Measures of Airway Caliber

The diameter, or caliber, of the airways is of great value in the investigation or diagnosis of obstructive airways diseases. Techniques used to evaluate airway caliber generally center on analysis of the rate of expiratory airflow, because airway obstruction tends to diminish expiratory flow.

1. Peak flow measurements. Perhaps the simplest measurement of expiratory airflow involves the use of a peak flowmeter. Subjects inspire maximally (i.e., to total lung capacity) and expire rapidly and maximally to residual volume into the mouthpiece of the instrument that provides a measurement of the peak expiratory flow. These instruments are simple to operate and often are provided to asthmatic patients for self-measurement and documentation of their ventilatory function.

2. Forced expiratory flow measurements. Spirometric techniques are used to measure the time course of expired volumes. The same ventilatory

Figure 7 Forced expiratory flow maneuvers in normal and obstructed airways. A maximal expiratory effort from total lung capacity results in a rapid expiration of air from the lungs, the volume of which is equivalent to the forced vital capacity (FVC) and the rate of which is dependent on the caliber of the airways. The volume of air expired in the first second of a maximal expiratory effort is the FEV10. In subjects with obstructed airways, air flow is retarded as reflected in a smaller FEV10 (FEV10ob) than in subjects with normal airways (FEV10).

Figure 7 Forced expiratory flow maneuvers in normal and obstructed airways. A maximal expiratory effort from total lung capacity results in a rapid expiration of air from the lungs, the volume of which is equivalent to the forced vital capacity (FVC) and the rate of which is dependent on the caliber of the airways. The volume of air expired in the first second of a maximal expiratory effort is the FEV10. In subjects with obstructed airways, air flow is retarded as reflected in a smaller FEV10 (FEV10ob) than in subjects with normal airways (FEV10).

maneuvers are conducted as those described for peak flow measurements. The subject exhales rapidly and maximally into the mouthpiece of the spirometer. This results in a trace similar to that shown in Fig. 7. The volume expired in the first second is termed the forced expiratory volume (FEV10), and the total volume expired is the forced vital capacity (FVC). FEV10 may be normalized to account for the body size, sex, and age of the subject and thereby permit comparison with "normal" estimates. Alternatively, the ratio FEV10: FVC can be calculated. In subjects with normal airways, this ratio is approximately 0.8; under conditions of airway obstruction, the ratio is less than 0.8.

3. Airway resistance and dynamic lung compliance. Measurement of expiratory flow represents a simple, noninvasive means of estimating airway caliber. However, these measurements are relatively insensitive to changes in peripheral airway caliber. More complicated measures of caliber include airway resistance (RL) and dynamic lung compliance (Cdyn). Airway resistance is thought to measure the caliber of the larger airways, such as the bronchi and bronchioles. Dynamic lung compliance measures the elasticity of the peripheral airways and is given as the change in volume of the lungs for a given change in pressure distending the alveoli. Measurement of these parameters in subjects involves highly specialized equipment (e.g., whole-body plethysmograph and pneumotachygraph) and can be invasive, i.e., requiring the placement of an intraesophageal balloon for intrathoracic pressure measurements. A precise description of the means of estimating these parameters is beyond the scope of the present discussion, but interested readers are directed to Ref. 52.

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