Spatial differences of the duration of ventricular late fields in the signal-averaged magnetocardiogram in patients with ventricular late potentials

Aspects of left ventricular morphology outperform left ventricular mass for prediction of QRS duration

BACKGROUND

The knowledge of the case-specific normal QRS duration in each individual is needed when determining the onset, severity and progression of the heart disease. However, large interindividual variability even of the normal QRS duration exists. The aims of the study were to develop a model for prediction of normal QRS complex duration and to test it on healthy individuals.

METHODS

The study population of healthy adult volunteers was divided into a sample for development of a prediction model (n = 63) and a testing sample (n = 30). Magnetic resonance imaging data were used to assess anatomical characteristics of the left ventricle: the angle between papillary muscles (PM(A)), the length of the left ventricle (LV(L)) and left ventricular mass (LV(M)). Twelve-lead electrocardiogram (ECG) was used for measurement of the QRS duration. Multiple linear regression analysis was used to develop a prediction model to estimate the QRS duration. The accuracy of the prediction model was assessed by comparing predicted with measured QRS duration in the test set.

RESULTS

The angle between PM(A) and the length of the LV(L) were statistically significant predictors of QRS duration. Correlation between QRS duration and PM(A) and LV(L) was r = 0.57, P = 0.0001 and r = 0.45, P = 0.0002, respectively. The final model for prediction of the QRS was: QRS(Predicted)= 97 + (0.35 x LV(L)) - (0.45 x PM(A)). The predicted and real QRS duration differed with median 1 ms.

CONCLUSIONS

The model for prediction of QRS duration opens the ability to predict case-specific normal QRS duration. This knowledge can have clinical importance, when determining the normality on case-specific basis.

Magnetocardiographic mapping of QRS fragmentation in patients with a history of malignant tachyarrhythmias

BACKGROUND

The identification of patients at increased risk for ventricular tachycardia or ventricular fibrillation (VT/VF) and sudden cardiac death has consequences for therapeutic options and thus may reduce mortality in patients with coronary artery disease (CAD).

HYPOTHESIS

We hypothesized that the intra-QRS fragmentation in magnetocardiographic recordings is increased in patients with CAD and with a history of VT/VF.

METHODS

Multichannel magnetocardiography (MCG) was carried out in 34 healthy controls, 42 patients with CAD without a history of VT/VF, and 43 patients with CAD and with a history of VT/VF. The intra-QRS fragmentation was quantified by a new fragmentation score. Its spatial distribution was investigated using two-dimensional (2-D) contour maps according to the sensor position of the 49-channel magnetogradiometer.

RESULTS

Patients with CAD and with a history of VT/VF had significantly increased QRS fragmentation compared with patients with CAD without VT/VF or controls (72.9+/-37.5, 48.5+/-14.3, and 42.5+/-7.8, respectively: p <0.05). The area of high fragmentation in 2-D contour maps was twice as large in patients with than in those without a history of VT/VF (represented by the number of MCG channels with high fragmentation: 26.3+/-15.5 vs. 12.4+/-9.9, p<0.0001). Patients prone to VT/VF could be identified with a sensitivity of 64% and a specificity of 90%.

CONCLUSION

In patients with CAD and with a history of VT/VF, intra-QRS fragmentation is increased and the area of high fragmentation in 2-D contour maps is enlarged. These findings may be helpful in identifying patients with CAD at risk for malignant tachyarrhythmias.

A method for detecting myocardial abnormality by using a total current-vector calculated from ST-segment deviation of a magnetocardiogram signal

A simple method to determine the state of ischaemia or fibrosis of myocardial cells has been developed. This method uses the ST wave of 64-channel magnetocardiogram (MCG) signals to calculate three parameters from the current-arrow map of the normal component signal of the MCG. One parameter is a total current vector that is obtained through summation of all current arrows. Another is a variance current vector calculated from the differential vector of two total current vectors at different times. The third is a flatness factor between the magnitude of the total current vector and the variance current vector. The three parameters are independent of the distance between the heart and the gradiometers. We measured the MCG signals of 29 healthy subjects, twenty patients with coronary artery disease (ten with previous myocardial infarction (MI) and ten with angina pectoris (AP)), and eight patients with cardiomyopathy (four with hypertrophic cardiomyopathy (HCM), three with dilated cardiomyopathy (DCM), and one with restrictive cardiomyopathy (RCM)). With our method, none of the healthy subjects tested positive for myocardial abnormalities, while 80% of the MI patients, 50% of the AP patients, and 100% of the cardiomyopathy patients tested positive. Although further testing is needed, we feel this simple technique enables easy diagnosis of myocardial damage.

