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Cardiac Imaging |
1 From the Departments of Radiology (E.M.M., Y.H.K., H.P.M.) and Biostatistics and Bioinformatics (J.E.H.), Duke University Medical Center, Durham, NC. Received February 25, 2003; revision requested April 29; revision received June 19; accepted August 8. Address correspondence to E.M.M., Department of Radiology, Box 57, M. D. Anderson Cancer Center, Houston, TX 77030 (e-mail: emarom@di.mdacc.tmc.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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MATERIALS AND METHODS: Thin-section (2.5-mm collimation) contrast material–enhanced CT scans of 201 consecutive patients obtained over a 3-month period for diagnosis of pulmonary embolism (n = 197), pulmonary vein stenosis (n = 2), or aortic injury (n = 2) were routinely reviewed in transverse and (if necessary) coronal and coronal-oblique imaging planes. A classification was formulated based on both the number of venous ostia on each side and the drainage patterns of pulmonary veins. The frequency of each pattern was determined, and association with atrial arrhythmia was assessed with the 
2 and Fisher exact tests.
RESULTS: Most patients (n = 142, 71%) had two ostia on the right side for upper and lower lobe veins. Fifty-six patients (28%) had three to five ostia on the right side, which were due to one or two separate middle lobe vein ostia in 52 (26%) patients. Three patients (2%) had a single venous ostium on the right side. Most patients (n = 173, 86%) had two ostia on the left side for upper and lower lobe veins. The remainder (n = 28, 14%) had a single ostium. There was no significant association between any particular venous drainage pattern and atrial arrhythmia; however, patients with a separate ostia for the right middle lobe pulmonary vein(s) tended to have a higher frequency of atrial arrhythmia than those with other patterns (P = .053).
CONCLUSION: A classification system to succinctly describe pulmonary venous drainage patterns was developed. Right-sided venous drainage was more variable than left-sided venous drainage. One-quarter of patients had more than two venous ostia on the right side.
© RSNA, 2004
Index terms: Computed tomography (CT), thin-section, 945.12918 • Pulmonary arteries, flow dynamics, 945.12944 • Pulmonary veins, CT, 945.12918 • Radiofrequency (RF) ablation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Variations in the number and course of pulmonary veins were thought to be rare (8) and, until recently, were the subject of only case reports (8–13). Recently, however, in a small series it was found that variations in pulmonary venous anatomy were seen in 36% of patients (4) and that ectopic beats could arise from these anomalous veins (6). This greater than expected variability in pulmonary venous anatomy could substantially alter success rates of radiofrequency ablation, as ectopic foci may go untreated in variant veins. Indeed, success rates of radiofrequency ablation are not uniform and are currently 62%–75% for paroxysmal atrial fibrillation (1,4,5,7) and 22%–68% for chronic atrial fibrillation (3,5). For these reasons, cross-sectional imaging (magnetic resonance [MR] imaging or computed tomography [CT]) may be performed prior to radiofrequency ablation to map the pattern of pulmonary venous drainage and to identify variant veins (4).
The purpose of our study was to evaluate the frequency of variability in pulmonary venous anatomy and to classify the various drainage patterns of the pulmonary veins, as depicted with thin-section chest CT.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Patients were referred for CT examinations to exclude acute pulmonary embolism (n = 196), chronic pulmonary embolism (n = 1), aortic injury (n = 2), or pulmonary vein stenosis (n = 2) after radiofrequency ablation. Our Institutional Review Board waived the requirement for patient consent and approved this retrospective study.
Imaging and Data Collection
For the purposes of this study, we reviewed images from only the first CT examination in the three patients who underwent two examinations. All CT examinations were performed with a QXi Light-Speed CT scanner (GE Medical Systems, Milwaukee, Wis). Images were obtained during a single breath hold from the top of the left diaphragm to the top of the aortic arch by using 2.5-mm collimation, 15-mm per rotation table speed, and 0.8-second rotation. A total of 150 mL of the noniodinated contrast material iopamidol (Isovue 300; Bracco Diagnostics, Princeton, NJ), which has an iodine concentration of 300 mg/mL, was administered with a power injector at a rate of 4 mL/sec through an 18- or 20-gauge catheter into an antecubital vein. Imaging commenced after a delay of 20–25 seconds.
Two thoracic radiologists (E.M.M. and H.P.M.) who had 6 and 11 years of experience, respectively, and were blinded to specific patient history retrospectively reviewed the CT studies at a free-standing workstation with the ability to display three-dimensional images (Vitrea 2; Vital Images, Minneapolis, Minn) in consensus. CT studies were primarily viewed in the transverse plane, but they were also viewed in the coronal or coronal-oblique planes when questions arose regarding number, location, or origin of veins. These views were used with approximately two-thirds of the CT examinations. The drainage pattern for each patient was documented with detailed written and diagrammatic descriptions and was assigned an alphanumeric descriptor for right and left drainage patterns, as will be described later. After review of CT studies, medical records were reviewed by one thoracic radiologist (E.M.M.) to document electrocardiographic findings obtained at the time of CT and any history of cardiac arrhythmia.
