Publications

2018

  • [DOI] M. Schmidt, M. Baumert, H. Malberg, and S. Zaunseder, “Iterative Two-Dimensional Signal Warping—Towards a Generalized Approach for Adaption of One-Dimensional Signals,” Biomedical Signal Processing and Control, vol. 43, pp. 311-319, 2018.
    [Bibtex]
    @article{schmidt_iterative_2018,
    title = {Iterative Two-Dimensional Signal Warping\textemdash{{Towards}} a Generalized Approach for Adaption of One-Dimensional Signals},
    volume = {43},
    issn = {1746-8094},
    doi = {10.1016/j.bspc.2018.03.016},
    abstract = {The assessment of subtle morphological changes in noisy signals is a common challenge in the field of biomedical signal processing. Concerning the electrocardiogram (ECG), it may yield novel risk factors for cardiac mortality. Here, we describe an iterative two-dimensional signal warping algorithm (i2DSW), which morphological analyses even in case of noise ratios. i2DSW adapts a generalized iterative template adaptation process that yields a more flexible template and allows for better fitting of subtle variations of signal shapes. Moreover, the template segmentation is not dependent on signal morphology. We test its performance, by measuring beat-to-beat repolarization variability in simulated and clinical ECG. Simulation studies show higher robustness of i2DSW in presence of typical ECG artefacts compared to previously proposed methods including the existing two-dimensional warping technique (26\% improvement). Comparison of short-term ECG recorded in normal subjects versus patients with myocardial infarction (MI) confirmed increased repolarization variability in MI patients (p \< 0.0001). Results obtained with long-term ECG show improved waveform adaptation of i2DSW (overall 19\%, up to 33\%). The assessment of subtle morphological changes by i2DSW may yield novel and more robust risk factors for cardiac mortality. By avoiding a fixed template segmentation, the generalized design of i2DSW has the potential to be also powerful in the application to other quasi-periodic signals.},
    journal = {Biomedical Signal Processing and Control},
    author = {Schmidt, Martin and Baumert, Mathias and Malberg, Hagen and Zaunseder, Sebastian},
    month = may,
    year = {2018},
    keywords = {ECG,QT interval,QT variability,2DSW,i2DSW,Warping},
    pages = {311-319},
    file = {C:\\Users\\martin\\Zotero\\storage\\YEVSWUAU\\Schmidt et al. - 2018 - Iterative two-dimensional signal warping—Towards a.pdf;C:\\Users\\martin\\Zotero\\storage\\FD9Q2M38\\S1746809418300740.html}
    }
  • [DOI] M. Schmidt, M. Baumert, T. Penzel, H. Malberg, and S. Zaunseder, “Nocturnal Ventricular Repolarization Lability Predicts Cardiovascular Mortality in the Sleep Heart Health Study,” American Journal of Physiology-Heart and Circulatory Physiology, vol. 316, iss. 3, p. H495-H505, 2018.