Identification of post-myocardial infarction patients with ventricular tachycardia by time-domain intra-QRS analysis of signal-averaged electrocardiogram and magnetocardiogram

A new time-domain analysis method, which quantifies ECG/MCG intra-QRS fragmentation, is applied to parts of the QRS complex to identify post-myocardial infarction patients with ventricular tachycardia. Three leads of signal-averaged electrocardiograms and nine leads of magnetocardiograms were band-pass filtered (74 Hz to 180 Hz). The filtered signals showed fragmentation in the QRS region, which was quantified by the number of peaks M and a score S, that is the product of M and the sum of the peak amplitudes. Both parameters were determined for the first 80 ms of the QRS complex and the total QRS complex in each channel. For classification, the mean-values of the parameters M and S of the three electrical leads and the nine magnetic leads were calculated. Late potential and late field analyses were performed for the same signals. 31 myocardial infarction patients were included, 20 of them with a history of documented ventricular tachycardia (VT). Identification of VT patients using the SAECG led to better results (sensitivity 95%, specificity 91%) considering the entire QRS complex than with the standard late potential analysis suggested by Simson (sensitivity 90%, specificity 73%). For the SAMCG and the entire QRS complex results using the parameters S and M are also better (sensitivity 95%, specificity 100%) than for the late field analysis (sensitivity 90% and specificity 100%). For the first 80 ms, the performance of the parameters M and S is only slightly decreased.

Value of magnetocardiographic QRST integral maps in the identification of patients at risk of ventricular arrhythmias

It has been shown that regional ventricular repolarization properties can be reflected in body surface distributions of electrocardiographic QRST deflection areas (integrals). We hypothesize that these properties can be reflected also in the magnetocardiographic QRST areas and that this may be useful for predicting vulnerability to ventricular tachyarrhythmias. Magnetic field maps were obtained during sinus rhythm from 49 leads above the anterior chest in 22 healthy (asymptomatic) control subjects (group A) and in 29 patients with ventricular arrhythmias (group B). In each subject, the QRST deflection area was calculated for each lead and displayed as an integral map. The mean value of maximum was significantly larger in the control group A than in the patient group B (1,626+/-694 pTms vs. 582+/-547 pTms, P<0.0001). To quantitatively assess intragroup variability in the control group A and intergroup variability of the control and patient groups, we used the correlation coefficient r and covariance sigma. These indices showed significantly less intragroup than intergroup variation (e.g., in terms of sigma, 28.0x10(-6)+/-12.3x10(-6) vs. 3.4x10(-6)+/-12.5x10(-6), P<0.0001). Each QRST integral map was also represented as a weighted sum of 24 basis functions (eigenvectors) by means of Karhunen-Loeve transformation to calculate the contribution of the nondipolar eigenvectors (all eigenvectors beyond the third). This percentage nondipolar content of magnetocardiographic QRST integral maps was significantly higher in the patient group B than in the control group A (13.0%+/-9.1 % vs. 2.6%+/-2.0%, P<0.0001). Discriminations between control subjects and patients with ventricular arrhythmias based on magnitude of the maximum, covariance sigma, and nondipolar content were 90.2%, 90.2%, and 86.3% accurate, with a sensitivity of 89.7%, 93.1%, and 75.9%, and a specificity of 90.9%, 86.4%, and 100%. We have shown that magnitude of the maximum and indices of variability and nondipolarity of the magnetocardiographic QRST integral maps may predict arrhythmia vulnerability. This finding is in agreement with earlier studies that used body surface potential mapping and suggests that magneticfield mapping may also be a useful diagnostic tool for risk analysis.

Magnetocardiographic analysis of the two-dimensional distribution of intra-QRS fractionated activation

The spatial distribution of high-frequency components in magnetic signals during the QRS complex of the human heartbeat was investigated. Cardiomagnetic signals were recorded simultaneously using 49 first-order magnetogradiometer channels of a multi-SQUID system with a low noise power density. The QRS fragmentation score S, as a measure of the fragmentation of the bandpass-filtered QRS complex, was examined for its sensitivity and specificity to discriminate 34 healthy volunteers, 42 post-myocardial infarction patients and 43 patients with coronary heart disease and with a history of malignant sustained ventricular tachycardia or ventricular fibrillation. The multichannel information was visualized by two-dimensional mapping of the score values of the single channels. By averaging the score values for the seven central channels, S7, the score values of all 49 channels, S49, and calculating the standard deviation for all 49 channels, D49, a higher sensitivity and specificity for detecting patients with ventricular tachycardia (VT) or ventricular fibrillation (VF) was reached than by analysis of a single channel. Combination of these parameters furnishes a sensitivity of 90% and a specificity of 70% for identifying patients prone to VT/VF. The results were compared with diagnostic information obtained from the QRS duration of the signal as well as with results obtained by modified QRS integral mapping.

