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

ABSTRACT

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)