Sniff nasal inspiratory pressure in patients with moderate-to-severe chronic obstructive pulmonary disease: learning effect and short-term between-session repeatability.

BACKGROUND Sniff nasal inspiratory pressure (SNIP) is a non-invasive measure of inspiratory muscle function often used as an outcome measure in clinical studies. An initial period of familiarisation with the test is recommended to minimise the learning effect. The repeatability of SNIP in patients with chronic obstructive pulmonary disease (COPD) is currently unknown. OBJECTIVES The aim of this study was to assess the between-session repeatability of SNIP over a 3-week period in moderate-to-severe COPD patients and compare it with that of maximal inspiratory (PI max) and expiratory pressure (PE max). METHODS Twenty-one patients (13 males) with a mean forced expiratory volume in 1 s (FEV1) of 38% of predicted (SD: 15) and FEV1/forced vital capacity of 34.3% (SD: 10.4) performed SNIP and PI max and PE max manoeuvres on 3 different sessions (S1, S2 and S3) 3-7 days apart. SNIP was performed at functional residual capacity (FRC), and PI max was performed at FRC and at residual volume (RV) to explore volume-dependent differences in the learning effect between sessions and PE max from total lung capacity. RESULTS The intra-class correlation coefficient (ICC) for SNIP was the highest of the three measures: S1-S3 ICC (95% CI) SNIP: 0.96 (0.88-0.94); PI max at FRC 0.82 (0.63-0.92); PI max at RV: 0.89 (0.78-0.95), and PE max: 0.96 (0.92-0.98), and had the lowest mean change between sessions [mean S2 - S1: 2.1(p = 0.4) and S3 - S2: -0.3 (p = 0.9)]. CONCLUSIONS SNIP is repeatable over a period of 3 weeks in medically stable, moderate-to-severe COPD patients. In our study, 2 sessions were adequate to learn how to perform the test.