Hyun Jae Baek

A Smart Health Monitoring Chair for Nonintrusive Measurement of Biological Signals

We developed nonintrusive methods for simultaneous electrocardiogram, photoplethysmogram, and ballistocardiogram measurements that do not require direct contact between instruments and bare skin. These methods were applied to the design of a diagnostic chair for unconstrained heart rate and blood pressure monitoring purposes. Our methods were operationalized through capacitively coupled electrodes installed in the chair back that include high-input impedance amplifiers, and conductive textiles installed in the seat for capacitive driven-right-leg circuit configuration that is capable of recording electrocardiogram information through clothing. Photoplethysmograms were measured through clothing using seat mounted sensors with specially designed amplifier circuits that vary in light intensity according to clothing type. Ballistocardiograms were recorded using a film type transducer material, polyvinylidenefluoride (PVDF), which was installed beneath the seat cover. By simultaneously measuring signals, beat-to-beat heart rates could be monitored even when electrocardiograms were not recorded due to movement artifacts. Beat-to-beat blood pressure was also monitored using unconstrained measurements of pulse arrival time and other physiological parameters, and our experimental results indicated that the estimated blood pressure tended to coincide with actual blood pressure measurements. This study demonstrates the feasibility of our method and device for biological signal monitoring through clothing for unconstrained long-term daily health monitoring that does not require user awareness and is not limited by physical activity.

Improved elimination of motion artifacts from a photoplethysmographic signal using a Kalman smoother with simultaneous accelerometry

A photoplethysmography (PPG) signal provides very useful information about a subjects hemodynamic status in a hospital or ubiquitous environment. However, PPG is very vulnerable to motion artifacts, which can significantly distort the information belonging to the PPG signal itself. Thus, the reduction of the effects of motion artifacts is an important issue when monitoring the cardiovascular system by PPG. There have been many adaptive techniques to reduce motion artifacts from PPG signals. In the present study, we compared a method based on the fixed-interval Kalman smoother with the usual adaptive filtering algorithms, e.g. the normalized least mean squares, recursive least squares and the conventional Kalman filter. We found that the fixed-interval Kalman smoother reduced motion artifacts from the PPG signal most effectively. Therefore, the use of the fixed-interval Kalman smoother can reduce motion artifacts in PPG, thus providing the most reliable information that can be deduced from the reconstructed PPG signals.

Nonintrusive Biological Signal Monitoring in a Car to Evaluate a Driver’s Stress and Health State

Nonintrusive monitoring of a drivers physiological signals was introduced and evaluated in a car as a test of extending the concept of ubiquitous healthcare to vehicles. Electrocardiogram, photoplethysmogram, galvanic skin response, and respiration were measured in the ubiquitous healthcare car (U-car) using nonintrusively installed sensors on the steering wheel, drivers seat, and seat belt. Measured signals were transmitted to the embedded computer via Bluetooth(R) communication and processed. We collected and analyzed physiological signals during driving in order to estimate a drivers stress state while using this system. In order to compare the effect of stress on physical and mental conditions, two categories of stresses were defined. Experimental results show that a drivers physiological signals were measured with acceptable quality for analysis without interrupting driving, and they were changed meaningfully due to elicited stress. This nonintrusive monitoring can be used to evaluate a drivers state of health and stress.

Nonintrusive Biosignal Measurement System in a Vehicle

Measurement of drivers state is important in both daily healthcare and prevention of car accidents. Existing measurement methods, however, are too complex and uncomfortable to measure for everyday. A nonintrusive biosignal monitoring system is demanded for increasing the accessibility. In this paper, we proposed a nonintrusive measurement system integrated in a vehicle for recording electrocardiographic (ECG) signals and tested the performance of the system. The system consists of dry electrodes attached to the steering wheel and a wireless communication module using a Bluetooth device. ECG signals measured by our system were compared with those by a conventional system employing wet electrodes during real city road driving. Various parameters used for HRV analysis in time and frequency domain showed no significant differences between the two simultaneously measured ECG signals. The result implies the possibility that developed system could have a potential to monitor health information in a vehicle instead of commercial equipments.

