Airway resistance, FRC, and RV cannot be measured with spirometry and require other techniques, such as body plethysmography. Helium dilution can also measure TLC and RV by determining the magnitude of dilution of inhaled helium in the air within the lung, but may underestimate air trapping.
Abnormal results on pulmonary function testing can beused to categorize obstructive airway disease (low flow rates and increased RV or FRC) or a restrictive defect (low FVCand TLC, with relative preservation of flow rates and FRC).When the FEV1 and flow rates are decreased to a greaterextent than the FVC, then airway obstruction is likely; however, a proportional decrease in FVC, FEV1, and flow rates suggests a restrictive lung defect. The mean midexpiratoryflow rate (FEF25–75%) is a more sensitive measure of small airways disease than the FEV1, but is also more variable. Pulmonary function testing can detect reversible airway obstruction characteristic of asthma with a significant improvementin FEV1 (>12%-15%) or in FEF25–75% (>25%) followinginhalation of a bronchodilator. Spirometry is also useful forlongitudinal patient management. The peak expiratory flow rate (PEFR) can be obtained with a simple handheld deviceand may be useful for home monitoring of older childrenwith asthma. However, it is highly dependent on patienteffort, and values must be interpreted with caution. Inhalation challenge tests using methacholine, histamine, or cold, dry air are used to assess airway hyperreactivity, but require sophisticated equipment and special expertise and should be performed only in a pulmonary function laboratory with experienced technicians.
Endoscopic Evaluation of the Airways
Endoscopic evaluation of the upper airways (nasopharyngoscopy) is performed with a flexible fiberoptic nasopharyngoscope to assess adenoid size, patency of the nasal passages, and abnormalities of the glottis. It is especially useful in evaluating stridor and assessing vocal cord motion/function, and it does not require sedation. Endoscopic evaluation of the subglottic space and intrathoracic airways can be done with either a flexible or rigid bronchoscope under anesthesia. Bronchoscopy is useful in identifying airway abnormalities (stenosis, malacia, endobronchial lesions, excessive secretions) and in obtaining airway samples for culture (bronchoalveolar lavage), especially in immunocompromised patients. Rigid bronchoscopy is the method of choice for removing foreign bodies from the airways and performing other interventions, and flexible bronchoscopy is most useful as a diagnostic tool and for obtaining lower airway cultures. Transbronchial biopsies are rarely performed in children. There are few absolute contraindications to bronchoscopy. Relative contraindications include bleeding diatheses, thrombocytopenia (<50,000/cm3), and clinical conditions when the patient is too unstable to tolerate the procedure.
Examination of Sputum
Sputum specimens may be useful in evaluating lower respiratory tract infections, but they are difficult to obtain in young children. In addition, an expectorated specimen may not provide a representative sample of lower airway secretions. Specimens containing large numbers of squamous epithelial cells either are not from the lower airways or are heavily contaminated with upper airway secretions and may yield misleading results. Sputum in patients with lower respiratory tract bacterial infections often contains polymorphonucleated leukocytes and one predominant organism on culture. If sputum cannot be obtained, then bronchoalveolar lavage specimens may be used for microbiologic diagnosis in selected situations. In patients with CF who cannot produce sputum, specially processed throat cultures are often used as surrogates for lower airway cultures.