Analysis of the inspiratory flow-volume curve. Should it always precede the forced expiratory maneuver?

The inspiratory flow-volume (FV) curve can be used to identify patients with upper airway obstruction, air trapping, and restriction. Current computed pulmonary function testing equipment often mandates a forced expiratory maneuver (FEM) immediately prior to the forced inspiratory maneuver (standard method). We evaluated the inspiratory FV curve with and without an antecedent FEM in 119 subjects referred for pulmonary function testing. The subjects were divided into four groups by grading the degree of airway obstruction using confidence intervals of the FEV1/FVC percent predicted minus the actual FEV1/FVC percent measured from the best FEM according to Intermountain Thoracic Society recommendations. The forced inspiratory vital capacity (FIVC), forced inspiratory flow 50 (FIF50), and peak inspiratory flow (PIF) from the inspiratory FV curve with an antecedent FEM was compared with the FIVC, FIF50, and PIF without an antecedent FEM in each category of obstructive lung disease. The FIVC without the antecedent FEM was significantly larger than that with an antecedent FEM by 170 ml (p < 0.002) in subjects with severe airway obstruction, but was not significantly different in the other groups. The FIF50 was not significantly different in any group, but approached significance in both normal subjects and subjects with severe obstruction. The PIF was not significantly different in any group, but approached significance in the normal subjects, order for patients with severe obstructive airway disease to generate a valid forced inspiratory FV curve, it should be obtained without an antecedent FEM. When a plateau of the inspiratory FV curve is encountered, we suggest that is useful to generate the inspiratory FV curve prior to the FEM and to analyze its flow and volume characteristics independent of the FEM. The "best" inspiratory FV curve should then be displayed with the "best" FEM for proper evaluation of the FV loop.