Evaluation of the Normal Inspiratory Flow-Volume Curve

Abstract

Introduction

The normal inspiratory flow-volume (FV) curve has not been extensively characterized. Acceptability criteria have been proposed but not thoroughly evaluated, and formal recommendations have not been published. This study was done to further characterize the inspiratory FV curve and to evaluate the provisional guidelines in a population of normal subjects.

Methods

The design for the study is a retrospective review of pulmonary function reports. The setting for the study was a university teaching hospital and tertiary care referral center. Subjects (patients) were ≥ 18 years old, had no pulmonary symptoms or disease, and had a normal expiratory spirometry between December 1993 and December 1994.

Results

One hundred twenty-one studies were evaluated. There were 91 men in the group, with a mean age of 52.1 years (± 16.7 years; range, 19-92 years). In 62.8% (n = 76) of cases, the maximum inspiratory flow (MIF) did not occur during the 'best effort' expiratory curve (largest sum of forced expiratory volume in one second + forced expiratory vital capacity [FVC]). The best forced inspiratory flow at 50% of vital capacity (FIF₅₀) was ≥95% of the MIF (on same effort) in only 62.8% of cases (n = 76). The forced inspiratory vital capacity (FIVC) on any effort was ≥ FVC in only 44.6% of subjects (n = 54). The FIVC was ≥ 90% of the FVC on the effort containing the best MIF in 87.6% of cases (n = 106). The inspiratory FV curve containing the largest MIF was parabolic with one inflection point in only 53.1% of cases in which there was agreement in the interpretation of curve shape between 2 of the authors.

Conclusions

Evaluation of inspiratory FV data from only the best effort expiratory curve misses a significant portion of maximal MIF and FIF₅₀ values. Therefore, spirometry equipment should be designed to allow the technician to choose the best inspiratory FV loop. On the basis of our findings, FIF₅₀ ≥ 95% MIF, FIVC ≥ FVC, or a parabolic shape with only one inflection point are not optimal criteria for a normal or acceptable inspiratory FV curve when the inspiratory effort is performed immediately following a forced expiratory maneuver. However, FIVC ≥ 90% FVC may be useful as a criterion of acceptability. Last, it is possible that recording inspiratory and forced expiratory maneuvers separately will result in loops that are more consistent with expectations. Further evaluation using larger populations is needed before definitive recommendations can be made.