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Objective: A large number of atrial fibrillation (AF) detectors have been published in recent years, signifying that the comparison of detector performance plays a central role, though not always consistent. The aim of this study is to shed needed light on aspects crucial to the evaluation of detection performance. Methods: Three types of AF detector, using either information on rhythm, rhythm and morphology, or segments of ECG samples, are implemented and studied on both real and simulated ECG signals. The properties of different performance measures are investigated, for example, in relation to dataset imbalance. Results: The results show that performance can differ considerably depending on the way detector output is compared to database annotations, i.e., beat-to-beat, segment-to-segment, or episode-to-episode comparison. Moreover, depending on the type of detector, the results substantiate that physiological and technical factors, e.g., changes in ECG morphology, rate of atrial premature beats, and noise level, can have a considerable influence on performance. Conclusion: The present study demonstrates overall strengths and weaknesses of different types of detector, highlights challenges in AF detection, and proposes five recommendations on how to handle data and characterize performance.

Objective: The clinical significance of QT interval adaptation to heart rate changes has been poorly investigated in atrial fibrillation (AF), since QT delineation in the presence of f-waves is challenging. Therefore, the objective of the present study is to investigate new techniques for QT adaptation estimation in permanent AF. Methods: A multilead strategy based on generalized periodic component analysis is proposed for QT delineation, involving a spatial, linear transformation which emphasizes Twave periodicity and attenuates f-waves. QT adaptation is modeled by a linear, time-invariant filter, whose impulse response describes the dependence between the current QT interval and the preceding RR intervals, followed by a memoryless, possibly nonlinear, function. The QT adaptation time lag is determined from the estimated impulse response. Results: Using simulated ECGs in permanent AF, the transformed lead was found to offer more accurate QT delineation and time lag estimation than did the original ECG leads for a wide range of f-wave amplitudes (the time lag estimation error was found to be -0.2+/-0.6 s for SNR = 12 dB). In a population with chronic heart failure and permanent AF, the time lag estimated from the transformed lead was found to have the strongest, statistically significant association with sudden cardiac death (SCD) (hazard ratio = 3.49), whereas none of the original, orthogonal leads had any such association. Conclusions: Periodic component analysis provides more accurate QT delineation and improves time lag estimation in AF. A prolonged adaptation time of the QT interval to heart rate changes is associated with a high risk for SCD. Significance: This study demonstrates that SCD risk markers, originally developed for sinus rhythm, can also be used in AF, provided that Twave periodicity is emphasized. The time lag is a potentially useful marker for identifying patients at high risk for SCD, guiding clinicians in adopting effective therapeutic decisions. IEEE

Objective The aim of this study is to compare the performance of two electrocardiogram (ECG) lead-space reduction (LSR) techniques in generating a transformed ECG lead from which T-wave morphology markers can be reliably derived to non-invasively monitor blood potassium concentration ( [ K + ] ) in end-stage renal disease (ESRD) patients undergoing hemodialysis (HD). These LSR techniques are: (1) principal component analysis (PCA), learned on the T wave, and (2) periodic component analysis ( π CA), either learned on the whole QRST complex ( π C B ) or on the T wave ( π C T ). We hypothesized π CA is less sensitive to non-periodic disturbances, like noise and body position changes (BPC), than PCA, thus leading to more reliable T wave morphology markers. Methods We compared the ability of T wave morphology markers obtained from PCA, π C B and π C T in tracking [ K + ] in an ESRD-HD dataset, including 29 patients, during and after HD (evaluated by correlation and residual fitting error analysis). We also studied their robustness to BPC using an annotated database, including 20 healthy individuals, as well as to different levels of noise using a simulation set-up (assessed by means of Mann–Whitney U test and relative error, respectively). Results The performance of both π C B and π C T -based markers in following [ K + ] -variations during HD was comparable, and superior to that from PCA-based markers. Moreover, π C T -based markers showed superior robustness against BPC and noise. Conclusion Both π C B and π C T outperform PCA in terms of monitoring [ K + ] in ESRD-HD patients, as well as of robustness against BPC and low SNR, with π C T showing the highest stability for continuous post-HD monitoring. Significance The usage of π CA (i) increases the accuracy in monitoring dynamic [ K + ] variations in ESRD-HD patients and (ii) reduces the sensitivity to BPC and noise in deriving T wave morphology markers.

