Limitations of sniff nasal pressure in patients with severe neuromuscular weakness

Abstract

Background: Inspiratory muscle strength in patients with neuromuscular disorders can be assessed using sniff inspiratory nasal pressure (Pn sn) and maximum inspiratory mouth pressure (Plmax). However, the relative merits of Pnsn against Plmax are not known in patients with severe neuromuscular disease. Objective: To investigate whether severity of disease modifies the relation between Pn sn and Plmax. Methods: Vital capacity (VC), Pn sn, and Plmax were measured in 258 patients with neuromuscular disorders. Results: Data were analysed from 241 patients, 17 being unable to perform Plmax or Pnsn manoeuvres. The correlation between Pnsn and Plmax was +0.94 (p<0.0001 ), with a mean (SD) difference between Pnsn and Plmax of -4.8 (21.2) cm H2O (the limits of agreement were 37.6 and -47.2 cm H2O). VC (% predicted) was positively correlated with Pn sn/Plmax (r = +0.86; p<0.0001), with a lower Pn sn/Plmax value in patients with a VC <40% of predicted than in those with a VC >40% (0.80 (0.35) v 1.04 (0.41); p<0.0001). Conclusions: Plmax is greater than Pnsn in patients with a severe restrictive ventilatory defect caused by neuromuscular disease. Pn sn may not accurately reflect inspiratory muscle strength in such patients and it is thus advisable to use both tests.