Tracheobronchial airway stenting has been an established and effective tool for central airway obstruction for many years. Appropriate size selection for airway stents can be challenging and varies between different types of stents. Accurate sizing is essential to reduce complications, especially migration, as some stent series report up to a 28% migration rate.1 There is no definitive technique for accurately selecting stent diameter and length. Current techniques for estimating the correct stent diameter include measurements from computed tomographic (CT) scans, use of airway balloons of standard sizes, relationship relative to flexible or rigid scope diameter, or use of an airway stent sizing device.2–5 This can be particularly challenging in irregular-shaped airways or in tracheobronchial malacia. Although CT scans do provide additional information regarding lung and mediastinal anatomy, there may be significant time intervals between imaging and the airway stent procedure, inadequate assessment of dynamic airway changes, or the need to avoid recurrent radiation in younger patients with nonmalignant disease.
Although radial-probe endobronchial ultrasound (EBUS) predated convex-probe EBUS by many years, the clinical utility and prevalence of convex-probe EBUS quickly surpassed that of radial probe. Radial-probe EBUS has been used for numerous applications, including assessment of mediastinal structures, guidance for transbronchial needle aspiration, determination of malignant airway wall invasion, and selection of appropriate therapeutic modality.6 More recently, it has also been used for the characterization and localization of peripheral pulmonary nodules.7
In the April 2011 issue of the Journal of Bronchology and Interventional Pulmonology, Nobuyama et al8 reported their experience with a novel use of radial-probe EBUS to facilitate selection of airway stent length and diameter, in an effort to overcome some of the limitations in current alternative stent sizing techniques.8 Although successful stent placement was not the intent of their study, they attempted to correlate diameter measurements between radial EBUS with those made using CT scans at 212 anatomic sites along malignant and benign airway stenoses in 31 patients. They demonstrated significant correlation in both malignant (n=160 sites) and benign (n=52 sites) disease and radial EBUS and CT correlation at normal anatomic sites. The investigators reported that radial EBUS resulted in a change in stent size selection in 8 of the 31 patients.
This radial EBUS approach does offer theoretical advantages over traditional techniques. These include the ability to measure both length and diameter, real-time assessment, and more importantly allowing measurement of airway diameter distal to the obstruction when it is not possible to pass a bronchoscope across the lesion. This distal airway diameter measurement might assist with stent diameter selection and perhaps provide some information regarding the viability of the distal airways, which can be challenging and critical to determining whether a patient will benefit from airway stenting.
The exact technique by which airway diameters were measured on CT scans or with radial-probe EBUS (ie, short axis) is not reported nor is the manner in which the measured anatomic sites on CT were determined to be the exact same measured anatomic sites by radial EBUS. Thus, the reporting of this information may be useful. There is also no information given regarding the time frame between CT and radial-probe EBUS, but one may assume they were relatively close together given the significant diameter correlation. As the primary outcome was to compare CT scan and radial-probe EBUS airway measurements, no information regarding the stent placements in the 31 patients is reported. Demonstration of satisfactory or reduced stent complication and migration rates with this technique would be a logical next step. In future studies, it would be useful to know whether the radial-probe EBUS assessment of the airway patency distal to a central airway obstruction correlates to a patent distal airway after stent placement.
The use of this technique in tracheobronchial malacia deserves special mention. Determination of stent size in this population can be particularly challenging due to dynamic airway changes, abnormal airway shape, and often large airway diameters. Radial EBUS theoretically offers an accurate way to measure tracheobronchial malacia airways by maximally expanding the airway with the balloon to obtain the measurement. However, in this study using a BS-20-26-R radial probe (Olympus, Tokyo, Japan) the maximum short-axis diameter that could be obtained was 15.9 mm. Thus, this may not be adequate to measure larger tracheal diameters.
A similar probe “pullback” measuring technique for airway obstruction has also been reported using anatomic optical coherence tomography (aOCT).9 Real-time measurements of 14 patients with benign and malignant central airway obstruction were correlated to CT measurements and used as a basis for therapeutic decisions such as airway stenting. Although aOCT and radial-probe EBUS are not without limitations, they will hopefully form a solid foundation on which continued investigation will lead to the ideal real-time assessment tool for airway dimensions and decisions regarding therapeutic interventions.
1. Gildea TR, Downie G, Eapen G, et al. A prospective multicenter trial of a self-expanding hybrid stent in malignant airway obstruction. J Bronchol. 2008;15:221–224
2. Christian R, Wahju A, Gilbert F, et al. Computed tomography measurements for airway stent insertion in malignant airway obstruction. J Bronchol Intervent Pulmonol. 2010;17:22–28
3. Rooney CP, Ferguson JS, Barnhart W, et al. Use of 3-dimensional computed tomography reconstruction studies in the preoperative assessment of patients undergoing balloon dilatation for tracheobronchial stenosis. Respiration. 2005;72:579–586
4. Williamson JP, James AL, Phillips MJ, et al. Quantifying tracheobronchial tree dimensions: methods, limitations, and emerging techniques. Eur Respir J. 2009;34:1–14
5. Downie G, Huang YY, McGuire FR. Measurement accuracy of a novel stent-sizing device in the porcine airway. J Bronchol. 2006;13:9–11
6. Shirakawa T, Ishida A, Miyazu Y, et al. Endobronchial ultrasound for difficult airway problems. In: Clinical Chest Ultrasound: From the ICU to the Bronchoscopy Suite (Progress in Respiratory Research)
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7. Yoshikawa M, Sukoh N, Yamakazi K, et al. Diagnostic value of endobronchial ultrasonography with a guide sheath for peripheral pulmonary lesions without x-ray fluoroscopy. Chest. 2007;131:1788–1793
8. Nobuyama S, Kurimoto N, Matsuoka S, et al. Airway measurements in tracheobronchial stenosis using endobronchial ultrasonography during stenting. J Bronchol Intervent Pulmonol. 2011;18:128–132
9. Williamson JP, McLaughlin RA, Phillips MJ, et al. Feasibility of applying real time optical imaging during bronchoscopic interventions for central airway obstruction. J Bronchol Intervent Pulmonol. 2010;17:307–316