Comparison of current density viability imaging at rest with FDG-PET in patients after myocardial infarction.

The assessment of myocardial viability is a major diagnostic challenge in patients with coronary artery disease (CAD) after myocardial infarction. Novel threedimensional current density (CD) imaging algorithms use high-resolution magnetic field mapping to determine the electrical activity of myocardial segments at rest. We, for the first time, compared CD activity obtained with several algorithms to 18-F-fluoro-deoxyglucose positron emission tomography (FDG-PET) in evaluation of myocardial viability. Magnetic field maps were obtained in nine adult patients (pt) with CAD and a history of infarction. The criterion for non-viable myocardium was an FDG-PET uptake with less than 45% of the maximum in the respective segments. CD imaging was applied to the left ventricle by using six different methods to solve the inverse problem. Mean CD activity was calculated for a close meshed grid of 90 locations of the left ventricle. A cardiologist compared bull's eye plots of CD and FDG-PET activity by eye. Spearman's correlation coefficients and specificity at a given level of sensitivity (70%) were calculated. Bull's eye plots revealed a significant correlation of CD/PET in 5 pt and no correlation in 3 pt. One pt had a negative correlation. The six different CD reconstruction methods performed similar. While CD reconstruction has the principal potential to image viable myocardium, we found that the reconstructed CD magnitude was low in scar segments but also reduced in some segments with preserved metabolic activity under resting conditions. New vector measurement techniques, the use of additional stress testing and advances in mathematical methodology are expected to improve CD imaging in future.

Magnetocardiography based spatiotemporal correlation analysis is superior to conventional ECG analysis for identifying myocardial injury.

Electrocardiogram (ECG) particular from tiny, non Q-wave myocardial infarction may lack striking infarct pattern. Spatiotemporal correlation analysis (SCA) of multichannel magnetocardiogram (MCG) is a high-resolution "magnifying glass" to analyze homogeneity of repolarization. SCA involves full 4D spatiotemporal information to identify abnormalities as empirically done by eye in conventional ECG-but on an advanced level of analysis. We compared the discriminatory performance of SCA to ECG analysis in identifying myocardial infarction. Eleven SCA parameters were taken from signal averaged 31-channel MCG and compared to infarct indicators of ECG's in 178 subjects: 108 patients (76 males, mean age 62 years) after myocardial infarction (16-64 d) and 70 controls (36 males, mean age 46 years). SCA improves the detection of myocardial injury: in 72.5% ECG (sensitivity 68.6%, specificity 56%) and in 80.2% SCA parameters (sensitivity 72.6%, specificity 64%) separated patients from controls. SCA is applicable for the analysis of de- and repolarization of cardiac mapping data.

31P-MR spectroscopic imaging in hypertensive heart disease.

Hypertensive heart disease (HHD) causes structural changes (e.g., fibrosis) that result in diastolic and systolic myocardial dysfunction. Alterations of (31)P metabolism and cardiac energy impairments were assessed in patients with HHD by MR spectroscopy (MRS) and correlated with left ventricular systolic function. Thirty-six patients with HHD and 20 healthy controls (mean age 35.2+/-10.7 years) were examined with (31)P-MRS at 1.5 T by using an ECG-gated CSI sequence. Twenty-five patients (mean age 64.3+/-9.3 years) had diastolic dysfunction, but preserved systolic function (HHD-D), whereas 11 patients (62.3+/-11.4 years) suffered from additional impaired systolic function (HHD-S). In both patient groups, the PCr/gamma-ATP ratio was lower than in the controls (controls: 2.07+/-0.17; P<0.001), and in HHD-S was lower than in HHD-D (1.43+/-0.21 vs. 1.65+/-0.25; P=0.012). PCr/gamma-ATP ratios were linearly correlated with LVEF (Pearson's r: 0.39; P=0.025). In the HHD-S group, the PDE/gamma-ATP ratio was significantly lower (0.56+/-0.36) than in the controls (1.14+/-0.42; P=0.001). In contrast to the group of HHD-D patients, whose slightly decreased PCr/gamma-ATP ratios compared to controls may be explained by age differences, the more distinct changes observed in HHD-S patients indicate an altered energy metabolism. The observed metabolic changes were related to functional impairments, as indicated by a reduced LVEF. Reduced PDE/ATP ratios indicate changes in the phospholipid metabolism.

