Stimulus-response curves of the lung inflation cardio-depressor reflex

In order to determine the relationship between extent of lung expansion and reflex depression of cardiovascular function in dogs, we used a preparation in which the left lung, isolated in situ, was subjected to a series of inflations ranging between 5 and 55 cm H2O (60 and 800 ml) before and following left cervical vagotomy. The threshold level of left lung inflation that would cause bradycardia and hypotension was 15 cm H2O transpulmonary pressure (63 ml) when the preceding level of inflation pressure was lower, and 10 cm H2O (102 ml) when the preceding level was higher. Increasing inflation pressure and volume above threshold produced a graded fall in heart rate and blood pressure until maximum expansion was reached at 40 cm H2O (778 ml). Maximum expansion caused a transient 45% fall in heart rate and 30% fall in blood pressure. Division of the ipsilateral (left) cervical vagosympathetic trunk eliminated these responses to unilateral lung inflation confirming the predominant, if not exclusive, afferent pathways. These data suggest that the lungs, as a function of the degree of expansion, impart a control over the neural regulation of the cardiovascular system.