Impulse oscillometry system (IOS) assesses respiratory system resistance (Rrs) at different frequencies and differentiates between central and peripheral components of airway obstruction. Increased Rrs at a low oscillation frequency (5Hz) reflects an increase in total respiratory resistance and at a higher frequency (20 Hz) reflects an increased resistance in central airways. This study was set out to evaluate the accuracy of IOS and compare IOS measurements with body plethysmographic airway resistance (Raw) and spirometric values in the diagnosis of central airway obstruction (CAO).
Thirty-two patients with CAO as confirmed by a bronchoscopy or a dynamic computed tomography of the chest and 18 healthy subjects underwent consecutive IOS measurements of Rrs at frequencies of 5 and 20 Hz (Rrs5, Rrs20, respectively), body plethysmographic Raw, forced expiratory volume in 1 second (FEV1), and peak expiratory flow rate (PEF). Receiver operating curve was established to determine the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), accuracy, and likelihood ratio (LR) of IOS in identifying CAO.
Nineteen of 32 patients (60%) had tracheobronchostenosis from previous tuberculosis infection. Two of these 19 patients had concurrent tracheobronchomalacia. Ten patients (31%) had endobronchial obstruction from tumor. Three patients (9%) had tracheobronchomalacia. An Rrs20 greater than the upper limit of normal (ULN) was interpreted as having CAO with the sensitivity of 69%; specificity of 94%; PPV of 96%; NPV of 63%; accuracy of 78%, and LR of 12.4. Significant correlations were found between Rrs5 and percentage of predicted FEV1 (FEV1%, r = −0.61), percentage of predicted PEF (PEF%, r = −0.75), specific Raw (sRaw, r = 0.64), and specific conductance (sGaw, r = −0.64). Rrs20 was not related to any of these measurements except for PEF% (r = −0.4).
Airway obstruction is indicated when Rrs5 increases. Increased Rrs20 measured by IOS suggests an obstruction in the central airways in various diseases.
IOS is helpful in providing an additional means to conventional methods such as body plethysmography, spirometry and flow-volume loop in assessing airflow obstruction.
Prapaporn Pornsuriyasak, No Financial Disclosure Information; No Product/Research Disclosure Information