Single-point measurements of maximal inspiratory pressure (MIP) are frequently used to suggest muscle weakness in clinical practice. Although there is a large variability in “mean” predicted MIP depending on the chosen reference values, it remains unclear whether those discrepancies actually impact on the prevalence of weakness, i.e., MIP below the lower limit of normal.
1729 subjects (50.1% males, aged 20 to 94) who underwent MIP measurements in a clinical laboratory comprised the study group. MIP was predicted according to the most cited regression equations as of August 2015. Pre-test probability of weakness was defined by a cluster of clinical and physiological variables.
Prevalence of weakness ranged from 33.4 % (Enright et al.) to 66.9 % (Neder et al.). Black and Hyatt, Bruschi et al. and Neder et al. (set 2 equations) agreed well in indicating weakness (kappa (95% CI) ranging from 0.81 (0.79-0.83) to 0.83 (0.81-0.85); p<0.01) There was a closer agreement between higher pre-test probability of weakness and low MIP according to set 2 compared to Wilson et al., Enright et al. and Harik-Khan et al. (set 1 equations). Thus, a significant fraction of subjects with abnormal MIP according to set 1 but preserved MIP according to set 2 had higher pre-test probability of weakness (p<0.05).
The choice of MIP reference values strongly impacts on the prevalence of weakness. Some specific equations relate better to clinical and physiological indicators of weakness suggesting that they might be particularly useful to screen subjects for advanced respiratory neuromuscular assessment.