Devy Widjaja

Application of Kernel Principal Component Analysis for Single-Lead-ECG-Derived Respiration

Recent studies show that principal component analysis (PCA) of heartbeats is a well-performing method to derive a respiratory signal from ECGs. In this study, an improved ECG-derived respiration (EDR) algorithm based on kernel PCA (kPCA) is presented. KPCA can be seen as a generalization of PCA where nonlinearities in the data are taken into account by nonlinear mapping of the data, using a kernel function, into a higher dimensional space in which PCA is carried out. The comparison of several kernels suggests that a radial basis function (RBF) kernel performs the best when deriving EDR signals. Further improvement is carried out by tuning the parameter that represents the variance of the RBF kernel. The performance of kPCA is assessed by comparing the EDR signals to a reference respiratory signal, using the correlation and the magnitude squared coherence coefficients. When comparing the coefficients of the tuned EDR signals using kPCA to EDR signals obtained using PCA and the algorithm based on the R peak amplitude, statistically significant differences are found in the correlation and coherence coefficients (both ), showing that kPCA outperforms PCA and R peak amplitude in the extraction of a respiratory signal from single-lead ECGs.

Inhalation/Exhalation ratio modulates the effect of slow breathing on heart rate variability and relaxation.

Slow breathing is widely applied to improve symptoms of hyperarousal, but it is unknown whether its beneficial effects relate to the reduction in respiration rate per se, or, to a lower inhalation/exhalation (i/e) ratio. The present study examined the effects of four ventilatory patterns on heart rate variability and self-reported dimensions of relaxation. Thirty participants were instructed to breathe at 6 or 12 breaths/min, and with an i/e ratio of 0.42 or 2.33. Participants reported increased relaxation, stress reduction, mindfulness and positive energy when breathing with the low compared to the high i/e ratio. A lower compared to a higher respiration rate was associated only with an increased score on positive energy. A low i/e ratio was also associated with more power in the high frequency component of heart rate variability, but only for the slow breathing pattern. Our results show that i/e ratio is an important modulator for the autonomic and subjective effects of instructed ventilatory patterns.

Cardiorespiratory information dynamics during mental arithmetic and sustained attention

An analysis of cardiorespiratory dynamics during mental arithmetic, which induces stress, and sustained attention was conducted using information theory. The information storage and internal information of heart rate variability (HRV) were determined respectively as the self-entropy of the tachogram, and the self-entropy of the tachogram conditioned to the knowledge of respiration. The information transfer and cross information from respiration to HRV were assessed as the transfer and cross-entropy, both measures of cardiorespiratory coupling. These information-theoretic measures identified significant nonlinearities in the cardiorespiratory time series. Additionally, it was shown that, although mental stress is related to a reduction in vagal activity, no difference in cardiorespiratory coupling was found when several mental states (rest, mental stress, sustained attention) are compared. However, the self-entropy of HRV conditioned to respiration was very informative to study the predictability of RR interval series during mental tasks, and showed higher predictability during mental arithmetic compared to sustained attention or rest.

Stress classification by separation of respiratory modulations in heart rate variability using orthogonal subspace projection

The influence of respiration on the heart rate is a phenomenon known as respiratory sinus arrhythmia. However, effects of respiration are often ignored in studies of heart rate variability. In this paper, we take respiratory influences into account by separating the tachogram in two components, one related to respiration and one residual component, using orthogonal subspace projection. We demonstrate that it is important to remove respiratory influences during classification of rest and mental stress. Using merely the original tachogram, the classification accuracy is 57.13%, while the use of the residual tachogram results in an almost perfect classification (accuracy = 97.88%).

Heart Rate Variability in Newborns with Hypoxic Brain Injury

In neonatal intensive care units, there is a need for continuous monitoring of sick newborns with perinatal hypoxic ischemic brain injury (HIE). We assessed the utility of heart rate variability (HRV) in newborns with acute HIE undergoing simultaneous continuous EEG (cEEG) and ECG monitoring. HIE was classified using clinical criteria as well as visual grading of cEEG. Newborns were divided into two groups depending on the severity of the hypoxic injury and outcome. Various HRV parameters were compared between these groups, and significantly decreased HRV was found in neonates with severe HIE. As HRV is affected by many factors, it is difficult to attribute this difference solely to HIE. However, this study suggests that further investigation of HRV as a monitoring tool for acute neonatal hypoxic injury is warranted.

Cardiorespiratory Dynamic Response to Mental Stress: A Multivariate Time-Frequency Analysis

Mental stress is a growing problem in our society. In order to deal with this, it is important to understand the underlying stress mechanisms. In this study, we aim to determine how the cardiorespiratory interactions are affected by mental arithmetic stress and attention. We conduct cross time-frequency (TF) analyses to assess the cardiorespiratory coupling. In addition, we introduce partial TF spectra to separate variations in the RR interval series that are linearly related to respiration from RR interval variations (RRV) that are not related to respiration. The performance of partial spectra is evaluated in two simulation studies. Time-varying parameters, such as instantaneous powers and frequencies, are derived from the computed spectra. Statistical analysis is carried out continuously in time to evaluate the dynamic response to mental stress and attention. The results show an increased heart and respiratory rate during stress and attention, compared to a resting condition. Also a fast reduction in vagal activity is noted. The partial TF analysis reveals a faster reduction of RRV power related to (3 s) than unrelated to (30 s) respiration, demonstrating that the autonomic response to mental stress is driven by mechanisms characterized by different temporal scales.