We investigated the ability of time-warping-based ECG-derived markers of T-wave morphology changes in time ($$d_{w}$$ d w ) and amplitude ($$d_a$$ d a ), as well as their non-linear components ($${d_w^{{\mathrm{NL}}}}$$ d w NL and $${d_a^{\mathrm{NL}}}$$ d a NL ), and the heart rate corrected counterpart ($$d_{w,c}$$ d w , c ), to monitor potassium concentration ($$[K^{+}]$$ [ K + ] ) changes ($$\Delta [K^+]$$ Δ [ K + ] ) in end-stage renal disease (ESRD) patients undergoing hemodialysis (HD). We compared the performance of the proposed time-warping markers, together with other previously proposed $$[K^{+}]$$ [ K + ] markers, such as T-wave width ($$T_w$$ T w ) and T-wave slope-to-amplitude ratio ($$T_{S/A}$$ T S / A ), when computed from standard ECG leads as well as from principal component analysis (PCA)-based leads. 48-hour ECG recordings and a set of hourly-collected blood samples from 29 ESRD-HD patients were acquired. Values of $$d_w$$ d w , $$d_a$$ d a , $${d_w^{\mathrm{NL}}}$$ d w NL , $${d_a^{\mathrm{NL}}}$$ d a NL and $$d_{w,c}$$ d w , c were calculated by comparing the morphology of the mean warped T-waves (MWTWs) derived at each hour along the HD with that from a reference MWTW, measured at the end of the HD. From the same MWTWs $$T_w$$ T w and $$T_{S/A}$$ T S / A were also extracted. Similarly, $$\Delta [K^+]$$ Δ [ K + ] was calculated as the difference between the $$[K^{+}]$$ [ K + ] values at each hour and the $$[K^{+}]$$ [ K + ] reference level at the end of the HD session. We found that $$d_{w}$$ d w and $$d_{w,c}$$ d w , c showed higher correlation coefficients with $$\Delta [K^+]$$ Δ [ K + ] than $$T_{S/A}$$ T S / A —Spearman’s ($$\rho$$ ρ ) and Pearson’s (r)—and $$T_w$$ T w —Spearman’s ($$\rho$$ ρ )—in both SL and PCA approaches being the intra-patient median $$\rho \ge 0.82$$ ρ ≥ 0.82 and $$r \ge 0.87$$ r ≥ 0.87 in SL and $$\rho \ge 0.82$$ ρ ≥ 0.82 and $$r \ge 0.89$$ r ≥ 0.89 in PCA respectively. Our findings would point at $$d_{w}$$ d w and $$d_{w,c}$$ d w , c as the most suitable surrogate of $$\Delta [K^+]$$ Δ [ K + ] , suggesting that they could be potentially useful for non-invasive monitoring of ESRD-HD patients in hospital, as well as in ambulatory settings. Therefore, the tracking of T-wave morphology variations by means of time-warping analysis could improve continuous and remote $$[K^{+}]$$ [ K + ] monitoring of ESRD-HD patients and flagging risk of $$[K^{+}]$$ [ K + ] -related cardiovascular events.

