Short-term variations in oscillatory and spirometric lung function parameters of non-asthmatic adults

Oscillatory respiratory resistance (Rrsfo) at 8 Hz was compared to flow-volume spirometry with regard to immediate, within-day, day-to-day and week-to-week variations in seven female and four male non-smoking, non-asthmatic volunteers. The lung functions were measured at 08:00, 12:00, 16:00 and 20:00 h on each of the four study days, i.e. two consecutive days in two consecutive weeks. During each visit there were three immediately repeated measurements of Rrsfo, followed by three spirometric recordings. The intra-subject coefficient of variation (Coeffvar) for the immediately repeated measurements was largest for Rrsfo (11.8%). When a simple reliability index (+/- 2 SD of the differences between the repeated measurements) was applied to the Rrsfo data, the Coeffvar reduced to 7.5%. In spirometry, the airflow parameters at defined lung volumes showed larger immediate variations (MEF50 = 5.6%, MEF25 = 8.3%, MMEF = 4.4%) than FEV1 (1.5%) and PEF (3.2%). The within-day variations were larger than the day-to-day or week-to-week variations, and the variations were largest in Rrsfo. A significant diurnal pattern was shown in spirometric parameters but not in Rrsfo. About 38% of the total variance in Rrsfo was due to variation within subjects, while the corresponding proportions in spirometric parameters were 1.8-18.4%. In conclusion, Rrsfo showed larger intrasubject variations than the spirometric parameters at all time intervals. Application of a simple reliability index and standardization of the time of day of the measurement reduced the variations and improved the quality of the Rrsfo data.