Effect of Ventricular Premature Contraction on
QT Interval Duration in Patients with Previous
Myocardial Infarction and Ventricular Arrhythmias

K Szydlo, M Trusz-Gluza, AWnuk-Wojnar, D Urbanczyk, K Wita

I^st Dept. of Cardiology, Silesian Medical Academy, Katowice, Poland

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1.  Introduction

The QT interval duration, especially prolonged, is regarded as a one of the factors of higher risk of the malignant ventricular arrhythmias such as ventricular tachycardia (VT) or ventricular fibrillation (VF). The QT/RR relationship is still not clear. It is modifying by many factors such as heart rate (HR), sympatho-vagal activity or intrinsic state of myocardium in patients with the history of myocardial infarction (MI) (e.g. regional ischemia or ion channel activity in the region surrounding the scar). Changes in the heart rate are regarded as a most potent factor, which provides to the phenomenon of the QT adaptation- hysteresis lasting up to few minutes. But it was also shown that during stable sinus rhythm the prolongation of the QT interval could be observed even after one premature beat.

The purposes of study were to analyze if any changes of QT interval could be observed in postextrasystolic beats in patients (pts) with benign (No VT/VF) and malignant (VT/VF) arrhythmias, and if there are related to the prematurity of premature ventricular contraction (PVC).

2.  Methods

The study population consisted of 80 pts with the history of MI (>30 days). There were: 40 pts with benign arrhythmia (No VT/VF) (25males, 58±7 yrs, EF- 45±7%, all treated with beta-blockers) and 40 pts with malignant ventricular arrhythmias (VT/VF) (all qualified to the implantation of ICD; 36 males, 57±11 yrs, EF- 42±11%, treated with amiodarone- 31 pts or beta-blockers- 9 pts). QT was calculated manually from the Holter recording's strips (one for each patient, always the same channel, between 9-12 a.m. during stable sinus rhythm 50-70 bpm, one sinus beat before and 4 after PVC). Duration of the coupling interval (CI), postextrasystolic pause (PP) were also measured with calculation of the premature index (PI). Both groups were divided into two subgroups: a cut-off point was established using the median value of PI in each group: for No VT/VF: Me= 0.64 and for VT/VF: Me= 0.71. The difference between QT of preceding (QT-1) and first sinus beat following (QT+1) PVC was also calculated- delta QT (dQT).

3.  Results

Table 1.    Basic ECG data of whole population.

	HR (bpm)	CI (msec)	PP (msec)	PI	dQT (msec)
No VT/VF	63±10	582±93	1210±200	0.64±0.11	8±19
VT/VF	56±9	635±157	1171±0.15	0.71±0.15	26±41*

           *- p<0.05 for No VT/VF vs VT/VF

There were no significant differences in gender, ejection fraction or treatment between analysed groups and subgroups. Basic ECG parameters did not differ either. Patients with lower PI (<Me) were characterised by higher QT duration after PVC, significantly pts with VT/VF. This subgroup had also the highest increase of postextrasystolic QT duration both in the value of dQT and of QT intervals in the next sinus beats.

It may indicate greater disturbances of the repolarization process, which could be related to the intrinsic state of the myocardium in these patients. Different electrophysiological properties in the peri-scar region or greater disturbances in microvascular flow may be responsible for the occurrence of malignant ventricular arrhythmias.

Table 2.    QT intervals in the study subgroups according to the median of PI (inter-group analysis).

*- p<0.01 for VT/VF<Me vs others dQT.

**- p<0.01 for No VT/VF<Me vs VT/VF<Me and for No VT/VF>Me vs others

Table 3.    Comparison of QT duration of the sinus beat before PVC (QT-1) and preceding beats (QT+1 to QT+4) in study subgroups. (p values are given).

	QT+1	QT+2	QT+3	QT+4
No VT/VF<Me	ns	ns	ns	ns
No VT/VF>Me	ns	ns	ns	ns
VT/VF<Me	<0.005	<0.005	<0.005	<0.005
VT/VF>Me	ns	ns	ns	ns

4.  Conclusion

Single PVC provokes significant prolongation of QT interval especially in patients with malignant ventricular arrhythmias with low prematurity index. It may indicate greater disturbances of the repolarisation process in these patients. Further investigation in this field is absolutely needed.