    [Bibtex]
    @article{schmidt_nocturnal_2018,
    title = {Nocturnal Ventricular Repolarization Lability Predicts Cardiovascular Mortality in the {{Sleep Heart Health Study}}},
    volume = {316},
    issn = {0363-6135},
    doi = {10.1152/ajpheart.00649.2018},
    abstract = {The objective of the present study was to quantify repolarization lability and its association with sex, sleep stage, and cardiovascular mortality. We analyzed polysomnographic recordings of 2,263 participants enrolled in the Sleep Heart Health Study (SHHS-2). Beat-to-beat QT interval variability (QTV) was quantified for consecutive epochs of 5 min according to the dominant sleep stage [wakefulness, nonrapid eye movement stage 2 (NREM2), nonrapid eye movement stage 3 (NREM3), and rapid eye movement (REM)]. To explore the effect of sleep stage and apnea-hypopnea index (AHI) on QT interval parameters, we used a general linear mixed model and mixed ANOVA. The Cox proportional hazards model was used for cardiovascular disease (CVD) death prediction. Sex-related differences in T wave amplitude (P $<$ 0.001) resulted in artificial QTV differences. Hence, we corrected QTV parameters by T wave amplitude for further analysis. Sleep stages showed a significant effect (P $<$ 0.001) on QTV. QTV was decreased in deep sleep compared with wakefulness, was higher in REM than in NREM, and showed a distinct relation to AHI in all sleep stages. The T wave amplitude-corrected QTV index (cQTVi) in REM sleep was predictive of CVD death (hazard ratio: 2.067, 95\% confidence interval: 1.105\textendash{}3.867, P $<$ 0.05) in a proportional hazards model. We demonstrated a significant impact of sleep stages on ventricular repolarization variability. Sex differences in QTV are due to differences in T wave amplitude, which should be corrected for. Independent characteristics of QTV measures to sleep stages and AHI showed different behaviors of heart rate variability and QTV expressed as cQTVi. cQTVi during REM sleep predicts CVD death.NEW \& NOTEWORTHY We demonstrate here, for the first time, a significant impact of sleep stages on ventricular repolarization variability, quantified as QT interval variability (QTV). We showed that QTV is increased in rapid eye movement sleep, reflective of high sympathetic drive, and predicts death from cardiovascular disease. Sex-related differences in QTV are shown to be owing to differences in T wave amplitude, which should be corrected for.},
    number = {3},
    journal = {American Journal of Physiology-Heart and Circulatory Physiology},
    author = {Schmidt, Martin and Baumert, Mathias and Penzel, Thomas and Malberg, Hagen and Zaunseder, Sebastian},
    month = dec,
    year = {2018},
    pages = {H495-H505},
    file = {C:\\Users\\martin\\Zotero\\storage\\ZB2VCUM6\\Schmidt et al. - 2018 - Nocturnal ventricular repolarization lability pred.pdf;C:\\Users\\martin\\Zotero\\storage\\5PZP78R6\\ajpheart.00649.html}
    }

2017

  • M. Schmidt, S. Zaunseder, H. Malberg, M. Baumert, and H. Bonnemeier, "Verfahren zur Formanalyse eindimensionaler Messkurven," , iss. DE102014100609B4, 2017.
    [Bibtex]
    @patent{schmidt_verfahren_2017,
    title = {{Verfahren zur Formanalyse eindimensionaler Messkurven}},
    nationality = {DE},
    language = {de},
    assignee = {Mathias Baumert, Hendrik Bonnemeier, Hagen Malberg, Martin Schmidt, Sebastian Zaunseder},
    number = {DE102014100609B4},
    author = {Schmidt, Martin and Zaunseder, Sebastian and Malberg, Hagen and Baumert, Mathias and Bonnemeier, Hendrik},
    month = nov,
    year = {2017},
    note = {rechtskr\"aftig erteilt}
    }
  • [DOI] M. Baumert, "Beat-to-Beat QT Interval Variability and Autonomic Activity," in ECG Time Series Variability Analysis, 1 ed., H. F. Jelinek, D. J. Cornforth, and A. H. Khandoker, Eds., Boca Raton: {CRC Press}, 2017, pp. 403-412.
    [Bibtex]
    @incollection{baumert_beat--beat_2017,
    address = {Boca Raton},
    edition = {1},
    series = {Engineering and {{Medicine}}},
    title = {Beat-to-{{Beat QT Interval Variability}} and {{Autonomic Activity}}},
    isbn = {978-1-4822-4347-5},
    abstract = {The QT interval of a body surface electrocardiogram (ECG) reflects the depolarization and repolarization processes across the ventricular myocardium 20.2.},
    language = {en},
    booktitle = {{{ECG Time Series Variability Analysis}}},
    publisher = {{CRC Press}},
    author = {Baumert, Mathias},
    editor = {Jelinek, Herbert F. and Cornforth, David J. and Khandoker, Ahsan H.},
    month = sep,
    year = {2017},
    pages = {403-412},
    file = {C:\\Users\\martin\\Zotero\\storage\\EGQG773U\\9781315372921-20.html},
    doi = {10.4324/9781315372921-20}
    }

2016

  • [DOI] S. Y. Bonabi, F. {El-Hamad}, A. Müller, M. Dommasch, A. Steger, Georg Schmidt, and M. Baumert, "Recording Duration and Short-Term Reproducibility of Heart Rate and QT Interval Variability in Patients with Myocardial Infarction," Physiol. Meas., vol. 37, iss. 11, p. 1925, 2016.
