To find the fibrillation threshold under rapidly changing conditions a method of its determination with long-lasting stimuli (100--300 msec) far exceeding the period of the highest vulnerability is described. The figures of the fibrillation threshold found by way of stimulating the heart with short and long impulses were partically identical. Intravenous administration of 4 mg/kg Lidocaine resulted in an increase of the fibrillation threshold in the experimental dogs, without changing the projection of the stimulus producing the fibrillation onto the force-interval curve and at the same time increasing the excitability of the myocardium. Intravenous 1 mg/kg Propranolol elevated the fibrillation threshold causing a parallel rightward shift in the projection of the stimulus on the force-interval curve and decreasing the exitability of the heart.
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