The reduction in LF and HF during exercise at a heart rate of 100 beats/min, which is not characterised by increased plasma catecholamine concentrations, and during atropine infusion suggests that heart rate variability in the supine state is largely influenced by vagal activity. The additional reduction in LF during exercise at 150 beats/min and during catecholamine infusion may reflect a negative feedback of circulating catecholamines on the sympathetic control of heart rate.

QT dispersion (QTd) describes the heterogeneity of ventricular repolarization on the basis of the temporal range of QT intervals as measured in the 12-lead ECG. We examined the spatial distribution of QTd using multichannel magnetocardiograms (MCGs), which noninvasively register changes in magnetic field strength at 37 sites over the heart. As in ECG, the MCG signal in each channel may be used to measure QT interval. By calculating QT deviation from QTmin at each site, one can reconstruct the spatial distribution of QTd. Analysis of spatial QTd in ten healthy subjects and ten patients after acute myocardial infarction (MI) showed clear differences in spatial distribution. The healthy subjects generally displayed shorter QT intervals along a line corresponding to the approximate position of the septum with longer intervals in plateaus in the upper right and lower left. Spatial QTd of the post-MI patients deviated from this pattern, often displaying a sharp rise in QT duration over specific areas, which could be related to functional and morphological disturbances. The quantification of local irregularities as well as the overall pattern on the basis of a smoothness index allowed better discrimination between healthy subjects and post-MI patients than QTd. Distribution patterns of QTd which reflect local repolarization alterations may thus represent a more differentiated marker for pathology and risk.

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To determine the value of alternation of QRS morphology in determining the site of origin of sustained narrow QRS supraventricular tachycardia (SVT), we retrospectively studied 163 distinct tachycardias in 161 patients (ages 4 to 91 years) in whom the site of origin of SVT was proven by intracardiac electrophysiologic study. Sustained SVT was defined as lasting longer than 30 sec. Narrow QRS was defined as QRS width less than 0.12 sec. Atrial fibrillation and flutter were excluded. The presence or absence of QRS altemation was judged at least 10 sec after initiation of SVT. Circus movement tachycardia with anterograde AV node conduction and a retrograde accessory AV pathway was seen in 89 patients (58 with Wolff-Parkinson-White syndrome, 31 with concealed accessory pathway); intra-AV nodal reentrant tachycardia (AVNT) was present in 57 cases, and 17 tachycardias were atrial in origin. QRS alternation was present in 36 of 163 cases (22%). In only eight of these 36 did RR interval length alternation accompany alternation in QRS morphology. Thirty-three of 36 (92%) tachycardias with QRS alternation were circus movement tachycardias. Two were atrial in origin and one was AVNT. We conclude that the presence of QRS alternation during sustained narrow QRS SVT is highly indicative of a retrograde accessory AV pathway in the tachycardia circuit. Circulation 68, No. 2, 368-373, 1983. ACCURACY in determining the site of origin of supraventricular tachycardia (SVT) with the 12-lead electrocardiogram (ECG) is important for correct treatment of the arrhythmia. Having seen electrical alternans of the QRS complex in patients with SVT, we wondered whether this finding could be of help in determining the site of origin of the tachycardia. Therefore we undertook a study to evaluate the diagnostic value of QRS alternation in patients with sustained SVT and a narrow QRS complex. Patients and methodsAll electrocardiographic tracings of all patients studied for SVT in Maastricht, The Netherlands, between February 1977 and December 1982 were retrospectively analyzed. In all patients the mechanism and site of origin of sustained narrow QRS tachycardia had been ascertained by an intracardiac electrophysiologic study, which included programmed electrical stimulation of the heart. Sustained tachycardia was defined as last- 368ing longer than 30 sec, and narrow QRS was defined as a QRS width of less than 0.12 sec. Atrial fibrillation and atrial flutter were excluded. Our methods of stimulation, recording, and analysis of tracings have been previously described. ' The site of origin (intra-AV nodal reentrant tachycardia, atrial tachycardia, or orthodromic circus movement tachycardia with anterograde AV node conduction and a retrograde accessory atrioventricular pathway) was determined according to previously defined criteria.2 A total of 161 patients met the entry criteria and were included in the study population. There were 71 women and 90 men (mean age 39 years). Two patients had more than one type of tachycardia and as a res...