Showing papers in "Physiological Measurement in 2000"

A direct comparison of wet, dry and insulating bioelectric recording electrodes.

Abstract: Alternatives to conventional wet electrode types are keenly sought for biomedical use and physiological research, especially when prolonged recording of biosignals is demanded. This paper describes a quantitative comparison of three types of bioelectrode (wet, dry and insulating) based on tests involving electrode impedance, static interference and motion artefact induced by various means. Data were collected simultaneously, and in the same physical environment for all electrode types. Results indicate that in many situations the performance of dry and insulating electrodes compares favourably with wet electrodes. The influence of non-stationary electric fields on shielded dry and insulating electrode types was compared to wet types. It was observed that interference experienced by dry and insulating electrode types was 40 dB and 34 dB less than that experienced by wet electrode types. Similarly, the effect of motion artefact on dry and insulating electrodes was compared to wet types. Artefact levels for dry and insulating electrodes were significantly higher than those for wet types at the beginning of trials conducted. By the end of the trial periods artefact levels for dry and insulating types were lower than wet electrodes by an average of 8.2 dB and 6.8 dB respectively. The reservations expressed in other studies regarding the viability of dry and insulating electrodes for reliable sensing of biosignals are not supported by the work described here.

Electrical impedance tomography (EIT) in applications related to lung and ventilation: a review of experimental and clinical activities.

Abstract: This review article is a summary of the publications dealing with the pulmonary applications of electrical impedance tomography (EIT). Original papers on EIT lung imaging published over 15 years are analysed and several aspects of the performed EIT measurements summarized. Information on the type of the EIT device and electrodes used, the studied transverse thoracic planes, the data acquisition rate, the number of studied animals, normal subjects or patients, the kind of lung pathology, the performed ventilatory manoeuvres and other interventions, as well as the applied reference techniques, is given. The type of the generated pulmonary EIT images and the quantitative analysis of the EIT data are described. Finally, the major results achieved are presented, followed by an analysis of the perspectives of EIT in clinical applications. A comparative analysis of the EIT hardware and the quality of the evaluation tools was not performed.

Magnetic induction tomography: experimental realization

Abstract: Magnetic induction tomography (MIT) is a new non-contacting technique for visualization of the electrical impedance distribution inside inhomogeneous media. A measuring system for MIT has been developed. An oscillating magnetic field is applied in the system as a sounding agent. The system is designed mainly for biomedical applications. Experiments demonstrate that with proper selection of measurement conditions it is possible to use the phase shifts between inductor and detector signals for image reconstruction by filtered backprojection along magnetic lines. Measurements with saline filled phantoms having various spatial distributions of conductivity were carried out and images were reconstructed. The experiments have demonstrated the applicability of MIT for medical imaging and diagnostics.

A comparison of modified Howland circuits as current generators with current mirror type circuits

Abstract: Multi-frequency electrical impedance tomography (EIT) systems require stable voltage controlled current generators that will work over a wide frequency range and with a large variation in load impedance. In this paper we compare the performance of two commonly used designs: the first is a modified Howland circuit whilst the second is based on a current mirror. The output current and the output impedance of both circuits were determined through PSPICE simulation and through measurement. Both circuits were stable over the frequency ranges 1 kHz to 1 MHz. The maximum variation of output current with frequency for the modified Howland circuit was 2.0% and for the circuit based on a current mirror 1.6%. The output impedance for both circuits was greater than 100 kohms for frequencies up to 100 kHz. However, neither circuit achieved this output impedance at 1 MHz. Comparing the results from the two circuits suggests that there is little to choose between them in terms of a practical implementation.

Similarity in bilateral photoplethysmographic peripheral pulse wave characteristics at the ears, thumbs and toes

Abstract: The characteristics of the photoplethysmography (PPG) pulse signal are body site specific, with pulses from the various peripheral sites showing differences in pulse transit time, strength and shape, and variation of each over time The aim of this study was to determine the similarity in the right-left pulse characteristics for a group of normal subjects with pulses obtained simultaneously from six peripheral sites (cars, thumbs and big toes) A multi-site photoplethysmography pulse measurement and analysis system is described and comprised six sets of pulse probes and amplifiers, arranged and electronically matched in pairs for the right and left side comparisons Two sets of data were collected: firstly a set of validation data to test the electronic matching of right left channels, and secondly a set of physiological data for pulses from 40 normal subjects The right-to-left side similarity in pulse waveform shape at the three segmental levels (ears, thumbs and toes) was assessed using two types of analysis: root mean square error (RMSE) providing a measure of differences, and cross correlation analysis providing a measure of the degree of similarity Very low levels of RMSE and correlation coefficients close to unity were obtained for the system validation data, demonstrating good right-left channel matching The RMSE was an order of magnitude lower than that calculated for the normative physiological data, where median RMSE levels were between 0053 and 0060 relative to a peak-to-peak pulse amplitude of unity The median correlation coefficients were greater than 098 for all three segmental levels studied, with the maximum values approaching those of the validation data We have shown that pulses from the right and left sides of normal subjects are highly correlated at each segmental level We have obtained a normative range of pulse data, with which specific (vascular) patient groups can be compared

