Continuous pulmonary C-fiber stimulation produces sustained reflex cardiovascular depression

We used an isolated perfused in situ left lung preparation to evaluate the hemodynamic response to specific pulmonary C-fiber stimulation with capsaicin in two groups of anesthetized dogs. In one group (n = 6), continuous administration of capsaicin at 5 micrograms.kg-1.min-1 into the isolated pulmonary circulation produced sustained (20 min) reflex decreases in mean arterial pressure, cardiac output, maximal left ventricular rate of pressure over time, and heart rate. These effects were abolished by vagotomy. In a second group (n = 5) of atropinized dogs, a mild hemorrhage was first performed in each dog during control conditions (capsaicin vehicle administration). The hemorrhage was performed by removing 10% of blood volume over 3 min and then replacing this blood 3 min later. After reinfusion and stabilization, the mild hemorrhage was repeated during continuous administration of capsaicin (6.0 +/- 1.0 microgram.kg-1.min-1) into the isolated pulmonary circulation. The decrease of mean arterial pressure in response to hemorrhage was significantly greater during capsaicin administration than during control conditions. Lung denervation prevented the effect of capsaicin administration on the response to hemorrhage. We concluded that continuous pulmonary C-fiber stimulation produces sustained cardiovascular depression and compromises the hemodynamic response to mild hemorrhage in dogs. This latter effect may represent a central alteration of baroreflex gain.