Background: End-stage renal disease (ESRD) patients undergoing hemodyalisis therapy (HD) experience blood potassium ([K+]) variations that are reflected on the T-wave (TW) morphology. Methods: We evaluated the performance of different lead space reduction (LSR) methods: principal component analysis (PCA), maximising the TW energy, and two derived versions of periodic component analysis (πCA) named πCA B and πCA T , maximising the QRST or TW beat periodicity. We applied these methods to 12-lead electrocardiogram (ECG) from 24 ESRD-HD patients. Then, we derived three markers of TW morphology changes (d u w , d w and d ^ w,c ), comparing an average TW derived every 30 min with that at the HD end, from the PCA, πCA B and πCA T based leads having the highest TW energy content. Similarities between these three methods were assessed by using Bland-Altman plots and the linear fitting error (∊) evaluated from the 12 th to the 44 th h of ECG recordings after the HD onset. Results: All series of d u w , d w and d ^ w,c values showed good degree of mutual agreement (median bias ≤. 0.5 ms) and a small deviation from linearity in the [K+] increasing stage (median ∊ ≤ 3.3 ms). Conclusions: PCA andπCA can be used interchangeably to track TW changes in ESRD-HD patients, in this type of low noise contamination ECG recordings.

Purpose: Prolonged weightlessness exposure generates cardiovascular deconditioning, with potential implications on ECG circadian rhythms. Head-down (- 6°) tilt (HDT) bed rest is a ground-based analogue model for simulating the effects of reduced motor activity and fluids redistribution occurring during spaceflight. Our aim was to evaluate the impact of 60-day HDT on the circadianity of RR and ventricular repolarization (QTend) intervals extracted from 24-h Holter ECG recordings, scheduled 9 days before HDT (BDC-9), the 5th (HDT5), 21st (HDT21) and 58th (HDT58) day of HDT, the 1st (R + 0) and 8th (R + 7) day after HDT. Also, the effectiveness of a nutritional countermeasure (CM) in mitigating the HDT-related changes was tested. Methods: RR and QTend circadian rhythms were evaluated by Cosinor analysis, resulting in maximum and minimum values, MESOR (a rhythm-adjusted mean), oscillation amplitude (OA, half variation within a night-day cycle), and acrophase (?, the time at which the fitting sinusoid's amplitude is maximal) values. Results: RR and QTend MESOR increased at HDT5, and the OA was reduced along the HDT period, mainly due to the increase of the minima. At R + 0, QTend OA increased, particularly in the control group. The ? slightly anticipated during HDT and was delayed at R + 0. Conclusion: 60-Day HDT affects the characteristics of cardiac circadian rhythm by altering the physiological daily cycle of RR and QTend intervals. Scheduled day-night cycle and feeding time were maintained during the experiment, thus inferring the role of changes in the gravitational stimulus to determine these variations. The applied nutritional countermeasure did not show effectiveness in preventing such changes.

Body acceleration due to heartbeat-induced reaction forces can be measured as mobile phone accelerometer (m-ACC) signals. Our aim was to test the feasibility of using m-ACC to detect changes induced by stress by ultra-short heart rate variability (USV) indices (standard deviation of normal-to-normal interval&mdash, SDNN and root mean square of successive differences&mdash, RMSSD). Sixteen healthy volunteers were recruited, m-ACC was recorded while in supine position, during spontaneous breathing at rest conditions (REST) and during one minute of mental stress (MS) induced by arithmetic serial subtraction task, simultaneous with conventional electrocardiogram (ECG). Beat occurrences were extracted from both ECG and m-ACC and used to compute USV indices using 60, 30 and 10s durations, both for REST and MS. A feasibility of 93.8% in the beat-to-beat m-ACC heart rate series extraction was reached. In both ECG and m-ACC series, compared to REST, in MS the mean beat duration was reduced by 15% and RMSSD decreased by 38%. These results show that short term recordings (up to 10 s) of cardiac activity using smartphone&rsquo, s accelerometers are able to capture the decrease in parasympathetic tone, in agreement with the induced stimulus.