Noninvasive measurements of cardiac high-energy phosphate metabolites in dilated cardiomyopathy by using 31P spectroscopic chemical shift imaging.

Dilated cardiomyopathy (DCM) is accompanied by an impaired cardiac energy metabolism. The aim of this study was to investigate metabolic ratios in patients with DCM compared to controls by using spectroscopic two-dimensional chemical shift imaging (2D-CSI). Twenty volunteers and 15 patients with severe symptoms (left ventricular ejection fraction, LVEF<30%) and ten patients with moderate symptoms (LVEF>30%) of DCM were investigated. Cardiac 31P MR 2D-CSI measurements (voxel size: 40x40x100 mm3) were performed with a 1.5 T whole-body scanner. Measurement time ranged from 15 min to 30 min. Peak areas and ratios of different metabolites were evaluated, including high-energy phosphates (PCr, ATP), 2,3-diphosphoglycerate (2,3-DPG) and phosphodiesters (PDE). In addition, we evaluated how PCr/ATP ratios correlate with LVEF as an established prognostic factor of heart failure. The PCr/gamma-ATP ratio was significantly decreased in patients with moderate and severe DCM and showed a linear correlation with reduced LVEFs. PDE/ATP ratios were significantly increased only in patients with severe DCM as compared to volunteers. Applying 31P MRS with commonly-available 2D-CSI sequences is a valuable technique to evaluate DCM by determining PCr/ATP ratios noninvasively. In addition to reduced PCr/ATP ratios observed in patients suffering from DCM, significantly-increased PDE/ATP ratios were found in patients with severe DCM.

Central venous catheters--the inability of 'intra-atrial ECG' to prove adequate positioning.

BACKGROUND: The classic increase in P wave size, known as 'P-atriale', is a widely accepted criterion for determination of proper positioning of central venous catheter tips. Recent transoesophageal echocardiography (TOE) studies did not confirm intra-atrial position despite advancing the central venous catheter further than indicated by ECG guidance. We postulate that the pericardial reflection rather than the entry into the right atrium corresponds to the ECG changes. In order to test our hypothesis we sought to determine the anatomical substrate for the electrical changes in an animal study. Subsequently, a modified version of the study was undertaken in man and is also reported. METHODS: In six juvenile pigs the left external jugular vein and right carotid artery were cannulated. A triple-lumen central venous catheter was positioned by ECG guidance using a Seldinger wire as an exploring electrode. The venous and arterial catheters were suture fixed 2 cm beyond the onset of an increase in P wave size. The corresponding anatomical catheter tip position was determined by open exploration of the vessels and the heart. Subsequently the catheter tip position (during advancement) of a pulmonary artery catheter and the corresponding electrical ECG changes were examined in 10 patients during open chest cardiac surgery. RESULTS: All catheters-arterial and venous, in animals and humans-revealed an increase in size of the P wave as well as the QRS complex. All venous catheters were positioned in the superior vena cava, beyond the pericardial reflection but outside the right atrium. All arterial catheters were positioned in the ascending aorta thus also beyond the pericardial reflection. CONCLUSIONS: The start of an increase in P wave size does not correspond with the entrance of the right atrium. The anatomic equivalent for the electrophysiological changes of the ECG is the pericardial reflection. ECG guidance is unable to distinguish between venous and arterial catheter position.

[Quantitative analysis of cardiovascular interactions in patients with cardiomyopathy and after myocardial infarct]. / Quantitative Analyse cardiovaskulärer Interaktionen bei Patienten mit Kardiomyopathie und nach Myokardinfarkt.

