Laryngotracheal stenosis refers to a heterogeneous myriad of rare disorders that cause abnormal narrowing of the central air passageways between the larynx and carina.1 The problem is rare and challenging and may be congenital or acquired. The majority of cases of acquired laryngotracheal stenosis result from endotracheal intubation; the reported incidence of tracheal stenosis following tracheostomy and laryngotracheal intubation ranges from 0.6% to 21% and 6% to 21%, respectively2–4 but only 1–2% of these patients are symptomatic or have severe stenosis.5–7

Bronchoscopy is considered the gold standard for the diagnosis and assessment of airway abnormalities, including those due to endotracheal intubation.8 Although it provides direct visualization of the airway, it is an invasive procedure with its inherent risks.

Inspection of the maximal expiratory and inspiratory flow volume loops is currently the simplest and most widely used method to detect the presence of upper airway obstruction (UAO) including tracheal stenosis. Spirometry, however, is not always feasible in patients with UAO, due to patient-related factors that do not allow or interfere with an optimal forced expiratory manoeuvre (e.g. severe dyspnoea, cough, fatigue, language barrier, cooperation and cognitive impairment). Furthermore, significant changes in spirometry appear relatively late in the course of the upper airway narrowing process9 and spirometry values may not correlate with the degree of airway narrowing.10,11 These observations indicate the need for a method that can detect UAO and specifically tracheal stenosis in high risk patients.

Impulse oscillometry (IOS) was developed by Michaelson et al.12 and was based on the physiologic concepts of the forced oscillation technique (FOT) originally described by Dubois et al. in 1956.13 This non-invasive method requires minimal patient collaboration and provides an estimate of respiratory system impedance and its components (resistance and reactance) at different frequencies. The data can be collected rapidly and the equipment is easy to operate.14 Compared to forced expiration parameters, IOS could play an important role in cases of tracheal stenosis, even in stable and normally breathing patients, considering the important distortions of the airway, which are deeply affected by forced manoeuvres.

In view of the considerable burden caused to the patient by bronchoscopy and the time needed for the examination, we looked for a non-invasive way of quantifying airway obstruction that could serve as a potential screening and follow-up method. So the aim of the present study was to assess the correlation between IOS measurements and airway narrowing evaluated by bronchoscopy. We also assessed the correlation between IOS and spirometry values and compared those parameters in the patients submitted to interventional bronchoscopy before and after the procedure.

We have proposed a simple diagnostic tool from IOS, and looked for its validity for tracheal stenosis detection, depending on its severity. So far X5% decreases with increasing tracheal stenosis. We also found that FEV1 (marginal significance) and FEV1/PEF decrease with increasing stenotic area and PEF and PEF% decrease with increasing length of the stenosis.

IOS has been employed to assess peripheral airway obstruction in children and adults with COPD or asthma. There are few studies about its usefulness in detecting UAO.15–18 In our study X5% was correlated with the tracheal narrowing, decreasing with the increasing stenotic area. Reactance is a reflection of pulmonary elastic capacitance and inertia,19 and this correlation could be based on the expression of the variable capacitance of the cartilaginous upper airway according to the extension of each lesion. In the study by Handa et al., they assessed the correlations between IOS measurements, symptoms, and type of airway narrowing, before and after interventional bronchoscopy.16 After interventional bronchoscopy, all IOS measurements significantly improved, especially R5 and X20. Changes in dyspnoea score correlated with R5, R5-20Hz, and X5.

