Journal Article

Characterization of the non-linear rate-dependency of QT interval in humans

G. Malfatto, M. Facchini and A. Zaza

in EP Europace

Published on behalf of European Heart Rhythm Association of the European Society of Cardiology (ESC)

Volume 5, issue 2, pages 163-170
Published in print April 2003 | ISSN: 1099-5129
Published online April 2003 | e-ISSN: 1532-2092 | DOI: http://dx.doi.org/10.1053/eupc.2002.0297
Characterization of the non-linear rate-dependency of QT interval in humans

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Aims

Repolarization has rate-dependent and rate-independent components. A function considering such components separately was validated in canine Purkinje fibres and applied to the QT/RR relation in humans.

Methods and Results

Action potential duration (APD) was measured in Purkinje fibres during steady-state pacing at different cycle lengths (CL) and after prolonged quiescence (APD0). The APD/CL relationship was expressed by this function: APD=APDmax*CLS/(CL50S+CLS), where APDmax (APD extrapolated at infinite CL) is a rate-independent measure of repolarization, CL50 (CL at which 50% of APDmax is achieved) and S evaluates the rate dependency of APD. The same function was used to fit the QT/RR relation in 46 normal subjects (20 males, 26 females) and in 7 amiodarone-treated subjects undergoing a bicycle stress test. RR and QT (V5) were measured at the end of each load step; QTc (Bazett's formula) was obtained at rest. The APD/CL and QT/RR relations were equally well expressed by the function with high correlation coefficients (R≥0·90). In Purkinje fibres, APDmax was 461±37 ms, CL50 was 394±54 ms and S was 0·98±0·11. APDmax and APD0 correlated (R=0·96) and were similar. The corresponding values in humans were: QTmax 432±63 ms, RR50 345±60 ms and S 2·6±0·8. While QTc and QTmax were longer in females, RR50 and S were similar between genders. Amiodarone increased QTc, QTmax and RR50 and decreased S. In QTmax and QTc distributions generated by pooling data from treated and untreated subjects, 86% of treated subjects were correctly identified by QTmax and 28% by QTc.

Conclusions

Canine and human repolarization showed a saturating dependency on cycle length, described by the proposed function. Gender and amiodarone independently affected QTmax, RR50 and S: therefore they might reflect specific ionic mechanisms. Finally, QTmax identified drug-induced repolarization abnormalities in individual subjects better than QTc.

Keywords: Heart rate; QT interval; ventricular repolarization

Journal Article.  0 words. 

Subjects: Cardiovascular Medicine

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