The aim of this study was to detect pathological changes of the complex autonomous regulation of ecg, blood pressure and respiration (synchronously recordings of 30 minutes) in patients with cardiac diseases. We investigated the cardiovascular and cardiorespiratory linear and nonlinear interactions in 10 patients with dilative cardiomyopathy (dcm) and in 10 patients after myocardial infarction (mi). We compared results from the classical linear correlation function analysis with results from the mutual information method (transinformation) for analysis of nonlinear and linear interactions. Both methods yield high significant parameters (p < 0.01). Thus, we found both linear and nonlinear interactions with partly different specificity in patients with dcm and mi compared to healthy subjects (ref).

[Effect of post-extrasystole potentiation on blood pressure and heart rate regulation]. / Einfluss der postextrasystolischen Potenzierung auf Blutdruck und Herzfrequenzregulation.

Noninvasive finger arterial blood pressure and ECG were recorded for 30 min in patients with idiopathic dilated cardiomyopathy (IDC) and healthy control subjects (CON) for the investigation of blood pressure and heart rate regulation following a premature ventricular complex (PVC). The potentiation of blood pressure amplitude of the postextrasystolic beat was fivefold increased in IDC compared with CON (48.7 +/- 32.6% vs. 9.8 +/- 5.4%, p < 0.01). PVC starts a baroreflex response, which last about 10 s in physiological regulation. In contrast, the baroreflex response in IDC is immediately suppressed by the augmented potentiation of the blood pressure amplitude from the first postextrasystolic beat. Thus, the regulation after PVC is determined by the PVC itself and postextrasystic potentiation.

[Afterload and blood pressure amplitude in dilated cardiomyopathy]. / Nachlast und Blutdruckamplitude bei dilatativer Kardiomyopathie.

The beat-to-beat variability of the diastolic blood pressure induces small variations in the afterload of the left ventricle. These variations influence myocardial contractility, and thus blood pressure amplitude. We assessed the interdependence of blood pressure and changes in the afterload. We continuously recorded blood pressure (duration 200 s, at rest) in 20 patients with dilated cardiomyopathy (ejection fraction 32 +/- 13%, left ventricular diameter 67 +/- 8 mm) and in 20 healthy volunteers. Interbeat intervals, diastolic pressures, systolic pressure amplitudes and mean slopes of systolic pressure amplitudes were measured. Correlation coefficients (r) were calculated to assess the interdependence of blood pressure amplitudes/mean systolic slopes and the preceding diastolic pressures/interbeat intervals, respectively. In healthy volunteers we found a strong interdependence between blood pressure amplitude and the preceding diastolic pressures (r = 0.62 +/- 0.21 and 0.47 +/- 0.22). Higher diastolic pressures were followed by higher blood pressure amplitudes, and by steeper slopes of the systolic peaks. In patients with dilated cardiomyopathy, such interdependence was significantly lower (r = 0.33 +/- 22 and r = 0.28 +/- 0.35), and in patients with severely reduced left ventricular function (ejection fraction < 32%) was only marginal (r = 0.23 +/- 0.27 and 0.21 +/- 0.44, respectively). The forces of the isovolumetric contraction necessary to initiate the ejection phase of the left ventricle depend on the afterload, i.e. on the diastolic pressure. The responses of amplitude and slope of the systolic blood pressure to small changes in the afterload make it possible to assess left ventricular contractility. The latter is impaired in dilated cardiomyopathy.

[Modified interaction of blood pressure and heart rate in idiopathic dilated cardiomyopathy]. / Veränderte Interaktion von Blutdruck und Herzfrequenz bei idiopathischer dilatativer Kardiomyopathie.

