Magnetocardiographic functional localization using current multipole models.

High-resolution magnetocardiographic mapping was applied to localize the ventricular preexcitation site in ten patients suffering form Wolff-Parkinson-White syndrome. Three different source models were tested, consisting of the dipole and quadrupole moments in a general multipole expansion. Noninvasive localizations were performed by computations based on measured magnetic maps without a priori assumptions of the source location and without imposing any constraints. In all cases, the computed results were compared with invasive localization results obtained by catheter mapping technique. Preoperative catheterization localizes the atrial end of the accessory pathway, while our method localizes the ventricular preexcitation site. Of the models used, the average three-dimensional difference between the invasive localization results and the HR-MCG results was smallest 2.9 cm for the source model consisting of the magnetic dipole. The preexcitation site was very deep in all cases. The current dipole alone was inaccurate in estimating the source depth, but inclusion of the quadrupole moments improved the results. Two of the patients underwent surgery to interrupt the accessory pathway, which provided further validation for the noninvasive localizations.

Neuropsychologic and cardiovascular effects of clemastine fumarate under pressure.

Allergic rhinitis and mild respiratory infections have been widely accepted as temporary contraindications for fitness to dive. Nonetheless, several sport and professional divers use antihistamines to ease ear, nose, and throat (ENT) problems, especially for opening tubal ostium. Some divers know they are unfit to dive, but for a variety of reasons (e.g., money or short holiday) they try to clear their ears. Thus, the use of antihistaminic drugs (like clemastine fumarate) is common during diving. This double-blind, crossover study indicates that this special antihistamine does not increase the sedative effects of nitrogen narcosis, nor does it increase the level of cardiac arrhythmias. Liberal use of antihistamines while diving cannot be recommended because of possible complications connected with different preparations and the temporary limitations they impose on the diver.

Sensitivity limits in biomagnetic measurements.

It is important to understand the character and the contribution of thermal magnetic noise in designing the measurement site and the instrumentation for biomagnetic measurements. The ultimate limit of the sensitivity is the thermal noise due to the object under study. In the case of the human body, it has been estimated to be about 0.1/square root of Hz. Magnetically shielded rooms are necessary for ultrasensitive biomagnetic measurements of human subject, but they also generate external noise which in some cases may become detectable. This noise problem can be avoided if the innermost walls are constructed of magnetically soft ferromagnetic material. Close to the conducting walls the thermal noise is higher than at the centre. Thus, the shielded room should be relatively large in size. The gantry and other things inside the room may contain metal parts, which can cause excess noise. The intensity depends on the conductivity, geometry, location and movement of these parts. In comparison to bioelectric studies, this inductive noise coupling demands extra attention. In most biomagnetic measurements performed inside a magnetically shielded room, the limiting factor of the sensitivity is the thermal noise caused by electrically conducting thermal shielding used inside the cryogenic measurement dewar. Fortunately, it is possible to reduce the noise contribution arising from the superinsulation in the dewar by careful design. The properties and dynamics of the SQUIDs are well understood nowadays. Studies of the nonlinear character in the coupling between the SQUID and external detection coil have made it possible to reduce the noise contribution of the sensor itself.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of thrombolytic therapy on the high-resolution electrocardiogram after myocardial infarction.

The effects of thrombolytic treatment was studied in 109 consecutive patients 9-11 days after their first acute myocardial infarction by high-resolution electrocardiography (ECG), 24 h Holter monitoring, exercise test and radionuclide ventriculography. Thirty-seven patients were treated with intravenous thrombolytic agents. Thrombolytic treatment was assessed by clinical criteria to be successful in 22 patients and probably successful in 12 patients. Thrombolysis failed in three patients and 72 patients did not receive thrombolytic treatment (control group). Measurements made on the high-resolution and filtered (60 Hz high-pass) vectormagnitude complex included the total duration, the duration of the potential less than 40 microV, the root mean square (RMS) voltage in 10 ms intervals over the first 50 ms and RMS voltage of the last 40, 50 and 60 ms. The filtered QRS duration was significantly shorter in reperfused patients compared with the control group (83 +/- 10 vs 89 +/- 12 ms; P = 0.017). In inferior infarcts (n = 57) the filtered QRS duration was 83 +/- 11 ms in reperfused and 89 +/- 10 ms in non-reperfused patients (P = 0.044), but in anterior infarcts (n = 52) there was no difference. The RMS voltage of the initial 50 ms of the QRS was higher in the reperfused than in non-reperfused anteroseptal infarcts (38 +/- 14 v 23 +/- 10 microV; P = 0.022). Patients successfully treated with thrombolytic agents within the first 2 h had higher RMS voltage of the terminal 40 ms of the QRS than patients treated within 2-4 h (38 +/- 17 vs 27 +/- 17 microV; P = 0.03).(ABSTRACT TRUNCATED AT 250 WORDS)

High-resolution and signal-averaged electrocardiography to separate post-myocardial infarction patients with and without ventricular tachycardia.