Fragmentation of bandpass-filtered QRS-complex of patients prone to malignant arrhythmia

The structure of high-frequency components of electric and magnetic signals from the heart during the depolarisation phase is investigated. After averaging and broadband filtering with a binomial bandpass filter (37 Hz-90 Hz), the fragmentation of the QRS-complex is quantified. The number of extrema M and a new score value S are calculated from the signals of three electrical leads and one magnetic lead of 23 healthy subjects, 23 patients with coronary heart disease (CHD) without reported event of ventricular tachycardia or fibrillation at the time of measurement, and eight patients with CHD who have suffered from malignant tachycardia. For the parameter M, the sensitivity and specificity for healthy subjects against patients with CHD and ventricular tachycardia for the magnetic lead (the best electric lead) are 100% (75%) and 100% (100%). For the magnetic lead (best electric lead) and parameter S, the sensitivity and specificity are 100% (75%) and 95.6% (100%).

[Magnetocardiographic diagnostic of late fields. Current state and future perspectives]

The occurence of ventricular late potentials in the signal averaged surface ECG is an indicator for slow electrical excitation in myocardial tissue prone to arrhythmias. Signal averaged surface ECG is performed for identification of patients with ventricular late potentials who have a high risk for life threatening arrhythmias. Since every electrical field is combined with a magnetic field, established methods of the signal averaged surface ECG were applied on recorded magnetocardiograms of patients. The incidences and details of late ventricular activity in the signal averaged electrocardiogram (ventricular late potentials) were compared with those in the magnetocardiogram (ventricular late fields). A close correlation of the two different methods was found. The magnetocardiogram has the option for the determination of the site of origin of magnetic signals three-dimensionally. In the future, this method may help to find areas of slow conduction for ablative procedures to cure patients with a high risk for malignant arrhythmias.

Magnetocardiography and cardiac risk

Risk evaluation is a challenging problem in clinical cardiology. Recently, the development of new therapeutic strategies for malignant cardiac arrhythmias and ischemia has urged the need for more accurate screening methods of risk patients The purpose of this review is to summarize the current scientific evidence on the applicability of a new method, high-resolution magnetocardiography (HR-MCG), in identification of cardiac patients at risk of malignant ventricular arrhythmias and ischemic episodes. In recent years different methods for recognizing the electromagnetic abnormalities indicating the increased risk have been used with promising results. At present, the following conclusions can be made: 1) MCG can reliably identify patients prone to malignant ventricular arrhythmias after myocardial infarction as well as in cardiomyopathy, in long QT syndrome, and in operated congenital heart disease. 2) Several analysis methods seem to work: high-pass filtering, relative smoothness score and magnetic field map trajectory plots. 3) Detection and localization of acute and chronic ischemia seems technically feasible and may be one of the most important new clinical applications of the method. 4) Larger clinical series are needed to optimize these techniques and to evaluate their feasibility in the clinics. 5) Prognostic studies should also be started as soon as possible. There are already many multichannel MCG measurement systems available in hospitals to enable clinical studies.ZUSAMMENFASSUNG: Die Risikoabschätzung stellt in der klinischen Kardiologie ein schwieriges Problem dar. Kürzlich hat die Entwicklung neuer Strategien bei malignen kardialen Arrhythmien und der Ischämie die, Notwendigkeit an exakteren Untersuchungsmethoden bei Risikopatienten unterstrichen. Diese Ubersicht soll dem Zweck dienen, die derzeitige wissenschaftliche Anwendbarkeit einer neuen Methode, der hochauflösenden Magnetokardiographie (HR-MCG) bei Herzpatienten nachzuweisen, bei denen ein Risiko des Auftretens maligner Kammerarrhythmien und ischämischen Episoden beseht. Im Verlauf der letzten Jahre sind mit vielversprechenden Resultaten verschiedene Methoden zum Nachweis elektromagnetischer Störungen entwickelt worden, die auf ein erhöhtes Risiko hinweisen. Momentan können daraus die folgenden Schlussfolgerungen gezogen werden: 1) Die MCG kann das erhöhte Risiko bei den Patienten zuverlässig aufzeigen, die einen Herzinfarkt durchgemacht haben oder an einer Kardiomyopathie, einem langen QT-Syndrom oder einer operierten, kongenitalen Herzerkrankung leiden. 2) Es scheinen verschiedene Auswertungsmethoden zu funktionieren: Hochpassfiltrierung, Relative Smoothness Score und Magnetkarten-Trajektaufzeichnungen. 3) Nachweis und Lokalisation der akuten und chronischen Ischämie erscheinen technisch möglich zu sein und können eine der wichtigsten neuen klinischen Untersuchungsmethoden darstellen. 4) Umfangreichere klinische Studien sind erforderlich, um die Optimierung dieser Methoden zu erreichen und ihre Eignung unter klinischen Bedingungen aufzuzeigen. 5) So bald als möglich sollten ausserdem prognostische Studien eingeleitet werden. Es existieren in Krankenhäusern bereits einige Multikanal-MCG-Messgeräte, die sich zur Durchführung klinischer Studien eignen.

Biomagnetic neurosensors

Non-invasive measurement of the neuromodulatory activity of certain analytes is now possible through the use of biomagnetic stimulation and detection techniques. The timely development of room-temperature instrumentation and of more effective techniques for coupling neurons to transducers are the critical elements for rapid progress in this field.