The expiratory phase of respiration is commonly described as prolonged in patients with Chronic Obstructive Pulmonary Disease (COPD). The goal of this study was to quantify the degree of this expiratory sound prolongation in patients with COPD as compared to normals.
We studied 38 patients with COPD and 43 normals using a multichannel lung sound analyzer (Stethographics, STG1602) as previously reported. The ratio of the duration of inspiration to expiration (R1) was measured on the time-amplitude plots from the microphone placed over the trachea.
R1 averaged 0.67±0.16 in the COPD patients and 0.83±0.16 in the normals (p<0.0001). The frequency distribution of R1 is shown in Figure 1. R1 equal to or less than 0.6 was observed in 39% of COPD patients, but only in 9% of normals. R1 equal to or less than 0.5 was observed in 18% of COPD patients, but only in 2% of normals. The sensitivity of R1 equal to or less than 0.6 was 0.39, the specificity was 0.91, and the positive predictive value was 0.79.
As expected a relatively prolonged expiratory phase was more common in COPD than normals. Values of R1 less than 0.6 were particularly more common in COPD than normals. However, low R1 values were seen in some normals and relatively high values were seen in some COPD patients.
A simple test using a lung sound analyzer that requires little patient cooperation can identify the presence of prolonged expiration consistent with COPD. Although the test has relatively low sensitivity, the relatively high specificity can help guide the selection of patients for further evaluation.
Raymond Murphy, Grant monies (from industry related sources) The research was supported in part by a grant from Stethographics, Inc.; Shareholder Dr. Murphy is founder and CMO of Stethographics, Inc.