Measures of Airway Resistance

The physical principles just described can be used to predict the main site of airway resistance in the bronchial tree, and this can be verified on experiments with isolated lungs or models of lungs. Figure 7 shows that the main site of airway resistance is the large airways, even though radius has a dominant influence on resistance (cf. Poiseuille's law). This is because the small cross-sectional area in the upper airway branches results in high velocities and turbulent flow. Small airways contribute less than 20% of airway resistance because their large number reduces their total resistance. Unfortunately, many airway diseases start in the small airways, where resistance changes have to be very large before they can be detected by physiologic tests. This level of the bronchial tree is called the silent zone.

Airway resistance can also be approximated by solving Ohm's law with measurements of flow and the pressure gradient between alveoli and the mouth. Flow is relatively easy to measure with a spirometer (see Chapter 18, Fig. 6) or a pneumotachometer (a physiologic gas flow meter). Mouth pressure is also easy to measure, but PA cannot be measured directly. PA can be measured indirectly with a body plethysmograph by solving Boyle's law for lung pressure instead of volume (see Chapter 18, Fig. 8).

PA can also be estimated from Ppl measurements. Figure 8 shows two plots for Ppl: The solid line is the pressure during resting ventilation, and the dashed line is the pressure that could maintain the given lung volume without flow. The dashed line is the same data used to plot static compliance curves (see Fig. 3). The

Expiration

Expiration

Intrapleural Pressure Lung Volume

FIGURE 8 Lung volume, pressures, and flow during the breathing cycle. The solid line is the intrapleural pressure during ventilation; the broken line is the intrapleural pressure that would occur with no flow and the same lung volume. The additional intrapleural pressure change during flow (shaded area) equals alveolar pressure and is necessary to overcome airway and tissue resistance.

FIGURE 8 Lung volume, pressures, and flow during the breathing cycle. The solid line is the intrapleural pressure during ventilation; the broken line is the intrapleural pressure that would occur with no flow and the same lung volume. The additional intrapleural pressure change during flow (shaded area) equals alveolar pressure and is necessary to overcome airway and tissue resistance.

shaded area between the solid and dashed lines shows the additional Ppl change necessary to overcome resistance to flow. This pressure difference equals PA at any time during flow if there is no tissue resistance (which is a good approximation; see later discussion).

The plots of PA in Fig. 8 illustrate two important points. First, these pressures are small for resting breathing in healthy individuals. PA changes most during maximal flow rates, near the midpoints of inspiration or expiration, but the change is only about 1 cm H2O. This results in normal airway resistance values of only 1.5-2.0 (cm H2O • sec)/L. However, even such low resistances require substantially larger PA for high ventilatory flow rates (e.g., during heavy exercise). Second, the PA and flow curves have essentially the same shape in healthy resting individuals. This is expected if airway resistance is constant throughout the breathing cycle. However, as described later, several physiologic and pathologic factors may affect airway resistance, even within a single breath.

Measuring airway resistance from PA is too difficult for routine diagnosis, so forced expiratory volumes (FEV) are more commonly measured as indexes of airway resistance. For example, FEV10 is the volume that can be exhaled with maximum effort in the first 1.0 sec of an expiration.

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