    [Bibtex]
    @article{bonabi_recording_2016,
    title = {Recording Duration and Short-Term Reproducibility of Heart Rate and {{QT}} Interval Variability in Patients with Myocardial Infarction},
    volume = {37},
    issn = {0967-3334},
    doi = {10.1088/0967-3334/37/11/1925},
    abstract = {Beat-to-beat variability of the QT interval (QTV) measured on surface ECG has emerged as a potential marker for ventricular repolarization instability and has been used along with heart rate variability (HRV) to predict arrhythmic risk. Since measurement modalities of QTV have not been standardized, the objective of this study was to investigate the effect of ECG recording duration on QTV as well as HRV. Using a database of 30 min ECG recorded from 500 patients with acute myocardial infraction during rest, we extracted RR and QT interval time series and estimated different HRV and QTV metrics over windows of varying length. Analysis of variance (ANOVA) and intra-class correlation analyses were computed to investigate the effect of recording length on consistency and short-term reproducibility of HRV and QTV variables. Good consistency (non-significant ANOVA results) and short-term reproducibility (intra-class correlation coefficients $>$0.8) were demonstrated for all but standard deviation based metrics when at least 200 beats were included in the estimation. In conclusion, QTV can be quantified from resting ECG with good short-term consistency and reproducibility that is comparable to that of HRV.},
    language = {en},
    number = {11},
    journal = {Physiol. Meas.},
    author = {Bonabi, Safa Yaghini and {El-Hamad}, Fatima and M\"uller, Alexander and Dommasch, Michael and Steger, Alexander and {Georg Schmidt} and Baumert, Mathias},
    year = {2016},
    pages = {1925}
    }
  • [DOI] M. Baumert, "Measurement of T Wave Variability in Body Surface ECG," Journal of Electrocardiology, vol. 49, iss. 6, pp. 883-886, 2016.
    [Bibtex]
    @article{baumert_measurement_2016,
    title = {Measurement of {{T}} Wave Variability in Body Surface {{ECG}}},
    volume = {49},
    issn = {0022-0736},
    doi = {10.1016/j.jelectrocard.2016.07.014},
    abstract = {Lability in the ventricular repolarization process has been associated with an increased risk of experiencing ventricular tachycardia or fibrillation. A number of risk predictors have been devised that quantify beat-to-beat variability in the T wave morphology of body surface ECG. Initial studies have suggested that measurement of T wave variability may yield important prognostics markers of cardiac mortality, but approaches and experimental designs vary. The aim of this contribution is to provide an overview of existing techniques as well as discuss some of the methodical considerations.},
    number = {6},
    journal = {Journal of Electrocardiology},
    author = {Baumert, Mathias},
    month = nov,
    year = {2016},
    keywords = {ECG,Repolarization,T wave variability},
    pages = {883-886},
    file = {C:\\Users\\martin\\Zotero\\storage\\4JQ8FTLY\\S0022073616300929.html}
    }
  • [DOI] M. Schmidt, M. Baumert, H. Malberg, and S. Zaunseder, "T Wave Amplitude Correction of QT Interval Variability for Improved Repolarization Lability Measurement," Front. Physiol., vol. 7, 2016.
