Closure of Peripheral Airways: Demonstration by Regional Studies of 133Xe Clearance

The closure of peripheral airways with low pulmonary volume in young normal subjects has been the subject of numerous controversies. Research on the regional distribution of ventilation, with different volumes, has tended to demonstrate the existence of this phenomenon, and recently it has been confirmed by studies of respiratory mechanics in man and histological studies in dogs. Apart from the usual methods of studying regional distribution, two simple investigations using 133Xe make it possible to measure and demonstrate the closure of peripheral airways. Measurement of closing volume: for this study a single scintillation counter is used to measure the radioactivity of expired air, after inspiration in the residual volume of a bolus of 133Xe followed by inspiration of air up to the total pulmonary volume. The results show a fresh rise in the expired radioactivity after the familiar ''alveolar plateau''. This is due to the fact that in this terminal part of the curve a larger contribution is made by air from the alveoli of the upper lobes, rich in 133Xe, while some alveoli of the base lobes where the air passages are closed are no longer participating in expiration. Hence the end of the alveolar plateau enables us to define the ''closure volume''. Demonstration of airway closure by regional analysis of 133Xe clearance curves: two scintillation counters are placed laterally near a subject in dorsal decubitus, one in the retrosternal (upper) and the other in the prevertebral (lower) position. 133Xe dissolved in saline solution and introduced by peripheral venous injection moves in the first passage from the pulmonary capillaries towards the alveoli. The activity, expressed as percentage of maximum activity, is studied as a function of time. In a young subject the rate of decay shown by the upper counter is slower than that shown by the lower counter, because of the familiar fact that the lower zones ventilate better than the upper zones. In older subjects, on the other hand, though the lower zones are well perfused, one observes a plateau due to residual activity on the lower counter: during normal respiration (current volume) some of the lower peripheral airways close and 133Xe is trapped below. Only energetic respiratory movements can then modify the thoracic pressures and reopen the airways so that 133Xe clearance can be completed. (author)