Standardized Nasal Inspiratory Pressure Measurement

Prior to SNIP performance, anthropometric and other measurements were recorded, including height (Seca, Germany), weight (Marsden, UK), blood pressure, heart rate and SpO₂; the last three measurements were recorded using a Vital Signs Monitor 300 (Welch Allyn, USA). MIP and maximal expiratory pressure (MEP) were recorded using a MicroRPM device (Care Fusion, UK) and variability was less than 20% [3 (link)]. For MIP, subjects exhaled to residual volume before inspiring through a mouthpiece until recording a maximum value, and for MEP, subjects inhaled to total lung capacity before expiring through the device [3 (link), 18 (link)]. Pulmonary function tests (PFTs) were performed using a Micro Loop spirometer (Care Fusion, UK), and repeated until meeting British Thoracic Society criteria [23 ].
SNIP was measured using the MicroRPM device whilst subjects remained seated upright with both feet on the floor. Instructions were given to breathe normally between tests, and, on cue, exhale to functional residual capacity before sharply sniffing inwards with the mouth closed [18 (link)]. With a subset of ten subjects (five males), SNIP was measured via a probe inserted into the right nostril, with the left non-occluded. Subjects performed three sets of ten repeats; during each, either a 30, 60 or 90 s rest was given between repeats (Fig. 1). The order of tests was randomized for each subject. Data from these experiments determined that 30 s was an appropriate rest interval for all remaining experiments. After an interval of at least one week, all 51 subjects performed four sets of 20 SNIP tests (Fig. 1). Each set used a different technique to measure SNIP; via the right nostril with the left non-occluded (RNLNO), the right nostril with the left occluded (RNLO), the left nostril with the right non-occluded (LNRNO) or the left nostril with the right occluded (LNRO). The order of tests was randomized for each subject by giving them four numbered cards, where each number corresponded to one of the four techniques, and asking the subjects to pick these in a sequence (whilst the cards were face down). The contralateral nostril was occluded by subjects placing their thumb over their nostril for the RNLO and LNRO techniques.Fig. 1

Subject recruitment and study design. RNLO: right nostril, left occluded; RNLNO: right nostril, left non-occluded; LNRO: left nostril, right occluded; LNRNO: left nostril, right non-occluded