    [Bibtex]
    @article{schmidt_t_2016,
    title = {T {{Wave Amplitude Correction}} of {{QT Interval Variability}} for {{Improved Repolarization Lability Measurement}}},
    volume = {7},
    issn = {1664-042X},
    doi = {10.3389/fphys.2016.00216},
    abstract = {Objectives: The inverse relationship between QT interval variability (QTV) and T wave amplitude potentially confounds QT variability assessment. We quantified the influence of the T wave amplitude on QTV in a comprehensive dataset and devised a correction formula. Methods: Three ECG datasets of healthy subjects were analyzed to model the relationship between T wave amplitude and QTV. To derive a generally valid correction formula, linear regression analysis was used. The proposed correction formula was applied to patients enrolled in the Evaluation of Defibrillator in Non-Ischemic Cardiomyopathy Treatment Evaluation trial (DEFINITE) to assess the prognostic significance of QTV for all-cause mortality in patients with non-ischemic dilated cardiomyopathy. Results: A strong inverse relationship between T wave amplitude and QTV was demonstrated, both in healthy subjects (R$^2$ = 0.68, p $<$ 0.001) and DEFINITE patients (R$^2$ = 0.20, p $<$ 0.001). Applying the T wave amplitude correction to QTV achieved 2.5-times better group discrimination between patients enrolled in the DEFINITE study and healthy subjects. Kaplan-Meier estimator analysis showed that T wave amplitude corrected QTVi is inversely related to survival (p $<$ 0.01) and a significant predictor of all-cause mortality. Conclusion: We have proposed a simple correction formula for improved QTV assessment. Using this correction, predictive value of QTV for all-cause mortality in patients with non-ischemic cardiomyopathy has been demonstrated.},
    language = {English},
    journal = {Front. Physiol.},
    author = {Schmidt, Martin and Baumert, Mathias and Malberg, Hagen and Zaunseder, Sebastian},
    year = {2016},
    keywords = {ECG,QT interval variability,T wave amplitude,risk stratification,Definite},
    note = {00006}
    }
  • [DOI] M. Baumert, M. Schmidt, S. Zaunseder, and A. Porta, "Effects of ECG Sampling Rate on QT Interval Variability Measurement," Biomedical Signal Processing and Control, vol. 25, iss. Supplement C, pp. 159-164, 2016.
    [Bibtex]
    @article{baumert_effects_2016,
    title = {Effects of {{ECG}} Sampling Rate on {{QT}} Interval Variability Measurement},
    volume = {25},
    issn = {1746-8094},
    doi = {10.1016/j.bspc.2015.11.011},
    abstract = {Beat-to-beat variability of the QT interval (QTV) has been used as a marker of repolarization lability and sympathetic activation. The aim of this study was to establish ECG sampling rate requirements for reliable QT interval variability measurement. We measured QTV in high resolution simulated (1000Hz) and real ECG (1600Hz; in the supine position during rest and during sympathetic activation upon standing), using time and frequency domain metrics as well as measures of symbolic dynamics for complexity assessment. We successively halved the sampling rate and investigated its effect on the QTV metrics. Reduction in sampling rate below 400Hz and 500Hz, respectively, resulted in a significant overestimation of QTV variability and also affected complexity measurement of QTV. QTV increased during standing compared to the supine measurement. At 100Hz, the posture related change in QTV was completely masked by the measurement noise introduced by the low sampling rate. In conclusion, ECG sampling rates of 500Hz yields a reliable QTV measurement, while sampling rates of 200Hz and below should be avoided.},
    number = {Supplement C},
    journal = {Biomedical Signal Processing and Control},
    author = {Baumert, Mathias and Schmidt, Martin and Zaunseder, Sebastian and Porta, Alberto},
    month = mar,
    year = {2016},
    keywords = {Sampling rate,ECG,QT interval},
    pages = {159-164},
    file = {C:\\Users\\martin\\Zotero\\storage\\FNRHHRQV\\Baumert et al. - 2016 - Effects of ECG sampling rate on QT interval variab.pdf;C:\\Users\\martin\\Zotero\\storage\\KTG83Q7R\\Baumert et al. - 2016 - Effects of ECG sampling rate on QT interval variab.pdf;C:\\Users\\martin\\Zotero\\storage\\HVRFX9R6\\S1746809415001962.html;C:\\Users\\martin\\Zotero\\storage\\NQV5XCV6\\S1746809415001962.html;C:\\Users\\martin\\Zotero\\storage\\RCMR79G7\\S1746809415001962.html}
    }

2015

  • [DOI] F. {El-Hamad}, E. Lambert, D. Abbott, and M. Baumert, "Relation between QT Interval Variability and Muscle Sympathetic Nerve Activity in Normal Subjects," Am. J. Physiol. - Heart Circ. Physiol., vol. 309, iss. 7, p. H1218-H1224, 2015.
