Repolarization abnormality in idiopathic ventricular fibrillation with respect to QT-RR relation

M. Sugao, A. Fujiki, K. Mizumaki, T. Tsuneda, K. Nishida, M. Sakabe, H. Inoue
The Second Department of Internal Medicine,
Toyama Medical and Pharmaceutical University, Toyama, Japan
2630 Sugitani, Toyama 930-0194, Japan

Abstract: The purpose of this study was to clarify the characteristics of QT-RR relation in idiopathic ventricular fibrillation (IVF).  Methods: The study group consisted of 7 males with IVF (4 Brugada type and 3 non-Brugada type) who had had recent episodes of VF and 7 healthy age-matched males.  The relation between QT and RR interval was analyzed from 24-hour Holter ECG tape using both automatic measurement system (SCM6000 Fukuda-denshi) and manual measurement with tangential method.  From QT-RR linear regression line, QT extrapolated to RR of 0.6 and 1.5 sec was determined.  Results: The QT-RR slope did not differ between IVF and control group.  However, both QT (0.6) and QT (1.5) were significantly shorter in IVF than in control.  Conclusion: Abnormality of I_to may affect not only ST-segment elevation but also QT-RR relation in IVF. 

INTRODUCTION

Idiopathic ventricular fibrillation (IVF) has been recognized as the cause of sudden unexplained nocturnal death.  In patients with IVF several mechanisms responsible for the electrocardiographic pattern (ST-segment and J wave elevation) have been reported.  Recent studies ^{1, 2)} indicate that the characteristic ECG pattern is related to the presence of a prominent transient outward current (I_to) in the epicardial layer and genetic abnormalities of sodium channel gene (SCN5A).  We hypothesized that these abnormalities of ionic currents may affect not only ECG pattern but also repolarization dynamics.  Hence, we evaluated 24-hour QT-RR relation in patients with IVF.  

METHODS

The study group consisted of 7 males with IVF (4 Brugada type and 3 non-Brugada type) who had had recent episodes of VF and 7 healthy age-matched males.  The relation between QT and RR interval was analyzed from 24-hour Holter ECG tape using both automatic measurement system (SCM6000 Fukuda-denshi) and manual measurement with tangential methods.  From QT-RR linear regression line, QT extrapolated to RR of 0.6 and 1.5 sec was determined.  Statistics: Results are presented as mean ± SD.  The correlation between QT interval and RR was expressed by linear regression line (QT = A [RR] + B; where A is slope and B is intercept) and compared using covariance analysis.  The unpaired data were compared by Student’s t-test.  Statistical significance was set at p<0.05.

TABLE I
QT-RR relation in patients with IVF

	IVF	Control
Age (yo)	42.3 ±9.0	43.3± 3.5
Slope	0.109 ±0.026	0.146 ±0.047
Intercept	0.270± 0.031	0.265 ±0.039
R2	0.566 ±0.181	0.671 ±0.19
QT (0.6)	0.335±0.016*	0.353±0.014
QT (1.5)	0.432± 0.017**	0.485 ±0.034

*p<0.05, **p<0.005, vs. Control, QT(0.6): QT extrapolated to RR of 0.6 sec, QT(1.5): QT extrapolated to RR of 1.5 sec, IVF: idiopathic ventricular fibrillation

RESULTS

The QT-RR slope tended to be smaller in IVF than in the control group but the difference was not statistically significant (p=0.084).  However, both QT (0.6) and QT (1.5) were significantly shorter in IVF than in the control group and the difference was significantly greater for QT(1.5).  

DISCUSSION

From 24-hour QT-RR linear regression line the QT interval in IVF was shorter than control group.  The degree of shortening was greater at longer RR intervals.  Both longer cycle lengths and increased vagal activity increase I_to and reduce I_Ca.  These changes in ionic current may facilitate ST-segment elevation and shorten the QT interval at night.  This repolarization characteristic in IVF may be related to the nocturnal occurrence of VF episodes. 

REFERENCES

[1]           I. Gussak, C. Antzelevitch, P. Bjerregaard, et al. The Brugada syndrome: clinical, electrophysiologic and genetic aspects, J Am Coll Cardiol, 33, 5-15, 1999.

[2]           Q. Chen, G.E. Kirsch, D, Zhang, et al. Genetic basis and molecular mechanism for idiopathic ventricular fibrillation, Nature, 392, 293-296,1998.