BACKGROUND: Neurovegetative and haemodynamic changes impact on the regulation pattern of blood pressure and heart rate in patients with heart failure. We studied these patterns and their interactions in patients with idiopathic dilated cardiomyopathy (IDC) and in healthy subjects (REF). METHODS: We continually measured the heart rate and blood pressure (Portapres device) in twenty-five supine IDC patients (age: 51 +/- 13 y; left ventricular end-diastolic diameter 67 +/- 11 mm; ejection fraction 30 +/- 11%) and in twenty-seven REF (age: 50 +/- 11 y) Recording time was 30 minutes. The heart rate (HR) of each beat and the systolic blood pressure (SYS) of the subsequent beat were measured. Code numbers (symbols) were assigned to the beat-to-beat changes in HR and SYS (increase: 1; decrease: 0). The frequencies of the symbols sequences of three successive beats were counted. In this way we obtained a matrix consisting of eight (two to the power of three) HR and SYS combinations: 000, 100, 010, 001, 111, 110, 011 and 101. We then counted the frequencies of the different combinations of the symbol sequences in HR and SYS (2(3) x 2(3) = 64 combinations). The relative frequencies of symbol patterns appearing in HR, SYS and in the combined analysis of HR and SYS, were compared for IDC and REF using the T-test for independent samples. RESULTS: Significant differences were seen between IDC and REF. The HR patterns 101 and 010 were more frequent in IDC than in REF patients (11.1 +/- 4.7 vs. 7.7 +/- 2.9%, p = 0.003, and 16.1 +/- 6.3 vs. 11.7 +/- 4.9%, p = 0.008). This finding was even more marked in the analysis of the SYS patterns 101 and 010 (11.0 +/- 7.4 vs. 8.2 +/- 2.9%, p < 0.001, and 11.6 +/- 7.4 vs. 5.4 +/- 2.7%, p < 0.001). Non-alternating patterns were more frequent in REF (e.g. 000HR & 111SYS: 4.6 +/- 3.3 vs. 2.9 +/- 2.4%, p = 0.03). CONCLUSIONS: We demonstrated significant interaction of the regulation patterns of blood pressure and heart rate, as also their interactions in IDC. Opposed changes in HR and SYS mediated by the baroreflex, became superimposed by alternans phenomena in IDC. The pattern analysis of changes in HR and SYS detects these disturbances of neurovegetative short-term control.

High frequency intra-QRS signals in idiopathic dilated cardiomyopathy.

We extracted and quantified high frequency intra-QRS signals in idiopathic dilated cardiomyopathy (IDC). In IDC the analysis of late potentials in the terminal QRS complex often fails in predicting clinical events because of intraventricular conduction abnormalities and the absence of a circumscribed arrhythmogenic substrate. Therefore, new approaches are required to assess the electrical state of the myocardium. We investigated 21 patients suffering from IDC with (n = 14) and without (n = 7) bundle branch block. High resolution 31 lead magnetocardiograms were filtered with a 67 point 4th order Savitzky-Golay filter. The difference of the measured and filtered signals was calculated (67-200 Hz). The spatio-temporal properties and the areas under the curves of the resulting high frequency intra-QRS signals (IQCs) were studied. We detected IQCs in all patients. The patients had individual patterns regarding the temporal and spatial properties of the IQCs during depolarisation. The IQCs predominantly appeared in the initial portion of the QRS. The ratios of the areas under the curves of the IQCs and the measured signals were linearly correlated to the left ventricular enddiastolic diameter (r = 0.71, significance 0.0012). In IDC the ventricular depolarization is accompanied by individual spatial and temporal patterns of high frequency intra-QRS signals. They can be studied non-invasively from body surface mapping data with the algorithm used in this study. This provides access to the assessment of the electrical status in patients with IDC.

Exercise capacity and Doppler pressure measurements in symptomatic peripheral arterial obstructive disease.

