Inspiratory flow-volume curve evaluation for detecting upper airway disease

The 2005 American Thoracic Society/European Respiratory Society guidelines on spirometry emphasize examination of the inspiratory curve of the flow-volume loop for evidence of intrathoracic or extrathoracic upper airway obstruction. We sought to determine how frequently evaluations are performed for abnormal inspiratory curves.

We retrospectively reviewed all examinations performed in our pulmonary function testing laboratory over a 12-month period (n = 2,662). In patients with normal spirometry or a mild restrictive defect, we inspected the inspiratory curves for truncation, flattening, or absent loop. With patients who had an abnormal inspiratory curve, we examined 3 flow-volume loops to determine if more than one loop showed an inspiratory abnormality, and to assess changes in the mid-flow ratio (ratio of forced expiratory flow at 50% of the forced expiratory volume to forced inspiratory flow at 50% of the forced inspiratory volume), and we used the loop that had the best inspiratory and expiratory curves. We reviewed the medical records for underlying disease processes and evidence of upper airway evaluation.

One hundred twenty-three patients (4.6%) had an abnormal inspiratory curve. Sixty-nine (56%) of those 123 patients had inspiratory abnormalities on > 2 flow-volume loops. Evaluation of the inspiratory abnormality was undertaken in only 17% of all patients, and 30% of patients who had consistently abnormal inspiratory curves. A specific etiology was identified in 52% of the evaluated patients. Vocal cord dysfunction was the most frequent diagnosis. Utilizing the loop that had the combination of the best inspiratory and expiratory curves decreased the mid-flow ratio from 3.07 +/- 1.63 to 1.77 +/- 1.15.

An abnormal inspiratory curve in the presence of otherwise normal spirometry should prompt an evaluation for the etiology. If one of the flow-volume inspiratory curves shows an abnormality, all the inspiratory curves from that PFT session should be reviewed, and if more than one inspiratory curves is abnormal, both anatomical and functional evaluation should be undertaken for intrathoracic and extrathoracic upper airway obstruction.