OBJECTIVE: To assess the effects of different durations of simulated microgravity exposure on ventricular repolarization (VR) in terms of T-wave alternans (TWA) as well as to test whether an increase in VR heterogeneity could be detected once normal gravity was restored. APPROACH: A total of 63 healthy volunteers were recruited in several head-down bed-rest (HDBR) experiments in the context of the European Space Agency bed-rest strategy. TWA is evaluated during the night period using ambulatory ECG recordings, before, during and after long- (60 d), mid- (21 d) and short- (5 d) duration HDBR by the long-term averaging technique. MAIN RESULTS: 5-21 d of exposure to simulated microgravity by means of the HDBR model do not lead to a significant increase of cardiac electrical instability in healthy myocardial substrates up to the point of eliciting TWA on the surface ECG. However, TWA indices increased after long-term HDBR exposure, once normal gravity was re-established, indicative of incipient electrical instability on VR at the conclusion of 60 d of HDBR. SIGNIFICANCE: The results of this work underline the importance of focusing future research on immediate effects after long-term microgravity exposure, both simulated by HDBR or from space mission scenarios, once partial gravity conditions are re-established. A deeper insight in the understanding of human body reactions in these scenarios results crucial in the design of future long-duration spaceflight missions, to mitigate any potential risk that can limit astronaut's performance.

Cardiac resynchronization therapy (CRT) is widely used in heart failure patients with left bundle branch block (LBBB). However, the high false-positive rates obtained with the conventional LBBB criteria limit the effectiveness of this therapy. This has yielded to the definition of a new stricter criteria for diagnosis. The aim of this work was to develop and assess a fully-automatic algorithm for strict LBBB diagnosis. Twelve-lead, high-resolution, 10-second ECGs from 602 patients enrolled in the MADIT-CRT trial were available. Data were labelled for strict LBBB by 2 experts and divided into training (n=300) and validation (n=302, blind annotations to the investigators) sets for assessing algorithm performance. After QRS detection, a wavelet-based delineator was used to detect individual Q-R-S waves, QRS onsets and ends, and identify the type of QRS pattern on each standard lead. Then, multilead QRS boundaries were determined in order to compute the QRS width. Finally, an automatic algorithm for notch/slur detection within the QRS complex was applied based on the same wavelet approach used for delineation. In the validation set, LBBB was diagnosed with a sensitivity and specificity of Se=92.9% and Sp=65% (Acc=79%, PPV=73.9% and NPV=89.6%). Results confirmed an accurate diagnosis of strict LBBB based on a fully-automatic extraction of temporal and morphological QRS features.

Introduction Not only repolarization, but also depolarization ECG indexes reflect the progression of ischemic injury. The aim was to assess the QRS duration and morphology dynamics during the prolonged coronary occlusion and their association with the myocardial area at risk (MaR) and final infarct size (IS). Methods In pigs, myocardial infarction was induced by inflation of an angioplasty balloon in the left descending artery (LAD), and ECG was continuously recorded. QRS duration was calculated on a beat-to-beat basis during the occlusion period. Single photon emission computed tomography (SPECT) was performed for the assessment of MaR, and IS was assessed by magnetic resonance imaging (MRI). Results All animals developed an anteroseptal infarction with MaR 40 ± 9% and IS 23 ± 7%. Two peaks of QRS widening were found in all animals: the early peak immediately after LAD occlusion and the late one 17.7 ± 4.1 min later. No association was found between MaR and IS and either QRS width or the degree of QRS widening at the early peak. QRS duration on the late peak correlated with both MaR (r = 0.61; p = 0.007) and IS (r = 0.55; p = 0.018). Conclusion The QRS widening at the late peak, but not at the early peak, is associated with the size of myocardial injury, suggesting different underlying mechanisms.

