Assessment of spirometry and impulse oscillometry in relation to asthma control.

Abstract

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. 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. 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. Spirometry or IOS measurements were equally useful as potential markers of asthma control in persistent asthmatic patients.