BACKGROUND: Doppler pressure measurements are a useful diagnostic tool in peripheral arterial obstructive disease. The aim of our study was to determine whether these pressure values do predict the degree of impairment of the walking capacity in symptomatic patients. PATIENTS AND METHODS: We compared the claudication distances (CDI: initial claudication distance, CDA: absolute claudication distance) of 939 patients (63 +/- 11 years) with stable intermittent claudication (Fontaine IIb) with the ankle pressure values at rest (APR) and after exercise (APE), with the ankle/brachial pressure index at rest (ABIR) and after exercise (ABIE), and with the ratio (ABIRATIO = ABIE/ABIR). Ankle systolic pressures were obtained using an 8 MHz Doppler probe. CD was measured by a treadmill test at constant-load conditions (3 km/hr; inclination 12%). Brachial systolic pressures were obtained using an automated blood pressure monitor. The values of the objectively worse leg were correlated with CDI and CDA. RESULTS: Low Doppler pressure values were not accompanied by significantly shorter walking distances in symptomatic patients. The resting pressure values (APR, ABIR) did not correlate with the claudication distances (CDI: 54 +/- 31 m; CDA: 87 +/- 41 m). For the exercise values (APE, ABIE), even a very slight inverse correlation with the claudication distances was found. In addition, the correlation between the pressure index ratio and the walking distances (ABIRATIO vs. CDI: r = -0.25, p < 0.01; ABIRATIO vs. CDA: r = -0.20, p < 0.01) was inverse, too, but slightly more pronounced. CONCLUSIONS: In patients with intermittent claudication the ankle artery pressures and the indices derived from these pressure values do not predict the walking distance. Therefore, the decision for angioplasty or bypass surgery should be made with regards to the impairment of quality of life rather than Doppler pressure values.

Localization of late potential sources in myocardial infarction.

INTRODUCTION: Late potentials (LP) are markers of arrhythmogenic events after myocardial infarction (MI). The localization of LP sources would help to identify arrhythmogenic myocardium. The purpose of this study was to localize these LP sources from non-invasive body surface mapping data. METHODS AND RESULTS: Six patients were investigated with cardiac MRI and signal averaged 62-lead magnetocardiography after MI. Three of them were suffering from sustained ventricular tachycardia (VT). Sophisticated computer algorithms were used in order to compute the current density on the surface of the left ventricle. We compared these current density distributions for the entire QRS complex and the high frequency LP signals. In the three patients which had premature ventricular complexes (PVCs) we localized the exit sites of these arrhythmias. We found a close matching of the low current density areas based on the QRS complexes and the high current density areas based on the LP signals. These areas predominantly corresponded to sites of the infarctions. Exit sites of PVCs were located close to these areas. CONCLUSIONS: By means of sophisticated computer algorithms we were able to localize LP sources. This would be useful in steering catheter ablation and coronary revascularization therapies. However, the method has to be proven with the help of invasive mapping in a larger number of patients.

Methods for the computational localization of atrio-ventricular pre-excitation syndromes.

BACKGROUND: The site of atrioventricular pre-excitation can roughly be estimated with the help of schemes basing on a few number of electrocardiogram (ECG) leads. Computer algorithms have been developed which utilize the body surface mapping of the pre-excitation signal for the localization purpose. We tested several new algorithms. METHOD: A patient suffering from Wolff-Parkinson-White syndrome was investigated prior the catheter ablation. The body surface mapping was performed with a 62-lead magnetocardiograph. The site of pre-excitation was calculated by using different methods: the dipole method with fixed and moving dipoles, the dipole scan on the endocardium, and different current density methods (L1 norm method, L2 norm method, low resolution electromagnetic tomography (LORETA) method, and maximum entropy method). Three-dimensional (3D) magnetic resonance imagings (MRIs) of the heart were used to visualize the results. The source positions were compared to the site of catheter ablation. RESULTS: The accessory pathway was successfully ablated left laterally. This site was correctly identified by the conventional dipole method. By scanning the entire endocardial surface of the heart with the dipole method we found a circumscribed source area. This area too, was located at the lateral segment of the atrio-ventricular grove. The current density methods performed differently. Whereas the L1 norm identified the site of pre-excitation, the L2 norm, the LORETA method and the maximum entropy method resulted in extended source areas and therefore were not suited for the localization purpose. CONCLUSION: The dipole scan and the L1 norm current density method seem to be useful additions in the computational localization of pre-excitation syndromes. In our single case study they confirmed the localization results obtained with the dipole method, and they estimated the size of the suspected source region.

[Imaging of coronary vascular abnormality with aneurysm and arteriovenous fistula using MRI bolus tracking]. / Darstellung einer Koronargefässanomalie mit Aneurysma und arteriovenöser Fistel mittels MRT-Bolustracking.

