© 2005 American Thoracic Society
Some Methodological Considerations Pertaining to Sniff Nasal Inspiratory Pressure (SNIP)To the Editor:I read with interest the article by Dr. Morgan and colleagues (1) establishing the value of sniff nasal inspiratory pressure (SNIP) for predicting survival in patients with amyotrophic lateral sclerosis (ALS). The aim of this letter is to raise some methodological points which departed from standard practice in this paper. As described in several studies (2–4) and as recommended by the ATS/ERS statement on respiratory muscle testing (5), SNIP is measured in one nostril during a maximal sniff performed through the contralateral nostril, which is left open. In this way, the maneuver is natural and easy for most subjects. In their study, Morgan and coworkers occluded the contralateral nostril, and the patients therefore performed a "gasp" rather than a sniff (5). During such an occluded inspiratory effort, the pressure measured in the upper airways often exceeds the simultaneous intrathoracic pressure, and thereby overestimates inspiratory muscle strength (6). This is likely due to a concomitant contraction of orofacial muscles. Figure 1 suggests that this happened in three subjects as their SNIP exceeded sniff transdiaphragmatic pressure. It is standard to measure SNIP as the peak pressure during a short, sharp sniff (2–5). Unlike maximal inspiratory pressure, there is no need to consider the value sustained for longer than one second, which will underestimate the SNIP in comparison with other studies. In view of the units of measure used for this test, the appropriate term is "sniff nasal inspiratory pressure" rather than "force." As well, the authors introduced the measurement of nasal pressure during an expiratory effort and called it "sniff nasal expiratory force." This is unfortunate because a sniff is an inspiratory effort only. Nevertheless, these considerations in all likelihood do not invalidate the important message of this study.
Service de Pneumologie, CHUV, Lausanne, Switzerland FOOTNOTES Conflict of Interest Statement: J.-W.F. does not have a financial relationship with a commercial entity that has an interest in the subject of this manuscript. REFERENCES
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