    [Bibtex]
    @article{el-hamad_relation_2015,
    title = {Relation between {{QT}} Interval Variability and Muscle Sympathetic Nerve Activity in Normal Subjects},
    volume = {309},
    copyright = {Copyright \textcopyright{} 2015 the American Physiological Society},
    issn = {0363-6135, 1522-1539},
    doi = {10.1152/ajpheart.00230.2015},
    abstract = {Beat-to-beat variability of the QT interval (QTV) is sought to provide an indirect noninvasive measure of sympathetic nerve activity, but a formal quantification of this relationship has not been provided. In this study we used power contribution analysis to study the relationship between QTV and muscle sympathetic nerve activity (MSNA). ECG and MSNA were recorded in 10 healthy subjects in the supine position and after 40$^\circ$ head-up tilt. Power spectrum analysis was performed using a linear autoregressive model with two external inputs: heart period (RR interval) variability (RRV) and MSNA. Total and low-frequency power of QTV was decomposed into contributions by RRV, MSNA, and sources independent of RRV and MSNA. Results show that the percentage of MSNA power contribution to QT is very small and does not change with tilt. RRV power contribution to QT power is notable and decreases with tilt, while the greatest percentage of QTV is independent of RRV and MSNA in the supine position and after 40$^\circ$ head-up tilt. In conclusion, beat-to-beat QTV in normal subjects does not appear to be significantly affected by the rhythmic modulations in MSNA following low to moderate orthostatic stimulation. Therefore, MSNA oscillations may not represent a useful surrogate for cardiac sympathetic nerve activity at moderate levels of activation, or, alternatively, sympathetic influences on QTV are complex and not quantifiable with linear shift-invariant autoregressive models.},
    language = {en},
    number = {7},
    journal = {Am. J. Physiol. - Heart Circ. Physiol.},
    author = {{El-Hamad}, Fatima and Lambert, Elisabeth and Abbott, Derek and Baumert, Mathias},
    month = oct,
    year = {2015},
    pages = {H1218-H1224},
    file = {C:\\Users\\martin\\Zotero\\storage\\AYSJWUPS\\El-Hamad et al. - 2015 - Relation between QT interval variability and muscl.pdf;C:\\Users\\martin\\Zotero\\storage\\NW3JRAIP\\H1218.html},
    pmid = {26276814}
    }

2014

  • [DOI] M. Schmidt, M. Baumert, H. Malberg, and S. Zaunseder, "QT Interval Extraction by Two-Dimensional Signal Warping," in Biomedical Engineering / Biomedizinische Technik, Hannover, 2014, pp. 154-158.