Classification of Pulmonary Venous Drainage Patterns
Over the course of the study, we developed and refined a system for classifying and describing pulmonary venous anatomy (Figs 1, 2) on cross-sectional images. The classification is based on both the number of venous ostia on each side and the drainage patterns of the pulmonary veins. Two or three alphanumeric characters are used to describe each pattern: The uppercase letter denotes the side of drainage (L = left, R = right), the number corresponds to the number of venous ostia on that side, and the lowercase letter (when present) denotes a particular variation of that pattern.
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2 test. Several patterns were also grouped together for comparison purposes (ie, R2a and R2b; R3a, R4a, R4b, and R5; and L2a and L2b). Associations between the grouped drainage patterns and atrial arrhythmias were assessed with the Fisher exact test. A P value of less than .05 was considered to indicate a significant difference. The Fisher exact test was used to assess the relationship between drainage patterns (individual and grouped) and sex. Logistic regression analysis was used to assess the joint effect of sex and drainage pattern on atrial arrhythmia. |
RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Right Pulmonary Venous Drainage Patterns
Right pulmonary venous drainage patterns were classified and were distributed as shown in Table 1. Most patients (n = 136, 68%) had the expected anatomy of two atrial ostia for upper and lower lobe veins, with the middle lobe vein joining the upper lobe vein. The remaining 65 (32%) patients had variant anatomy. The most common variation was independent drainage of the middle lobe vein(s) directly into the left atrium (Fig 3) and was seen in 52 (26%) patients.
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Left Pulmonary Venous Drainage Patterns
Left pulmonary venous drainage patterns were classified and distributed as shown in Table 2. Less anatomic variability was seen in the left atrium, with 173 (86%) patients having two ostia for the upper and lower lobe veins (Fig 4). A common trunk forming one ostium in the left atrium was seen in the remaining 28 (14%) patients. An unusual pattern was observed in one patient: A lingular vein drained into the proximal inferior pulmonary vein, and then both veins drained into the superior pulmonary vein to form a large common trunk that emptied into the atrium. This patient was included in the group of patients with an L1b pattern.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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Focal ablation of specific arrhythmogenic foci has proved to be the less popular technique for several reasons. First, there may be several arrhythmogenic foci in one vein. Unless all foci are identified and ablated, treatment may not be successful (15). Second, arrhythmogenic foci may be present in more than one pulmonary vein (4,14). This may account for the high recurrence rates reported with focal ablation (1). Third, ectopic electrical activity in the pulmonary vein may be infrequent or absent during the electrophysiologic study, even when provocative pharmacologic or pacing procedures are performed (1,4,14). Fourth, the catheters used are fixed in size and shape. This can lead to difficulties with catheter manipulation (4,5) and may account for the significantly longer fluoroscopic times reported with this technique (15), which leads to increased radiation dose to both the patient and the operator. Fifth, complications—primarily pulmonary vein stenosis—occur more frequently with this technique (5,15). For these reasons, complete electrical isolation of all pulmonary veins by creating circumferential lesions at their ostia has become the more popular and widely performed procedure.
It has been shown that the best results are achieved with isolation of three or four veins (1,2), although recurrence rates of up to 50% are still reported (1,3–5,7,18). This high recurrence rate may be due to the fact that all pulmonary vein ostia are not identified and treated at the time of the procedure. It has been shown that arrhythmogenic foci do occur in anomalous veins and that ablation of these veins can be used to treat atrial arrythmia successfully (6). Thus, it would seem that mapping the pulmonary veins and identifying anomalous veins prior to the procedure could be beneficial.
Initially, we were somewhat skeptical of the early reports that suggested a high frequency of variant venous anatomy on the right side (6). As noted earlier, variations in pulmonary venous drainage were not well described until catheter-directed radiofrequency ablation became an important treatment for atrial arrythmia. For example, a search of the pathology and surgery literature failed to reveal any specific descriptions of variant pulmonary venous anatomy. Thoracic surgeons, however, are well aware of anatomic variants such as drainage of the right middle lobe into the inferior pulmonary vein, which can lead to devastating results during right lower lobectomy (19). The results of our study confirm that there is indeed substantial variation in pulmonary venous anatomy, particularly on the right side. In our series, 32% of patients had variant right-sided anatomy and 25% had a separate orifice for the right middle lobe vein. The frequency of variant pulmonary venous anatomy in our series is similar to the range (31%–38%) in a number of studies that used ultrasonography (4), MR imaging (6,20), and CT (21).
Most prior studies of radiofrequency ablation have focused on the identification and mapping of the four primary pulmonary veins (1,3,15,16,18). This is because preablation assessment has usually been performed with either echocardiography or conventional angiography. These modalities may not, however, optimally define the often complex pulmonary venous anatomy (22–25). For these reasons, cross-sectional imaging with either MR or CT may be requested prior to the ablation procedure. Although MR imaging is probably better than echocardiography or planar angiography for assessment of the pulmonary veins, the cardiac gating that is required for such studies can be difficult and time consuming. Artifacts due to arrhythmia and claustrophobia also occur.
