Laura Burattini

Beat-to-Beat Repolarization Variability in LQTS Patients with the SCN5A Sodium Channel Gene Mutation

Current techniques evaluating beat‐to‐beat variability of repolarization rely on accurate determination of T wave endpoints. This study proposes a T wave endpoint‐independent method to quantify repolarization variability in a standard 12‐lead ECG using a wavelet transformation. Our method was used to identify repolarization variability in long QT syndrome patients (LQTS) with the SCN5A sodium channel gene mutation. Using wavelet transformations based on the second Gaussian derivative, we evaluated repolarization variability in 11 LQTS patients with the mutation, 13 noncarrier family members, and 28 unrelated healthy subjects. Time‐domain repolarization variability parameters (SDRTo, SDRTm) and wavelet parameters describing temporal (beat‐to‐beat) variability of repolarization in time (TVT) and in amplitude (TVA) were analyzed. Reproducibility of wavelet parameters and relationship of wavelet‐based variability with heart rate and preceding RR interval were investigated. The wavelet‐based method quantified beat‐to‐beat variability of the entire repolarization segment (regardless of QT interval identification) providing insight into variability in repolarization morphology. Our method showed that SCN5A carriers have significantly increased repolarization variability in amplitude (23%± 14% vs 8 ± 4%, P < 0.001) and in time (14 ± 17 ms vs 3 ± 2 ms, P < 0.004) compared to noncarriers. Variability of repolarization amplitude was found to be heart rate dependent with variability decreasing with increasing heart rate. Relative error describing reproducibility of TVA and TVT was ± 5% and ± 10%, respectively. Our method quantifies repolarization variability in amplitude and in time without the need to identify T or U wave endpoints. Wavelet‐detected repolarization variability contributes to phenotypic identification of SCN5A carriers, with more pronounced beat‐to‐beat variability in repolarization amplitude than in time.

Adaptive Match Filter Based Method for Time vs. Amplitude Characterization of Microvolt ECG T-Wave Alternans

To develop a new method for non-invasive identification of patients prone to ventricular tachyarrhythmia and sudden cardiac death, an adaptive match-filter (AMF) was applied to detect and characterize T-wave alternans (TWA) in 200 coronary artery diseased (CAD) patients compared with 176 healthy (H) subjects. TWA was characterized in terms of duration (TWAD), amplitude (TWAA), and magnitude (TWAM, defined as the product of TWAD times TWAA). A criterion derived from these parameters, estimated over the H-population, allowed discrimination between a risk (TWA+) and a normality (NO TWA) zone in the TWAD-TWAA plane. To gain further ability to discriminate among different risk levels, the TWA+ zone was divided into four sub-zones respectively characterized by low duration and low amplitude (LDLA), low duration and high amplitude (LDHA), high duration and low amplitude (HDLA), and high duration and high amplitude (HDHA). With our methodology, 21 CAD-patients (10.5%) were identified as TWA+, 9 falling in the LDLA zone, 4 in the HDLA, 7 in the LDHA, and 1 in the HDHA. These results are in agreement with clinical expectations and pave the way to further clinical follow-up studies finalized to analyze pathophysiological implications and risk factors associated to each TWA+ zone.

Comparative analysis of methods for automatic detection and quantification of microvolt T-wave alternans

Microvolt T-wave alternans (TWA), consisting of every-other-beat changes in ECG T-wave morphology, is an index of susceptibility to malignant ventricular arrhythmias, requiring automatic techniques to be identified. Five of these, namely, fast-Fourier-transform spectral method (FFTSM), complex-demodulation method (CDM), modified-moving-average method (MMAM), Laplacian-likelihood-ratio method (LLRM) and adaptive-match-filter method (AMFM), were applied here to simulated and sample clinical data. The aim was to compare individual methods ability to properly identify stationary and time-varying TWA, avoiding false-positive detections. The MMAM provided false-positive TWA when applied to simulated ECGs affected by amplitude variability, but TWA. Stationary TWA was properly quantified by the MMAM and, occasionally, underestimated by all other methods. The AMFM properly identified time-varying TWA. By contrast, the FFTSM detected not-stationary TWA as stationary, the MMAM introduced a time-delay in the estimated TWA-amplitude signal, while the CDM and LLRM were reliable only in the presence of slow-varying TWA. Altogether, the AMFM accomplished the best compromise between the needs to avoid false-positive TWA and to detect and characterize true-positive TWA. Results of our simulation approach were useful to explain different TWA levels measured by each competing methods applied to sample Holter ECGs from healthy subjects and coronary artery disease patients.

