Electrocardiographic response of digoxin-toxic fascicular tachycardia to Fab fragments: implications for tachycardia mechanism.

The electrocardiographic response of digoxin-induced fascicular tachycardia to Fab fragments was evaluated in two patients. In addition, we documented the response of the fascicular tachycardia to spontaneous premature ventricular depolarizations during different tachycardia rates, the response to a nonsustained episode of ventricular tachycardia, and the mode of spontaneous initiation and termination of short-lived episodes of the tachycardia during the treatment process. The following findings were noted: slowing of the tachycardia in response to Fab administration; change in the morphologic characteristics of the tachycardia from multiform to uniform; resetting of the tachycardia by spontaneous premature ventricular depolarization with the return cycle equal to the observed tachycardia cycle length; acceleration of the tachycardia in response to five beats of a faster nonsustained ventricular tachycardia; and initiation and termination of the tachycardia, both by spontaneously occurring premature ventricular depolarizations and in the absence of premature ventricular depolarizations. Both tachycardias resolved completely within 20 and 40 minutes, respectively, of Fab administration. We conclude that Fab administration can promptly resolve fascicular tachycardias precipitated by digoxin toxicity and that the observed electrocardiographic phenomena strongly suggest triggered activity as the electrophysiologic mechanism of fascicular tachycardia in man.