Objective: Atrial fibrillation (AF) rhythm gives rise to an irregular response in ventricular activity, preventing the use of standard ECG-derived risk markers based on ventricular repolarization heterogeneity under this particular condition. In this study, we proposed new indices to quantify repolarization variations in AF patients, assessing their stratification performance in a chronic heart failure population with AF. Methods: We developed a method based on a selective bin averaging technique. Consecutive beats preceded by a similar $RR$ interval were selected, from which the average variation within the ST-T complex for each $RR$ range was computed. We proposed two sets of indices: 1) the 2-beat index of ventricular repolarization variation, ( ${\mathrm{I}_{\rm{V2}}}$ ), computed from pairs of stable consecutive beats; and 2) the 3-beat indices of ventricular repolarization variation, computed in triplets of stable consecutive beats ( ${\mathrm{I}_{\rm{V3}}}$ ). Results: These indices showed a significant association with sudden cardiac death (SCD) outcome in the study population. In addition, risk assessment based on the combination of the proposed indices improved stratification performance compared to their individual potential. Conclusion: Patients with enhanced ventricular repolarization variation computed in terms of the proposed indices were successfully associated to a higher SCD incidence in our study population, evidencing their prognostic value. Significance: using a simple ambulatory ECG recording, it is possible to stratify AF patients at risk of SCD, which may help cardiologists in adopting most effective therapeutic strategies, with a positive impact in both the patient and healthcare systems.

Objective : We proposed and evaluated a method for correcting possible phase shifts provoked by the presence of ventricular premature contractions (VPCs) for a better assessment of T-wave alternans (TWA). Methods: First, we synthesized ECG signals with artificial TWA in the presence of different noise sources. Then, we assessed the prognostic value for sudden cardiac death (SCD) of the long-term average of TWA amplitude (the index of average alternans, $IAA$ ) in ambulatory ECG signals from congestive heart failure (CHF) and evaluated whether it is sensitive to the presence of VPCs. Results: The inclusion of the phase correction after VPC in the processing always improved estimation accuracy of the $IAA$ under different noisy conditions and regardless of the number of the VPCs included in the sequence. It also presented a positive impact on the prognostic value of $IAA$ with increased hazard ratios (from 17% to 29%, depending of the scenario) in comparison to the noninclusion of this step. Conclusion: The proposed methodology for $IAA$ estimation, which corrects for the possible phase reversal on TWA after the presence of VPCs, represents a robust TWA estimation approach with a significant impact on the prognostic value of $IAA$ for SCD stratification in CHF patients. Significance: An accurate TWA estimation has a potential direct clinical impact on noninvasive SCD stratification, allowing better identification of patients at higher risk and helping clinicians in adopting the most appropriate therapeutic strategy.

Prolonged exposure to weightlessness is known to induce cardiovascular deconditioning. The aim of this work was to assess changes in T-wave alternans (TWA) induced by 60 days exposure to simulated microgravity using the −6° head-down bed-rest model (HDBR) and the potential effectiveness of a jump training countermeasure. We hypothesized that TWA could be able to reflect these changes, if they exist. Twenty-four healthy men were recruited at the German Aerospace Center (12 in control group, 12 with applied countermeasure), from which ECG signals were acquired before (PRE), at day 21 of HDBR (HDT21), before the end of HDBR (HDT57), and the day after its conclusion (POST). The index of average alternans (IAA), quantifying the average TWA amplitude, was computed using a fully automated algorithm based on periodic component analysis and the Laplacian likelihood ratio method. A significant increase in HR was found at HDT57 and POST compared to PRE. The IAA showed an increasing trend at POST (median (25th;75th percentile): 0.467(0.381;0.574) μV) compared to PRE (0.337(0.204;0.437) μΥ p=0.18) only in the control group, although no significant differences along HDBR were found. In conclusion, long-term exposure to simulated microgravity did not induce significant alterations in electrical instability measured in terms of nocturnal TWA.

The aim of this work was to test if the smartphone's embedded triaxial accelerometer can be used to extract respiratory frequency information from the chest movements during a controlled breathing protocol. Respiratory signals from 10 young volunteers were recorded simultaneously, by two smartphones (iPhone 4s and 6s; sampling frequency ∼100 Hz), positioned one on the sternum and one on the belly, while in supine posture. At the same time, a belt transducer was used to acquire the reference respiratory signal. A controlled breathing protocol, consisting of four consecutive phases of 12 respiratory cycles each (respiratory frequencies at 0.25, 0.17, 0.125 and 0.1 Hz), was imposed through the visualization of a moving bar on a display. After low-pass filtering (fc=0.5 Hz), the respiratory signal was obtained from both smartphones, and respiratory frequency derived for each phase. Compared to the belt transducer, the resulting error was lower than 2% for each imposed respiratory frequency, for both smartphones' positions, with better results obtained for the smartphone positioned above the belly.

