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Narrow QRS Complex Tachycardia with Alternating Shorterand Longer R-R Cycles: What is the Mechanism?

ANDRAS VERECKEI, M.D.

From the Third Department of Internal Medicine, Semmelweis University, School of Medicine, Budapest, Hungary

Case Presentation

A 19-year-old woman without organic heart disease pre-sented with sudden-onset tachycardia. Figure 1 shows amagni� ed view of ECG recordings from leads V1 to V3 inwhich the P waves were best seen, recorded during thetachycardia. There is a narrow QRS complex tachycardiawith alternating R-R cycles. What is the mechanism of thealternating R-R cycles and of the tachycardia?

Commentary

The ECG (Fig. 1) shows a narrow QRS complex tachy-cardia at a rate of 204 beats/min with alternating longer (310msec) and shorter (280 msec) R-R cycles and a normal QRSaxis. The P-R interval of the longer R-R cycles is 150 msecand that of the shorter R-R cycles is 120 msec. The R-Pintervals of all R-R cycles are � xed at 160 msec. Conse-quently the P-P intervals also are alternating at 310 and 280msec, the same cycle lengths as those of the R-R intervals.The alternating longer and shorter P-R intervals are consis-tent with alternating conduction through slow and fast AVnodal pathways.

The differential diagnosis includes AV nodal reentranttachycardia (AVNRT), orthodromic AV reentrant tachycar-dia (AVRT), and atrial tachycardia with alternating slowand fast AV nodal pathway conduction. In the case of atrialtachycardia, a � xed P-P interval with alternating longer andshorter P-R, R-P, and R-R intervals would be expected.However, in the present case, the R-P interval is � xed andthe P-R, R-R, and P-P intervals alternate, indicating thatatrial tachycardia (or sinus tachycardia) cannot be the mech-anism.

There usually is a reciprocal relationship between theP-R and R-P intervals in AVNRT, due to the physiology ofAV nodal conduction. Therefore, the � xed R-P intervals inthe face of changing P-R intervals are not consistent withAVNRT.

By exclusion, the most likely possibility is orthodromicAVRT with alternating conduction down fast and slow AVnodal pathways. This is consistent with the � xed R-P inter-val, which occurs in the presence of alternating longer and

shorter P-R, R-R, and P-P intervals. A � xed VA intervalregardless of changing tachycardia cycle lengths is typicalof a tachycardia that uses a concealed bypass tract, becauseaccessory pathways usually do not display decremental con-duction.1

Orthodromic AVRT often has an R-P/P-R ratio ,1.However, when the tachycardia cycle length is ,300 msec,as in the case here, the R-P/P-R ratio can be .1. Thus, theconstant R-P interval is more important than the R-P/P-Rratio in the diagnosis of orthodromic AVRT.2

QRS alternans is observed, which has been consideredby some to be strongly suggestive of AVRT.3 ,4 However, itssigni� cance is unclear, because it may occur during su-praventricular tachycardias of other mechanisms, such asAVNRT. It appears that QRS alternans is not diagnostic ofAVRT but instead is a rate-related phenomenon.5 ,6

Acknowledgment: The author thanks Jozsef Tenczer, M.D., for reviewingthe manuscript.

References

1. Josephson ME, Wellens HJJ: Electrophysiologic evaluation of su-praventricular tachycardia. Cardiol Clin 1997;15:567-586.

2. Josephson ME: Clinical Cardiac Electrophysiology: Techniques andInterpretation. Second Edition. Lea & Febiger, Philadelphia, 1993, p.235.

J Cardiovasc Electrophysiol, Vol. 13, pp. 835-836, August 2002.

Address for correspondence: Andras Vereckei, M.D., Third Department ofMedicine, Semmelweis University School of Medicine, Budapest,Kutvolgyi .ut 4, Hungary, 1125. Fax: 36-1-225-0196; E-mail:vereckei@kut.sote.hu

Figure 1. Magni�ed view of recordings from 12-lead ECG leads V1 to V3.

835Reprinted with permission fromJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Volume 13, No. 8, August 2002

Copyright ©2002 by Futura Publishing Company, Inc., Armonk, NY 10504-0418

3. Green M, Heddle B, Dassen W, Wehr M, Abdollah H, Brugada P,Wellens HJJ: Value of QRS alternation in determining the site oforigin of narrow QRS supraventricular tachycardia. Circulation 1983;68:368-373.

4. Chen SA, Tai CT, Chiang CE, Chang MS: Role of the surfaceelectrocardiogram in the diagnosis of patients with supraventriculartachycardia. Cardiol Clin 1997;15:539-565.

5. Miles WM, Zipes DP: Atrioventricular reentry and variants: Mecha-

nisms, clinical features and management. In Zipes DP, Jalife J, eds:Cardiac Electrophysiology: From Cell to Bedside. Third Edition. WBSaunders, Philadelphia, 2000, pp. 488-504.

6. Miller JM, Hsia HH, Rothman SA, Buxton AE: Ventricular tachycar-dia versus supraventricular tachycardia with aberration: Electrocardio-graphic distinctions. In Zipes DP, Jalife J, eds: Cardiac Electrophys-iology: From Cell to Bedside. Third Edition. WB Saunders,Philadelphia, 2000, pp. 696-705.

836 Journal of Cardiovascular Electrophysiology Vol. 13, No. 8, August 2002