Comments on "A novel high input impedance front-end for capacitive biopotential measurement".

A front-end for biopotential sensing in wearable medical devices has been recently proposed which is claimed to provide 100 GΩ input impedance by manually matching two resistor pairs in a positive- and a negative-feedback loop around an operational amplifier (op amp); the cost being that the equivalent input noise voltage doubles with respect to a simple non-inverting amplifier. The ECG acquired with capacitive (sic) electrodes through a cotton shirt is presented as a proof of the performance of the proposed circuit. It turns out, however, that the analysis ignores op amp's input capacitance hence the effort to achieve a very high input resistance seems futile. Further, cotton is highly hygroscopic hence not an appropriate dielectric, so that there is no proof that the electrodes tested were actually capacitive. This comment addresses these two problems and some additional conceptual and methodological inaccuracies found in the paper.

A Practical Approach to Electrode-Skin Impedance Unbalance Measurement.

Unbalance between electrode-skin impedances is a major problem in biopotential recordings, leading to increased power-line interference. This paper proposes a simple, direct method to measure that unbalance at power-line frequency (50-60Hz), thus allowing the determination of actual recording conditions for biopotential amplifiers. the method is useful in research, amplifier testing, electrode design and teaching purposes. It has been experimentally validated by using both phantom impedances and real electrode-skin impedances.

The QT Scale: A Weight Scale Measuring the QTc Interval.

INTRODUCTION: Despite the strong evidence of the clinical utility of QTc prolongation as a surrogate marker of cardiac risk, QTc measurement is not part of clinical routine either in hospital or in physician offices. We evaluated a novel device ("the QT scale") to measure heart rate (HR) and QTc interval. METHOD: The QT scale is a weight scale embedding an ECG acquisition system with four limb sensors (feet and hands: lead I, II, and III). We evaluated the reliability of QT scale in healthy subjects (cohort 1) and cardiac patients (cohorts 2 and 3) considering a learning (cohort 2) and two validation cohorts. The QT scale and the standard 12-lead recorder were compared using intraclass correlation coefficient (ICC) in cohorts 2 and 3. Absolute value of heart rate and QTc intervals between manual and automatic measurements using ECGs from the QT scale and a clinical device were compared in cohort 1. RESULTS: We enrolled 16 subjects in cohort 1 (8 w, 8 m; 32 ± 8 vs 34 ± 10 years, P = 0.7), 51 patients in cohort 2 (13 w, 38 m; 61 ± 16 vs 58 ± 18 years, P = 0.6), and 13 AF patients in cohort 3 (4 w, 9 m; 63 ± 10 vs 64 ± 10 years, P = 0.9). Similar automatic heart rate and QTc were delivered by the scale and the clinical device in cohort 1: paired difference in RR and QTc were -7 ± 34 milliseconds (P = 0.37) and 3.4 ± 28.6 milliseconds (P = 0.64), respectively. The measurement of stability was slightly lower in ECG from the QT scale than from the clinical device (ICC: 91% vs 80%) in cohort 3. CONCLUSION: The "QT scale device" delivers valid heart rate and QTc interval measurements.

On the effect of body capacitance to ground in tetrapolar bioimpedance measurements.

Tetrapolar bioimpedance measurements on subjects have long been suspected of being affected by stray capacitance between the subjects' body and ground. This paper provides a circuit model to analyze that effect in the frequency range from 100 Hz to 1 MHz in order to identify the relevant parameters when impedance is measured by applying a voltage and measuring both the resulting current and the potential difference between two points on the surface of the volume conductor. The proposed model includes the impedance of each electrode and the input impedance of the differential voltage amplifier. When common values for the circuit parameters are assumed, the simplified model predicts: 1) a frequency-independent gain (scale factor) error; 2) inductive artifacts, that is, the measured impedance increases with increasing frequency and may include positive angle phases; and 3) resonance that can affect well below 1 MHz. In addition to the stray capacitance to ground, relevant parameters that determine those errors are the capacitance of the "low-current" electrode and the input capacitance of the differential voltage amplifier. Experimental results confirm those theoretical predictions and show effects from several additional resonances above 1 MHz that also depend on body capacitance to ground.

Multi-signal bathroom scale to assess long-term trends in cardiovascular parameters.

This paper describes the circuits and signal processing techniques that convert an electronic bathroom scale intended for bioimpedance analysis (BIA) into a compact system to acquire the electrocardiogram (ECG), the ballistocardiogram (BCG), and the impedance plethysmogram (IPG) using only plantar measurements. The signal processing methods proposed rely on the higher quality of the IPG as compared to the ECG and BCG and they enhance the signal-to-noise ratio (SNR) of these two signals, which otherwise could be too poor in non-controlled environments. The system is suitable for long-term periodic monitoring of cardiovascular function.

Heart and respiratory rate detection on a bathroom scale based on the ballistocardiogram and the continuous wavelet transform.

