The R-on-T phenomenon during supraventricular tachycardia in childhood

J. ELECTROCARDIOLOGY, 9 (2) 171-174

The R-on-T Phenomenon During Supraventricular Tachycardia in Childhood BY RUEYJ. SUNG, M.D., HENRY GELBAND, M.D., F.A.C.C., ROBERTJ. MYERBURG, M.D., AGUSTIN CASTELLANOS, M.D., F.A.C.C. AND DOLORES M. TAMER, M.D., F.A.C.C.

the case ofa 5 year old child in whom the R-on-T phenomenon was clearly related to recurrent e s p i s o d e s of s u p r a v e n t r i c u l a r t a c h y c a r d i a (SVT). Since the R-on-T phenomenon may trigger fatal ventricular tachycardia and/or ventricular fibrillation, its possible role in the pathogenesis of sudden death in children needs to be further investigated.

SUMMARY The electrocardiographic R-on-T phenomenon was observed in the absence of Wolff-Parkinson-White syndrome during recurrent episodes of supraventricular tachycardia (SVT) with rates between 240 to 275 per minute in a 5 year old boy. The rapid ventricular response during SVT corroborates the recent finding of a short refractory period of the A-V node in children. The d e m o n s t r a t i o n o f the R-on-T phenomenon in this child may be important in attempting to understand the pathogenesis of sudden death in children.

CASE REPORT A 5 year old boy with episodic chest discomfort, weakness, and pallor related to paroxysmal SVT of two years duration was admitted for cardiac evaluation. Physical findings were unremarkable, and there was no evidence of an organic cardiac defect. The electrocardiogram (ECG) revealed marked sinus arrhythmia with rates varying from 70 to 105/rain; the QRS duration was 0.06 sec; the P-R interval was 0.12 sec and the corrected QT interval 0.39 sec (Fig. 1). Bouts of SVT occurred spontaneously at rates between 220 to 275/min and their initiation was usually preceded by an atrioventricular (A-V) junctional escape beat. The R-on-T phenomenon was frequently observed in association with aberrant ventricular conduction during SVT (Fig. 2). Echocardiography and cardiac catheterization including cineangiography revealed no abnormalities. During cardiac catheterization, electrophysiological studies were performed. At the time of the study, the child was not receiving any medications. A tripolar electrode catheter was introduced from the right femoral vein and positioned across the septal leaflet of the tricuspid valve for recording the His bundle electrogram (HBE). ~ Another bipolar electrode catheter was inserted through the left femoral vein and placed in the high right atrium for recording a high right atrial electrogram (HRA). Simultaneously HBE, HRA, and surface ECG lead I, II, and III were displayed on a multichannel oscilloscopic photographic recorder (Electronics for Medicine, DR-8, White Plains, New York), and recorded at a paper speed of 100 mngsec. The definitions and techniques of measurement of P-R, P-A,

In 1940, Wiggers and Wegria I established the presence of a vulnerable period of late ventricular systole in the dog heart, during which an electrical shock of suitable intensity could precipitate ventricular fibrillation. This vulnerable period corresponds in time to the midportion of the electrocardiographic T wave. 2-4 Clinically, Smirk and Palmer 2 later noted that interruption of the R wave by the succeeding QRS complex, i.e., the "R-on-T phenomenon," was associated with a high incidence of sudden death in patients with significant myocardial disease. The occurrence of this phenomenon in the pediatric population has been seldom mentioned; we are, therefore, reporting herein

From the Divisions of Pediatric and Adult Cardiology, Departments of Pediatrics and Medicine, University of Miami School of Medicine, Miami, Florida. Reprint requests to: Dr. Henry Gelband, Department of Pediatrics, University of Miami School of Medicine, P.O. Box 520875, Miami, FL 33152. 171

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Fig. 1: The 12 lead ECG reveals sinus arrhythmia with rates varying from 70 to 105/min, PR and QTc intervals of 0.12 and 0.39 sec respectively, QRS axis of + 15~ and QRS duration of 0.06 sec. Fig. 2: The rhythm strip with simultaneous recording of leads II, V2 and V6 reveals a short bout of supraventricular tachycardia (SVT) at a rate of 220/rain initiated by an A-V junctional escape (the third beat). During SVT, aberrant ventricular conduction and the R-on-T phenomenon (most pronounced in the beat marked with an arrow) are observed.

