Frequency dependence of total respiratory resistance in early airway disease

Patients who develop frequency dependence of lung compliance will theoretically have frequency dependence of pulmonary resistance. We investigated the ability of the simpler, noninvasive measurement of frequency dependence of total respiratory resistance to identify subjects with frequency dependence of compliance. Ten healthy nonsmokers, 14 asymptomatic smokers, and 6 patients with obstructive airway disease were studied. Frequency dependence of total respiratory resistance was determined by the superimposed oscillating airflow technique at 3 to 9 cycles per sec, and frequency dependence of lung compliance was determined by measurements at 10 to 80 breaths per min. Spirometry, airway resistance, closing volume, and closing capacity were also measured. Frequency dependence of lung compliance and total respiratory resistance were closely correlated (P less than 0.001, r = 0.82), but closing volume, closing capacity, spirometry, and airway resistance could not be used to identify subjects with abnormal frequency dependence of lung compliance. Measurements of frequency dependence of total respiratory resistance and lung compliance, total respiratory resistance at 3 cycles per sec, and closing volume minus expiratory reserve volume were able to distinguish significantly between the smokers and the nonsmokers, but spirometry, closing volume, closing capacity, and airway resistance could not. These data indicate that in asymptomatic smokers and subjects with obstructive airway disease, frequency dependence of lung compliance can be predicted from measurements of frequency dependence of total respiratory resistance. These two tests appear to have equivalent sensitivity and selectivity in detecting the uneven time constants in the airways of asymptomatic smokers.