Effect of vagal stimulation on collateral flow resistance in dog lungs

Vagal effects on steady-state resistance of pathways within a segment of lung (Rss) were studied in open-chest dogs using a wedged catheter technique. To prevent adrenergic bronchodilation, dogs were pretreated with propranolol. Rss was determined with vagi intact or sectioned and during electrical stimulation of right and left vagi. Following determination of Rss, gas flow into the segment was arrested, segmental pressure decay recorded, and the time for 90% decay (T90) measured. Stimulation of the vagus supplying the lung with the wedged catheter (ipsilateral) increased Rss and T90 at low (2-4 cmH2O) transpulmonary pressures (Pao) and T90 at high (5-9 cmH2O) Pao. Vagal sectioning and contralateral stimulation had minimal effects. Atropine abolished vagal effects. We modeled the segment as two series compartments, one with a rapid time constant and one with a longer time constant. At both low and high Pao, vagal stimulation prolonged the slow time constant. If the fast compartment represents small airways and the slow compartment collateral pathways, collaterals may not be effective in maintaining ventilation to lung segments distal to a vagally constricted bronchus.