Spectral components of heart rate variability determined by wavelet analysis

Abstract: Spectral components of heart rate variability (HRV) are determined in the time-frequency domain using a wavelet transform. Based on the finer estimation of low-frequency content enabled by the logarithmic resolution of the wavelet transform, corrections of spectral intervals, already defined by Fourier and model based methods, are proposed. The characteristic peaks between 0.0095 and 0.6 Hz are traced in time and four spectral intervals are defined, I (0.0095-0.021 Hz), II (0.021-0.052 Hz), III (0.052-0.145 Hz) and IV (0.145-0.6 Hz), within which peaks are located for all subjects included. These intervals are shown to be invariant regardless of the age and the state of the system. We also show that the frequency and power of the spectral components are related to age, AMI and particularly to type II diabetes mellitus.

Multifrequency electrical impedance imaging: preliminary in vivo experience in breast

Abstract: We have deployed a recently completed spectroscopic electrical impedance tomography (EITS) imaging system in a small series of women (13 participants accrued to date) in order to investigate the feasibility of delivering EITS breast examinations on a routine basis. Hardware is driven with sinusoidally varying spatial patterns of applied voltage delivered to 16 electrodes over the 10 kHz to 1 MHz spectral range using a radially translating interface which couples the electrodes to the breast through direct contact. Imaging examinations have consisted of the acquisition of multi-channel measurements at ten frequencies on both breasts. Participants lie prone on an examination table with the breast to be imaged pendant in the electrode array that is located below the table. Examinations were comfortable and easy to deliver (about 10 minutes per breast including electrode-positioning time). Although localized near-surface electrode artefacts are evident in the acquired images, several findings have emerged. Permittivity images have generally been more informative than their conductivity counterparts, except in the case of fluid-filled cysts. Specifically, the mammographically normal breast appears to have characteristic absolute EITS permittivity and conductivity images that emerge across subjects. Structural features in the EITS images have correlated with limited clinical information available on participants with benign and malignant abnormality, cysts and scarring from previous lumpectomy and follow-up radiation therapy. Several cases from this preliminary experience are described.

Determination of stroke volume by means of electrical impedance tomography.

Abstract: ECG-gated electrical impedance tomography (EIT) is a non-invasive imaging technique, developed to monitor blood volume changes. This study is the first in comparing this non-invasive technique in measuring stroke volume with established techniques. The objective of this study was to validate EIT variables derived from the EIT images with paired obtained stroke volume measurements by thermodilution and MRI. After right cardiac catheterization, EIT measurements were performed in 25 patients. Regression analysis was used to analyse the relation between the EIT results and stroke volume determined by thermodilution. From the regression line an equation was derived to estimate stroke volume (in ml) by EIT. A strong correlation was found between EIT and stroke volume measured by the thermodilution method (r = 0.86). In a group of 11 healthy subjects this equation was validated to MRI. The mean and standard deviation of the difference between EIT and MRI was 0.7 ml and 5.4 ml respectively. These data indicate that EIT is a valid and reproducible method for the assessment of stroke volume.

Image correlation method for measuring blood flow velocity in microcirculation: correlation 'window' simulation and in vivo image analysis.

Abstract: To elucidate the function of the microcirculation system, it is very important to know the blood flow velocity and its distribution in the microvessels. We have developed an automated system for measuring blood flow velocity in microcirculation by image correlation. The `window' in the image correlation method is equivalent to the sensors in various other measurement methods. We performed simulations with virtual blood flow images consisting of random dots before measuring actual ones, and examined the optimum window shape and size. We found that by reducing the size of a circular window to the size of erythrocytes we could measure in vivo blood flow images with high accuracy. We recorded them with a high-speed video camera system at high temporal resolution, and measured the velocity in microvessels of normal Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR). SHR had higher blood velocity than WKY even though the vessel diameters were the same. Using this method to measure the blood flow velocity profile at the bent corner of SHR's arteriole at the heart systole, we found that erythrocytes flow faster at the inner side of the bend, so the vessel wall was exposed locally to higher shear stress in the hypertensive condition.

Comparison of five algorithms for the detection of ventricular fibrillation from the surface ECG

Abstract: The introduction and widening application of automatic external defibrillators (AEDs) present very strong requirements for external ECG signal analysis. Highly accuratc discrimination between shockable and non-shockable rhythms is required, with sensitivity and specificity aimed to approach the maximum values of 100%. We undertook an assessment of the performance of five detection algorithms, selected from among several others for their good published results. Test signals were 71 8 s ECG episodes on sinus rhythm and 90 8 s episodes on ventricular fibrillation, which were taken from the well known ECG-signal databases of the American Heart Association (AHA) and the Massachusetts Institute of Technology Beth Israel Hospital (MIT-BIH-'cudb' and 'vfdb' files). The purpose of this study is to assess the accuracy of the algorithms with signals other than those used for their development. An expected reduction of the sensitivity and specificity was found. The results could be used for further assessment, e.g. of noise and artefact sensitivity, for comparison with newly developed algorithms, etc.

