B. Milan Horáček

QT interval variability on the body surface

To assess the effects of measurement methodology on QT determinations and to define the spectrum of QT values, including interlead variability, on the body surface, we measured QT in each of 120 simultaneously-recorded, signal-averaged ECG leads in 10 normal subjects and 14 patients with QT prolongation (lead II QTc greater than 440). Two separate, but related, methods of QT measurement were utilized. Method A was a relatively conventional technique in which ST-T offset was defined as the time instant of return of the T wave to a P-P baseline, or as the point of U-on-T intersection. Method B was a more rigorous method, which defined ST-T offset in a similar manner, and in addition discarded from analysis all QT values from leads with monophasic ST-T waveform in which the QT values were greater than the longest QT from leads with definite U waves. Method B was utilized to minimize factitious prolongation of QT by inapparent U-on-T. By both methods the mean body surface QTc values were significantly greater (p less than 0.001) in the patient group (482 +/- 65 [S.D.] msec, method A; 447 +/- 43 msec, method B), than in the normal subject group (399 +/- 14 msec, method A; 396 +/- 12, method B). Interlead QTc variability (difference between the longest and shortest QT) was considerable with both methods and in both study groups. Expressed as percent of average body surface values, the mean interlead QTc variability in normal subjects averaged 22 percent with method A and 19 percent with method B; in the patient group, however, it averaged 32 percent with method A and only 18 percent with method B. In absolute terms, the mean variability in the patient group with method A (155 +/- 62 msec) was significantly greater (p less than 0.001) than that of the normal group (89 +/- 33 msec); with method B, interlead variability was the same (p = NS) in the normal (76 +/0 27 msec) and patient groups (80 +/- 44 msec). This latter finding suggests the possibility that the repolarization abnormality in patients with QT prolongation may occur relatively uniformly throughout the ventricular myocardium. Thus, measurement techniques are important in multiple-lead QT determinations. Although reduced by techniques designed to minimize factitious QT prolongation, interlead QT variation is considerable over the torso surface, in both normal subjects and patients with repolarization abnormalities.(ABSTRACT TRUNCATED AT 400 WORDS)

Identification of best electrocardiographic leads for diagnosing myocardial infarction by statistical analysis of body surface potential maps

This study describes a practical approach for the extraction of diagnostic information from body surface potential maps. Body surface potential map data from 361 subjects were used to identify optimal subsets of leads and features to distinguish 184 normal subjects from 177 patients with myocardial infarction (MI). Multivariate analysis was performed on 120-lead data, using as features instantaneous voltage measurements on time-normalized QRS and STT waveforms. Several areas on the map, most of which were located outside the precordial region, contained leads with important discriminant features; 2 of the 3 limb leads (aVR and aVF) also exhibited high diagnostic capability. A total of 6 features (mostly STT measurements) from 3 locations accounted for a specificity of 95% and a sensitivity of 95%; these were the right subclavicular area, the left posterior axillary region and the left leg. As a comparison, the same number of features from the standard 12-lead electrocardiogram yielded a sensitivity of 88% for a specificity of 95%. To investigate the repeatability of the results, the entire population was separated into a training set (100 normal subjects and 100 patients with MI) and a testing set (84 normal subjects and 77 patients with MI); computing a discriminant function on the training set and applying it to the testing set only moderately deteriorated the diagnostic classification. It is concluded that this approach achieves efficient information extraction from body surface potential maps for improved diagnostic classification.

The inverse problem of electrocardiography: A solution in terms of single- and double-layer sources on the epicardial surface