Gender differences in the myoelectric activity of lower limb muscles in young healthy subjects during walking

Abstract The present study was designed to achieve a comprehensive analysis of gender-related differences in the myoelectric activity of lower limb muscles during normal walking at self-selected speed and cadence, in terms of muscle activation patterns and occurrence frequencies. To this aim, statistical gait analysis (SGA) of surface EMG signal from tibialis anterior (TA), gastrocnemius lateralis (GL), rectus femoris (RF), biceps femoris (BF) and vastus lateralis (VL) was performed in 11 female (F-group) and 11 male (M-group) age-matched healthy young adults. SGA is a recent methodology performing a statistical characterization of gait, by averaging spatio-temporal and sEMG-based parameters over numerous strides. Findings showed that males and females walk at the same comfortable speed, despite the significantly lower height and higher cadence detected in females. No significant differences in muscle onset/offset were detected between groups. The analysis of occurrence frequencies of muscle activity showed no significant differences in BF and RF, between groups. Conversely, in F-group, compared with M-group, GL, TA and VL showed a significantly higher occurrence frequency in the modalities with a high number of activations, and a significantly lower occurrence frequency in the modalities with a low number of activations. These findings indicate a propensity of females for a more complex recruitment of TA, GL and VL during walking, compared to males. The observed differences recommend the suitability of developing electromyographic databases, separated for males and females.

Assessment of Physiological Amplitude, Duration, and Magnitude of ECG T-Wave Alternans

Background: An association between T‐wave alternans (TWA) and malignant ventricular arrhythmias is generally recognized. Because relatively low levels of TWA have also been observed in healthy (H) subjects, the question arises as to whether these are ascribable to noise and artifacts, or can be given the relevance of a physiological phenomenon characterizing a preclinical condition.

Automatic microvolt T-wave alternans identification in relation to ECG interferences surviving preprocessing

The aim was to investigate the effect of interferences surviving preprocessing (residual noise, baseline wanderings, respiration modulation, replaced beats, missed beats and T-waves misalignment) on automatic identification of T-wave alternans (TWA), an ECG index of risk for sudden cardiac death. The procedures denominated fast-Fourier-transform spectral method (FFTSM), complex-demodulation method (CDM), modified-moving-average method (MMAM), Laplacian-likelihood-ratio method (LLRM), and adaptive-match-filter method (AMFM) were applied to interferences-corrupted synthetic ECG tracings and Holter ECG recordings from control-healthy subjects (CH-group; n=25) and acute-myocardial-infarction patients (AMI group; n=25). The presence of interferences in simulated data caused detection of false-positive TWA by all techniques but the FFTSM and AMFM. Clinical applications evidenced a discrepancy in that the FFTSM and LLRM detected no more than one TWA case in each population, whereas the CDM, MMAM, and AMFM detected TWA in all CH-subjects and AMI-patients, with significantly lower TWA amplitude in the former group. Because the AMFM is not prone to false-positive TWA detections, the latter finding suggests TWA as a phenomenon having continuously changing amplitude from physiological to pathological conditions. Only occasional detection of TWA by the FFTSM and LLRM in clinics can be ascribed to their limited ability in identifying TWA in the presence of interferences surviving preprocessing.

Noninvasive fetal electrocardiography: an overview of the signal electrophysiological meaning, recording procedures, and processing techniques.

Noninvasive fetal electrocardiography (fECG), obtained positioning electrodes on the maternal abdomen, is important in safeguarding the life and the health of the unborn child. This study aims to provide a review of the state of the art of fECG, and includes a description of the parameters useful for fetus clinical evaluation; of the fECG recording procedures; and of the techniques to extract the fECG signal from the abdominal recordings.

