The view that d.c. cardioversion can be hazardous in individuals receiving cardiac glycosides is based on studies reporting that the shock provoked serious ventricular arrhythmias (refractory ventricular tachycardia or fibrillation). Indeed, toxic levels of digitalis can lower the threshold for postshock ventricular arrhythmias. Usual therapeutic digoxin levels do not increase the risk of serious postshock ventricular arrhythmias. The following approach is recommended when considering d.c. cardioversion in digitalized individuals.
1. Electrolyte imbalances should be corrected, fever suppressed, and hypoxia and anxiety treated before d.c. cardioversion.
2. When there is overt digitalis toxicity, elective d.c. cardioversion is not performed.
3. Under all circumstances, the smallest energy that is likely to be effective is used. We start with 25 to 50 J, with subsequent increments of 25 to 50 J as needed. Digoxin-specific antibody (Fab fragments) for life-threatening toxicity
Because of the widespread tissue binding of digoxin, hemodialysis or hemoperfusion is of limited value for prompt reversal of life-threatening toxicity. Advanced digitalis intoxication should be treated with purified digoxin-specific polyclonal antibody fragments (Fab fragments) obtained from sheep immunized with digoxin. The following protocol for administration of Fab fragments is recommended (A.n..tm..a.n.eLa.l 1.99O; .Hlcke.y eLal 1991).
1. The dose of Fab fragments is calculated to be equal on a mole-for-mole basis to the amount of digoxin or digitoxin in the patient's body, estimated from the medical history, determinations of serum digoxin or digitoxin concentrations, or both (Table 1). Examples of the calculation of the body load of digoxin to be neutralized are shown in Tab].§..,2 (Antman.efa/ 1990).
Table 1 Calculation of the equimolar dose of digoxin-specific Fab fragments
Table 2 Examples of calculation of the equimolar dose of digoxin-specific Fab fragments
2. The Fab fragments should be administered intravenously through a 0.22-^m membrane filter over 15 to 30 min, unless the gravity of the clinical situation demands more rapid infusion.
3. Serum potassium should be monitored at least every hour for the first 4 to 6 h after Fab treatment.
4. Typically, the total extracellular digoxin concentration rises dramatically, but such digoxin is pharmacologically inactive because only the unbound form can associate with the inhibitory site on Na+,K+-ATPase. For this reason, as well as because of technical problems imposed on assay systems by the presence of high-affinity Fab fragments, measurements of serum digoxin concentration are not reliable indicators of the state of digitalization for about 1 to 2 weeks after Fab administration.
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