Objective The aim of this work was to evaluate the potential changes in the ballistocardiogram (BCG) signal induced by different respiratory patterns and posture, by using the dynamic time warping (DTW) technique. Approach BCG signals were recorded in a group of 20 healthy volunteers, simultaneously with an electrocardiogram (ECG). Two recordings, one in a supine (SUP) and one in a standing (ST) position, including spontaneous breathing, two 1 min apneas (at full and empty-lungs, respectively) and 30 s of Valsalva, were analyzed. A warped averaged waveform was computed for each phase, from which amplitude and temporal parameters were extracted to characterize each condition. Main results Variations were found in both amplitude and duration of BCG-derived parameters among manoeuvres, especially when compared to spontaneous breathing, suggesting a complex interaction between intra-thoracic pressure changes acting on venous return, together with the autonomic nervous system modulation on heart rate. The effect of a hydrostatic pressure gradient elicited by postural conditions was also evident. Significance Posture and respiratory manoeuvres affect the BCG signal in different ways, probably as a result of changes induced in preload and afterload. This supports the need to define separate normality ranges for each posture and/or breathing conditions, as well as the importance of applying specific manoeuvres to highlight any pathological response in the computed BCG parameters.

The feasibility of measuring stress-related parameters by ultra-short variability (USV) indices calculated from the ballistocardiographic signal acquired by the mobile phone accelerometers (m-BCG) positioned on the navel was tested, and its accuracy compared with gold standard ECG-derived indices. The m-BCG was acquired in six healthy volunteers while in supine position, during spontaneous breathing (CTRL) and during 1 minute of mental stress (MS) induced by arithmetic serial subtraction task. Beat occurrence was independently and automatically extracted from both ECG and m-BCG signals, to compute USV parameters in 30 s intervals, during both the CTRL and MS. Linear regression and Bland-Altman analyses between RR series and m-BCG derived beat-to-beat measurements (JJ series) showed very high correlation (r2>0.97), no bias, and narrow limits of agreement (±2SD < ±34 ms) for both CTRL and MS. A significant decrease (p=0.03 Wilcoxon test) in beat duration, SDNN and RMSSD was found in MS compared to CTRL, in both RR and JJ variability series, underlying the ability of m-BCG in capturing the decrease in parasympathetic tone in agreement with the induced stimulus.

Mobile phones offer the possibility to monitor and track health parameters. Our aim was to test the feasibility and accuracy of measuring beat-to-beat heart rate using smartphone accelerometers by recording the vibrations generated by the heart during its function and transmitted to the chest wall, i.e. the so-called seismocardiographic signal (SCG). Methods: 9 healthy male volunteers were studied in supine (SUP) and in standing (ST) posture. A smartphone (iPhone6, Apple) was positioned on the thorax (POS1) to acquire SCG signal. While supine, a second smartphone was positioned on the navel (POS2). The SCG signal was recorded for 3 minutes during spontaneous respiration, synchronous with 3-leads ECG. Using a fully automated algorithm based on amplitude thresholding after rectification, the characteristic peak of the SCG signal (IVC) was detected and used to compute beat-to-beat heart duration, to be compared with the corresponding RR intervals extracted from the ECG. Results: A 100% feasibility of the approach resulted for POS1 in SUP, while 89% in POS2, and 78% for POS1 in ST. In supine, for each smartphones' position, the automated algorithm correctly identified the cardiac beats with >98% accuracy. Linear correlation (r2) with RR was very high (>0.98) in each posture and position, with no bias and narrow limits of agreement. Conclusions: The obtained results proved the feasibility of the proposed approach and the robustness of the applied algorithm in measuring the beat-to-beat heart rate from smartphone-derived SCG, with high accuracy compared to conventional ECG-derived measure.