An approach to the inverse problem of electrocardiography that involves an estimation of the electric potentials (double-layer equivalent sources) on the hearts epicardial surface from the electrocardiographic potentials that are measurable on the body surface has received considerable attention. This report deals with a heretofore unexplored extension of this approach, one that yields, in addition to the electric potentials on the epicardial surface, the normal components of their gradients (single-layer equivalent sources). We show that this formulation has at least three advantages over the formulation in term of epicardial potentials alone: (1) single-layer equivalent sources, which reflect the flow of current across the epicardial surface, are well suited for the imaging of regional ischemia and infarction; (2) the transfer matrix linking the epicardial and body-surface potentials for this formulation is less ill conditioned than that for the formulation in terms of potentials alone; (3) the input vector for inverse calculations consists of spatially filtered (rather that directly measured and therefore noise) body-surface potentials. To establish the feasibility of this new formulation of the inverse problem and to compare it with the formulation in terms of potentials alone, we used a realistically shaped boundary-element model of human torso. By calculating singular values less ill conditioned. We then directly calculated epicardial and body-surface potentials for a single dipole located centrally and for three simultaneously active dipoles located eccentrically in the torsos heart region and used these results to test three methods that are prerequisites of a successful inverse solution: Tikhonov regularization, linearly constrained least squares, and an L-curve method. The feasibility of the new formulation was demonstrated by the fact that the method based on the linearly constrained least squares improved on overregularized Tikhonov solutions over a wide range of regularization parameters, and it yielded solutions that were more accurate than the best-possible Tikhonov solutions. Moreover, the L-curve solution procedure, which requires no a priori information about the solution, yielded slightly underregularized, but accurate, estimates for the optimal regularization parameter and the corresponding best-possible Tikhonov solution. Our results also showed that replacing--in the interest computational economy--quadrature formulas for the planar triangles with various approximate formulas for the nodes of the model reduces the accuracy of the inverse solution.

Complementary nature of electrocardiographic and magnetocardiographic data in patients with ischemic heart disease

High resolution body surface potential maps (BSPM) and magnetic field maps (MFM) for study groups consisting of 11 Q wave and 11 non Q wave myocardial infarct (MI) patients as well as 9 normal subjects, were recorded in a magnetically and electrically shielded room. A control group of 22 normal subjects provided group mean normal time integral maps for selected QRST time intervals. The difference between magnitudes of extrema in each map defined the normal mean data range R for that time interval. The root mean square sum of the differences between the time integral map of a study subject and the normal group-mean map provided an estimate of individual map variability, V. Subsequent calculation of group-mean map variability, V, and group-mean normalized variability, V/R, for specific time intervals of the cardiac cycle, were used to test the abilities of BSPM and MFM techniques to distinguish between the normal and MI study groups. Results indicate that BSPM V/R differences between MI and normal groups are most pronounced during Q wave and Q zone activity; between inferior MIs and normals (p less than 0.05) and between anterior MIs and normal (p less than 0.01). Significant differences in MFM V/R occur during repolarization; between inferior MIs and non Q wave MIs (p less than 0.05), between anterior MIs and normals (p less than 0.05), between non Q wave MIs and normals (p less than 0.05) and between all MIs and normals (p less than 0.01). It is concluded that high resolution BSPM and MFM provide complementary means of discriminating between normal subjects and MI patients.

Activation Dynamics in Anisotropic Cardiac Tissue via Decoupling

Bidomain theory for cardiac tissue assumes two interpenetrating anisotropic media—intracellular (i) and extracellular (e)—connected everywhere via a cell membrane; four local parameters σi,eℓ,t specify conductivities in the longitudinal (ℓ) and transverse (t) directions with respect to cardiac muscle fibers. The full bidomain model for the propagation of electrical activation consists of coupled elliptic–parabolic partial differential equations for the transmembrane potential vm and extracellular potential φe, together with quasistatic equations for the flow of current in the extracardiac regions. In this work we develop a preliminary assessment of the consequences of neglecting the effect of the passive extracardiac tissue and intracardiac blood masses on wave propagation in isolated whole heart models and describe a decoupling procedure, which requires no assumptions on the anisotropic conductivities and which yields a single reaction–diffusion equation for simulating the propagation of activation. This reduction to a decoupled model is justified in terms of the dimensionless parameter ∈ = (σℓiσte − σtiσℓe)/(σℓi+σℓe)(σti+σte). Numerical simulations are generated which compare propagation in a sheet H of cardiac tissue using the full bidomain model, an isolated bidomain model, and the decoupled model. Preliminary results suggest that the decoupled model may be adequate for studying general properties of cardiac dynamics in isolated whole heart models.

