The application of subspace preconditioned LSQR algorithm for solving the electrocardiography inverse problem

Regularization is an effective method for the solution of ill-posed ECG inverse problems, such as computing epicardial potentials from body surface potentials. The aim of this work was to explore more robust regularization-based solutions through the application of subspace preconditioned LSQR (SP-LSQR) to the study of model-based ECG inverse problems. Here, we presented three different subspace splitting methods, i.e., SVD, wavelet transform and cosine transform schemes, to the design of the preconditioners for ill-posed problems, and to evaluate the performance of algorithms using a realistic heart-torso model simulation protocol. The results demonstrated that when compared with the LSQR, LSQR-Tik and Tik-LSQR method, the SP-LSQR produced higher efficiency and reconstructed more accurate epcicardial potential distributions. Amongst the three applied subspace splitting schemes, the SVD-based preconditioner yielded the best convergence rate and outperformed the other two in seeking the inverse solutions. Moreover, when optimized by the genetic algorithms (GA), the performances of SP-LSQR method were enhanced. The results from this investigation suggested that the SP-LSQR was a useful regularization technique for cardiac inverse problems.     

Wavelet analysis of high-resolution signal-averaged ECGs in postinfarction patients

The authors present an original method for the discrimination of patients prone to ventricular tachycardia. The wavelet transform, which is a new time-scale technique suitable for transient signal detection, was applied to bipolar unfiltered X, Y, Z signal-averaged electrocardiograms in 20 postinfarction patients with sustained ventricular tachycardia, in 20 myocardial infarction patients without ventricular tachycardia, and in 10 healthy subjects. An improved automated algorithm for the detection and localization of sharp variations of the signal, based on coherent detection of the local maxima of the wavelet transform, was developed. A risk stratification method, based on the detection of at least one singularity at or after a point defined with reference to the QRS onset, was assessed. The optimum cutoff point, found 98 ms after the onset of QRS, provides a specificity of 90% and a sensitivity of 85%. The authors conclude that wavelet analysis makes it possible, in this group of patients, to discriminate those with ventricular tachycardia. It yields better results than those obtained from the conventional time-domain approach.     

Asymptomatic hyperglycaemia is associated with increased intimal plus medial thickness of the carotid artery

Summary Atherosclerotic changes have not been demonstrated directly in asymptomatic hyperglycaemic non-diabetic subjects, although high mortality due to coronary heart disease has been reported. We measured arterial wall thickness non-invasively, in order to directly demonstrate atherosclerosis of the carotid arteries of hyperglycaemic non-diabetic subjects and to evaluate its risk factors.The thicknesses of the intimal plus medial complex (IMT) of the carotid arteries of 112 asymptomatic hyperglycaemic non-diabetic subjects (aged 22–81, 95 males and 17 females) were compared with those of 55 healthy male subjects and 211 non-insulin-dependent NIDDM male diabetic patients. The subjects were subgrouped into impaired glucose-tolerant (IGT) subjects who had a 2-h glycaemic level of more than 7.8 mmol/l, and non-IGT subjects whose 2-h glycaemic levels were within 6.7–7.7 mmol/l.Non-IGT and IGT subjects showed significantly greater IMTs than age-matched healthy males and showed no significant differences compared to age-matched NIDDM patients. Multivariate analysis demonstrated that the risk factors for IMT of non-IGT and IGT subjects were age and systolic blood pressure. According to data on the accumulation of atherogenic risks (hypertension, dyslipidaemia, and smoking), IMT increased linearly in non-IGT and IGT subjects. However, non-IGT and IGT subjects without hyperlipidaemia, hypertension, or smoking risk still had significantly greater IMT than age-matched normal males (1.019±0.063 vs 0.770±0.111 mm, p<0.05). Prevalence of ECG-indicated coronary heart disease was significantly higher in hyperglycaemic non-diabetic subjects and NIDDM with increased carotid arterial wall thickness (IMT 1.1 mm) than in those without increased thickness (IMT<1.1 mm). Asymptomatic hyperglycaemic non-diabetic subjects have increased thickness of their carotid arteries compared to age-matched male NIDDM patients. As one of several independent risk factors, mild hyperglycaemia advances atherosclerosis, which leads to coronary heart disease.Abbreviations IMT Intimal plus medial complex - NIDDM non-insulin-dependent diabetes mellitus - IGT impaired glucose tolerance - CHD coronary heart disease - T-Chol serum total cholesterol - HDL-C high-density lipoprotein cholesterol - TG serum triglycerides     

ST-recovery loop of exercise-induced ST deviation in the identification of coronary artery disease: which parameters should we measure?

