Spirometric diagnosis of upper airway obstruction.

We evaluated the results of routine spirometry in patients with well-documented upper airway obstruction (UAO) to determine if this readily available form of pulmonary function testing could reliably identify patients with this abnormality. Our results indicate that, although individual standard spirometric indexes could not identify these patients, ratios derived from these indexes provided excellent discrimination between patients with UAO and patients with a variety of lung diseases. A ratio of maximal voluntary ventilation to forced expiratory volume in 1 s of less than 25 was present in ten (66%) of 15 patients with UAO and only one (1%) of 100 comparison patients. A ratio of forced inspiratory flow between 25% and 75% of the vital capacity to forced expiratory flow between 25% and 75% of the vital capacity of less than 1 was found in 12 (80%) of 15 patients with UAO, but in only four (4%) of 100 comparison patients. All patients with UAO had one ratio abnormal. We conclude that spirometry remains a valuable procedure in the diagnosis of UAO.