Maximal inspiratory pressure often has been used to evaluate the strength or fatigue of the inspiratory muscles. Increased lung volume has been shown to effect maximal inspiratory pressure. If the measurements are taken from different lung volumes, some inferences drawn from changes in maximal inspiratory pressure may be mistaken. The authors quantitatively examine the effects of changing lung volumes on the development of maximal inspiratory pressure. Fifty-nine healthy subjects performed maximal inspiratory pressure maneuvers at lung volumes ranging from residual volume to 2.5 L above residual volume. At each lung volume, the subjects inhaled maximally against a closed-end tube and the inspiratory pressure was measured. The results of the study indicate that maximal inspiratory pressure decreased with increased lung volume, and that the decrease began with lung volumes as little as 0.5 L above residual volume. Further 0.5 L increases in lung volume each led to significant additional decreases in maximal inspiratory pressure. Therefore, the authors conclude that the response of decreased maximal inspiratory pressure with increased lung volume is a continuous one, which begins at very low lung volumes. The authors argue that it is very important to standardize the lung volume at which the pressure is measured when performing maximal inspiratory pressure tests. With the fluctuations inherent in functional residual capacity (particularly while the person is concentrating on breathing) residual volume may be a better position from which to make maximal inspiratory pressure measurements.
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