Frequent ventricular ectopic activity without underlying cardiac disease: Analysis of 45 subjects

Forty-five subjects, aged 2 weeks to 62 years, who presented with frequent (greater than 100/day) ventricular ectopic beats (VEBs) and without evidence of underlying cardiac disease were studied. The spectrum of ventricular dysrhythmia was assessed by 24-hour ambulatory electrocardiography and exercise tolerance test. Sinus rhythm was the prevailing rhythm in all subjects. VEB frequency averaged 444 +/- 454 per hour (range 0 to 1,863) over the 24-hour monitoring period and was not significantly different during waking or sleeping periods. There was no simple correlation of VEB frequency with prevailing sinus rate (r = -0.0006; p = not significant [NS]). The prevalence of complex VEBs (multiform, R-on-T and repetitive) was relatively high (18 of 45 patients), and was equally distributed about the median VEB frequency of 314 VEBs/hour (7 of 18 versus 11 of 18; NS). Of the 43 subjects who had exercise tests, 37 had VEBs during the preexercise rest phase, compared with only 11 at peak exercise (p less than 0.0001). To assess the short-term natural history of the VEBs, 27 subjects had repeat clinical examinations and 24-hour electrocardiograms at a mean interval of 8 months. All remained well. Although there was considerable individual temporal variability of VEB frequency in this subgroup, there was no significant change in group mean values (415 +/- 409 VEBs/hour initially versus 401 +/- 383 VEBs/hour at follow-up study; NS). The relative temporal constancy of VEB frequency in the group as a whole was also reflected in a high linear correlation of VEB frequency at initial and follow-up studies (r = 0.816; p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of posture and respiration on body surface electrocardiogram

Abstract To define more fully the effects of posture and respiration on electrocardiographic (ECG) patterns, 120-lead body surface potential maps (BSPM) were recorded in 36 normal subjects (aged 21 to 48 years) during cyclic respiration in both supine and upright positions; and at static end-tidal inspiration, functional residual capacity (FRC), total lung capacity (TLC) and residual volume (RV). In addition, BSPMs were recorded at TLC and RV during the Valsalva and Muller maneuvers, respectively. P-wave, QRS and ST-segment time integrals were evaluated. From supine to upright position, there was an inferior torso shift of P-wave and QRS distributions, but no change in amplitude of their maximal or minimal values; ST-segment distributions were spatially unaltered, but there was a significant (p ST segment > QRS (p Thus, resting tidal volume respiration has little effect on body surface ECG patterns in normal adults. However, large volume respiration and posture change may substantially alter ECG body surface distributions and should be considered in states involving either factor.

Inverse Solution Mapping of Epicardial Potentials Quantitative Comparison With Epicardial Contact Mapping

