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Ann Thorac Surg 2004;77:1216-1221
© 2004 The Society of Thoracic Surgeons

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Original article: general thoracic

Measurement of tracheal size in children with congenital heart disease by computed tomography

^a Department of Medical Imaging, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan
^b Department of Surgery, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan
^c Department of Pediatrics, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan
^e Department of General Examination, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan
^d Department of Diagnostic Radiology, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan

Accepted for publication August 15, 2003.

^* Address reprint requests to Dr Lee, Department of Medical Imaging, National Taiwan University Hospital and National Taiwan University College of Medicine, 7 Chung-Shan South Rd, Taipei, 100 Taiwan
e-mail: jacklee{at}ha.mc.ntu.edu.tw

	Abstract

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
BACKGROUND: To establish a reference of tracheal size in children with congenital heart disease to allow detection of airway stenosis.

METHODS: We conducted a retrospective study using existing data from children referred for computed tomography (CT) scanning. From January 1999 to February 2001, 540 consecutive children with congenital heart disease who received electron beam CT scanning at our hospital were considered eligible for inclusion in the study. After exclusion criteria were considered, 99 children (50 girls and 49 boys; aged 4 days to 16 years 10 months) were enrolled in the study. Tracheal width was measured at three levels on CT images. The relationship between tracheal width and the patient's height, weight, age, and sex were analyzed by multiple regression and formula transformation.

RESULTS: Height was the most effective parameter for predicting the transverse diameter of the intrathoracic trachea, and tracheal size could be predicted based on height using the established equations. In addition, tracheal width increased from the thoracic inlet to the carina. Reference curves based on the subject's height were created for convenient use.

CONCLUSIONS: Tracheal stenosis in children with congenital heart disease may be diagnosed by comparing the size of the trachea of the individual to the 95% confidence interval of predicted values based on the patient's height.

	Introduction

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Tracheal stenosis must be considered in any child who has recurrent pneumonia or episodes of stridor, cyanosis, and wheezing [1]. These symptoms may appear at birth or develop within months as the child outgrows the stenotic airway's capacity to maintain adequate ventilation. Children with tracheal stenosis have structural features that also make their airway more vulnerable to occlusion, and a small amount of edema or mucus will markedly narrow the airway [2]. However, some patients with tracheal stenosis are asymptomatic until mechanical insults (eg, endotracheal intubation) cause mucosal edema that aggravates airway occlusion. Awareness of this possible complication is crucial to prevent complications in the perioperative period, particularly in children with cyanotic congenital heart disease in which arterial oxygen saturation is frequently compromised. Absence of reference data for airway size in children with congenital heart disease, especially in infants, leads to underestimation of this possibility and results in occasional complications during the perioperative period. Because growth in children with congenital heart disease may be impaired, data for airway size gathered from healthy children may not be applicable. The purpose of this study was to establish a convenient reference curve and formula to estimate the size of the trachea in children and infants with congenital heart disease.

	Material and methods

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Patients
We conducted a retrospective study using existing data from children referred by their pediatric cardiologist for computed tomography (CT) scanning. From January 1999 to February 2001, 540 consecutive children with congenital heart disease who received electron beam CT scanning at our hospital were considered eligible for inclusion in the study. In every case, signed, informed consent was obtained after the procedure was fully explained to the patients or their family. Exclusion criteria were used to help prevent statistical bias from any inherited or acquired abnormalities that could cause narrowing of the airway. Patients were excluded if they had any respiratory symptoms or signs (eg, cough, wheezing, or stridor); a history of sternotomy, which could cause airway deformity due to fibrosis; lungs that were not clear either on chest roentgenography or CT images, which could result in airway narrowing due to inflammation; or space-occupying structures in the surrounding area (eg, left pulmonary artery sling or aberrant subclavian artery), which could cause airway narrowing by external compression. We excluded patients with anomalies of the great arteries (eg, transposition of great arteries, double outlet of the right ventricle, tetralogy of Fallot, hypoplastic or coarctation of aorta, or main pulmonary artery atresia) because, from an embryologic viewpoint, airways and great vessels may originate from the same brachial arch [3]. We also excluded patients with a collapsible airway resulting in significant narrowing (more than 50%) in sequential images of different respiratory phases (ie, tracheomalacia) and patients with obvious segmental tracheal narrowing (ie, focal tracheal stenosis). It is important to note that children with Down syndrome, in whom postoperative stridor is common, were not included in the group of children in this study.

Electron beam computed tomography technique
All subjects underwent electron beam CT (Imatron C-150L; South San Francisco, CA) with electrocardiogram gating. All images were obtained at the end-diastolic phase of the cardiac cycle in the high-resolution program. Slice thickness was 3 mm. Electron beam CT was performed from the level of the lung apex to the diaphragm without gaps. The table increment was 2 mm in infants for slice overlapping. The actual total imaging time, excluding positioning the patients, was less than 2 minutes. The total radiation dose was approximately 5 to 7 mSv, which is a similar dosage to an adult portable supine chest roentgenogram. Nonionic iodinated contrast medium (2 to 3 mL/kg; Ultravist 370; Schering, Berlin, Germany), used to allow evaluation of the cardiovascular structures simultaneously, was delivered using a power injector.