Coronary arteriovenous fistulas are very rare findings and predominantly drain into the right atrium or the right ventricle. Echocardiography and x-ray coronary angiography are established methods in the detection of this anomalies. It is reported on a patient, in whom an anomaly with dilatation of the circumflex coronary artery, aneurysma and drainage over an arteriovenous fistula in the right ventricle was observed. By means of magnetic resonance tomography with bolustracking technique the vessel course and the dynamics of perfusion are noninvasive shown beside the imaging using TSE-sequence. The method of multiple injections of 2 ml Gd-DTPA makes the bolustracking technique to an ideal tool for the determination of the individual vessel course, the haemodynamics and draining direction of an coronary anomaly with fistula.

Reproducibility of HTS-SQUID magnetocardiography in an unshielded clinical environment.

A new technology has been developed which measures the magnetic field of the human heart (magnetocardiogram, MCG) by using high temperature superconducting (HTS) sensors. These sensors can be operated at the temperature of liquid nitrogen without electromagnetic shielding. We tested the reproducibility of HTS-MCG measurements in healthy volunteers. Unshielded HTS-MCG measurements were performed in 18 healthy volunteers in left precordial position in two separate sessions in a clinical environment. The heart cycles of 10 min were averaged, smoothed, the baselines were adjusted, and the data were standardized to the respective areas under the curves (AUC) of the absolute values of the QRST amplitudes. The QRS complexes and the ST-T intervals were used to assess the reproducibility of the two measurements. Ratios (R(QRS), R(STT)) were calculated by dividing the AUC of the first measurement by the ones of the second measurement. The linear correlation coefficients (CORR(QRS), CORR(STT)) of the time intervals of the two measurements were calculated, too. The HTS-MCG signal was completely concealed by the high noise level in the raw data. The averaging and smoothing algorithms unmasked the QRS complex and the ST segment. A high reproducibility was found for the QRS complex (R(QRS)=1.2+/-0.3, CORR(QRS)=0.96+/-0.06). Similarly to the shape of the ECG it was characterized by three bends, the Q, R, and S waves. In the ST-T interval, the reproducibility was considerably lower (R(STT)=0.9+/-0.2, CORR(STT)=0.66+/-0.28). In contrast to the shape of the ECG, a baseline deflection after the T wave which may belong to U wave activity was found in a number of volunteers. HTS-MCG devices can be operated in a clinical environment without shielding. Whereas the reproducibility was found to be high for the depolarization interval, it was considerably lower for the ST segment and for the T wave. Therefore, before clinically applying HTS-MCG systems to the detection of repolarization abnormalities in acute coronary syndromes, further technical development of the systems is necessary to improve the signal-to-noise ratio.

Temporal properties of high frequency intra-QRS signals in myocardial infarction and healthy hearts.

The prevalence of late potentials after myocardial infarction depends on the site of the infarction. This may be caused by the different activation onsets of the anterior and inferior myocardial segments. Therefore, in anterior infarcts the high frequency signals may be concealed within the QRS whereas in the inferior infarcts they last beyond the end of the QRS. We compared the timing and the spatial patterns of high frequency intra-QRS signals (IQSs) in the different infarction sites. We investigated 14 patients with anterior infarcts, 17 patients with inferior infarcts, and 10 healthy subjects. 31-lead magnetocardiograms were recorded in left precordial position and averaged. The QRS signals were smoothed with a Savitzky-Golay filter. The smoothed QRS signals were subtracted from the measured ones. The difference of the signals (frequency band of about 60-200 Hz) representing the high frequency components was quantified. The percentage of the high frequency signals was calculated for the entire QRS, for the first and for the second half, respectively. We found that in patients with anterior infarcts the high frequency components predominantly appeared in the first half of the QRS whereas in inferior infarcts these components predominantly appeared in the second half of the QRS. The different infarction sites were associated with different spatial patterns of the high frequency signals on the body surface. In healthy subjects there was not such a preferential association of time intervals and high frequency signals. Late potentials are the special case of high frequency signals appearing in the terminal QRS. It is the general property of the myocardium to generate high frequency signals associated with the depolarization of infarcted tissue. The timing of such signals and the spatial distribution patterns on the body surface may help to identify the location of the sources.

