Breathing response to lung congestion with and without left heart distension.

This study compared the effect of lung congestion with and without left heart (LH) distension on breathing frequency (fr) and discriminated among responses mediated by myelinated and nonmyelinated vagal afferents. Cardiopulmonary bypass perfusion of anesthetized dogs was used to isolate reflexes. The following three groups were prepared: 1) lung vessels pressurized by pumping into the main pulmonary artery (MPA); 2) lungs and fibrillating LH pressurized by pumping into MPA while draining from LH; 3) lungs congested by occluding several pulmonary veins while holding cardiac output constant. Congestion of lungs alone in groups 1 and 3 depressed fr. Congestion of lungs and distension of LH (group 2) caused transient depression of fr but a steady-state excitation. Cooling cervical vagi to 8 degrees C prevented depression of fr by congestion in all groups. In groups 1 and 2, in which MPA pressure was higher than in group 3, congestion during vagal cooling stimulated breathing. I conclude that lung congestion may stimulate fr via C-fiber afferents, but this may be overcome by a depressor effect via myelinated afferents. Simultaneous LH distension may reflexly stimulate breathing and overcome the lung depressor reflex.