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A-H and HLV intervals have been previously described.6 During the past 14 months this patient has b e e n t r e a t e d with p r o p r a n o l o l (10 rag/day) without any f u r t h e r clinical s y m p t o m a t o l o g y or electrocardiographic evidence of a tachyarrhythmia. RESULTS During sinus a r r h y t h m i a with a cycle length b e t w e e n 650 and 820 msec, the P-R interval was 120 msec. The P-A, A-H, and H-V intervals w e r e respectively 25, 60 and 35 msec (Fig. 3). These values are normal for his age. 7,s Several bouts of SVT w e r e observed during the study. Each p a r o x y s m was initiated with an A-V junctional escape beat (Fig. 4). The r a t e of the t a c h y c a r d i a r a n g e d b e t w e e n 240 and 275/min (cycle length of 250 to 220 msec), and during tachycardia, the arrival of excita-

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Fig. 4: Simultaneous recording of surface leads I, II, III, high right atrial electrogram (HRA) and His bundle electrogram (HBE). A sinus impulse (the second P wave) coincides with an A-V junctional escape thereby initiating a short run of supraventricular tachycardia. Details, see text. Aberrant ventricular conduction and the R-on-T phenomenon (the last QRS complex falling on the T wave upstroke of the preceding beat as indicated by an arrow) are observed. A = atrial deflection; H = His bundle deflection. Paper speed 100 mm/sec. All measurements are expressed in milliseconds. tion at the His-Purkinje system and ventricles was early enough to produce both aberrant ventricular conduction and inscription of the last R wave on top of the preceding T wave (R-on-T phenomenon) (Fig. 4).

DISCUSSION The mechanism of SVT in this case appeared to be an A-V junctional reciprocating tachycardia. Moe and Mendez 9 have postulated that the upper portion of the A-V node can be functionally dissociated into two separate pathways, alpha ( a ) and beta ( ~ ), with different functional properties. These two pathways in turn converge upon a final common pathway to merge with the bundle of His. As depicted in the ladder diagram of Fig. 4, retrograde propagation of an A-V junctional escape beat could result in complete refractoriness of the ~ pathway and partial refractoriness of the a pathway. The preceding sinus impulse (second P wave) is blocked in the /3 p a t h w a y and conducts through the ~ pathway with considerable delay. If this delay in transmission of the impulse is sufficient so that the ~ pathway is now excitable, reentry may occur initiating a reciprocating tachycardia. It has been assumed that the physiological conduction delay in the A-V node prevents supraventricular impulses from reaching the yenJ. ELECTROCARDIOLOGY, VOL. 9, NO. 2, 1976

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tricle before its recovery from normal excitation. 3,1~ The R-on-T phenomenon, therefore, is not expected to occur during sinus tachycardia or supraventricular tachyarrhythmias. However, an appropriately timed premature atrial impulse in the presence of apparently normal A-V nodal function, may occasionally be transmitted to the ventricle during its vulnerable period, triggering ventricular tachycardia and/or ventricular fibrillation. Descriptions of such examples have been reported in dogs during acute experimental coronary artery occlusion 11 and in a patient following open heart surgery. 12 Delayed recovery of ventricular excitability such as in patients with congenital prolongation of the Q-T interval has also been postulated to be predisposed to this development. 13,14 Additionally, the presence of an accessory pathway completely or partially bypassing the A-V node may compromise the role of the A-V node as a protective barrier. The R-on-T phenomenon may thus occur when a supraventricular impulse is propagated by the accessory pathway early enough to reach the ventricles at a time when ventricular recovery is incomplete. The potentially hazardous consequences due to this phenomenon have been postulated in patients with Wolff-ParkinsonWhite (W-P-W) 15,16 and Lown-Ganong-Levine (L-G-L) 17 syndromes. Our 5 year old boy had no electrocardiographic evidence of any preexcitation syndrome. Echocardiography as well as cardiac catheterization with cineangiography demonstrated no cardiac abnormality. Although the possibility of an accessory pathway partially bypassing the A-V node, as in patients with L-G-L syndrome, could not be completely ruled out, 17 his P-R and A-H intervals are normal for his age. 7,8,~8 With the advent of the catheter technique for recording intracardiac electrical events, it has become feasible to evaluate the functional properties and conduction time of the specialized conduction system in humans.5-7,19 Children are known to be able to sustain SVT at much higher rates than adults. ~9 Significantly faster intracardiac conduction timeS, is and shorter refractory period of the A-V node 2~ have been demonstrated in children. Rapid atrial pacing at a rate as fast as 270/rain with 1:1 conduction to the ventricles has also been reported in pediatric patients. 22 Thus, the rapid spontaneous SVT of 240-275/rain in our patient corresponds with these findings. In young mammals, Preston et aP ~ have demonstrated that premature atrial beats can propagate to the ventricle during its vulnerable period, triggering ventricular fibrillation. They have attributed this to the fact that the immature A-V nodal tissue has a shorter re-

