ORIGINAL CONTRIBUTION Basic Res Cardiol 95: 424 – 430 (2000) © Steinkopff Verlag 2000 Petri Haapalahti Markku Mäkijärvi Petri Korhonen Panu Takala Juha Montonen Yrjö Salorinne Lasse Oikarinen Matti Viitasalo Lauri Toivonen Magnetocardiographic QT dispersion during cardiovascular autonomic function tests Abstract QT dispersion is considered to reflect nonhomogeneity of ventricular repolarization. The autonomic nervous system modulates QT interval duration, but the effect may not be spatially homogenous. Magnetocardiography (MCG) regis- ters the weak magnetic fields generated by myocardial electric currents with high localizing accuracy. We studied the effects of rapid cardiovascular autonomic nervous adjustment on QT dispersion in MCG. Ten healthy male volunteers were monitored during deep breathing, the Valsalva maneuver, sustained handgrip, hyper- ventilation, the cold pressor test and mental stress. 67 MCG channels and 12 ECG leads were recorded simultaneously. A computer algorithm was used for QT inter- val measurements. QT dispersion was defined as maximum – minimum or standard deviation of the QT peak and QT end intervals. In MCG the QT end dispersion increased during deep inspiration compared with deep expiration (96 ± 19 ms v 73 ± 27 ms, p = 0.05). Magnetic QT dispersion tended to increase during the bradycardia phase of the Valsalva maneuver, but the change was obvious only for QT end (55 ± 26 ms v 76 ± 29 ms, p < 0.05). Other tests had no significant effect on QT dispersion, not even the cold pressor test, although it causes strong sympathetic activation. Mag- netic and electric QT peak and QT end intervals correlated closely (r = 0.93 and 0.91), whereas the QT dispersion measures showed no correlation. In conclusion, magnetic QT dispersion is not modified by rapid changes in autonomic tone, but maneuvers involving deep respiratory efforts and changes in ventricular loading affect QT dispersion measurements. Key words Automatic measurement – autonomic nervous system – magnetocardiography – QT dispersion – QT interval P. Takala · J. Montonen Laboratory of Biomedical Engineering and the BioMag Laboratory Helsinki University of Technology Y. Salorinne Department of Clinical Physiology and Nuclear Medicine Helsinki University Central Hospital P. Haapalahti, MD () · M. Mäkijärvi P. Korhonen · L. Oikarinen · M. Viitasalo L. Toivonen Division of Cardiology Department of Medicine Helsinki University Central Hospital Haartmaninkatu 4 00290 Helsinki, Finland E-mail: petri.haapalahti@pp.fimnet.fi Received: 4 April 2000 Returned for revision: 2 May 2000 Revision received: 20. June 2000 Accepted: 10 July 2000 BRC 235 Introduction Increased dispersion of the QT interval duration in the 12-lead ECG is associated with arrhythmia susceptibility (2, 5, 7, 13). Heart rate and autonomic tone are important determinants of QT interval duration. Changes in heart rate produce rather pre- dictable changes in the QT interval, but when autonomic reflexes are involved, the relation becomes more complex (1, 6, 16). Sudden strong autonomic nervous influences affect myocardial vulnerability to arrhythmia, and this might be related to autonomic effects on ventricular repolarization. Several reports have been published on autonomic influence on QT interval duration (1, 4, 6), but the dynamic behavior of QT dispersion during autonomic adjustment has received little attention (9). Magnetocardiography (MCG) measures the weak magnetic fields generated by myocardial electric currents. MCG may, due to its greater sensitivity to currents tangential to the myocardial wall, recognize phenomena not detected on the ECG (15). MCG has been used to assess QT dispersion. In