On the other hand, CT is easily tolerated by almost all patients, and with the latest generation of multi–detector row spiral CT scanners, the scanning time usually lasts less than 10 seconds. Although our study was not designed to determine the optimal CT technique for depicting pulmonary veins, our experience suggests that optimal depiction requires fairly thin sections (2.5 mm or less) and adequate intravenous contrast material enhancement. The level of contrast material opacification is probably not as critical for analyzing pulmonary venous anatomy as it is for imaging the pulmonary arteries for filling defects. In fact, contrast material may not be absolutely necessary, and it can be eliminated in patients with severe allergies.
We did not find nontransverse views to be particularly helpful in patients with conventional anatomy. In patients with variant anatomy, however, coronal or coronal-oblique views were particularly useful for trying to decide if the veins converged into a single large orifice or into two adjacent ostia. By using a computer workstation, complex venous anatomy can be easily deciphered with multiplanar two- or three-dimensional reconstructions and displayed for review by referring clinicians. By obtaining detailed anatomic knowledge prior to radiofrequency ablation, the overall procedure time and radiation exposure may be substantially decreased (4,26). With this technique, it is also possible to measure the diameter and surface areas of the venous ostia. This information affects the type of catheter—a multipolar loop catheter like the Lasso (Biosense Webster, Diamond Bar, Calif)—that is used for radiofrequency ablation (5,15,16).
If preablation cross-sectional imaging becomes routine, and because of the complexity of pulmonary venous anatomy, we believe that a classification system for the succinct description of the pattern of drainage is important. Our system uses three alphanumeric characters to describe the pattern of drainage. The first character describes the side. The second character describes the number of venous ostia on that side and is the most important descriptor for preablation planning. The third character describes particular variations from the basic pattern. Our classification system is flexible and can be easily modified to account for variations that we did not encounter. We believe that such a system will be useful for succinctly communicating information about the often-complex pulmonary venous anatomy to referring clinicians.
We were interested in determining if there was an association between any particular venous drainage pattern and atrial arrhythmia. This question arose because Tsao et al (6) reported a high frequency of variation in right middle lobe venous drainage in patients with refractory atrial fibrillation. In our series, about one-quarter of the patients had separate ostia for the middle lobe vein(s); however, most did not have atrial fibrillation, and we found no statistically significant association between this drainage pattern and atrial arrhythmia. Although right-sided venous drainage was generally more variable than left-sided drainage in patients in our study, we found that 28 (14%) patients had only one venous ostium on the left side. Again, we found no statistically significant association between this pattern and atrial arrhythmia.
Our study has several limitations. First, because we evaluated primarily patients undergoing CT to exclude pulmonary embolism, our results might not be applicable to the whole population; however, we have no reason to believe that pulmonary venous anatomy should differ substantially between patients in whom pulmonary embolism is suspected and the general population. Second, we had a very small group of patients with atrial arrhythmias. This could have limited our ability to detect an association between arrhythmia and pulmonary venous drainage patterns. Third, concern may arise as to the inclusion of patients with chest abnormalities that might alter pulmonary venous configuration; however, we specifically excluded patients with central pulmonary, pleural, or hilar abnormalities. Thus, the veins were not obscured in patients included in the study. Fourth, because our classification was refined as the study progressed, concern may arise that classification was inconsistent; however, because we recorded a detailed description and drawing for each case that we reviewed, we do not believe this is an important issue. Fifth, we had no reference standard for validating our CT findings. Postmortem examination is arguably the best standard for this type of study, but such data were not available. Angiography is probably not an optimal standard because complex pulmonary venous anatomy is not always best demonstrated with planar imaging. Furthermore, pulmonary venous angiography was not performed in any of our patients. Because venous anatomy is often so complex, cross-sectional imaging techniques such as MR imaging or CT, combined with multiplanar or three-dimensional reconstruction capabilities, should be the best techniques for depiction, short of direct visual inspection. Our findings are further validated by the fact that the frequency of variation we found was similar to that in other reports (4,6,20,21). Further studies will be required, however, to validate our classification system and to analyze inter- and intraobserver variability.
In conclusion, our results confirm that there is substantial variation in pulmonary venous anatomy. Right-sided venous drainage is considerably more variable than left-sided venous drainage. One-quarter of patients have at least one separate ostium for the right middle lobe veins. We propose a flexible classification system that succinctly describes these variations in pulmonary venous anatomy and that facilitates communication with referring clinicians.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantors of integrity of entire study, E.M.M., H.P.M.; study concepts and design, all authors; literature research, E.M.M., Y.H.K.; clinical studies, E.M.M., H.P.M.; data acquisition and analysis/interpretation, E.M.M., Y.H.K., H.P.M.; statistical analysis, S.E.H.; manuscript preparation and definition of intellectual content, E.M.M., H.P.M., Y.H.K.; manuscript editing, E.M.M., H.P.M.; manuscript revision/review, E.M.M., H.P.M., Y.H.K.; manuscript final version approval, E.M.M., H.P.M.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODS RESULTS DISCUSSIONREFERENCES |
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