Diuretic induced change in lung water assessed by electrical impedance tomography.

Abstract: Monitoring patients with left ventricular failure can be difficult. Electrical impedance tomography (EIT) produces cross-sectional images of changes in the impedance of the thorax. We measured changes in the electrical impedance of the lung in nine volunteers following a diuretic challenge. The hypothesis was that lung impedance would increase with diuretic induced fluid loss. Heart rate, blood pressure and urine output were also recorded. After diuretic the mean urine output was 1220 ml compared with 187 ml after placebo. Following diuretic administration, mean thoracic impedance increased by 13.6% (p < 0.01) and lung impedance increased by 7.8% (p < 0.05). Taken as a group there was a correlation between overall impedance change and total urine output. However, for each individual, the time course of change in impedance and urine output did not correlate significantly. Our findings show that EIT may offer a better guide to the response of the lung to diuretic treatment than simply measuring urine output. The urine output is neither specific nor sensitive in the assessment of lung water. Mean lung impedance, however, is largely determined by lung water. The study showed that lung impedance can be recorded at supra-normal values. EIT may help in the management of patients with excess lung water.

Real-time three-dimensional electrical impedance imaging

Abstract: Electrical impedance tomography is a technology for producing images of internal body structures based upon electrical measurements made from electrodes on the body surface. Typically a single plane of electrodes is used, seeking to reconstruct a cross-section of the body. Yet the majority of image reconstruction algorithms ignore the three-dimensional (3D) characteristics of the current flow in the body. Actually, a substantial amount of current flows out of the electrode plane, creating distortions in the resulting images. This paper describes a reconstruction algorithm, ToDLeR, for solving a linearized 3D inverse problem in impedance imaging. The algorithm models the body as a homogeneous cylinder and accounts for the 3D current flow in the body by analytically solving for the current flow from one or more layers of electrodes on the surface of the cylinder. The algorithm was implemented on the ACT3 real-time imaging system and data were collected from a 3D test phantom using one, two and four layers of electrodes. By using multiple planes of electrodes, improved accuracy in any particular electrode plane was obtained, with decreased sensitivity to out-of-plane objects. A cylindrical target located vertically more than 8 cm below a single layer of 16 electrodes, and positioned radially midway between the centre and the boundary, produced an image that had 35% of the value obtained when the target was in the electrode plane. By adding an additional layer of 16 electrodes below the first electrode plane, and using 3D current patterns, this artefact was reduced to less than 10% of the peak value. We conclude that the 3D algorithm, used with multiple planes of electrodes, reduces the distortions from out-of-plane structures in the body.

Movement artefact rejection in impedance pneumography using six strategically placed electrodes

Abstract: In this paper, we have proposed a technique for reducing movement artefacts in impedance pneumography by placing six electrodes at appropriate locations and suitably combining the measurements obtained. The strategy for electrode placement was based on the observation that the electrodes appeared to slide over the rib cage along with the skin, during movement. A volume conductor model of the thoracic cavity was developed and movement artefacts were simulated by shifting the electrodes to a different location on the surface. The impedance changes due to movement in one of the measurements of a 'symmetrical pair' were 180 degrees out of phase with respect to those observed in the other measurement of that pair. However, the impedance changes due to breathing were in phase in both these measurements. Thus, it was possible to reduce movement artefacts by taking a mean of these measurements without affecting the breathing related changes. The six electrodes could be configured into two such symmetrical pairs. The same observation was made in experimental data recorded from human subjects. This indicated that movement artefacts were caused by sliding of electrodes along with the skin and could be reduced by using the six-electrode configuration.

The application of electrical impedance tomography to reduce systematic errors in the EEG inverse problem--a simulation study.

Abstract: In this paper we propose a new method, using the principles of electrical impedance tomography (EIT), to correct for the systematic errors in the inverse problem (IP) of electroencephalography (EEG) that arise from the wrong specification of the electrical conductivities of the head compartments. By injecting known currents into pairs of electrodes and measuring the resulting potential differences recorded from the other electrodes, the equivalent conductivities of brain (sigma3), skull (sigma2) and scalp (sigma1) can be estimated. Since the geometry of the head is assumed to be known, the electrical conductivities remain as the only unknown parameters to be estimated. These conductivities can then be used in the inverse problem of EEG. The simulations performed in this study, using a three-layer sphere to model the head, prove the feasibility of the method, theoretically. