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CT assessment of tracheal carinal angle and its determinants

Department of Radiology, Pamukkale University Hospital, Denizli, Turkey

	Abstract

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The aim of this study was to investigate the effect of body habitus, dimensions of the thoracic cavity, location of the carina within the mediastinum, and left atrial size on tracheal carinal angle using CT scan. The study population was drawn from the patients referred to CT scan for various indications. A total of 120 patients (65 men and 55 women; age range 17–85 years; mean age 56 years) who denied a history of prior thoracic surgery, and in whom CT scan excluded pulmonary fibrosis, moderate or severe emphysema, atelectasis, intrathoracic mass or adenopathy, pericardial or pleural effusion were prospectively enrolled. The interbronchial (IBA) and subcarinal (SCA) angles were measured on coronal reformatted images. The presternal and retrovertebral fat thickness, the anteroposterior and transverse diameters of the thorax, the distances from carina to the sternum and to the vertebral column were obtained at the level of carina. Three orthogonal dimensions and the volume of the left atrium were also assessed. The mean interbronchial angle was 77°±13° (range 49–109°) and subcarinal angle was 73°±16° (range 34–107°). IBA positively correlated with the female gender (r=0.25, p=0.007), body mass index (r=0.28, p=0.002), presternal (r=0.40, p=0.001) and retrovertebral fat thickness (r=0.31, p=0.001). The interbronchial angle was significantly greater in obese patients compared with lean patients (p=0.02). Both IBA and SCA were positively correlated with the left atrial volume (r=0.40, p=0.001 and r=0.34, p=0.001, respectively), and its transverse and craniocaudal dimensions. The carina-vertebral column distance inversely correlated with IBA (r=–0.42, p=0.001) and SCA (r=–0.41, p=0.001). The size of the thoracic cavity did not show significant relation to tracheal bifurcation angle. Tracheal bifurcation angle ranges widely in normal subjects, and absolute measurements of the carinal angle is of little diagnostic value. In addition to left atrial enlargement, female gender, obesity and close situs of carina relative to vertebral column are associated with greater tracheal bifurcation angle.

	Introduction

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The angle of tracheal bifurcation on a posteroanterior chest radiograph may be widened due to both cardiac disease and mediastinal abnormalities. Despite this, it has long been accepted as a manifestation of left atrial enlargement [1, 2]. There are conflicting reports on the clinical value of this sign, and its utility remains controversial [1, 3]. All of the previous studies assessed the tracheal carinal angle on chest radiographs using a goniometer [1–7]. Helical CT enables the acquisition of volumetric images of the chest, and reliable measurements may be performed using reconstructed images on a workstation. To date no study has investigated the tracheal carinal angle using CT. In this study, helical CT scan was used to investigate whether or not the tracheal bifurcation angle is influenced by patient gender, age, body habitus, dimensions of the thoracic cavity, and left atrial size.

	Materials and methods

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Study population
The study population comprised 608 consecutive patients referred for a chest CT scan for various indications between December 2003 and November 2004. Patients with a history of prior cardiothoracic surgery, or in whom CT examination showed evidence of pulmonary fibrosis, moderate or severe emphysema, atelectasis, an intrathoracic mass or lymphadenopathy, a pericardial or pleural effusion, and patients who failed to hold their breath during examination were excluded from the study. The remaining 120 patients (65 men and 55 women; age range 17–85 years; mean age±standard deviation (SD), 56±15 years) with a virtually normal chest CT scan were prospectively enrolled. The body mass index (BMI) of an each patient was calculated by dividing the body weight by the square of the body height (kg m^–2), and patients with BMI greater than 25 kg m^–2 were classified as obese.

