Altered vagal and sympathetic control of heart rate in left ventricular dysfunction and heart failure

We investigated alterations in autonomic control of heart rate in conscious dogs with left ventricular (LV) dysfunction in the presence and absence of heart failure (HF) due to rapid pacing. In dogs with LV dysfunction but no HF, indexes of parasympathetic control decreased significantly after only 4 days of pacing. In dogs with fully developed HF, both vagal and sympathetic contributions were small. Vagomimetic doses of atropine increased both R-R interval (419 +/- 25 to 466 +/- 34 ms, P < 0.05) and standard deviation (SD) of the R-R interval (13 +/- 2 to 34 +/- 9 ms, P < 0.05). The digitalis glycoside deslanoside (Cedilanid-D) alone prolonged R-R interval (420 +/- 33 to 492 +/- 44 ms, P < 0.01) and tended to increase SD (14 +/- 4 to 28 +/- 8 ms, P = 0.08). After Cedilanid-D, low-dose atropine resulted in no significant further change in R-R interval or SD. These data indicate that changes in vagal control of heart rate become apparent at a very early developmental stage of LV dysfunction, and we speculate that this may provide important prognostic information in patients who are at risk for developing progressive myocardial dysfunction and HF.