Maximal Inspiratory Pressure: Discussion
The mean Pimax was consistent with values reported for similar groups of patients with moderate to severe COPD. The results of this study supported the notion that in naive COPD patients, performance of Pimax improves with practice of the Pimax maneuver. The third and fourth test of Pimax produced reliable data.
Learning Effect
Systematic differences between consecutive tests were relatively small but at the fourth test, Pimax was 17 percent higher than at the first test. Other investigators observed similar increases in Pimax during an initial learning period, but for most the observed improvements in Pimax were not statistically significant because they used smaller samples with inadequate statistical power to detect an effect of this magnitude. The study of Ringquist2 is the one exception. He studied a larger sample and found no statistically significant improvement with the second measure of Pimax. This apparent inconsistency with our results may be attributed to the number of trials employed with each test session and the level of motivation. Ringquist2 studied healthy military conscripts who were highly motivated and athletic as compared with our COPD patients. Moreover, he conducted approximately ten trials per session, whereas we conducted five trials. It is possible that the high level of motivation and the additional number of trials enabled subjects to learn the Pimax maneuver in a single session.
In other tests of muscle strength, the evidence for a learning effect is inconsistent. A learning effect was observed when testing a variety of muscle groups, including the following: strength of the wrist extensors and elbow extensors as measured by a hand-held dynamometer, strength of the wrist flexors as measured by a tensiometer, and hand grip strength as measured by a hand dynamometer. In contrast, no learning effect was observed in other studies when testing strength of the elbow flexors as measured by a hand-held dynamometer, strength of right elbow flexors and right shoulder abductors, and strength of the right wrist flexors. In these studies and studies of Pimax, inconsistencies may be accounted for by a number of factors, including the individuals familiarity with the required physical task and their prior athletic experience.
In becoming familiar with a strenuous physical task, people learn to ignore extraneous sensations and to perform the task with very little conscious thought. The physical task required to perform a Pimax maneuver from RV is unique and unlike any other physical activities performed in daily life. This could increase the amount of practice required to learn the task. We suspect that in becoming familiar with the Pimax maneuver, patients learned to relax and ignore the sensations of pressure in the head while focusing on sensations of muscle contraction in the chest wall. We observed that many patients appeared to become more relaxed and self-confident after the first test session.
Additionally, past athletic experience may influence patients mental attitude and their ability to concentrate on the task. Anecdotally, we observed that former athletes approached the task with greater intensity of purpose and with less fear of overexertion. This may account for some of the observed variability in performance.
Many patients demonstrated a learning effect of >10 cm H20. Clinically, it would be useful to predict which patients were most likely to demonstrate a large learning effect, but this was not possible given the data we collected. In our study, older patients were less likely to demonstrate a large learning effect by increasing their Pimax more than 10 cm H20. However, the relationship with age disappeared when the increases in Pimax were calculated as a percentage of actual Pimax, because older patients tended to generate lower values for Pimax. We suspect that the magnitude of the learning effect was influenced by other factors as described above.
Our data supported the need for at least five trials for each measure of Pimax. While most subjects generated their best Pimax within the first three trials, as many as 10 percent generated their best Pimax at the fifth trial of each test session. The question of conducting more than five trials with each test session was not addressed by this study. However, these inexperienced subjects required a total of 15 to 20 trials over three test sessions to establish peak performance, suggesting that more than five trials may be required per test session. This would be consistent with the findings of Fiz and coworkers in which a total of nine trials were needed to establish peak performance in a single test session with inexperienced COPD patients.