Ventricular fibrillation (VF) was electrically induced in cats, rabbits, guinea pigs and rats. In young animals of all species and also in old rats short transient VF (TVF) was followed by spontaneous reversion to normal sinus rhythm. Old cats, rabbits and guinea pigs had episodes of sustained VF (SVF) which did not terminate spontaneously; artificial electro-defibrillation was required.Dibenzepin converted the sustained response of old animals to a transient one. Verapamil antagonized the effect of dibenzepin in old animals and also changed the TVF in young animals to SVF. The effect of calcium on this response was the opposite of that of verapamil.It is concluded that the age of the animal is an important factor in determining whether the fibrillatory response of the heart will be transient or sustained. The age change may be related to an alteration in the properties of the cell membrane.
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