P. Siltanen

Late cardiac effects of mediastinal radiotherapy in patients with Hodgkin's disease

Twenty‐eight patients younger than age 40 years, treated for Hodgkins disease with mediastinal irradiation, were examined no less than 5 years after the irradiation in order to evaluate the frequency of cardiac abnormalities. Twelve patients (43%) had had some pericardial event after radiation: a diagnosed pericarditis, remarkably increased heart volume, or a conspicuous change of cardiac silhouette, suggesting pericardial fluid. On evaluation, 50% of the patients complained of symptoms, and 13 patients had to stop the exercise test on a low level because of chest pain, dyspnea, or general fatigue. In 13 patients some of the following abnormalities in the electrocardiogram (ECG) was found: right bundle branch block (four), first‐degree atrioventricular block (four), abnormal P terminal force (five), or a low voltage (two). In ten patients (38%) an increase of the pericardial fluid was seen in the echocardiogram, and in nine patients the right ventricle wall thickness had increased. In two patients a severe coronary artery disease was found. One died suddenly after an acute myocardial infarction (AMI), and the other had a large anterior AMI. Two patients with chronic pericardial fluid underwent partial pericardectomy. Two cases of mild pulmonary valve stenosis, one pulmonary subvalvular stenosis and two aortic valve deformities were discovered. Eight patients went through cardiac catheterization, and in all but one case the pressures were slightly elevated suggesting diminished diastolic compliance. In summary, 19 of 28 patients had some abnormal cardiac findings, but only three of them were serious ones.

Identification of patients with ventricular tachycardia after myocardial infarction by high-resolution magnetocardiography and electrocardiography

The value of time domain analysis of late fields in the high-resolution magnetocardiogram in the identification of myocardial infarction patients with ventricular tachycardia was investigated in 30 subjects: 10 patients with documented sustained ventricular tachycardia and old myocardial infarction, 10 patients with old myocardial infarction without complex ventricular arrhythmias, and 10 normal volunteers. The duration of the QRS complex in the magnetocardiogram was significantly longer in ventricular tachycardia patients compared to myocardial infarction patients (144 (SD, 33) vs 109 (SD, 8) ms; p = 0.004). The root-mean-square field of the last 60 ms of the QRS complex was smaller in ventricular tachycardia patients than in myocardial infarction patients (830 (SD, 650) vs 1,480 (SD, 730) fT, respectively; p = 0.047). Also, the duration of the low-amplitude signal less than 700 fT was longer in ventricular tachycardia patients than in myocardial infarction patients (47 (SD, 28) vs 28 (SD, 8) ms, respectively; p = 0.048). The sensitivity and specificity in identifying ventricular tachycardia patients were both 80%, and the positive and negative predictive values were 78% and 86%, respectively. High-resolution electrocardiography recorded during the same session performed slightly better: sensitivity 90%, specificity 90%, and positive and negative predictive values 90%. The signal-to-noise ratio of electrocardiogram was higher (approximately 2 x) than that of magnetocardiogram. It is concluded that the new magnetocardiographic technique seems helpful in screening patients at risk of ventricular arrhythmias after myocardial infarction. The results encourage further refinement of the technique and application in prospective studies.

Usefulness of QTc interval on the discharge electrocardiogram for predicting survival after acute myocardial infarction.

The corrected QT (QTc) interval was measured on the discharge electrocardiogram of 457 consecutive patients who had survived the first 28 days after a first acute myocardial infarction (AMI). The patients were followed for 4 years. The QTc interval was not related to long-term survival after the acute phase of AMI. Sixteen percent of the patients had a QTc interval above the normal upper limit of 440 ms. Of them, 71% survived 4 years and 77% with a shorter QTc interval survived (p = 0.31). When mortality per 100 patient-years was calculated for different QTc intervals, with 10 ms accuracy, no consistent relation between the 2 variables was seen. Results that indicate a strong relation between QTc-interval prolongation and sudden death after AMI should be reevaluated. The QTc interval is not a useful prognostic tool after AMI.

Localization of accessory pathways in Wolff-Parkinson-White syndrome by high-resolution magnetocardiographic mapping

Fifteen patients with Wolff-Parkinson-White syndrome were studied with standard 12-lead electrocardiogram, invasive electrophysiologic study, and high-resolution magnetocardiographic (MCG) mapping. In addition, intraoperative epicardial mapping was performed in seven surgically treated patients. The MCG characteristics of ventricular preexcitation for different locations of the atrioventricular accessory pathways were described in terms of morphology and field patterns. Three mathematical source models in semi-infinite conducting space were used for localization computations: the current dipole model, the truncated current multipole model and the magnetic dipole model. Finally, the localization results of MCG and invasive mappings and electrocardiograms were compared. The mean three-dimensional distance between the localization results obtained from MCG maps and electrophysiologic study was 3.9 cm for the magnetic dipole model, 4.8 cm for the truncated current multipole model, and 7.3 cm for the current dipole model. The corresponding distances in the seven intraoperatively mapped cases were 2.3 cm for the magnetic dipole model, 5.2 cm for the truncated current multipole model, and 6.3 cm for the current dipole model. In conclusion, noninvasive MCG mapping may significantly contribute to the invasive catheter mapping for optimal preoperative localization of preexcitation site and atrioventricular accessory pathways in Wolff-Parkinson-White syndrome.