Patients younger than 5 years old were routinely sedated with chloral hydrate 50 mg/kg and were sleeping quietly during their examinations. Intrathoracic tracheal diameter has been reported to remain the same at either maximal inspiration or expiration [4, 5]. However, if the patients were old enough to cooperate, nurses or technicians instructed them to maintain shallow respiration during scanning. Otherwise, natural respiration without crying was deemed acceptable in small children and infants.

Image analysis and quantification
The trachea was measured at the level of the thoracic inlet, at the level of the great vessels just arising from the arch, and at the supracarinal trachea separated from the carina by at least two sections (6 mm) to prevent the partial volume effect from carina. We only measured transverse widths because the tracheae of children are nearly round in cross section [6, 7] and it was most convenient in clinical application.

All electron beam CT images were displayed with a default air window (level = −680; window = 1600) for viewing. Quantification of tracheal sizes was performed by an analyzing program on electron beam CT scanner using a full width half maximum system, an approach well established for measuring size against air-containing cylinders [8]. Although the trachea is not truly perpendicular to the plane (X-Y plane) of the electron beam CT sections, it is perpendicular to the left-right axis (X-axis) by proper positioning of the patients. Therefore, trigonometric correction is not necessary because the transverse dimension is not affected.

Statistics
It was assumed that the variables age, sex, weight, and height of the patient may be correlated with tracheal size. Multiple regression analysis was used to analyze the factors that were significantly associated with the transverse diameter of the trachea at the thoracic inlet, aortic arch, and supracarinal levels. Models were created for predicting tracheal size within a 95% confidence interval by the explored minimal effective measurement with the highest level of data fitness (maximal adjusted R²). The initial model (model 1) used all four variables to calculate the correlation. Only the statistically significant measurements from model 1 were used in the next model (model 2). According to reports in the literature [6, 12], height and weight appear to be the most important measurements. We therefore used height, weight, and the square and cube of both height and weight in the next model (model 3) to improve the fitness of our data if our results agreed with the findings previously reported. However, because there is a correlation between weight and height, we were interested in using a single effective parameter, instead of two effective measurements, to simplify the calculation for clinical use (model 4). Because body surface area is calculated from weight and height, which were already being considered, this was not considered as a variable in the analyses. The homogeneity of the tracheal widths was also examined at the different levels using a nonparametric Wilcoxon test.

	Results

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A total of 99 children met the inclusion criteria for the study; their main diagnoses are listed in Table 1. The children ranged in age from 4 days to 16 years 10 months (average age 5.3 years; median age 3.8 years). There were 50 girls and 49 boys. The age distribution was as follows: 19 infants and 22 children between 1 and 3 years; 19 children between 3 and 6 years; 19 children between 6 and 11 years; and 20 children between 11 and 17 years. Although, on average, the boys were slightly taller and heavier than the girls, no significant difference was noted in height and weight between the two sexes (p < 0.05, t test).

The adjusted R² of these formulas (by only the height-related variables instead of the combination of height and weight variables) increased. In other words, we achieved a better model for estimating tracheal size than we originally predicted using height and weight. These models can be easily stored in a hand-held electronic personal data assistant to facilitate fast calculations of tracheal diameter.

View larger version (37K): [in this window] [in a new window]   Fig 1. The reference curves of the (A) thoracic inlet, (B) aortic arch, and (C) supracarinal levels tracheal width (cm) by patient's height (cm). The line in the center represents the mean value. The bilateral inner parallel lines are the 95% confidence limits for the regression line, and the bilateral outer lines are the 95% confidence limits for the data points. = boy; • = girl; = boy; = girl.

	Comment

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Children who receive surgical treatment for congenital heart disease are at the greatest risk of mortality in the acute postoperative period. Patients may experience respiratory failure, desaturation, and death despite successful correction of the cardiac problem. One of the unexpected causes of morbidity and mortality is from unrecognized insidious tracheobronchial stenosis during the perioperative period [3, 13]. Magnified airway (high kilovoltage filtered) chest roentgenography has been used preoperatively to evaluate the size of the airway [13] in children with heart disease, but the accuracy of this technique is not always reliable [14]. Bronchography is a conventional diagnostic modality that visualizes the tracheobronchial tree, but it may hasten further respiratory decompensation in cases with congenital heart disease, which can be life threatening [1, 2, 15]. Bronchoscopy can directly visualize the trachea but is invasive [16]. This procedure is frequently performed under general anesthesia, particularly when using rigid instrumentation for size measurement [17]. Neither of the two above-mentioned procedures are suitable for screening.

