The objective is to determine whether maximal inspiratory pressure (P(imax)) measurement is more sensitive than vital capacity (VC) measurement to detect acute respiratory muscle failure considering a theoretical curvilinear relationship between volume and pressure.Review of VC and P(imax) of all patients hospitalized in ICU for Guillain-Barré syndrome (GBS) and myasthenia gravis (MG) exacerbation.84 consecutive caucasian patients between 19- and 70-years-old hospitalized in intensive care unit from April 2008 to December 2010, for MG exacerbation (44 patients) and GBS (40 patients). The regression curve between VC and P(imax) was linear rather than exponential (r = 0.599, P0.0001). The contingency table demonstrated agreement between VC and P(imax) (χ(2)= 26.7, P = 0.0001), with similar number of patients having abnormal P(imax) associated to normal VC and normal P(imax) associated to abnormal VC (9 (10.7%) vs. 8 (9.5%) respectively). Six of the patients developed an important decrease of VC from normal value to less than 60% of the predicted value and did not present evident curvilinear relationship between VC and P(imax) during this follow-up.Because the regression between VC and P(imax) was linear rather than curvilinear, P(imax) was not more sensitive than VC for early detection of respiratory muscle failure in patients hospitalized in ICU for GBS and MG exacerbation. Therefore, VC remains well suited to assess acute respiratory muscle failure and P(imax) gives poor additional information.