This study aimed to characterize the ST-recovery loop and assess which range of heart rates (HRs) best discriminates between patients with and without significant coronary artery stenosis. Bicycle exercise tests were undertaken in 44 men and 18 women with coronary artery disease (CAD) and in 59 controls (26 men, 33 women) in the same age range with no signs of CAD. The ST level and the ST-segment slope were continuously monitored, and changes from rest to peak exercise and to 4 min after exercise, respectively, were calculated. Plotting the ST level against HR gives the STHR loop, characterized by the normalized area (NA(alpha)) circumscribed by the ST level during and after exercise from alpha% to 100% of the HR range. Eight values of alpha between 20% and 90% were investigated, and chest and extremity leads were investigated separately. Optimal alpha was found to be < or =70% in men and < or =30% in women. Change in ST-segment slope was the only parameter that gave significant additional discriminating power in both men and women once the area had been taken into account. We conclude that NA(alpha) for extremity and chest leads have similar weights, and that a substantial part of the STHR loop should be taken into consideration, especially in women. NA(30) was superior to end-exercise ST-depression and STHR loop orientation (as defined by the sign of NA(90)) in both men and women, and to ST/HR index in men, in identifying CAD.     

Late QRS Activity in Signal‐Averaged Magnetocardiography,Body Surface Potential Mapping,and Orthogonal ECG in Postinfarction Ventricular Tachycardia Patients

Background: Delayed electrical activity necessary for re‐entrant ventricular tachycardia (VT) is detectable noninvasively with high resolution techniques. We compared high resolution signalaveraged analysis of magnetocardiography (MCG), body surface potential mapping (BSPM), and orthogonal three‐lead ECG (SA‐ECG) in the identification of patients prone to VT after myocardial infarction (Ml). Methods: Patients with remote myocardial infarction and cardiac dysfunction were studied, 22 with (VT group) and 22 without VT (control group). MCG with seven channels and BSPM with 63 and SA‐ECG with three orthogonal leads were registered. After signal‐averaging and highpass filtering, three time domain analysis (TDA) parameters describing late electrical activity were computed: QRS duration (QRSd), root mean square amplitude (RMS) of the last 40 ms of QRS, and the duration of the low‐amplitude QRS end (LAS). Results: All parameters by each method were significantly different between the patients’groups. For example, LAS parameter in MCG was 59 (SD 22) ms in the VT group vs. 37 (SD 13) ms in controls (P < 0.001), 77 (SD 22) ms vs. 56 (SD 19) ms in BSPM (P = 0.002), and 60 (SD 24) ms vs. 39 (SD 22) ms in SA‐ECG (P = 0.005). The combination of LAS parameter in MCG and SA‐ECG resulted in improved performance in comparison to any single parameter with 95% sensitivity and 68% specificity. Conclusions: All three high resolution methods identified VT propensity among post‐Mi patients with cardiac dysfunction and between‐method differences were small. Information in MCG and SA‐ECG may be complementary and their combination could be of value in postinfarction arrhythmia risk assessment. A.N.E. 2002;7(4):389–398     

Amplitude of the Electrocardiographic QRS Complexes during and after Severe Pulmonary Edema

There is recent evidence that anasarca peripheral edema, irrespective of its etiology, attenuates ECG QRS potentials. Pulmonary edema (PE) also is thought to cause reduction in the amplitude of QRS complexes. The case reported herein is of a patient with severe PE, hypertension, left ventricular diastolic dysfunction, and no peripheral edema who did not show changes in the QRS complexes with the management of her pulmonary edema. Thus it appears that PE does not attenuate the amplitude of QRS complexes, and alleviation of this condition does not cause augmentation of QRS voltage. This is in contrast to alterations of peripheral edematous states in the setting of congestive heart failure, which result in changes in the QRS amplitude, as shown previously.     

Effects of tilt and exercise on signal-averaged electrocardiogram after acute myocardial infarction

To determine whether enhanced sympathetic activity could altera non-invasive index of cardiac instability, we analysed theeffects of 90° head-up tilt and submaximal exercise stresstest on high amplification signal-averaged electrocardiogramin 64 patients after acute myocardial infarction. At rest, ventricularlate potentials were detected in 25% of patients, characterizedby a significant prolongation of filtered QRS complex (137 ±3vs 115 ±2 ms) and of its components smaller than 40 fiV(38 ±2 vs 16 ±1 ms), as well as by a reduced rootmean square voltage calculated for the terminal 40 ms of QRScomplex (RMS40 voltage) (19 ± 1 vs 75 ± 9µV)in comparison to patients without micropotentials. Sympathetic activation induced by tilt caused a significantincrease in heart rate (from 67 ±3 to 79 ±3 beatsmin^–1) but did not modify either the incidence of ventricularlate potentials or the values of any of the signal-averagedelectrocardiogram parameters considered. In 19 patients, recordingswere also obtained during a submaximal bicycle exercise stresstest at a heart rate of 114 ±4 beats min^–1 andwith systolic arterial blood pressure at 153 ±6 mmHg.No effect on signal-averaged electrocardiogram parameters wasdetectable during this experimental intervention. These data indicate that after myocardial infarction, sympatheticactivation does not seem to modify signal-averaged electrocardiogramparameters.