Enhancing the estimation of blood pressure using pulse arrival time and two confounding factors

A new method of blood pressure (BP) estimation using multiple regression with pulse arrival time (PAT) and two confounding factors was evaluated in clinical and unconstrained monitoring situations. For the first analysis with clinical data, electrocardiogram (ECG), photoplethysmogram (PPG) and invasive BP signals were obtained by a conventional patient monitoring device during surgery. In the second analysis, ECG, PPG and non-invasive BP were measured using systems developed to obtain data under conditions in which the subject was not constrained. To enhance the performance of BP estimation methods, heart rate (HR) and arterial stiffness were considered as confounding factors in regression analysis. The PAT and HR were easily extracted from ECG and PPG signals. For arterial stiffness, the duration from the maximum derivative point to the maximum of the dicrotic notch in the PPG signal, a parameter called TDB, was employed. In two experiments that normally cause BP variation, the correlation between measured BP and the estimated BP was investigated. Multiple-regression analysis with the two confounding factors improved correlation coefficients for diastolic blood pressure and systolic blood pressure to acceptable confidence levels, compared to existing methods that consider PAT only. In addition, reproducibility for the proposed method was determined using constructed test sets. Our results demonstrate that non-invasive, non-intrusive BP estimation can be obtained using methods that can be applied in both clinical and daily healthcare situations.

Effect of confounding factors on blood pressure estimation using pulse arrival time

Two confounding factors were selected and analyzed in blood pressure estimation using pulse arrival time (PAT) for each individual. The heart rate was used as the confounding factor for the cardiac cycle, and the duration from the maximum derivative point to the dicrotic peak (TDB) in the photoplethysmogram was used as another confounding factor representing arterial stiffness. By considering these factors with PAT in multiple regression analysis, the performance of blood pressure estimation is enhanced significantly in the diastolic phase as well as in the systolic phase. The reproducibility of this method was also validated with formerly obtained regression equations from the training set. The correlation between estimated and measured blood pressure decreased a little, but the validity was still maintained (r congruent with 0.8). This shows the value of the method in non-intrusive blood pressure estimation for individual patients and may be useful for various applications.

Conductive Polymer Foam Surface Improves the Performance of a Capacitive EEG Electrode

In this paper, a new conductive polymer foam-surfaced electrode was proposed for use as a capacitive EEG electrode for nonintrusive EEG measurements in out-of-hospital environments. The current capacitive electrode has a rigid surface that produces an undefined contact area due to its stiffness, which renders it unable to conform to head curvature and locally isolates hairs between the electrode surface and scalp skin, making EEG measurement through hair difficult. In order to overcome this issue, a conductive polymer foam was applied to the capacitive electrode surface to provide a cushioning effect. This enabled EEG measurement through hair without any conductive contact with bare scalp skin. Experimental results showed that the new electrode provided lower electrode-skin impedance and higher voltage gains, signal-to-noise ratios, signal-to-error ratios, and correlation coefficients between EEGs measured by capacitive and conventional resistive methods compared to a conventional capacitive electrode. In addition, the new electrode could measure EEG signals, while the conventional capacitive electrode could not. We expect that the new electrode presented here can be easily installed in a hat or helmet to create a nonintrusive wearable EEG apparatus that does not make users look strange for real-world EEG applications.

Photoplethysmogram Measurement Without Direct Skin-to-Sensor Contact Using an Adaptive Light Source Intensity Control

We developed a chair-attached, nonintrusive photoplethysmogram (PPG) measuring system for everyday life, unconstrained monitoring using nonskin-contacting sensor-amplifier circuits capable of emitting suitable light intensity adaptable to clothing characteristics. Comparison between proposed and conventional systems showed reasonable correlation and root-mean-squared error levels, indicating its feasibility for unconstrained PPG monitoring.

Second Derivative of Photoplethysmography for Estimating Vascular Aging

Coronary artery disease (CAD) is one of the major causes of death. Therefore recurrent monitoring of arterial condition is important. The photoplethysmography (PPG) is simple and cost effective technique to measure blood volume change. As an application of PPG technique, the second derivative of photoplethysmography(SDPTG) was introduced for monitoring arterial condition. SDPTG consists of an a, b, c and d wave in systole and an e wave in diastole. The SDPTG wave pattern is determined by the proportions of the b, c, d, and e waves to the a wave. In the present study, it is confirmed that the b/a ratio and SDPTG-AI increased with age, and the c/a, d/a, and e/a ratios decreased with age. The informal method which substitutes formal SDPTG-AI was suggested.

The relationship between HRV parameters and stressful driving situation in the real road

The aim of this paper is to find a relation between HRV parameters of ECG and stressful driving situation in the real road through various tests. First, we gave a stress to the subject by increasing the driving speed using driving simulator in the laboratory. Second, the subject drove the vehicle on the real road. Through let him drove in the busy narrow street, we evoked stress. ECG and PPG signal was recorded from steering wheel and back cushion each unconsciously. The driving experiment with simulator was not suitable to reproduce actual traffic condition because the adaptation was occurred. On the other hand, in real driving test, the results show that the heart rate is increase, and SDNN, RMSSD, pNN50 were decreased. The heart rate is the strongest indicator and other parameters tend to be changed depend on a personal. The blood pressure had no significant changes. Our results show the good possibility of that the time parameters of the HRV could be used as an instantaneous stress indicator.