The mortality predictive power of discharge electrocardiogram after first acute myocardial infarction

The prognostic value of discharge ECG was studied in 457 patients after their first acute myocardial infarction. Thirteen different ECG variables were studied on the discharge ECG. When cumulative 4-year survival rates were calculated by standard life-table method for each variable individually, the following variables had statistically significant prognostic power: PTF (P terminal force), PTFA (P terminal frontal axis), AF (atrial fibrillation), ST depression, ST elevation, QRS duration, and the combination block (LBBB/RBBB + LAHB/LPHB). The variables with no statistically significant predictive power were: QTc, LBBB or RBBB, LAHB or LPHB, AV block, T wave angle, T negativity, and sigma R. The relative risks for the most important variables in the discrete life-table model were: PTF 3.4, QRS duration 3.3, ST depression 2.6, PTFA 2.5, and ST elevation 2.2. In further analysis a model with only three ECG variables (PTF, ST depression, and ST elevation) was developed which stratified the study population in categories with 1.9% to 75.5% estimated 4-year survival rates.

Magnetic Measurement of Cardiac Volume Changes

Magnetic susceptibility plethysmography (MSPG) is a non-invasive technique for detecting changes in the total cardiac volume. The MSPG signal is observed with a sensitive magnetometer when an external magnetic field is applied to the thorax. The signal is due to the differences in magnetic susceptibilities of intracardiac blood, heart muscle, and surrounding tissues. In this work the spatial distribution of the MSPG signal across the chest of normal subjects is presented. The ventricular ejection of blood was clearly exhibited in signals measured above the heart. Relatively large MSPG signals were also observed on the upper and lower thorax during the ventricular ejection, related to the blood flow in the aortic arch and in the abdominal aorta, respectively tively. The temporal behavior of the MSPG is compared with the results obtained from existing clinical methods. The origin of the MSPG signal is analyzed theoretically. When a fixed cardiac magnetic dipole model was used, overall change in cardiac volume as a function of time could be evaluated with reasonable accuracy when the proximity effect of the torso was taken into account.

Analysis of High Resolution MCG Recordings of Patients with Ventricular Tachycardia

Selection of patients prone to sustained ventricular tachycardia (VT), by detecting abnormal cardiac electric micropotentials called late potentials (LP), from high resolution (HR) ECG recordings has recently become to clinical practice. Observation of magnetic late field (LF) signals, corresponcling to electric LP:s, was first reported by Erne et al. in 1983.1 Since then only few studies have been published on late fields.

Magnetic measurements of cardiac mechanical activity

The origin of the magnetic susceptibility signal caused by mechanical activity of the heart is studied. Magnetic signals were measured with a second order SQUID (superconductor quantum interface device) gradiometer by applying a homogeneous external magnetic field with magnitude up to 260 A/m. Three experimental methods were used to separate the different components in the magnetic susceptibility signal. First, the thorax of the subject was surrounded by water solution of MnCl/sub 2/ with various magnetic susceptibilities, then a plastic bag containing a similar solution was placed on the chest of the subject, and finally the chest wall of the subject was immobilized during the measurements by using a special vacuum casting. The results suggest that usually about 65-80% of the signal measured near the heart originates directly from cardiac volume changes. This cardiac plethysmogram can be separated easily by immobilizing the subjects chest or by surrounding the chest with the liquid, whose magnetic susceptibility is about -5*10/sup -6/.<<ETX>>

High resolution electrocardiography and magnetocardiography: clinical application

A high-resolution (HR) surface ECG system is described, suitable for HR real-time (RT) recording and for signal averaging (SA). HR-ECG measurements were done in an electrically shielded room. The signal was amplified with a battery-driven 8-channel preamplifier, digitized with a 12(16)-bit A/D converter and stored in a microcomputer. The overall noise level of the system was below 1 µ Vp-p in the band 0.05–300 Hz. RT- and SA-magnetocardiographic (MCG) measurements were done in a magnetically shielded room using a very sensitive (5 fT/√VHz) DC-SQUID magnetic gradiometer. The output signal was transferred to a minicomputer for analysis. Examples of ECG and MCG recordings of cardiac micropotentials are presented, using both RT- and SA-techniques with special reference to late potentials (LP). The MCG technique was also utilized for localizing the Kent bundle in 6 cases of WPW syndrome. The recovery rate of LP was 37% in patients with recent myocardial infarction (AMI), and 72% in patients with ventricular tachycardia (VT) or ventricular fibrillation (VF). The mean duration of LP in the VT/VF-group was 27 ms, and the mean amplitude 17 µV. The His-Purkinje signal was detected in 60% of cases examined. The RT-ECG proved to be superior to the SA-ECG in detecting intermittent or inconstantly timed signals. Preliminary base-line data of 2 prospective studies are presented (10 patients of which with VT, 10 with recent AMI without VT and 10 healthy controls) using both RT-ECG and SA-ECG, the latter with both time and frequency domain analysis. The initial and terminal QRS notches of the low-gain ECG were most frequent in the VT group, and probably belong to the same category of depolarization abnormalities as LPs.

Comparison between measured and simulated magnetocardiograms in a case of anterolateral myocardial infarction

SummaryThe MCGs measured in a case of an anterolateral myocardial infarction were compared with the corresponding simulated MCGs. The most distinctive changes in the measured MCGs were observed in the upper and lower parts of the thorax as predicted by the simulation.RiassuntoSi confrontano i magnetocardiogrammi misurati in un caso di infarto miocardico anterolaterale con i corrispondenti magnetocardiogrammi simulati. I cambiamenti piú notevoli nell’MCG misurato si sono osservati nelle parti superiori e inferiori del torace come previsto dalla simulazione.