Effect of age on the cardiotoxic action of digitalis

The cardiotoxic effect of ouabain was explored in rabbits at different ages. The dose of ouabain necessary to produce ventricular tachycardia and death in 1-to 4-week-old rabbits was significantly greater than that required in older animals. In the 1 to 3-week-old animals, ventricular fibrillation occurred only once whereas in animals older than 3 weeks, fibrillation occurred in 23 of 31 animals. A similar pattern of sensitivity to ouabain was observed in bilaterally vagotomized animals. Neither the initial heart rate nor the potassium content of the ventricles in the different age groups appeared to have any influence on the change in the cardiotoxic dose of ouabain. Heart weight increased during the interval when the occurrence of fibrillation increased but it may not fully explain the alteration in the mode of death since fibrillation occurred in some of the younger animals with bilateral vagotomy before the heart weight increased. The catecholamine content of ventricles from 4-to 12-week-old rabbits was significantly greater than that in 1- or 3-week-old rabbits. The age-dependent decrease in the dose of ouabain necessary to produce ventricular tachycardia, ventricular fibrillation and death appeared to be associated with an increase oof ventricular norepinephrine content, although other factors may also play an important part in this response.