The use of conventional CT scanning for quantification of tracheal size was described in the early 1980s by Griscom and coworkers [6, 7, 10, 18]. At the time of those studies, scanning of one CT section required a minimum of 2 seconds. Thus, for a child with a respiratory rate of 30 breaths per minute, a 2-second scan time meant one respiratory cycle, so the presence of spatial motion artifacts made measurement inaccuracy inevitable. Breath holding was used to overcome this problem in cooperative cases, but was not possible in small children or infants. This explains why few infants were included in their studies. However, the airway of an infant is more vulnerable to occlusion than that of an older child [2]. Infants are at higher risk of perioperative complications if insidious tracheal stenosis is not identified, and younger patients with heart problems that require early surgical intervention usually have a more severe cardiovascular condition. Therefore, the need to establish criteria to confirm airway narrowing has become more important in younger patients with congenital heart disease.

Electron beam CT has the advantages of a short scanning time (0.1 second), high spatial resolution, and noninvasiveness. The short scanning time eliminates the presence of motion artifacts of the trachea during the scanning, even in small infants, so the measured data are more accurate and reliable. Our study population had more newborn infants and infants than previous reports [6, 7, 10, 18], which allows application of our results to younger patients.

Electron beam CT imaging allow diagnosis of localized narrowing of the airway and identification of tracheomalacia (eg, if the trachea is collapsible at different respiratory phases). However, if a relatively "small" trachea is suspected, there is no way to determine whether it is really narrow or if surgical correction at the same time of cardiac intervention should be considered [2, 15]. The absence of an airway reference size in children, particularly in infants, further adds to this problem. Our results provide a practical way to overcome this situation.

Although we excluded patients with a relatively soft airway with variable sizes in contiguous electron beam CT images and kept children sedated or quiet during their examinations, there is still a limitation of our study in that there may be visually nondetectable changes in the diameter of the trachea during breathing. This is an inherent limitation because it is difficult or impossible to have an infant or young child hold their breath. Although electron beam CT has limited worldwide usage, data created from this modality may be used as a reference for other conventional CT scanning. Furthermore, in patients with congenital heart disease, electron beam CT can clearly demonstrate cardiovascular and tracheobronchial information at the same time [19].

In the patients in this study, the aortic arch level of the trachea was slightly wider than the thoracic inlet level of the trachea, and the supracarinal level of the trachea was slightly wider than the aortic arch level of the trachea. This discrepancy was more obvious in older children. This finding contradicts one previous report that the size of the trachea changes little along the course from the thoracic inlet to the carina [7], and is also inconsistent with common assumptions that the distal part of the trachea is smaller than the proximal part. Our finding is consistent with Griscom's [10] report that the part of the supracarinal trachea broadens in preparation for branching to bilateral main bronchi, which is a reasonable explanation. Because of this finding, we used tracheal diameters at the level of the thoracic inlet, aortic arch, and supracarina to increase precision in a clinical setting instead of the averaged width of the whole trachea used by Griscom and Wohl [6].

Sex, age, height, and weight have all been reported to affect the diameter of the trachea. However, the close interrelationship between these variables must be considered. Taller children are usually heavier, and girls are frequently shorter than boys after puberty. In addition, patients with congenital heart disease ordinarily have poor growth, so that the reference created using normal subjects may not be 100% applicable. Similarly, our results should only be applied to children with congenital heart disease.

The results of our statistical analyses indicate that height and weight are the most significant predictive factors of tracheal size at all levels of the trachea. The relative predictive value of sex and age was comparatively low at all ages. For simplicity and practical convenience, we developed a predicting formula using a single independent variable (height) after careful statistical analysis. We also created reference curves of the trachea width at the three different levels.

Currently in our institution, we suggest electron beam CT to delineate possible airway narrowing in those cardiac patients with respiratory symptoms and signs. We also suggest electron beam CT for those patients at high risk of accompanying tracheal anomalies and possible stenosis [20]. However, our reference is even more useful for identifying insidious narrowing of the airway in children with no obvious respiratory symptoms and signs but who receive electron beam CT for other cardiovascular indications (eg, to detect the presence of a central pulmonary artery in tetralogy of Fallot). We suggest that only those children with congenital heart disease whose tracheal width is below the 95% confidence limit of the regression line will be at risk of tracheal stenosis. By using our reference, the diagnosis of tracheal stenosis could be made definitively, and correction of the narrow airway in conjunction with cardiac correction may be considered [2, 15]. Both situations require very gentle care of the airway. In particular, the anesthesiologist should be well informed of the condition of the airway preoperatively. In addition, all medical participants (cardiologists, surgeons, anesthesiologists, residents, and nurses) should be especially careful when manipulating the airway.

We hope that use of the predictive equations and reference curves that were developed in this study promotes patient care during the perioperative period and prevents unexpected airway complications in children with congenital heart disease.

	Acknowledgments

Top Abstract Introduction Material and methods Results Comment Acknowledgments References

 
This study is supported by a grant from the National Science Council of the Republic of China (NSC 88–2314-B-002–147). We thank Ritta Huang for her assistance in manuscript preparation.

	References

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