133Xenon Washout Scans following Parenteral ~ethacholine*

To determine the influence of bronchoconstriction on the distribution of ventilation during an asthma attack, pulmonary clearance of lssxenon was evaluated in four normal and eight asthmatic subjects within three to five minutes after intramuscular injection of methacholine. In asthmatics, administration of 4-10 mg methacholine resulted in a decrease of forced vital capacity of 28.5 2 5.1 (SE) percent and a decrease in expiratory flow at 60 percent vital capacity of 44.2 6.9 pemt (P < 0.001). The cumulative ventilation required to reach 50 percent of the pre-washout radioactivity increased from 3.6 2 A cute asthma is characterized physiologically by aidow obstruction and uneven distribution of ~entilation.l-~ The major mechanism causing airilow obstruction during the early phase of an acute asthma attack is thought to be constriction of the bronchial smooth mu~;e.~ As the asthmatic episode persists, mucosal edema and mucous plugging frequently contribute to airway narr~wing.~ Most studies of the distribution of ventilation during spontaneous asthma have been done when the attack was well-established.s.e-8 Thus, the uneven pattern of ventilation that has been observed may have been influenced by bronchial secretions. The purpose of this Ady was to evaluate the role of bronchoconstriction as a cause of uneven distribution of ventilation during the early phase of an acute asthma attack. ~ronchos~asrn -was induced by parenteral methacholine, aid shortly thereafter, regional ventilation was studied by measuring the