Heart beat detection in multimodal data using automatic relevant signal detection

Accurate R peak detection in the electrocardiogram (ECG) is a well-known and highly explored problem in biomedical signal processing. Although a lot of progress has been made in this area, current methods are still insufficient in the presence of extreme noise and/or artifacts such as loose electrodes. Often, however, not only the ECG is recorded, but multiple signals are simultaneously acquired from the patient. Several of these signals, such as blood pressure, can help to improve the heart beat detection. These signals of interest can be detected automatically by analyzing their power spectral density or by using the available signal type identifiers. Individual peaks from the signals of interest are combined using majority voting, heart beat location estimation and Hjorths mobility of the resulting RR intervals. Both multimodal algorithms showed significant increases in performance of up to 8.65% for noisy multimodal datasets compared to when only the ECG signal is used. A maximal performance of 90.02% was obtained on the hidden test set of the Physionet/Computing in Cardiology Challenge 2014: Robust Detection of Heart Beats in Multimodal Data.

Cardiovascular autonomic adaptation in lunar and martian gravity during parabolic flight

PurposeWeightlessness has a well-known effect on the autonomic control of the cardiovascular system. With future missions to Mars in mind, it is important to know what the effect of partial gravity is on the human body. We aim to study the autonomic response of the cardiovascular system to partial gravity levels, as present on the Moon and on Mars, during parabolic flight.MethodsECG and blood pressure were continuously recorded during parabolic flight. A temporal analysis of blood pressure and heart rate to changing gravity was conducted to study the dynamic response. In addition, cardiovascular autonomic control was quantified by means of heart rate (HR) and blood pressure (BP) variability measures.ResultsZero and lunar gravity presented a biphasic cardiovascular response, while a triphasic response was noted during martian gravity. Heart rate and blood pressure are positively correlated with gravity, while the general variability of HR and BP, as well as vagal indices showed negative correlations with increasing gravity. However, the increase in vagal modulation during weightlessness is not in proportion when compared to the increase during partial gravity.ConclusionsCorrelations were found between the gravity level and modulations in the autonomic nervous system during parabolic flight. Nevertheless, with future Mars missions in mind, more studies are needed to use these findings to develop appropriate countermeasures.

Anxious women do not show the expected decrease in cardiovascular stress responsiveness as pregnancy advances.

Altered stress responsiveness is a risk factor for mental and physical illness. In non-pregnant populations, it is well-known that anxiety can alter the physiological regulation of stress reactivity. Characterization of corresponding risks for pregnant women and their offspring requires greater understanding of how stress reactivity and recovery are influenced by pregnancy and womens anxiety feelings. In the current study, women were presented repeatedly with mental arithmetic stress tasks in the first and third pregnancy trimester and reported their trait anxiety using the state trait anxiety inventory. Cardiovascular stress reactivity in late pregnancy was lower than reactivity in the first pregnancy trimester (heart rate (HR): t(197)=4.98, p<.001; high frequency heart rate variability (HF HRV): t(196)=-2.09, p=.04). Less attenuation of stress reactivity occurred in more anxious women (HR: b=0.15, SE=0.06, p=.008; HF HRV: b=-10.97, SE=4.79, p=.02). The study design did not allow the influence of habituation to repeated stress task exposure to be assessed separately from the influence of pregnancy progression. Although this is a limitation, the clear differences between anxious and non-anxious pregnant women are important, regardless of the extent to which differing habituation between the groups is responsible. Less dampened stress reactivity through pregnancy may pose long-term risks for anxious women and their offspring. Follow-up studies are required to determine these risks.

Adaptation of autonomic heart rate regulation in astronauts after spaceflight

Background Spaceflight causes changes in the cardiovascular control system. The aim of this study was to evaluate postflight recovery of linear and nonlinear neural markers of heart rate modulation, with a special focus on day-night variations. Material/Methods Twenty-four-hour Holter ECG recordings were obtained in 8 astronauts participating in space missions aboard the International Space Station (ISS). Data recording was performed 1 month before launch, and 5 and 30 days after return to Earth from short- and long-term flights. Cardiovascular control was inferred from linear and nonlinear heart rate variability (HRV) parameters, separately during 2-hour day and 2-hour night recordings. Results No remarkable differences were found in the postflight recovery between astronauts from short- and long-duration spaceflights. Five days after return to Earth, vagal modulation was significantly decreased compared to the preflight condition (day: p=0.001; night: p=0.019), while the sympathovagal balance was strongly increased, but only at night (p=0.017). A few nonlinear parameters were reduced early postflight compared to preflight values, but these were not always statistically significant. No significant differences remained after 30 days of postflight recovery. Conclusions Our results show that 5 days after return from both short- and long-duration space missions, neural mechanisms of heart rate regulation are still disturbed. After 1 month, autonomic control of heart rate recovered almost completely.