Critical anatomical site in healthy subjects from observation of forced expiratory volume curves with external resistances

The internal diameter of the air passages narrows from the trachea to the periphery of the lungs. Because of this it should be possible to predict the site at which the greatest change in resistance would occur for a given flow rate using the orifice flow equation (P = p/Cý(1/A2ý 1/A1ý) Vý where p = density of air, C = discharge constant which depends on the roughness of the tube and on Reynolds number, V is the volume rate of flow and A1 and A2 are the cross sectional areas) and compare it with forced expiratory volume curves (FEV) in healthy subjects. Theoretical calculations showed that the 3rd-6th generation of the bronchial tree contributed about 40 percent to the total airway resistance. Direct measurements by previous workers showed that approximately 55 percent of the total pulmonary resistance was contributed by the 3rd-6th generation i.e. 3-6 mm internal diameter bronchi. Results of FEV curves in normal subjects against external resistance of known internal diameters will be presented. These results show that the critical orifice (i.e. the point at which greatest inflection begins in the FEV curve) is in the range of 6-9 mm internal diameter (around the 3rd generation of the tracheobronchial tree). An analog of the tracheobronchial tree is being prepared to simulate the resistance to breathing, and thereby provide a simple method of measuring airway resistance (AU)

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

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)