CT examination
CT of the chest was performed with a dual slice helical CT scanner (Mx8000; Philips Medical Systems, Cleveland, OH) during a single breath-hold using 2 mm x 5 mm collimation 120 kV, and 125 mAs. Table feed was 17.5 mm per 0.75 second of scanner rotation (23 mm s^–1), resulting in a pitch of 1.75:1.0. From the raw data of each acquisition, 6.5 mm-thick transverse sections were reconstructed with 3.2 mm increments. The field of view was 400 mm x 400 mm with an acquisition matrix of 512 x 512. If clinically necessary, 100–120 ml of iodine-based contrast material (300 mg I ml^–1) was injected through the antecubital vein at a rate of 3 ml s^–1 using a power injector and the scan initiated after a 20 second delay. All the CT images were transferred to a workstation for review (MxView_exp; release 4.01, Philips Medical Systems), and all measurements were performed by the same chest radiologist. The presternal and retrovertebral fat thicknesses were measured from the skin to the sternum and the spinous process of the vertebral body to the skin, respectively. The anteroposterior and transverse diameters of the thorax were measured as the greatest distance between two sides of the inner ribs. The distances from the carina to the sternum and to the vertebral column were also obtained to determine the position of the carina. All these measurements were performed at the level of the carina on mediastinal window settings (WW=350, WL=50). The interbronchial (IBA) and subcarinal (SCA) angles were measured electronically on 10 mm thick reformatted coronal images using a wider window settings (WW=500, WL=–100). The IBA was determined by the intersection of lines positioned along the central axis of both main bronchi over their lengths (Figure 1a). The SCA was measured by the intersection of the first few centimetres of inferior margins of main bronchi (Figure 1b). Three orthogonal dimensions of the left atrium were measured and its volume was calculated using the formula for an ellipse (0.5 x height x width x depth). Transverse and sagittal diameters were measured at the level of the aortic root avoiding the pulmonary veins. Based on the left atrial volume, the study group was divided into two groups; those with normal left atrial volume (<50 ml), and those with left atrial enlargement (50 ml).

View larger version (45K): [in this window] [in a new window]   Figure 1. Coronal reformatted image through the tracheal bifurcation. (a) Intenterbronchial angle is measured by the intersection of two lines positioned along central axes of the main bronchi. (b) Subcarinal angle is determined by the intersection of the inferior margins of the main bronchi.

Statistical significance was defined as a p-value of less than 0.05. Statistical software (SPSS for Windows, release 11.0; SPSS, Chicago, IL) was used.

	Results

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The overall quality of the reconstructed images were adequate, and did not hamper the measurement of bifurcation angles. The mean interbronchial angle was 77°±13° (range 49–109°) and subcarinal angle was 73°±16° (range 34–107°). There was an excellent correlation between IBA and SCA (r=0.88, p<0.001). The interbronchial angle was greater than the subcarinal angle in 80 (67%) of 120 cases, and smaller in 34 (28%) patients. Both angles were equal in 6 (5%) subjects. The relationship of the tracheal carinal angle with the patient's sex, BMI and left atrial volume is summarized in Table 1. Both angles were significantly greater in females than in males. The interbronchial angle was significantly larger in obese patients compared with lean patients (p=0.02). Both IBA and SCA were positively correlated with the left atrial volume (r=0.40, p<0.001 and r=0.34, p<0.001, respectively), and its transverse (r=0.38, p<0.001 and r=0.33, p<0.001, respectively) and craniocaudal dimensions (r=0.39, p<0.001 and r=0.37, p<0.001, respectively). IBA positively correlated with the female gender (r=0.25, p=0.007), BMI (r=0.28, p=0.002), presternal (r=0.40, p<0.001) and retrovertebral fat thickness (r=0.31, p=0.001). The carina-vertebral column distance inversely correlated with IBA (r=–0.42, p=0.001) and SCA (r=–0.41, p<0.001). Based on the distance between the carina and the vertebral column, those with smaller distances (<15 mm) had a greater IBA (83°±12°) and SCA (80°±15°) compared with those with greater distances (15 mm) (IBA, 74°±12°; SCA, 68°±15°, p<0.001). The size of the thoracic cavity did not show a significant association with IBA or SCA (p>0.05).

