Airway constriction in asthmatics determined from forced expiratory volume curves with external resistances

ABSTRACT

This study was undertaken to determine what role the level of bronchial constriction played in the dyspnoea of the asthmatic attack. Forced expiratory volume curves (FEV) of asthmatic patients were studied with and without external resistances (discs) of known internal diameters, during an attack, after bronchodilator therapy and during remission. The results show that the site of narrowing of the tracheobronchial tree was important. This "critical site" accounts for approximately 20-40 percentof the calculated equivalent resistance in an attack. From the matched FEV curves with external resistances it was observed that discs with internal diameters of 1.5-5.0 mm most frequently matched the asthmatic curve. On recovery disc of 5.0-9 mm matched the patient's curve. The data supports previously reported studies based on theoretical calculations and direct measurements which showed that the critical resistance to respiration occurs around the level of the 3rd-6th generation of the tracheobronchial tree with internal diameters of 3-8mm; it is of interest to note in this repect that endotracheal tubes have internal diameters in this same range (AU)