Chronic heart failure (CHF) and atrial fibrillation (AF) are worldwide leading causes of morbidity and mortality in elders, a large part due to sudden cardiac deaths (SCD). The high irregularity of ventricular response in AF patients makes the use of standard SCD-risk markers inappropriate in this target population. The aim of this study was twofold: i) to propose a new index, suitable for AF patients, able to quantify ventricular repolarization changes; and ii) to evaluate its prognostic value in a CHF population with AF. Holter ECG recordings from 176 consecutive CHF patients with AF (22 SCD) were analyzed. The index of T-wave variation (I TV ), quantifying the average T-wave changes in pairs of consecutive beats under stable rhythm conditions, was computed using a fully-automatic method. Survival analysis was performed considering SCD as an independent endpoint. I TV was higher for SCD than non-SCD victims (median (Q1;Q3): 24.9 (14.4;85.4) μV vs 17.1 (11.3;28.2) μV, p=0.06). In a survival analysis where the threshold was set on the third quartile of I TV values, I TV (+) outcome was successfully associated to SCD (Hazard Ratio (CI):3.22 (1.36, 7.58)per μV, p=0.008). In conclusion, we show in this work that Ijy stratifies CHF patients with AF according to their risk of SCD, with larger I TV associated to lower survival probability.

Average TWA activity has been shown to be an independent predictor of sudden cardiac death (SCD) in chronic hean failure (CHF) patients% flowerer the influence o f cireadian rhythms on TWA remains understudied. In this work, we assessed circadian TWA changes in a CHF population and evaluated whether the prognostic value of TWA indices Ls sensitive to the circadian pattern. Hotter ECG recordings from 626 consecutive CHF patients (52 SCD) were analyzed. The index of average altemans (IAA), quantiffing the average TWA level, was measured in 4 consecutive 6-hour intervals using a multi lead fully-cuttomated method. Survival analysis vvas performed considering SCD as an independent endpoint. IAA changed along the day,. with statistically significtmt lower values during the night than during daytime. This patterrt is similar to the one observed in the mean heart rate (HR). However, a low correlation (r= 18) was found between MA and HR in windows of 128 heats. After dichototnizatthn f patients based on the third quartile of IAA indices, IAA indices computed between hours 06-12 (IAA,) and 18-24 (1AA,) successfully predicted SCD (Hazard Ratio, HaR.-2.34( 1.33-4.13) per IN, and HaR:1.87(1.04-3.36) per ktV, respectively). In conclusion, circadian variation should be considered for SCD risk prediction.

Our aim was to test the feasibility of measuring beat-to-beat heart rate from vibrations of the body due to the cardiac activity by smartphone accelerometers. The signal recorded by the accelerometers has a similar morphology respect to the ballistocardiographic signal (BCG). This preliminary study was conducted on one healthy volunteer, with the ECG signal simultaneously acquired as reference. Systolic waves (H, I and J) on the longitudinal (head-foot) acceleration component were automatically detected and used to compute the beat-to-beat duration series (HH, II and JJ). Correlation and Bland-Altman analysis were used to test their accuracy against conventional RR series extracted from the ECG. All series showed high correlation and accuracy with RR, with best results obtained for JJ series (r2>0.90 CI = ±25 ms), despite the limited time resolution of the accelerometric signal. Smartphone's SCG-derived beat-to-beat quantification of heart rate appears feasible and accurate, thus opening new possibilities in using this signal for self-monitoring.

