Detecting Fixed Upper Airway Obstruction in Normal Subjects and COPD Patients

Objective: To propose a forced oscillation test modality for detecting fixed extrathoracic upper airway obstruction (UAO) and
compare its efficacy to spirometric UAO indices, even in subjects with additional peripheral airways obstruction. Methods: Ten
normal subjects and 10 COPD patients underwent impulse oscillometry and spirometry while being randomly presented with
hidden external orifices (diameters ranging 6−12mm). Airway resistance at 5Hz and 25Hz was averaged over 5 normal breathing
cycles (R), subtracted from that averaged over 5 rapid breathing cycles and divided by the flow rate difference to also obtain an
index of flow dependence of resistance (∆R/∆flow). Measurements and main results: In normals and COPD, R(25Hz) was not
flow rate dependent in the absence of an external orifice (i.e.,∆R/∆flow(25Hz) not significantly different from zero) and
∆R/∆flow(25Hz) showed steady increases with decreasing orifice diameter. Establishing the optimal cut−off value to detect an
orifice ≤10mm in normals and applying it to COPD, R(25Hz) yielded a [sensitivity,specificity] of [88%,93%] in normals and
[82%,100%] in COPD; ∆R/∆flow(25Hz) yielded a [sensitivity,specificity] of [96%,97%] in normals and [88%,100%] in COPD.
Combining both parameters further increased sensitivity and specificity to 94% and 100% in COPD. Corresponding numbers for
the best spirometric UAO index FEV1/PEF were [96%,83%] in normals and [30%,100%] in COPD. Conclusion: The forced
oscillation test,preferably performed at two breathing flow rates, provides resistance derived indices that are largely superior to
spirometric indices for fixed UAO detection. This is particularly true for UAO detection in subjects with additional peripheral airway
obstruction.