Paroxysmal atrial tachycardia with exit block

Extra AV nodal Wenckebach periodicity

Extra AV nodal Wenckebach periodicity was diagnosed in seven patients. The most frequent form of this conduction abnormality was an exit block. The underlying block was localized in the sinoatrial junction and in the atria in two patients; the AV junction and the ventricle were the site of the Wenckebach periodicity in one case each. In extra AV nodal exit block, the actual conduction delay is not seen on the ECG and the diagnosis is based on the progressive shortening of the P-P or R-R intervals followed by a pause which is less than twice the shortest P-P (R-R) interval depending on the level of the block. A Wenckebach periodicity in the bundle branches or within the reentry pathway each occurred in one patient. In these forms of Wenckebach periodicity, the diagnosis is established more readily because the conduction delay can be demonstrated on the surface ECG. The clinical significance of extra AV nodal Wenckebach periodicity is discussed.

Atrial flutter with exit block

The mechanism of atrial flutter is controversial. A 76-year-old woman with rheumatic heart disease was referred to our clinic with an unusual rhythm disturbance which initially appeared to be classic atrial flutter at a rate of 300 beats/min. Later tracings, however, demonstrated a rate exactly one-half that of the earlier ECGs, with an identical p-wave morphology and vector. This latter rhythm also behaved in a manner expected for a flutter mechanism in that both spontaneously and with carotid pressure high-degree atrioventricular block occurred without alteration of the underlying atrial mechanism. Finally, the two rates interchanged spontaneously over several days without any significant interval changes in medical therapy. These findings were initially explained as probable digoxin toxicity. The underlying mechanism, however, was more likely atrial flutter with exit block and in this patient may have represented another facet of her sick sinus syndrome. This unusual phenomenon is discussed in terms of previous reports and possible implications for the mechanism of atrial flutter.

Bi-morphic atrial flutter

A case of atrial flutter whose classic "saw-tooth" morphology, resistant to full doses of digoxin and quinidine, was changed to an unusual form by low energy direct current shock is presented. The atrial and ventricular rates remained identical in spite of the different F wave pattern before and after attempted cardioversions. 0.5 mg digoxin was given after cardioversion failed and the patient reverted to normal sinus rhythm. This case strengthens the theory of rapidly discharging atrial focus as a mechanism of atrial flutter and the case for a change in intra-atrial conductions as cause for change in atrial electrograms.

Complex atrial arrhythmias studied by suction electrode technique

Two elderly patients with coronary heart disease, chronic pulmonary disease, and complex atrial arrhythmias are presented. The suction electrode technique for right atrium monophasic action potential recording was used. This recording allows better analysis of the atrial arrhythmias and estimates the stability of focal activity.

Exit block in sustained atrial tachycardia in two children

The electrocardiographic findings in two children with sustained atrial tachycardia and exit block are reported. The exit block was dependent on the atrial pacemaker rate, being present only at rapid rates of discharge. In one case propranolol enhanced the exit block, and on an appropriate dose the heart rate was maintained within a well tolerated range.

Studies on left atrial automaticity in dogs

The site of origin and the sequence of atrial activation were studied in 21 open-chested dogs anesthetized with sodium pentobarbital. Bipolar plunge electrodes were used to record atrial electrograms simultaneously with surface lead II and His bundle electrogram. In sinus rhythm the sinus node was activated first, followed sequentially by Bachmann's bundle, right atrial appendix, left atrial appendix, posterior left atrium, and the proximal portion of the coronary sinus. In 15 dogs left atrial beats and rhythms of various rate were recorded, including instances of left atrial exit block and of rapid left atrial rhythms. In such beats one of the left atrial electrodes was activated first and was followed by Bachmann's bundle, sinus node, proximal portion of the coronary sinus, and by right atrial appendix. Left atrial beats were observed occurring (1) spontaneously; (2) as a result of vagal stimulation; (3) after destruction of the sinus node; (4) during ventricular pacing. Left atrial pacing through the electrode recording initial activity reproduced the sequence and configuration of atrial electrogram similar to that seen in the ectopic left atrial beats. These results demonstrate that in dogs ectopic activity frequently originates in the left atrium.

Supraventricular tachycardia with AV block

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Atrial tachysystole (flutter?) with apparent exit block

A case of atrial tachysystole with apparent exit block is reported in which the basic cycle length of atrial activity is 260 msec. Simultaneous right intra-atrial electrograms show cycle lengths of 510 to 780 msec which are roughly multiples of 260 msec establishing the diagnosis of exit block. Rapid atrial stimulation (1,200/min) to overdrive the rapid ectopic atrial pacemaker was only transiently successful in converting the arrhythmia to atrial fibrillation with a slower ventricular response. Two days later the rhythm converted to normal sinus rhythm (cycle length, 600 msec) with periods of sino-atrial block measuring 2,400 and 1,240 msec. Atrial rate further slowed so that the patient ultimately required a permanent demand pacemaker.

The case supports the theory of repetitive discharge from an ectopic atrial pacemaker to explain the mechanism producing atrial tachysystoles that simulate atrial flutter.