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Tracheal Collapse versus Tracheobronchomalacia: Normal Function versus Disease

We appreciate the comments by Drs. Baram and Smaldone regarding our article on the correlation of air trapping and tracheobronchomalacia (TBM) as long-term sequelae of bronchiolitis obliterans in mustard gas–exposed patients (1). Since we did not aim to explain tracheal anatomical abnormalities in our study, they were not mentioned in the manuscript. Nevertheless, some evidence shows that our finding was not just normal tracheal collapse. Normal and abnormal tracheal collapse have been described for years (2). In our study, TBM was considered to be present if the airway diameter decreased by more than 30% during expiration (3). The high degree of collapse presented in our study is not expected to occur in a functionally normal trachea and is more than a normal collapse.

We agree with Drs. Baram and Smaldone that flow limitation is not due to tracheal collapse, and tracheal collapsibility does not have to impede flow. In our study, flow limitation was located at the level of the lobar bronchi, and it led to air trapping visible on chest high-resolution computerized tomography. There is a strong relationship between severity and frequency of air trapping and the presence of TBM (4). Although patients with chronic obstructive pulmonary disease, symptomatic asthma, and obesity are often flow-limited, most of our subjects had approximately normal results in FEV₁ and FVC; only a minimal increase in residual volume was seen in most patients. Considering the age (mean age, 42 years) and the mild obstruction found in our series, tracheal collapse as a normal variation is not reasonable according to the Starling resistor model. We hypothesize that, in bronchiolitis obliterans, both air trapping and TBM are caused by a single underlying process affecting both lobar bronchi and large airways (i.e., weakness of the airway walls and supporting cartilages). It can be explained independently of pulmonary function testing results. We believe that detection of significant air trapping is suggestive of the presence of TBM, and when both air trapping and TBM exist, bronchiolitis obliterans should be considered.

Mostafa Ghanei and Ali Amini Harandi

Baqiyatallah University of Medical Sciences Tehran, Iran

FOOTNOTES

Conflict of Interest Statement: Neither author has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

REFERENCES

1. Ghanei M, Moqadam FA, Mohammad MM, Aslani J. Tracheobronchomalacia and air trapping after mustard gas exposure. Am J Respir Crit Care Med 2006;173:304–309.[Abstract/Free Full Text]
2. Gamsu G, Webb WR. Computed tomography of the trachea: normal and abnormal. AJR Am J Roentgenol 1982;139:321–326.[Abstract/Free Full Text]
3. Aquino SL, Shepard JA, Jinns LC, Moore RH, Halpern E, Grillo HC, McLoud TC. Acquired tracheomalacia: detection by expiratory CT scan. J Comput Assist Tomogr 2001;25:394–399.[CrossRef][Medline]
4. Zhang J, Hasegawa I, Hatabu H, Feller-Kopman D, Boiselle PM. Frequency and severity of air trapping at dynamic expiratory CT in patients with tracheobronchomalacia. AJR Am J Roentgenol 2004;182:81–85.[Abstract/Free Full Text]

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