The occurrence frequency: a suitable parameter for the evaluation of the myoelectric activity during walking

Many studies have recently addressed the quantification of the natural variability of myoelectric activity during walking, considering hundreds of strides. The availability of so many strides allows assessing a parameter seldom considered in classic surface EMG (sEMG) studies: the occurrence frequency, defined as the frequency each muscle activation occurs with, quantified by the number of strides in which a muscle is recruited with that specific activation modality. Aim of this study is to point out the occurrence frequency as a suitable parameter for the evaluation of the variability of the myoelectric activity during walking. This goal was pursued by means of the statistical gait analysis of sEMG signal acquired from Gastrocnemius Lateralis (GL) in six healthy subjects, with different characteristics. Results show that among these six subjects relevant differences were not detected in the temporal parameters, i.e., activation onset/offset instant and activation duration. In the same subjects, the values of the occurrence frequency ranged from 3% to 74% in the different activation modalities, indicating a large variability of this parameter. These findings show that occurrence frequency is able to provide further and different information with respect to classical temporal parameters. Thus, the occurrence frequency is proposed as a suitable parameter to support the classic temporal parameters in the evaluation of variability of myoelectric activity during walking.

Normative EMG patterns of ankle muscle co-contractions in school-age children during gait

PURPOSE The study was designed to assess the co-contractions of tibialis anterior (TA) and gastrocnemius lateralis (GL) in healthy school-age children during gait at self-selected speed and cadence, in terms of variability of onset-offset muscular activation and occurrence frequency. METHODS Statistical gait analysis, a recent methodology performing a statistical characterization of gait by averaging spatio-temporal and sEMG-based parameters over numerous strides, was performed in 100 healthy children, aged 6-11 years. Co-contractions were assessed as the period of overlap between activation intervals of TA and GL. RESULTS On average, 165±27 strides were analyzed for each child, resulting in approximately 16,500 strides. Results showed that GL and TA act as pure agonist/antagonists for ankle plantar/dorsiflexion (no co-contractions) in only 19.2±10.4% of strides. In the remaining strides, statistically significant (p<0.05) co-contractions appear in early stance (46.5±23.0% of the strides), mid-stance (28.8±15.9%), pre-swing (15.2±9.2%), and swing (73.2±22.6%). This significantly increased complexity in muscle recruitment strategy beyond the activation as pure ankle plantar/dorsiflexors, suggests that in healthy children co-contractions are likely functional to further physiological tasks as balance improvement and control of joint stability. CONCLUSIONS This study represents the first attempt for the development in healthy children of a normative dataset for GL/TA co-contractions during gait, achieved on an exceptionally large number of strides in every child and in total. The present reference frame could be useful for discriminating physiological and pathological behavior in children and for designing more focused studies on the maturation of gait.

Repolarization alternans heterogeneity in healthy subjects and acute myocardial infarction patients

An association between heterogeneity of repolarization alternans (RA) and cardiac electrical instability has been reported. Characterization of RA in health and identification of physiological RA heterogeneity may help discrimination of abnormal RA cases more likely associated to arrhythmic events. Thus, aim of the present study was the identification of a physiological RA region in terms of mean temporal location (MRAD) with respect to the T apex, and mean amplitude (MRAA), by application of our heart-rate adaptive match filter method to clinical ECG recordings from 51 control healthy (CH) subjects and 43 acute myocardial infarction (AMI) patients. Results indicate that RA occurring within the first half of the T wave is dominant in both CH and AMI populations (74.5% and 53.5% of cases, respectively; P<0.05). Definition of physiological RA region in the MRAD vs. MRAA plane (-83 ms ≤ MRAD ≤ 23 ms, 0≤ MRAA ≤ 30 μV) provided 0% and 32.6% abnormal RA cases among the CH subjects and AMI patients, respectively. We conclude that myocardial infarction may associate with an RA occurring early (MRAD<-83 ms) or late (MRAD >23 ms) along the JT segment, in addition or in alternative to an abnormally high RA amplitude (MRAA >30 μV).