Ballistocardiography is a non-invasive technique that yields information about the cardiovascular system that is not available in other external signals such as the electrocardiogram (ECG). In the last years, several research groups have obtained the ballistocardiogram (BCG) by using instrumentation methods simpler than those available in the 1950s and that did not progress because of their complexity as compared to ultrasound and other noninvasive techniques that are in common use nowadays. We describe a novel method for real-time robust heart- (HR) and respiratory- (RR) rate detection from a subject that stands on a common electronic bathroom scale. BCG signals from the scale are wirelessly sent to a PC where algorithms based on the continuous wavelet transform (CWT) extract the HR and the RR. HR results are compared to those obtained from the ECG. To better assess the RR results, subjects have been asked to synchronize their breathing rate to an on-screen bar-graph set at a constant rate of breaths per minute. This method to obtain the heart and respiratory rates is simple, compact, non-invasive and passive, and can be applied to any person able to stand on an electronic weighing scale, even if wearing shoes.

Heart rate detection from single-foot plantar bioimpedance measurements in a weighing scale.

Electronic bathroom scales are an easy-to-use, affordable mean to measure physiological parameters in addition to body weight. They have been proposed to obtain the ballistocardiogram (BCG) and derive from it the heart rate, cardiac output and systolic blood pressure. Therefore, weighing scales may suit intermittent monitoring in e-health and patient screening. Scales intended for bioelectrical impedance analysis (BIA) have also been proposed to estimate the heart rate by amplifying the pulsatile impedance component superimposed on the basal impedance. However, electronic weighing scales cannot easily obtain the BCG from people that have a single leg neither are bioimpedance measurements between both feet recommended for people wearing a pacemaker or other electronic implants, neither for pregnant women. We propose a method to detect the heart rate (HR) from bioimpedance measured in a single foot while standing on an bathroom weighting scale intended for BIA. The electrodes built in the weighing scale are used to apply a 50 kHz voltage between the outer electrode pair and to measure the drop in voltage across the inner electrode pair. The agreement with the HR simultaneously obtained from the ECG is excellent. We have also compared the drop in voltage across the waist and the thorax with that obtained when measuring bioimpedance between both feet to compare the possible risk of the proposed method to that of existing BIA scales.

Microphonics in biopotential measurements with capacitive electrodes.

Biopotential measurements with capacitive electrodes do not need any direct contact between electrode and skin, which saves the time devoted to expose and prepare the contact area when measuring with conductive electrodes. However, mechanical vibrations resulting from physiological functions such as respiration and cardiac contraction can change the capacitance of the electrode and affect the recordings. This transformation of mechanical vibrations into undesired electric signals is termed microphonics. We have evaluated microphonics in capacitive ECG recordings obtained from a dressed subject seated on a common chair with electrodes placed on the front side of the backrest of the chair. Depending on the softness of the backrest, the recordings may be clearly affected by the displacement of the thorax back wall due to the respiration and to the heart's mechanical activity.

Heart rate detection from plantar bioimpedance measurements.

In this paper, a novel technique for heart rate measurement on a standing subject is proposed that relies on electrical impedance variations detected by a plantar interface with booth feet, such as those in some bathroom weighting scales for body composition analysis. Heart-related impedance variations in the legs come from arterial blood circulation and are below 500 mOmega. To detect them, we have implemented a system with a gain in excess of 600, and whose fully differential AC input amplifier has a gain of 4.5 and a common-mode rejection ratio (CMRR) higher than 90 dB at 10 kHz. Differential coherent demodulation based on synchronous sampling yields a signal-to-noise ratio (SNR) of about 54 dB . The system sensitivity is 610 mV/Omega. The technique has been demonstrated on 18 volunteers, whose bioimpedance signal and ECG were simultaneously recorded. A Bland-Altman plot shows a mean bias of -0.2 ms between the RR time intervals obtained from these two signals, which is negligible. The technique is simple and user friendly and does not require any additional sensors or electrodes attached to the body, hence no conductive gel or skin preparation.

Electrostatic interference in contactless biopotential measurements.

Sensing biopotentials without contacting the subject's body could obviate skin preparation and allow the recording of the ECG without needing to remove clothing. However, body surface charges unrelated to bioelectric signals can interfere the record by contributing a signal that results from the distance variation between the body surface and the electrode used to detect electric potential. This paper shows that, for ungrounded human subjects, distance changes related to the impact of the heart on the thorax can yield electric potentials higher than those related to the ECG. Therefore, we dispute that non-contact biopotentials recordings obtained without any control of electrostatic charges can be attributed only to the ECG. Because earth grounding a patient is forbidden by electrical safety regulations, non-contact biopotential measurements ask for control measures to prevent the generation and accumulation of electrostatic charges, the same as conventional measurements.

A practical approach to electrode-skin impedance unbalance measurement.

Unbalance between electrode-skin impedances is a major problem in biopotential recordings, leading to increased power-line interference. This paper proposes a simple, direct method to measure that unbalance at power-line frequency (50-60 Hz), thus allowing the determination of actual recording conditions for biopotential amplifiers. The method is useful in research, amplifier testing, electrode design and teaching purposes. It has been experimentally validated by using both phantom impedances and real electrode-skin impedances.

Novel indices of ventricular repolarization to screen post myocardial infarction patients.