BACKGROUND Certain types of the early repolarization phenomenon, previously considered to be benign, have been reported to be associated with ventricular fibrillation (VF), both in populationbased studies and in the myocardial infarction (MI) settings. OBJECTIVE To analyze whether QRS widening and appearance of a J-wave pattern in experimental MI settings is predictive of VF. METHODS MI was induced in 32 pigs by 40-minute inflation of an angioplasty balloon in the left descending artery, and electrocardiogram was continuously recorded. Multilead QRS boundaries were computed, and QRS duration was calculated on a beat-to-beat basis during the occlusion period for each pig. An association between QRS widening and subsequent VF was studied using receiver operating characteristic curve analysis. Electrocardiograms at maximum QRS duration were reviewed for the presence of a J-wave pattern. RESULTS Sixteen animals had VF episodes during the occlusion period. Two peaks of QRS wideningwerefound inall animals:the first peak immediately on left descending artery occlusion and the second peak 19.1 4.0 minutes later. The magnitude of changes in the QRS width over time had significant interindividual differences. A QRS widening ofZ28 ms during a 3-minutetime window was observed in 14 animals and predicted impending VF (selectivity 80%, specificity 73%, positive predictive value 57%, and negative predictive value 89%; P ¼ .008). In 10 of 14 (71%) pigs, a J-wave pattern appeared at maximal QRS duration. The appearance of a J-wave pattern predicted VF with selectivity 80%, specificity 68%, positive predictive value 53%, and negative predictive value 88% (P ¼ .02). CONCLUSION Transient QRS widening, commonly associated with a J-wave pattern, appears to predict impending VF in acute ischemia settings and motivates further clinical studies for monitoring immediate risk of VF in MI.

Reports of ventricular arrhythmias during spaceflights raise the question of whether microgravity increases sudden cardiac death risk. We aimed at studying changes in T-wave alternans (TWA) together with Autonomic Nervous System activity during tilt-table (TT) testing before and after five days of -6° head-down bed-rest (HDBR), simulating exposure to microgravity. ECG signals were obtained in 22 males during TT, before and immediately after the end of HDBR, analyzed for multilead TWA detection and spectral analysis of heart rate variability. No differences in TWA indices before and after HDBR were found. However, subjects with lower orthostatic tolerance time after HDBR presented higher TWA values and unbalanced sympato-vagal response to TT already before HDBR.

Background T-wave alternans (TWA) is associated with prognosis after myocardial infarction (MI), however its link to the extent of ischemic injury has not been clarified. We analyzed the course of TWA and its relation to myocardial damage in experimental myocardial infarction. Methods In 21 pigs, infarction was induced by 40-minute long balloon inflation in LAD under continuous 12-lead ECG monitoring. TWA was assessed in a 32-beat sliding window, using periodic component analysis and the Laplacian Likelihood Ratio method. Myocardium at risk (MaR) and infarct size (IS) were evaluated by SPECT and magnetic resonance imaging respectively. Results TWA appeared at 7.2 ± 4.5 minutes of occlusion, reached its maximum at 12.7 ± 6.3 and lasted until 26.5 ± 9.2 minutes. The maximal level of TWA was associated with both MaR (r = 0.499, p = 0.035) and IS (r = 0.65, p = 0.004). Conclusion TWA magnitude is associated with both MaR and IS in experiment, which encourages further studies in clinical settings.

Our aim was to assess the effects of 21-days head-down (−6 degrees) bed-rest (BR) and effectiveness of sledge jump countermeasure (CM), on aortic flow and morphology by Phase Contrast (PC) MRI. Male subjects were enrolled at :envihub (DLR, Germany) in control (CTRL, N=12) or countermeasure (CM, N=12) groups. PC-MRI images were obtained before and after 21-days of BR, and analyzed with custom software. Semi-automated region growing and thresholding were applied to segment the aortic lumen, and to compute parameters from velocity images: area lumen (AL), flow velocity, stroke volume (SV), flow rate (Qpeak), time-to-peak flow, systolic duration and heart beat duration (RR). After 21 days, in CTRL significant decreases in SV (14%), Qpeak (5%) and AL (4%) were observed compared to baseline values. Conversely, for CM no changes were observed in these parameters, but only in RR (−8%). Cardiac adaptation to deconditioning due to immobilization resulted in a reduction of SV and Qpeak that might have induced a remodeling process in the ascending aorta, by shrinking of its lumen. The applied CM seemed to counteract at least partially these effects.