    [Bibtex]
    @inproceedings{schmidt_qt_2014,
    address = {Hannover},
    title = {{{QT}} Interval Extraction by Two-Dimensional Signal Warping},
    volume = {59},
    isbn = {1862-278X},
    doi = {10.1515/bmt-2014-4069},
    abstract = {We propose a novel two-dimensional warping technique to match two one-dimensional patterns. Our approach, referred
    to as two-dimensional signal warping (2DSW), extends the basic ideas of known warping techniques such as dynamic
    time warping and correlation optimized warping. By employing two-dimensional piecewise stretching 2DSW is able to
    capture inhomogeneous variations of one-dimensional signals in direction of abscissa and ordinate. In order to prove the
    applicability of our method we apply 2DSW for tracking changes in beat-to-beat variability in QT intervals (QTV). Using
    simulated data we demonstrate the robustness of our approach. Analysis of long-term ECG from the DEFINITE trial on
    circadian rhythms demonstrated the sensitivity of the proposed algorithm to track changes in the QT interval. Using
    repeated measure ANOVA and Holm\textendash{}Bonferroni corrected paired Student's t-test for post-hoc analysis we found
    statistically significant differences in QTV between 1 a.m. to 8 a.m. and 10 a.m. to 11 p.m.},
    booktitle = {Biomedical {{Engineering}} / {{Biomedizinische Technik}}},
    publisher = {{Walter de Gruyter}},
    author = {Schmidt, Martin and Baumert, Mathias and Malberg, Hagen and Zaunseder, Sebastian},
    month = oct,
    year = {2014},
    pages = {154-158}
    }
  • [DOI] S. Zaunseder, M. Schmidt, H. Malberg, and M. Baumert, "Measurement of QT Variability by Two-Dimensional Warping," in 2014 8th Conference of the European Study Group on Cardiovascular Oscillations (ESGCO), 2014, pp. 163-164.
    [Bibtex]
    @inproceedings{zaunseder_measurement_2014,
    title = {Measurement of {{QT}} Variability by Two-Dimensional Warping},
    doi = {10.1109/ESGCO.2014.6847570},
    abstract = {This contribution presents a novel warping method, two-dimensional signal warping (2DSW), for tracking beat-to-beat changes in time intervals from electrocardiograms. To evaluate the efficiency of 2DSW to capture subtle changes in the QT interval we apply 2DSW to the Physionet QT database. It is shown that 2DSW allows highly accurate tracking of QRS-onset and T-end, which renders the method useful for future clinical applications, in particular beat-to-beat variability analysis of ECG features.},
    booktitle = {2014 8th {{Conference}} of the {{European Study Group}} on {{Cardiovascular Oscillations}} ({{ESGCO}})},
    author = {Zaunseder, Sebastian and Schmidt, Martin and Malberg, Hagen and Baumert, Mathias},
    month = may,
    year = {2014},
    keywords = {Electrocardiography,medical signal processing,Algorithm design and analysis,bioelectric potentials,2DSW,ECG features,QRS-onset,QT variability measurement,electrocardiograms,high accurate tracking,physionet QT database,time intervals,two-dimensional signal warping method,variability analysis,Databases,Europe,Oscillators,Standards,Time measurement,electrocardiography},
    pages = {163-164},
    file = {C:\\Users\\martin\\Zotero\\storage\\R7N5YCWD\\6847570.html}
    }
  • [DOI] M. Schmidt, M. Baumert, A. Porta, H. Malberg, and S. Zaunseder, "Two-Dimensional Warping for One-Dimensional Signals—Conceptual Framework and Application to ECG Processing," IEEE Trans. Signal Process., vol. 62, iss. 21, pp. 5577-5588, 2014.
