Introduction: Guidelines advocate the use of spirometry to assess pulmonary function in asthmatic patients. Commonly used measures include forced expiratory volume in 1 s (FEV1), forced expiratory ratio (FEV1/FVC), and forced mid-expiratory flow between 25 and 75 % of forced vital capacity (FEF25-75). Impulse oscillometry (IOS) is an effort-independent test performed during tidal breathing. IOS may be used to assess the total and central airway resistance at 5 Hz (R5) and 20 Hz (R20), respectively, and hence derive the peripheral airway resistance from the difference (R5-R20). We compared spirometry and IOS as tests of global airway function (i.e., FEV1, FEV1/FVC, R5) and putative measures of small airways function (i.e., FEF25-75, R5-R20) and their relationship to oral steroid and short-acting beta-agonist (SABA) use as surrogates for long-term asthma control.
Methods: Spirometry and IOS measurements from physician-diagnosed asthmatics were linked to a health informatics database for oral steroid and SABA use 1 year prior to the index measurements.
Results: Four hundred forty-two patients had both spirometry and IOS, mean FEV1 = 86 % predicted, 94 % on ICS, median dose 800 µg/day. IOS and spirometry measures were equally predictive of impaired asthma control for both oral steroid and SABA use. For oral steroid use, the adjusted odds ratio, OR (95 % CI) is as follows: FEV1 < 80 %: 1.56(0.99-2.47), p = 0.056; FEV1/FVC < 0.70: 1.67(1.03-2.69), p = 0.037; FEF25-75 < 60 %: 1.84(1.18-2.86), p = 0.007; R5 > 150 %: 1.91(1.25-2.95), p = 0.003; and R5-R20 > 0.1 kPa L(-1) s 1.73(1.12-2.66), p = 0.013. For SABA use, the adjusted OR (95 % CI) is as follows: FEV1 < 80 %: 2.22(1.43-3.44), p < 0.001; FEV1/FVC < 0.70: 2.26(1.44-3.57), p < 0.001; FEF25-75 < 60 %: 2.51(1.65-3.82), p < 0.001; R5 > 150 %: 1.76(1.18-2.63), p = 0.006; and R5-R20 > 0.1 kPa L(-1) s: 2.94(1.94-4.46), p < 0.001.
Conclusion: Spirometry or IOS measurements were equally useful as potential markers of asthma control in persistent asthmatic patients.