Peripheral Lung Resistance in Normal and Asthmatic Subjects

Abstract

In obstructive lung disease, peripheral airways are a major site of pathologic abnormalities. However, resistance to airflow in small airways in the periphery of the lung accounts for only a small fraction of total airway resistance. Consequently, abnormalities of small airway function may not be readily detected using routine pulmonary function testing. In the present study, resistance of the peripheral lung was examined directly in six normal subjects and nine mildly asthmatic subjects. There were no significant differences between the normal and asthmatic groups in pulmonary function assessed by spirometry (FEV1, FVC) and body plethysmography (specific airway conductance). Direct measurements of peripheral lung function were made using a fiberoptic bronchoscope wedged into a subsegmental, right upper lobe bronchus. Using a double-lumen catheter inserted into the instrument channel of the bronchoscope, pressures (PB) produced by three or more different levels of gas flow (.ovrhdot.V) (5% CO2 in air) between 50 and 500 ml/min were measured. All pressure measurements were made at a constant lung volume (i.e., functional residual capacity) confirmed by monitoring transpulmonary pressure with an esophageal balloon. The pressure-flow relationship in both normal and asthmatic subjects could be approximated by a straight line through the origin, demonstrating these airways to be relatively nondistensible. Peripheral lung resistance (Rp) was defined by PB/.ovrhdot.V and averaged for three or more levels of flow. In contrast to spirometric results that showed no differences between the two groups, Rp was increased more than sevenfold in asthmatic subjects (0.069 .+-. .017 cm H2O/ml/min) (mean .+-. SEM) compared to normal subjects (0.009 .+-. .002 cm H2O/ml/min). Pretreatment of the asthmatic subjects (n = 5) with aerosolized isoproterenol did not decrease Rp to normal levels. These results demonstrate marked physiologic abnormalities in the peripheral lungs of asymptomatic asthmatic subjects and suggest that changes in the mechanical properties of these airways, although having a small influence on overall pulmonary function, may contribute to increased airway responsiveness in asthma.