    [Bibtex]
    @article{schmidt_two-dimensional_2014,
    title = {Two-{{Dimensional Warping}} for {{One}}-{{Dimensional Signals}}\textemdash{{Conceptual Framework}} and {{Application}} to {{ECG Processing}}},
    volume = {62},
    issn = {1053-587X},
    doi = {10.1109/TSP.2014.2354313},
    abstract = {We propose a novel method for evaluating the similarity between two 1d patterns. Our method, referred to as two-dimensional signal warping (2DSW), extends the basic ideas of known warping techniques such as dynamic time warping and correlation optimized warping. By employing two-dimensional piecewise stretching 2DSW is able to take into account inhomogeneous variations of shapes. We apply 2DSW to ECG recordings to extract beat-to-beat variability in QT intervals (QTV) that is indicative of ventricular repolarization lability and typically characterised by a low signal-to-noise ratio. Simulation studies show high robustness of our approach in presence of typical ECG artefacts. Comparison of short-term ECG recorded in normal subjects versus patients with myocardial infarction (MI) shows significantly increased QTV in patients (normal subject 2.36 ms $\pm$ 1.05 ms vs. MI patients 5.94 ms $\pm$ 5.23 ms (mean $\pm$ std), ). Evaluation of a standard QT database shows that 2DSW allows highly accurate tracking of QRS-onset and T-end. In conclusion, the two-dimensional warping approach introduced here is able to detect subtle changes in noisy quasi-periodic biomedical signals such as ECG and may have diagnostic potential for measuring repolarization lability in MI patients. In more general terms, the proposed method provides a novel means for morphological characterization of 1d signals.},
    number = {21},
    journal = {IEEE Trans. Signal Process.},
    author = {Schmidt, M. and Baumert, M. and Porta, A. and Malberg, H. and Zaunseder, S.},
    month = nov,
    year = {2014},
    keywords = {Electrocardiography,two-dimensional warping,Correlation,Cost function,Heuristic algorithms,Physiology,Signal processing algorithms,Vectors,Dynamic time warping,ECG,QT,QT interval,QT variability,signal processing,warping},
    pages = {5577-5588},
    file = {C:\\Users\\martin\\Zotero\\storage\\BYDNA77L\\6891378.html}
    }
  • [DOI] M. Baumert, B. Czippelova, A. Ganesan, M. Schmidt, S. Zaunseder, and M. Javorka, "Entropy Analysis of RR and QT Interval Variability during Orthostatic and Mental Stress in Healthy Subjects," Entropy, vol. 16, iss. 12, pp. 6384-6393, 2014.
    [Bibtex]
    @article{baumert_entropy_2014,
    title = {Entropy {{Analysis}} of {{RR}} and {{QT Interval Variability}} during {{Orthostatic}} and {{Mental Stress}} in {{Healthy Subjects}}},
    volume = {16},
    copyright = {http://creativecommons.org/licenses/by/3.0/},
    doi = {10.3390/e16126384},
    abstract = {Autonomic activity affects beat-to-beat variability of heart rate and QT interval. The aim of this study was to explore whether entropy measures are suitable to detect changes in neural outflow to the heart elicited by two different stress paradigms. We recorded short-term ECG in 11 normal subjects during an experimental protocol that involved head-up tilt and mental arithmetic stress and computed sample entropy, cross-sample entropy and causal interactions based on conditional entropy from RR and QT interval time series. Head-up tilt resulted in a significant reduction in sample entropy of RR intervals and cross-sample entropy, while mental arithmetic stress resulted in a significant reduction in coupling directed from RR to QT. In conclusion, measures of entropy are suitable to detect changes in neural outflow to the heart and decoupling of repolarisation variability from heart rate variability elicited by orthostatic or mental arithmetic stress.},
    language = {en},
    number = {12},
    journal = {Entropy},
    author = {Baumert, Mathias and Czippelova, Barbora and Ganesan, Anand and Schmidt, Martin and Zaunseder, Sebastian and Javorka, Michal},
    month = dec,
    year = {2014},
    keywords = {QT variability,Heart rate variability,stress,sympathetic,complexity,heart rate variability},
    pages = {6384-6393},
    file = {C:\\Users\\martin\\Zotero\\storage\\3VQU5QAF\\Baumert et al. - 2014 - Entropy Analysis of RR and QT Interval Variability.pdf;C:\\Users\\martin\\Zotero\\storage\\9KAD6PQZ\\Baumert et al. - 2014 - Entropy Analysis of RR and QT Interval Variability.pdf;C:\\Users\\martin\\Zotero\\storage\\YALNU8E8\\Baumert et al. - 2014 - Entropy Analysis of RR and QT Interval Variability.pdf;C:\\Users\\martin\\Zotero\\storage\\3Q5RDX8Q\\6384.html;C:\\Users\\martin\\Zotero\\storage\\8M9XM3C6\\htm.html;C:\\Users\\martin\\Zotero\\storage\\Y7CJ422F\\htm.html}
    }