Our data did not show any significant correlation between resistance and the percentage of tracheal narrowing. Horan et al. evaluated the utility of FOT in patients with central nervous system injuries, for detection and follow-up of post tracheostomy tracheal stenosis.20 FOT evaluations were compared to tracheal diameter before and after bronchoscopic tracheostenosis dilatation procedures. In this case series the authors found a strong inverse correlation of Rf and Z5 with stenosis diameter and a positive correlation with R5. Although forced oscillation R5 has been reported to increase in patients with UAO,20 its lack of specificity for UAO detection has also been recognized.21 In the case of stenosis patients with a smoking history, it is also unclear which part of R is attributable to tracheal obstruction and which part arises from more peripheral obstruction. Handa et al. found that subjects with variable UAO, R5–20Hz and X5 were similar to those in subjects with severe COPD and tidal expiratory flow limitation16; the usefulness of IOS was assumed to be based on the fact that the resistance obtained at a high frequency preferentially reflects the large central airways, and therefore a change in the frequency dependence of resistance can be considered to reflect the change in the large or small airways. Comparing patients with COPD and those without it we did not find significant differences in oscillometry results. In accordance with previous studies,16,17,20 we observed reductions in resistance and reactance in all patients after each successful dilatation, yet not statistically significant and only in three patients post-intervention R5 values fell within the limits of normal. It is possible that, after an intervention, the combination of residual obstruction and some mucosal swelling due to the procedure itself (e.g., granulation tissue, sputum impaction, development of oedema), may impair a complete resolution of resistance to normal values. Despite the limited options to practically and accurately measure the cross-sectional dimensions of subglottic stenosis and quantify the efficacy of treatment, new techniques have emerged. In a recent study the authors performed intraoperative long-range optical coherence tomography (LR-OCT) of the adult airway with objective comparison of pre-intervention and post-intervention luminal patency.22 OCT is a light-based imaging technique which provides high-resolution, cross-sectional imaging of biological tissue with up to 1–2mm depth. LR-OCT is a derivative of OCT that is optimized for intraluminal topographical mapping of large, hollow organs such as the airway.23,24 In the referred study the segmented axial images obtained were coalesced into an array and rendered into 3-D airway models which allowed the surgeon to measure post-dilation changes in the airway and quantify the immediate response to therapy.

Regarding spirometry, we found a positive correlation between FEV1 and FEV1/PEF and the degree of tracheal narrowing. In the studies that additionally to oscillometry also evaluated the correlation of spirometry values with tracheal stenosis size the latter ones did not correlate well.16,20 Other studies comparing tracheal size, lung function indices and body size in healthy subjects have been published; however, the findings were inconsistent. Some authors found significant correlations between some indices of tracheal size and of lung or body size25–27; however the relevant functional indices could differ depending on gender25 and there were inconsistencies on which lung function index was best correlated with tracheal size.25,26,28 Other authors found very poor correlations between lung function indices and tracheal size25,29,30 and they attributed this to the unequal growth pattern of the tracheobronchial tree and the lung parenchyma, which has been coined “dysanapsis”.30 The stenosis length was also evaluated and it correlated with PEF and PEF%. Similarly in the study by Jamaati et al., plethysmography variables directly correlated with the intensity and length of stricture.31 The predicting factors for complications after tracheal resection for benign stenosis were previously evaluated and resections longer than 4cm were associated with a dramatic rise in the rate of failure.32,33 Herein, PEF could also be seen as a potential predictor of more challenging surgical approach, predicting lower surgical success rate.

Our results showed a moderate to high correlation between spirometry parameters and IOS parameters mainly impedance and resistance. Similarly, Verbanck et al. found a direct correlation of resistance measurements pre- and postintervention to the corresponding spirometric indices of tracheal stenosis, regardless of respective area lumen size, confirming that spirometry parameters bear a distinct relationship to obstruction in the upper airway.17 These findings can be of be clinical relevance; for example, patients with tracheal stenosis and profound weakness or dyspnoea may be better candidates for impulse oscillometry than for spirometry, once they cannot perform forced expiratory manoeuvres. Also, IOS may be useful in cases in which spirometry is contraindicated (patients who had recently undergone surgery, who have had recurrent pneumothoraces or who spirometry-related bronchospasm is a concern).

We recognize some limitations in our study. First, the small sample size limits the statistical results. Second, the percentage of tracheal narrowing was visually estimated. However, our study design tried to overcome this limitation by having two different bronchoscopists confirming each measurement in all cases and agreeing with it. Lastly, dyspnoea scores were not applied to our population. Assessing dyspnoea in this type of patients can be important as it could be related to the severity of stricture as well as in treatment decision-making.16

It remains unclear whether IOS can be used for physiological assessment of patients with tracheal stenosis. Our study showed a weak correlation between X5%, FEV1 and FEV1/PEF and tracheal narrowing. However, stenosis length correlated with PEF making it a potential predictor of more complicated stenosis. Also, the strong correlation between IOS and spirometric values makes IOS a potential alternative in patients with suspected tracheal stenosis who are not able to perform spirometry. Taking into consideration the invasive nature of bronchoscopy, other evaluation method of tracheal stenosis is necessary. Larger scale studies on this subject would be of major importance to further clarify the role of IOS in this pathology.

The authors declare that there is no conflict of interest regarding the publication of this paper.