IJBEM, Vol. 5, No. 1, 2003

Vectorcardiographic Changes in Cardiac Memory
Induced by Ventricular Pacing in Man

Liliane Wecke^a, Fredrik Gadler^a, Mats Jensen-Urstad^a, Cecilia Linde^a, Gunilla Lundahl^b,
Michael R Rosen^c, Lennart Bergfeldt^a

^aDept. of Cardiology, Karolinska Institutet at Karolinska Hospital,Stockholm, Sweden
^bOrtivus AB, Stockholm, Sweden
^cColumbia University, NY, USA

Correspondence: L Wecke,Dept. of Cardiology, Karolinska Institutet at Karolinska Hospital, P.O. Box 110, S-171 76 Stockholm, Sweden.
E-mail: liliane.wecke@ks.se, phone +46 8 51772241

Abstract. Cardiac Memory (CM) can be induced by periods of altered ventricular depolarization, such as intermittent left bundle branch block, ventricular tachycardia, periods of ventricular preexcitation in WPW-patients and ventricular pacing. In this study the time for development of CM in patients paced endocardially in the right ventricular apex at physiologic rates was studied both with electro- (ECG) and vectorcardiography (VCG).

We studied 14 patients with normal intraventricular conduction receiving a DDDR-pacemaker because of Sick Sinus Syndrome. The pacemakers were programmed to a short AV-delay, so that maximum ventricular pacing was achieved, which was at least 90% confirmed by the pacemaker log. ECG and VCG were recorded prior to pacemaker implantation and one day post-implantation (9/14 pts), and then once a week up to 5-8 weeks with the pacemakers temporarily programmed in AAI. The pacemakers were then reprogrammed to a longer AV-delay to minimize ventricular pacing, and the patients were seen after 4-5 weeks.

A majority of the ECGs recorded one day post-implantation showed appearance of CM defined as new T-wave vectors tracking the paced QRS vector, as did all ECGs recorded during up to 8 weeks follow-up.

The T-wave vector angles, measured as azimuth and/or elevation, were markedly changed after one week of pacing, and remained stable for the rest of follow-up. All patients except one, showed a significant change in T-wave azimuth, mean difference 150 degrees, and 11/14 patients showed a significant change in T-wave elevation, mean difference 63 degrees. After 4-5 weeks pacing with a long AV-delay, CM was conserved in proportion to the amount (%) of ventricular pacing; the relation seemed to be linear.

Cardiac Memory develops within one week after the onset of ventricular pacing at physiologic rates, and is identified on the ECG as T-wave negativity. The VCG, which is a quantitative method, shows a large change in the T-vector angle. If pacing is discontinued cardiac memory will disappear within 4 weeks.