Maximal Inspiratory Pressure: Method
Maximal inspiratory pressure (Pimax) is commonly used to measure inspiratory muscle strength. It reflects the force-generating ability of the combined inspiratory muscles during a brief quasi-static contraction.
The Pimax test is simple to perform and usually measured during a single test session with several trials. However, reliability has not been fully addressed in the research literature, specifically with respect to quantifying systematic measurement error associated with learning effects and day-to-day fluctuations associated with technical and/or methodologic errors and biologic variability in performance. Several studies examined the issue and most reported high test-retest reliabilities for repeated measures and no significant increase in Pimax with repeated measurements in naive subjects. However, in most studies, reliability was described on the basis of two or three test sessions, making it difficult to separate the systematic measurement error associated with learning effects from the day-to-day fluctuations in performance.
We examined initial learning effects and test-retest reliability of Pimax as measured at four test sessions in patients with COPD using a method that can be easily employed in the office or clinic setting.
Data for this study were acquired from COPD patients during a 4-week period prior to participating in a pulmonary rehabilitation program. The study was approved by the appropriate institutional committees on human research and informed consent was obtained from all patients. Patients qualified for the study if they (1) had clinically stable COPD with moderate to severe airflow obstruction (FEV, <65 percent predicted), (2) had no clinical evidence of asthma and <30 percent improvement in FEV, with bronchodila-tors, and (3) had no exacerbations for at least 2 months prior to the study and remained free of respiratory tract infections during the 4 weeks of data collection. Patients were dropped from the study if they reported a respiratory tract infection during the 4-week period of data collection. Potential subjects were excluded if they had major health problems that might interfere with testing. All patients were naive in regard to respiratory muscle testing and had never performed tests of Pimax. One hundred twenty-nine patients enrolled in the pulmonary rehabilitation program during data collection and 91 of them provided usable data with Pimax measured at four separate sessions. All data reported herein were taken before initiating pulmonary rehabilitation.
Maxima] inspiratory pressures were measured with an aneroid pressure gauge (No. 2000-200 cm magnehelic pressure gauge, Dwyer Instruments, Michigan City, Ind) that measured pressures from 0 to — 200 cm H20 in increments of 5 cm H20. Its calibration was verified at regular intervals against a column of w^ater and found to be accurate within 3 cm of w ater throughout its full range. Sixty centimeters of pressure tubing connected the aneroid pressure gauge to a flange-type mouthpiece. An air leak was established with a small hole (diameter = 1.16 mm) in the adapter between the mouthpiece and rubber tubing.