Respiratory mechanics in upper airway obstruction

Respiratory mechanics were studied in 7 patients with vocal cord paralysis (Group A), 7 with laryngeal or sublaryngeal obstruction (Group B) an in 3 with intrathoracic tracheal obstruction (Group C). Group A showed decrease in FIV1.0, PIFR and VI50 when compared with corresponding expiratory parameters. Group B only showed a decrease in PIFR compared with PEFR. In all patients the high flow area near TLC in expiratory V/V curves was replaced by a flat flow pattern, extending variably into lower lung volumes. Predominant inspiratory flow reduction was present in Group A. Usually the normal expiratory plateau (IVPF curves) was absent even at very low lung volumes (about 25% VC) in the 8 patients studied (Group A, B, C). In Group B, inspiratory and expiratory resistances (IVPF curves) were similar up to about 50-60 cm H2O Palv; above this level inspiratory resistance was higher. The flow-pressure pattern (IVPF curves) of a normal subject with a relatively low resistance added at the mouth (9 mm i.d.) was very close to that of the resistance alone throughout inspiration. Above 60 cm H2O Palv expiratory resistance appeared to be lower, as in Group B. Calculated pressure drop due to convective acceleration across the resistance coincides with its pressure-flow relationship. These results do not support the hypothesis of inspiratory reduction in tracheal caliber as a contributing cause to inspiratory flow decrease. The difference PEFR-PIFR at highest Palv was inversely related to the degree of obstruction. Consequently, flow-volume curves of moderate sublaryngeal lesions may show disproportionate reductions in VI (like Group A) despite its fixed nature. Results obtained in six patients submitted to surgical treatment are discussed.