Mode of Spontaneous Onset and Termination of Supra-ventricular Tachyarrhythmias

the present study, a recording technique which ECG tracings from 16 patients with spontaneous Onset gives a high resohtion of the atrial waves has been and/or termination of supraventricular tachyarrhythmias used, thereby offering better possibilities for pre-(SVTA) were studied. Of these recordings, 13 were made cise assessment of p wave aberration and p-p in-with a special technique which gives a high resolution of the tervals. atrial waves. At the onset of SVTA, the first atrial wave invariably had an aberrant configuration. The coupling index (coupling interval (P-'P) preceding cycle length) was 0.50 or less in 9 of 15 cases but more than 0.60 in 4 cases. In the 5 cases of onset of atrial fibrillation, the intervals


INTRODUCTION
The mechanisms responsible for the initiation, maintenance and termination of supraventricular tachyarrhythmias (SVTA), have been the subject of continuing controversy and investigation. Experiments with electrical or chemical stimulation of the with spontaneous onset and/or termination of SVTA. In tracings with more than one bout of SVTA a representative one was selected for the study. The change from sinus rhythm to atrial fibrillation was studied in 5 tracings, to atrial flutter in 8 and to atrial tachycardia in 2. The reversion to sinus rhythm from atrial fibrillation was studied in 2 tracings, from atrial flutter in 6 and from atrial tachycardia in 2. In 3 patients the ECG was recorded with conventional amplification ( 1 mV=lO mm), lead V, and at least 3 other leads being recorded. In 13 patients a special recording technique was used (7), using three bipolar leads. The common reference electrode was placed at the midline of the angle of sternum and the three different electrodes were placed at the highest attainable point of the armpit in the left mid-axillary line (S 1). at the caudal end of the sternal body (S2) and at a point over the vertebral column at the transthoracic level of the sternal angle (S3). The three bipolar leads were recorded with conventional amplification, as well as with a tenfold greater amplification (0.1 mV=lO mrn), using a 3-channel differential preamplifier and a Mingograph 81 (Siemensatrium, while yielding important information concerning the electro-physiological characteristics of SVTA, have not solved the Problems of onset and Elema Ltd., For measurements, the system illustrated in Fig. I  were made with a conventional recording technique, which is not sufficiently sensitive to clearly (exhibit changes in the atrial wave configuration. In

RESULTS
The tracings of SVTA with the special recording technique permitted analysis of the atrial wave con-  At the onset of SVTA, the first atrial wave ('P) invariably had an aberrant configuration in comparison to the ordinary P wave. The following atrial waves (W"P) of the SVTA had the same configuration as 'P in the 2 S V T cases and in 6 of the 8 AFu cases. In the 5 AFi cases the configuration changed. In 9 of the I5 cases, the 'coupling index' (the ratio of coupling interval (P-'P) to preceding cycle length (P-P)) was 0.50 or less, while it was more than 0.60 in 1 AFi case, in 2 AFu cases and in 1 S V T case. Acceleration of the atrial activity after *P was seen in the 5 AFi cases, in 3 of the AFu cases and in none of the SVT cases. The final atrial wave frequency was always obtained within 30 s.
The termination of SVTA occurred in 5 of the 10 cases studied within 30 cs from the beginning of a QRS complex. The point of termination was de-' fined as the start of the P' wave plus the Pz-P1 interval. There was no deceleration of the last few atrial impulses before the termination and the form of the atrial waves did not change. The first atrial wave after termination of SVTA exhibited an ordinary configuration in all but one case.

DISCUSSION
When dealing with the mechanism of SVTA onset and maintenance, some authors have concluded that at least S V T and AFu, and possibly AFi, are evoked by an ectopic atrial focus with a high impulse-formation rate. Other authors have concluded that SVTA depends on the establishment of an atrial circus movement with or without engagement of the atrio-ventricular node. From experimental studies there is evidence that SVTA can result from either of these mechanisms. The arguments in favour of both hypotheses have been reviewed by Hecht et al. 1953 (4) patients. An atrial premature discharge preceded the onset of AFi in all 14 episodes. In 10 of these the premature P wave was immediately followed by AFi. In the other 4 cases the premature P wave appeared to initiate a brief run of S V T (less than 6 beats) which accelerated to AFi. The relative prematurity of ectopic atrial beats was expressed by calculating the coupling index. The coupling index was less than 0.50 in 9 of the 14 patients. Killip & Gault thus concluded that a spontaneously occurring atrial premature impulse may initiate AFi. When the coupling index is less than 0.50, the chance of ensuing AFi is high; when it is greater than 0.60, the chance of ensuing AFi is small.
Bennett & Pentecost (1) studied the onset of AFi in 8 patients with acute myocardial infarction. One intra-atrial and one surface lead were registered. A total of 32 episodes were studied, and on each occasion the arrhythmia was preceded by a premature atrial beat. The premature atrial beat was always followed by a rapid regular atrial tachycardia of variable duration with a rate of approximately 340 beats/min; on some occasions the tachycardia lasted as little as 1 or 2 s, but at other times the duration was up to 30 s.
Bigger & Goldreyer ( 2 ) studied the repeated onset of paroxysmal S V T in 5 patients. The S V T always began with an atrial premature depolarization (APD). Spontaneous episodes of S V T were initiated only by APDs occurring in the relative A-V refractory period, while electrical stimuli during the atrial vulnerable period did not elicit SVT. From these and other findings Bigger & Goldryer concluded that paroxysmal S V T is most often due to reentry utilizing the A-V conduction system.
The results of the present study in general correspond well with the results of the earlier reports. The first premature atrial wave was shown to be aberrant in all cases studied and the coupling interval was in most cases short. The coupling index, however, was more than 0.60 in 4 of our cases ( Fig.  2 shows one of these cases). In these cases the first premature atrial wave could scarcely have occurred during the vulnerable period of the atria or the relative refractory period of the A-V conduction system. In these cases, therefore, the onset of SVTA probably depended on a rapidly firing ectopic focus rather than on circus movement.
Our study of the termination phase indicates that there is not a successive modification of atrial activity during the last seconds of a SVTA: the SVTA always ended abruptly in our cases. Rytand (9) reported similarly for 3 cases of AFu, while Bennett & Pentecost ( l ) , in their study of AFi in patients with acute myocardial infarction, found that a change of atrial wave form in the intra-atrial lead preceded the cessation of AFi on all 28 occasions observed.
Several explanations for the interruption of SVTA have been offered, for example, exit block from a rapidly firing ectopic focus or interruption of a circus movement with or without engagement of the atrio-ventricular node. One possible cause of such interruption could be retrograde depolarization of the atrio-ventricular node. In the latter case, the SVTA should be expected to terminate a t or shortly after a QRS complex. This was observed in only 5 of our cases of AFi or AFu, so that this explanation is probably not the only one.
Our results thus indicate that the theory of circus movement does not explain the mode of spontaneous onset or termination of SVTA in all cases. Onset and maintenance of SVTA in some cases may perhaps be governed by different mechanisms, the onset being initiated from an atrial ectopic focus and a circus movement being responsible for the maintenance. The importance of the vulnerable period of the atria could be that one single ectopic and premature atrial impulse occurring during the vulnerable period of the atria may initiate a circus movement SVTA, while an ectopic atrial beat with a long coupling interval must be followed by repeated rapid discharges from the ectopic focus in order for SVTA to ensue. The functional conditions of the atria may then determine which kind of SVTA eventually results.