Cardioversion and Digitalis Drugs: Changed Threshold to Electric Shock in Digitalized Animals

The relationships between the action of digitalis drugs and the effects of cardioversion were studied in 44 dogs. Ventricular tachycardia provoked by either acetyl strophanthidin (AS) or ouabain could not be reverted to sinus rhythm by DC shock. When ventricular fibrillation was induced electrically during digitalis toxicity, defibrillation could be accomplished readily but the toxic arrhythmia due to digitalis remained unaltered. When ventricular fibrillation was induced by AS, it was also unaffected by repeated high energy discharges. In control animals the median energy to produce ventricular tachycardia by electrical discharge was 400 watt-seconds. Repeated electrical shocks did not change the level of energy required to produce ventricular tachycardia. However, following recovery from ouabain-induced ventricular tachycardia, the electrical threshold for ventricular tachycardia was 0.2 watt-second. A similar but less consistent response was noted after AS-induced ventricular tachycardia. The sensitization to electrical shock lasted for five to ten minutes after AS and for approximately one hour after recovery from ouabain toxicity. This same phenomenon of sensitization to electrical discharge was observed during digitalization. About 80% of the potentially toxic dose of ouabain had to be administered before the electrical threshold was lowered. The sensitivity to DC shock in the digitalized animal was not related to any specific phase of the cardiac cycle. Digitalization did not change the duration, location or energy limits of the ventricular vulnerable period. These experiments lead to the following conclusions of clinical importance: 1) Cardioversion will not be effective in arrhythmias induced by digitalis toxicity and 2) in the presence of overdigitalization cardioversion discharge may provoke serious disturbance of rhythm.