surface display, multiplanar reconstruction, and maximum intensity projection images are acquired in addition to the transverse images (9,12,25–29). Bypass graft patency is determined on the basis of graft enhancement after intravenous administration of contrast material (9–12). In analogy to the scanning of coronary arteries, acquisition of overlapping sections may improve the quality of three-dimensional reconstruction images of bypass grafts and thereby increase detectability of graft stenosis. The required increase in the total number of scans, however, necessitates use of a longer breath-hold period. Breathing artifacts and malpositioning of the imaging volume are considered to be the leading causes of reduced image quality and incomplete depiction of bypass grafts (9,7,13).
Our results indicate the entire course of coronary bypass grafts may be depicted on overlapping scans obtained with preoxygenation without an increase in breathing artifacts. However, we did not compare the frequency of breathing artifacts with preoxygenation to that without. Use of the preoxygenation device resulted in a significantly longer breath-hold interval in 40 of 45 patients but failed to prolong the breath-hold interval in five patients, possibly because of a lack of training with the device before the studies. Although most patients achieved a longer breath-hold interval with preoxygenation, the SD remained large in even the 40 patients; therefore, the need to breathe cannot be ruled out during the acquisition of 60 scans.
Preoxygenation may be useful in both nonenhanced and contrast-enhanced electron-beam tomography of coronary arteries, as well as in any imaging procedure in which a prolonged breath hold is desirable. The preoxygenation device consists of nonmetallic materials to eliminate beam hardening, which makes its use possible in magnetic resonance imaging. Use of the mouthpiece not used in our study may allow a longer lasting effect of preoxygenation on the breath-hold interval.
The distal anastomosis of a bypass graft to the RCA, which is typically the most caudal of the three main coronary vessels, is missed more frequently than that to the other coronary arteries. The anastomosis to the LAD is usually the most cranial. In our study, nearly half (49%) of all anastomoses to the coronary arteries were not depicted on group A scans, and all 19 anastomoses to the RCA were missed. Group B scans, acquired with the standard coverage of 12 cm in the z axis, did not depict the distal anastomosis of 19% of patent bypass grafts, and most (64%) grafts to the RCA were not completely depicted. Of anastomoses to the LAD and LCX, respectively, group A scans did not depict 24% and 73% and group B scans did not depict 2% and 20%. Consequently, a scanning volume larger than the standard 12 cm in the z axis is required to cover the entire course of all bypass grafts to the RCA and LCX, but the standard volume seems to be adequate for grafts to the LAD.
Use of prolonged scanning intervals necessitates use of increasing amounts of contrast material. In our study, a volume of 180 mL was administered. Other investigators administer between 120 and 160 mL of contrast material at flow rates of 3–4 mL/sec for coronary angiography (26,28–30) and bypass graft studies (6,9–11). In patients with low heart rates, scanning times can be effectively reduced by intravenously injecting atropine to speed up the heart rate, which would allow use of shorter breath-hold intervals and less contrast material. Atropine may cause anginalike symptoms, however, and should be used cautiously. No such symptoms occurred in our study group.
In conclusion, preoxygenation significantly prolonged the breath-hold interval in patients with coronary artery bypass grafts. Preoxygenation has the potential to reduce breathing artifacts and may improve scanning of coronary bypass grafts. As opposed to contiguous sections, overlapping sections may increase detection of bypass graft stenosis by improving the quality of three-dimensional reconstruction images, particularly of the crucial distal anastomosis. Acquisition of overlapping scans is useful, however, only when the total number of images acquired is markedly increased. Coverage of 12 cm in the z axis appears to be insufficient for the complete depiction of all coronary bypass grafts and may need to be augmented.
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ACKNOWLEDGMENTS
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We thank Alexia Lorimer, Marie-Luise Rode, and Simone Schwedler for their technical assistance with acquisition of the electron-beam tomographic images.
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FOOTNOTES
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Abbreviations: LAD = left anterior descending coronary artery,
LCX = left circumflex coronary artery,
RCA = right coronary artery
Author contributions: Guarantor of integrity of entire study, B.H.; study concepts and design, C.N.H.E., D.E.K.; definition of intellectual content, C.N.H.E., D.E.K., M.T.; literature research, C.N.H.E.; clinical studies, C.N.H.E., L.P., P.D., A.C.B.; data acquisition, C.N.H.E., T.H.W., S.H.; data analysis, C.N.H.E., T.H.W., D.E.K.; statistical analysis, C.N.H.E., S.H.; manuscript preparation, C.N.H.E., D.E.K.; manuscript editing, C.N.H.E., M.T.; manuscript review, M.T., G.B., B.H.
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ABSTRACT
INTRODUCTION