We propose novel indices of ventricular repolarization intervals, the JTp/JT, Tpe/JTp and Tpe/JT ratios. These indices have been compared with the duration of the ventricular repolarization intervals and other ratios in 17 normal subjects and 17 patients with old myocardial infarction. In the intervals and other ratios, the best separation between groups is obtained with the Tpe/QTp and Tpe/QT ratios with 94% sensitivity and 82% specificity, the proposed ratios increased sensitivity to 100% and specificity to 94%. These indices should be further tested to determine their usefulness in discriminating between OMI patients with and without susceptibility to ventricular arrhythmias.

A novel fully differential biopotential amplifier with dc suppression.

Fully differential amplifiers yield large differential gains and also high common mode rejection ratio (CMRR), provided they do not include any unmatched grounded component. In biopotential measurements, however, the admissible gain of amplification stages located before dc suppression is usually limited by electrode offset voltage, which can saturate amplifier outputs. The standard solution is to first convert the differential input voltage to a single-ended voltage and then implement any other required functions, such as dc suppression and dc level restoring. This approach, however, yields a limited CMRR and may result in a relatively large equivalent input noise. This paper describes a novel fully differential biopotential amplifier based on a fully differential dc-suppression circuit that does not rely on any matched passive components, yet provides large CMRR and fast recovery from dc level transients. The proposed solution is particularly convenient for low supply voltage systems. An example implementation, based on standard low-power op amps and a single 5-V power supply, accepts input offset voltages up to +/-500 mV, yields a CMRR of 102 dB at 50 Hz, and provides, in accordance with the AAMI EC38 standard, a reset behavior for recovering from overloads or artifacts.

Cuantificación de la variabilidad de la repolarización ventricular en pacientes con cardiopatía isquémica / Quantification of ventricular repolarization variability in patients with ischemic heart disease

Se propuso un nuevo indicador que se denomina: Relación de desviaciones estándar (DERI) para cuantificar la variabilidad de la repolarización ventricular (VRV). Este indicador tiene en cuenta la variación de las componentes rápidas y lentas de variación de los intervalos RR, RTm (pico de R a pico de T) y RTe (pico de R a final de T). Se ha evaluado en 20 sujetos sanos, 12 sujetos con cardiopatía isquémica (no infartados) y 13 sujetos que sufrieron infarto de miocardio agudo. El indicador DERI es menor en sujetos sanos que en infartados (p < 1,68 ´10-4 para el RTm y p < 4,46 ´ 10-4 para el RTe) o con cardiopatía isquémica sin infarto (p < 3,23 ´ 10-5 para el RTm y p < 0,02 para el RTe). Se concluyó que la respiración afecta el valor del DERI, razón por la cual se propone registrar al paciente en períodos largos donde su respiración sea más o menos uniforme, como por ejemplo durante el período de sueño(AU)

Cuantificación de la variabilidad de la repolarización ventricular en pacientes con cardiopatía isquémica / Quantification of ventricular repolarization variability in patients with ischemic heart disease

Se propuso un nuevo indicador que se denomina: Relación de desviaciones estándar (DERI) para cuantificar la variabilidad de la repolarización ventricular (VRV). Este indicador tiene en cuenta la variación de las componentes rápidas y lentas de variación de los intervalos RR, RTm (pico de R a pico de T) y RTe (pico de R a final de T). Se ha evaluado en 20 sujetos sanos, 12 sujetos con cardiopatía isquémica (no infartados) y 13 sujetos que sufrieron infarto de miocardio agudo. El indicador DERI es menor en sujetos sanos que en infartados (p < 1,68 ´10-4 para el RTm y p < 4,46 ´ 10-4 para el RTe) o con cardiopatía isquémica sin infarto (p < 3,23 ´ 10-5 para el RTm y p < 0,02 para el RTe). Se concluyó que la respiración afecta el valor del DERI, razón por la cual se propone registrar al paciente en períodos largos donde su respiración sea más o menos uniforme, como por ejemplo durante el período de sueño

AC-coupled front-end for biopotential measurements.

AC coupling is essential in biopotential measurements. Electrode offset potentials can be several orders of magnitude larger than the amplitudes of the biological signals of interest, thus limiting the admissible gain of a dc-coupled front end to prevent amplifier saturation. A high-gain input stage needs ac input coupling. This can be achieved by series capacitors, but in order to provide a bias path, grounded resistors are usually included, which degrade the common mode rejection ratio (CMRR). This paper proposes a novel balanced input ac-coupling network that provides a bias path without any connection to ground, thus resulting in a high CMRR. The circuit being passive, it does not limit the differential dc input voltage. Furthermore, differential signals are ac coupled, whereas common-mode voltages are dc coupled, thus allowing the closed-loop control of the dc common mode voltage by means of a driven-right-leg circuit. This makes the circuit compatible with common-mode dc shifting strategies intended for single-supply biopotential amplifiers. The proposed circuit allows the implementation of high-gain biopotential amplifiers with a reduced number of parts, thus resulting in low power consumption. An electrocardiogram amplifier built according to the proposed design achieves a CMRR of 123 dB at 50 Hz.