[Cardiorespiratory desynchronization after acute myocardial infarct]. / Kardiorespiratorische Desynchronisation nach akutem Myokardinfarkt.

The prognosis of cardiac diseases can be estimated from the variability of regulation parameters of the cardiovascular system. Changes in the variability of a regulation parameter causes disturbances in the synchronisation of interacting control loops. Conclusions about the severity of the underlying functional impairment can be drawn from these disturbances. This study investigates the synchronisation of the control loops of the heart rate and respiration (cardiorespiratory synchronisation, CRS) after acute myocardial infarction. We investigated 43 patients after myocardial infarction and 27 healthy controls. To quantify the CRS the synchronisation in phase of respiration and heart rate was assessed. The heart rate variability (HRV) was also assessed. Patients after myocardial infarction have a significantly reduced HRV and CRS. There is a non-linear relationship between HRV and CRS. Patients with left ventricular enlargement and reduced left ventricular ejection fraction (< or = 45%) significantly differed from the other infarct patients and controls in CRS but not in HRV. They had a marked degree of cardiorespiratory desynchronisation and were identified by a threshold value. CRS is a measure of the interaction of respiration control and heart rate control. After myocardial infarction, a reduction of the HRV can be observed. The desynchronisation of the control loops of respiration and heart rate especially appears in large infarcts. This can be quantitatively assessed by the method presented.

[Effect of choice of baseline correction interval on localization of electrical heart activity]. / Einfluss der Wahl des Basislinien-Korrekturintervalls auf die Lokalisation der elektrischen Herzaktivität.

The electric heart activity can be localised from body surface mapping data with computer algorithms. At higher heart rates the T and P waves merge. Thus, the offset can not be subtracted in the TP segment. We investigated 28 healthy volunteers with signal averaged 31-lead magnetocardiography. The offset of the baseline was determined in the TP-segment and in the PR-segment, respectively. The electrical heart activity was localised in the initial 30 ms of the QRS complex (Q), at the QRS maximum (R), and at the T wave maximum (T). The volume currents were considered by using a boundary element model with the compartments lungs and torso. The 3D positions of the dipoles, the dipole orientations, and the dipole strengths were calculated using the data preprocessed with two different offset correction intervals. The offsets of the TP and PR segments significantly differed one from another. The average deviations of the dipole localisation were within a few centimetres (Q: 20 +/- 31 mm, R: 6 +/- 13 mm, T: 14 +/- 30 mm). However, in a small number of subjects (Q: n = 5, R: n = 2, T: n = 5) we observed a deviation of more than 30 mm. These deviations were not linearly correlated to the differences in the baseline offsets. High resolution recordings continuously detect heart activity in the PR segment. The correction of the baseline in the PR segment instead of the TP segment may introduce artefacts in the source localisation and therefore should be avoided.

QRS amplitude and shape variability in magnetocardiograms.

In magnetocardiography, averaging of QRS complexes is often used to improve the signal-to-noise ratio. However, averaging of QRS complexes ignores the variation in amplitude and shape of the signals caused, for example, by respiration. This may lead to suppression of signal portions within the QRS complexes. Furthermore, for inverse source, reconstructions of dipoles and of current density distributions errors in the spacial arrangement may occur. To overcome these problems we developed a method for separating and selective averaging QRS complexes with different shapes and amplitudes. The method is based on a spline interpolation of the QRS complex averaged by a standard procedure. This spline function then is fitted to each QRS complex in the raw data by means of nonlinear regression (Levenberg-Marquardt method). Five regression parameters are applied: a linear amplitude scaling, two parameters describing the baseline drift, a time scaling parameter, and a time shift parameter. We found that both amplitude and shape of the QRS complex are influenced by respiration. The baseline shows a weaker influence of the respiration. The regression parameters of two neighboring measurement channels correlate linearly. Thus, selective averaging of a larger number of sensors can be performed simultaneously.