Effects of Vagal Blockade on Neurohumoral Systems in Conscious Dogs with Heart Failure

It has been hypothesized that a decreased activity of vagal afferents might contribute to the activation of neurohumoral systems in congestive heart failure. Therefore, we studied the effects of vagal nerve blockade by local anesthesia on neurohormones in six conscious dogs before and after induction of heart failure by rapid right ventricular pacing (250 beats/min, 10 days). In healthy dogs, vagal blockade significantly increased plasma vasopressin levels (from 1.5 +/- .6 to 13.7 +/- 10.5 pg/ml, p less than 0.02), without significantly affecting plasma catecholamines and renin. After 10 days of pacing, mean arterial pressure and cardiac output were decreased, right atrial and pulmonary arterial pressures and plasma levels of norepinephrine, dopamine, and atrial natriuretic peptide were increased. In this state, vagal blockade significantly increased plasma renin activity (from 1.52 +/- .43 to 3.18 +/- .54 ngAI/ml/h, p less than 0.02) and plasma vasopressin (from 4.2 +/- 3.3 to 89.1 +/- 54.9 pg/ml, p less than 0.02), this increase being significantly higher than in healthy dogs. We conclude that in these dogs with low cardiac output state, which resembles early heart failure, vagal afferent activity is increased and effectively suppresses renin and vasopressin. This does not exclude the possibility that in later stages of heart failure vagal afferent dysfunction may develop, resulting in neurohumoral disinhibition.