Which cardiac disturbances should be treated with digoxin immune Fab (ovine) antibody? Digoxin excess can produce characteristic bradyarrhythmias, tachyarrhythmias, and hyperkalemia. The bradyarrhythmias, which consist of disturbances in conduction and block at the level of the atrioventricular and sinus nodes, are mediated by a direct and vagotonic effect. The vagotonic effect of excess digoxin may also result in a marked slowing of the sinus rate in the setting of severe toxicity. Digoxin increases automatic and triggered electrical activity in atrial muscle, His-Purkinje system, and ventricular muscle, which predisposes to tachycardias. Many of the tachyarrhythmias are relatively specific for the toxic effects of digoxin. Atrial tachycardias with variable atrioventricular block, accelerated junctional rhythms (especially in the setting of atrial fibrillation), and fascicular tachycardias are characteristic digoxin toxic rhythms. Digoxin-specific antibody fragments should be considered the treatment of choice for any digoxin toxic arrhythmia associated with hemodynamic compromise or the threat of hemodynamic compromise. Hyperkalemia, when due to acute severe digoxin toxicity, is also an appropriate indication for digoxin-specific Fab fragment therapy. When assessing the risk:benefit ratio for using digoxin-specific Fab fragment therapy, one needs to determine, in addition to the electrocardiographic manifestations and patient's hemodynamic status (1) the severity of toxicity, as indexed by the amount ingested and/or the serum digoxin concentration; (2) the expected time course for reversal of toxicity, which is usually determined by the status of renal function; (3) the need for digoxin to provide ventricular rate control or improved ventricular contractility and therapeutic alternatives to digoxin; (4) the presence of a strong allergy history; (5) the presence of such factors as increased age and severity of heart disease that may predispose to digoxin toxicity. 