The unidirectional valve is the best method to determine maximal inspiratory pressure during weaning

OBJECTIVES: Although maximal inspiratory pressure (MIP) is used as an index of inspiratory muscular strength, there is no consensus on how to measure it. We compared, during weaning from mechanical ventilation, two methods of measurement to determine which shows the greater values (MIPbest) and is more reproducible. One method measured MIP when negative pressure was maintained for at least 1 s after a forceful expiration, and the other method measured MIP with a unidirectional expiratory valve (MIPuni). DESIGN: The study had a crossover design, and patients randomly performed three measurements of each method (t1). The procedure was repeated by the same observer after 20 min (t2). The maximal value in each method was considered. SETTING: ICU, Hospital A.C. Camargo, S?o Paulo, Brazil. PATIENTS: Fifty-four consecutive patients undergoing short-term mechanical ventilation who became eligible for the study when their physicians decided to restore spontaneous breathing. RESULTS: MIPbest values were arrived at using MIPuni 75% of the time either in tl or t2. MIPuni yielded a higher average of MIPbest values in t1 and t2 (p < 0.0001). The effort-to-effort coefficient of variation of one method compared with the other during t1 and t2 was similar (p > 0.2 for t1; p > 0.8 for t2). Also, when comparing tl and t2, the coefficients of variation were similar for each method (p > 0.62). CONCLUSIONS: Because MIPuni displayed the maximal values, it is the best method for estimating MIP in patients undergoing short-term mechanical ventilation. The reproducibility of consecutive measurements was similar between the methods, even after a short period of time.