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fractory period than that of the ventricle and has, accordingly, a less efficient filtering mechanism against progressively more p r e m a t u r e atrial beats. Therefore, it is conceivable t h a t in the absence of W-P-W syndrome, the R-on-T phenomenon could occur during rapid SVT or p r e m a t u r e atrial beats in children. This possibility s u p p o r t s t h e c o n c e p t (not clinically p r o v e n 22-24) that a r r h y t h m i a s may be a cause of sudden death in infants.

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11. CARROLL,SE, AHUJA,SP ANDMANNING,GW: The initiation of ventricular tachycardia and fibrillation in experimental coronary artery occlusion. Am J Cardiol 16:813, 1965 12. SAKAMOTO, T, YAMADA, T AND HIEJIMA, K: Ventricular fibrillation induced by conducted sinus or supraventricular beat. Circulation 48: 438, 1973 13. JAMES, T N: Congenital deafness and cardiac arrhythmias. Am J Cardio] 19:627. 1967 14. GARZA, LA, VICK, RL, NORA, JJ AND MCNAMARA, D G: Heritable Q-T prolongationwithout deafness. Circulation 4!:39, 1970 15. MOORE, E N, SPEAR, J F AND BOINEAU, J P: Electrophysiologica] studies on the preexcitation syndrome. (abst) Circulation 42, Suppl III-185, 1970 16. DREIFUS, L S, HAIAT, R, WATANABE, Y, ARRIAGA, J ANDREITMAN, N: Ventricular fibrillation. A possible mechanism of sudden death in patients with Wolff-Parkinson-White synd!ome. Circulation 43:520, 1971 17. CASTELLANOS, A, JR, VAGUEIRO, M C, BEFELER, B, AND MYERBURG, R J: Syndrome of short P-R, narrow QRS and repetitive supraventricular tachyarrhythmias: The possible occurrence of the R-on-T phenomenon and the limits of this syndrome. Europ J Cardiol (in press) 18. ROBERTS, N AND OLLEY, P: His bundle electrogram in children: Statistical correlation of the atrioventricular conduction times in children with their age and heart rate. Br Heart J 34: 1099, 1972 19. WIT, A L , WEISS, MB, BERKOWITZ, WD, ROSEN, KM, STEINER, C AND DAMATO, AN: Patterns of A-V conduction in the human heart. Circ Res 27:345, 1970 20. LINDE, LM, TURNER, SW AND AWA, S: Present status and treatment of paroxysmal supraventricular tachycardia. Pediatrics 50:127, 1972 21. GILLETTE, PC AND MULLINS, CE: Atrioventricular conduction in children: Response to rapid atrial stimulation and premature stimulation. Cardiovasc Dis, Bull Texas Heart Ins 2:41, 1975 22. KRONGRAD, E, STEEG, CN, WALDO, A L AND GERSONY, W: The response of the atrioventricular specialized conduction system (AVSCS) to rapid atrial stimulation (RAS) in children. (abst) Pediat Res 299:71, 1973 23. DAWES, G S: Sudden death in babies: Physiology of the fetus and newborn. Am J Cardiol 22: 469, 1968 24. JAMES, T N: Sudden death in babies: New observations in the heart. Am J Cardiol 22:479, 1968

J. ELECTROCARDIOLOGY, VOL. 9, NO. 2, 1976