Background—Catheter ablation of ventricular tachycardia (VT) is still one of the most challenging procedures in cardiac electrophysiology, limited, in part, by unmappable arrhythmias that are nonsustained or poorly tolerated. Calculation of the inverse solution from body surface potential mapping (sometimes known as ECG imaging) has shown tremendous promise and can rapidly map these arrhythmias, but we lack quantitative assessment of its accuracy in humans. We compared inverse solution mapping with computed tomography–registered electroanatomic epicardial contact catheter mapping to study the resolution of this technique, the influence of myocardial scar, and the ability to map VT. Methods and Results—For 4 patients undergoing epicardial catheter mapping and ablation of VT, 120-lead body surface potential mappings were obtained during implantable defibrillator pacing, catheter pacing from 79 epicardial sites, and induced VT. Inverse epicardial electrograms computed using individualized torso/epicardial surface geometries extracted from computed tomography images were compared with registered electroanatomic contact maps. The distance between estimated and actual epicardial pacing sites was 13±9 mm over normal myocardium with no stimulus-QRS delay but increased significantly over scar (P=0.013) or was close to scar (P=0.014). Contact maps during right ventricular pacing correlated closely to inverse solution isochrones. Maps of inverse epicardial potentials during 6 different induced VTs indicated areas of earliest activation, which correlated closely with clinically identified VT exit sites for 2 epicardial VTs. Conclusions—Inverse solution maps can identify sites of epicardial pacing with good accuracy, which diminishes over myocardial scar or over slowly conducting tissue. This approach can also identify epicardial VT exit sites and ventricular activation sequences.Background— Catheter ablation of ventricular tachycardia (VT) is still one of the most challenging procedures in cardiac electrophysiology, limited, in part, by unmappable arrhythmias that are nonsustained or poorly tolerated. Calculation of the inverse solution from body surface potential mapping (sometimes known as ECG imaging) has shown tremendous promise and can rapidly map these arrhythmias, but we lack quantitative assessment of its accuracy in humans. We compared inverse solution mapping with computed tomography–registered electroanatomic epicardial contact catheter mapping to study the resolution of this technique, the influence of myocardial scar, and the ability to map VT. Methods and Results— For 4 patients undergoing epicardial catheter mapping and ablation of VT, 120-lead body surface potential mappings were obtained during implantable defibrillator pacing, catheter pacing from 79 epicardial sites, and induced VT. Inverse epicardial electrograms computed using individualized torso/epicardial surface geometries extracted from computed tomography images were compared with registered electroanatomic contact maps. The distance between estimated and actual epicardial pacing sites was 13±9 mm over normal myocardium with no stimulus-QRS delay but increased significantly over scar ( P =0.013) or was close to scar ( P =0.014). Contact maps during right ventricular pacing correlated closely to inverse solution isochrones. Maps of inverse epicardial potentials during 6 different induced VTs indicated areas of earliest activation, which correlated closely with clinically identified VT exit sites for 2 epicardial VTs. Conclusions— Inverse solution maps can identify sites of epicardial pacing with good accuracy, which diminishes over myocardial scar or over slowly conducting tissue. This approach can also identify epicardial VT exit sites and ventricular activation sequences.

Temporal evolution of body surface map patterns following acute inferior myocardial infarction

We studied the evolution of body-surface potential map (BSPM) patterns in 32 patients following first acute inferior myocardial infarction. Initial BSPMs were obtained at a mean of 79 hours post-infarction; follow-up BSPMs, a mean of eight months post-infarction. Temporal area-of-difference maps, constructed by subtracting initial from follow-up group-mean BSPMs, revealed reciprocal changes over the superior and inferior torso for both Q-zone and ST-segment time-integral distributions. The temporal changes in Q-zone patterns were small but definite: over the inferior torso there was a relative gain in Q-zone values and, over the superior torso, a relative decrease. In contrast, there were marked spatial and quantitative changes of ST-segment distributions during the follow-up period. Over the superior torso, particularly anteriorly, there was a gain in ST-segment values; over the inferior torso, a decrease. With the small temporal changes in Q-zone time-integral distributions, individual Q-zone maps continued to reflect a pattern of inferior myocardial infarction at follow-up. In contrast, the marked temporal changes in ST-segment time-integral distributions resulted in individual map patterns at follow-up that were nearly indistinguishable from normal ST-segment maps. The relatively small changes in depolarization time-integral patterns during the early post-infarction period suggest that the Q-zone patterns of the acute phase of myocardial infarction reflect near-irreversible or completed myocardial damage. The marked normalization of repolarization time-integral patterns during the recovery phase suggests, however, that there are also considerable areas of myocardium-at-risk during the early phase of the infarction process which stabilize with time.

Electrocardiographic ST-Segment Changes During Acute Myocardial Ischemia

The recognition and management of patients with acute coronary syndromes has relied to a large extent on the standard 12-lead electrocardiogram (ECG) for assessing ST-segment changes associated with ischemia. The purpose of this review is to show both the capabilities and the limitations of the 12-lead ECG in recognizing ischemia, and to seek alternative electrocardiographic leads, optimized for detection of ischemia originating in different regions of the ventricular myocardium. Three such leads are proposed-based on the results obtained by electrocardiographic body-surface mapping performed during ischemia induced by balloon-inflation coronary angioplasty. A survey of recent clinical studies shows that the electrocardiographic manifestations of acute myocardial ischemia observed during coronary angioplasty are in agreement with the ST-segment measurements in admission ECGs of patients with acute myocardial infarction.