High-resolution and signal-averaged ECG, 24 h Holter recording and ejection fraction were used to separate post-myocardial infarction patients with and without ventricular tachycardia (VT) among 150 individuals: 26 patients with an old myocardial infarction and documented sustained VT, 104 patients with an acute myocardial infarction without sustained VT, who were followed-up for 2 years, and 20 healthy volunteers. Bipolar orthogonal XYZ leads were recorded, high-pass filtered at cut-off frequencies of 25, 40, 60, 80 and 100 Hz, and combined to vector magnitude square root of X2 + Y2 + Z2. The filtered QRS duration, the root-mean-square voltages of different time intervals and the durations of low amplitude signals under different thresholds, both from the initial and terminal QRS, were calculated. The sensitivity and specificity of each parameter alone and in every combination of two, three and four parameters (17 million different combinations) were computed both from non-averaged and averaged data. The best separation was achieved by 12 combinations all including four signal-averaged ECG parameters, with a sensitivity of 81% and a specificity of 79%. The parameters represented most were: filtered QRS duration at 25 Hz, RMS voltage of the last 50 ms at 25 Hz, terminal LAS duration at 80 Hz, and RMS voltage of the last 20 ms at 80 Hz. Parameters of the initial QRS complex did not improve either the sensitivity or the specificity of the method. In logistic regression analysis, the best combinations of four signal-averaged ECG parameters separated VT patients better (P < 0.001) than non-sustained ventricular tachycardia at Holter (P = 0.001); left ventricular ejection fraction (P = 0.01); or age (P = 0.006). Parameters calculated from averaged data gave better results than parameters calculated from non-averaged data.

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.

Magnetocardiographic localization of ventricular pre-excitation in a child with a congenital heart defect.

Magnetocardiographic mapping was performed on a 2-year-old boy who suffered from the Wolff-Parkinson-White syndrome in association with a complex congenital heart defect. The pre-excitation site was determined noninvasively from the measured cardiac magnetic fields. The location was in the same anatomic region as found by intraoperative epicardial mapping. This result shows that magnetocardiography can be helpful for determining an accessory pathway also in patients with grossly abnormal cardiac anatomy.

Magnetocardiographic localisation and modelling.

In our magnetocardiographic (MCG) localisation studies, two modelling approaches have been applied: (a) modelling the sources with dipole and quadrupole moments in a general multipole expansion and using a homogeneous, semi-infinite volume conductor, and (b) using a single current dipole source in a homogeneous, realistically shaped torso. Both approaches have been successfully applied in localising the premature ventricular excitation site in patients suffering from the Wolff-Parkinson-White syndrome. In addition, we have participated in developing a model of propagation of electrical activation in the ventricles. Anisotropic conductivity properties and spiral arrangement of myocardial fibres are included in the model.

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.

Non-invasive magnetocardiographic localization of ventricular pre-excitation in the Wolff-Parkinson-White syndrome using a realistic torso model.

This study was performed to evaluate the accuracy of magnetocardiography in non-invasive localization of the ventricular pre-excitation site in patients suffering from the Wolff-Parkinson-White (WPW) syndrome. Twelve WPW patients were studied, in whom the pre-excitation caused serious supraventricular arrhythmias refractory to drug therapy. Magnetocardiographic measurements were performed in a magnetically shielded room, and non-invasive localization was computed from preprocessed magnetic signals using a current dipole source in a realistically shaped digital torso. All patients underwent intra-operative multicatheter mapping and subsequent dissection of the accessory atrioventricular connection. The intra-operative localization results were marked on magnetic resonance images of the heart, where magnetocardiographic results were also superimposed to allow comparison. The average of the three-dimensional differences between the magnetocardiographic and the invasive results was 2.1 +/- 0.9 cm. In all cases, the computed localization result was in the same or adjacent anatomical region as the intra-operative result. The present results show that the magnetocardiographic method using a realistic torso model is capable of localizing pre-excitation sites with sufficient accuracy to provide extra information so that non-pharmacological therapeutic interventions can be applied.