	Discussion

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Widening of the tracheal bifurcation angle has long been accepted as a sign of left atrial enlargement, but conflicting data are present about its reliability [1–3]. In a series of 70 patients (35 with normal and 35 with enlarged left atrium), Taskin et al [2] compared atrial size with IBA and reported that an IBA greater than 90° is a satisfactory predictor of left atrial enlargement. Conversely, Murray et al [1] reported a considerable degree of overlap in the range of bifurcation angles measured in patients with normal and enlarged left atria, and concluded that an increased tracheal carinal angle is an insensitive and non-specific sign of left atrial enlargement. Although a significant correlation was found between left atrial size and the tracheal bifurcation angle, our results support the findings of Murray et al [1], as we encountered a significant degree of overlap in the carinal angle range in patients with or without left atrial enlargement. This can be explained by the fact that the left main bronchus is not displaced by a normal or mildly enlarged left atrium [1]. Another explanation is that the late development of left atrial enlargement cannot displace the bronchus as the tracheobronchial cartilages become rigid and non-compliant at an older age [8]. Furthermore the results of this study showed that left atrial size may not be the sole determinant of the tracheal bifurcation angle, because it is also influenced by the patient's sex, body habitus, and the situation of the carina relative to the vertebral column.

The greater value of the angle in female patients in this study is contradictory to most of the previous studies, which reported that the tracheal bifurcation angle is independent of patient's gender [3–5]. On the other hand, Murray et al [1] reported larger IBA and SCA values in female patients, but they did not assess its statistical significance. In our study, there was no difference in the BMI or left atrial size between men or women (p>0.05), but female patients had a greater presternal fat thickness (p<0.001), and a smaller carina-vertebral column distance (p=0.002), and as such the patient's gender may not be an independent variable.

We have shown that the location of the carina relative to the vertebral column, presternal and retrovertebral fat thicknesses influence the tracheal bifurcation angle. Proximity of the carina to the vertebral column widens the bifurcation angle. Presternal and retrovertebral fat thickness are positively correlated with the tracheal bifurcation angle. These two factors have not previously been investigated, but it is conceivable that these distances are manifestations of increased body weight, as increased fat within the subcarinal space may enlarge the bifurcation angle. However we have demonstrated that BMI weakly correlated with IBA, and did not show a significant correlation with SCA.

The dimensions of the thoracic cavity can potentially affect the bifurcation angle. A weak correlation between the bifurcation angle and the height or width of the thorax has been previously reported [3, 5]. In our study, there was no significant correlation of the carinal angle to either the transverse or sagittal diameter of the chest.

In conclusion, the tracheal bifurcation angle has a wide range in normal subjects, and the absolute measurement of the carinal angle is of little diagnostic value. Not only is it influenced by left atrial size, but also by the patient's gender, body habitus, and the position of the carina.

Received for publication February 18, 2005. Revision received March 22, 2005. Accepted for publication April 12, 2005.

	References

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1. Murray JG, Brown AL, Anagnostou EA, Senior R. Widening of the tracheal bifurcation on chest radiographs: value as a sign of left atrial enlargement. AJR Am J Roentgenol 1995;164:1089–92.[Abstract/Free Full Text]
2. Taskin V, Bates MC, Chillag SA. Tracheal carinal angle and left atrial size. Arch Intern Med 1991;151:307–8.[Abstract/Free Full Text]
3. Haskin PH, Goodman LR. Normal tracheal bifurcation angle: a reassessment. AJR Am J Roentgenol 1982;139:879–82.[Abstract/Free Full Text]
4. Alavi SM, Keats TE, O'Brien WM. The angle of tracheal bifurcation: its normal mensuration. AJR Am J Roentgenol 1970;108:546–9.[Abstract]
5. Coppola V, Vallone G, Coscioni E, Coppola M, Maraziti G, Alfinito M, Di Benedetto G. Normal value of the tracheal bifurcation angle and correlation with left atrial volume. Radiol Med (Torino) 1998;95:461–5.
6. Chen JT, Putman CE, Hedlund LW, Dahmash NS, Roberts L. Widening of the subcarinal angle by pericardial effusion. AJR Am J Roentgenol 1982;139:883–7.[Abstract/Free Full Text]
7. Kakeda S, Kamada K, Aoki T, Watanabe H, Nakata H. Postsurgical change in the tracheal bifurcation angle after upper lobectomy: radiographic evaluation. Acad Radiol 2003;10:644–9.[CrossRef][Medline]
8. Silber EN. Tracheal carinal angle in evaluating left atrial size. Arch Intern Med 1991;151:2096, 2100.[Abstract/Free Full Text]

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