PURPOSE: During a methacholine challenge test, a 20% decrease in FEV1 is considered diagnostic and the failure to decrease excludes reactive airways. However, FEV1 is not a sensitive indicator of airway resistance. Therefore, we measured airway resistance and reactance by IOS prior to spirometry during methacholine challenge in 100 patients referred to the PFT laboratory to diagnose or exclude reactive airways.
METHODS: Standard 5-dose methacholine challenge was performed following ATS protocol. Airway resistance and reactance were also measured by IOS (Viasys, CA) prior to spirometry.
RESULTS: All patients had normal baseline spirometry. By spirometry, 73 patients had a negative test. 27 had a 20% decrease in FEV1 with 19/27 having a PC20 FEV1 of < 4 mg/ml and in 8/27 the PC20 FEV1 was between 4 - 16 mg/ml. Of the 19 patients with PC20 FEV1 <4 mg, 8 had normal baseline IOS and were also positive by IOS. The remaining 11/19 had abnormal baseline IOS, but were also IOS positive. There were 4 patients with a 20% decrease in FEV1 without a significant change in IOS. There were 35 patient that had no decrease in FEV1, but had major changes in airway resistance (R5 Hz) and reactance that completely reversed (frequently below baseline levels) after Albuterol. 20/35 had normal baseline IOS, did not decrease their FEV1, and yet clearly developed reversible small airway site of resistance (% baseline values ±SEM R5Hz, R20Hz, X5, Fres, Ax; 199±9.6, 124±6.3, 341±29, 221±13, 1142±180, respectively) all of which returned to or below baseline after Albuterol. 15/35 had abnormal baseline IOS and methacholine induced worsening of a central site of resistance, more consistent with upper airway/vocal cord dysfunction.
CONCLUSIONS: During methacholine challenge, IOS provides additional information not obtained by simple spirometry.
CLINICAL IMPLICATIONS: By standard ATS criteria, a negative methacholine challenge does not exclude the presence of reactive airways. By standard ATS criteria, a positive methacholine challenge may not represent reactive airways, but may reflect methacholine - induced central site of resistance (i.e. likely vocal cord dysfunction). In order to confirm these results, video-laryngoscopy may be needed during methacholine challenge, but if confirmed, IOS may be able to replace methacholine video-laryngoscopy.
DISCLOSURE: The following authors have nothing to disclose: Larry Casey, Ellen Moran, Jason Gunn
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