Clinical scenarios: lower airway
This section is divided into three parts: chronic productive cough, recurrent wheezing lower respiratory tract infection, and recurrent radiologic shadowing. This approach has been found to be useful in personal practice, although some investigations that cut across clinical scenarios may be described inadequately or, conversely, there may be repetition. Specific conditions that are referred to in the differential diagnosis in these sections (eg, CF, PCD, and immunodeficiency) are discussed in the final sections of the article.
Many children are too young for pulmonary function testing, although increasingly techniques such as airway resistance using the interrupter technique (Rint)40 and the lung clearance index[41], [42] and [43] are applicable in preschool-age children. The shape of the tidal flow–volume loop may suggest an obstructive picture.29 If there are facilities to carry out spirometry, lung volumes, and carbon monoxide transfer, and the child is able to perform these tests, much useful information can be obtained. Pulmonary function testing provides only part of the clinical information, however, and the only specific diagnoses that can be made in the pulmonary function laboratory are exercise-induced asthma and hyperventilation syndromes.
Airway disease is suggested by the combination of hyperinflation (raised residual volume, functional residual capacity, and total lung capacity) with reduced 1-second forced expiratory volume (FEV1) and FEV1 to forced vital capacity (FVC) ratio. Elevated carbon monoxide transfer (DLco) is suggestive of asthma or bronchiectasis, whereas reduced DLco suggests parenchymal destruction as well as airflow obstruction (eg, the sequelae of chronic lung disease of prematurity). Parenchymal disease is suggested by reduced FEV1 and FVC, with a normal or elevated FEV1:FVC ratio and reduced residual volume, functional residual capacity, and total lung capacity. A high DLco per liter of accessible lung volume suggests chest wall restriction, such as scoliosis. An isolated reduction in DLco suggests pulmonary vascular disease, such as recurrent pulmonary embolism or primary pulmonary hypertension. An isolated elevation in DLco suggests recent pulmonary hemorrhage.
The shape of the flow–volume loop may give additional clues. Rigid tracheal obstruction is suggested by abrupt attenuation of flows in inspiration and expiration (see Fig. 1). Attenuation in only the expiratory limb suggests variable obstruction (eg, malacia) of the intrapulmonary trachea or large airways. Isolated attenuation of the inspiratory limb may be caused by variable obstruction of the extrathoracic trachea, inspiratory muscle (diaphragm) dysfunction, or stiff lungs. Marked variability, and in particular a very variably impaired inspiratory flow–volume curve, is highly suggestive of vocal cord dysfunction.44
Another important issue is the state of the child's immune system. In the following sections, it is assumed that the child does not have a known and diagnosed immune deficiency but is thought at presentation to have a normal immune system. The special problems of the immunodeficient child are discussed in a separate section or are referred to specifically where applicable. For example, recurrent infiltrates would be investigated very differently, and much more urgently, in a child who has pancytopenia after chemotherapy than in an otherwise well child who has similar radiologic appearances.
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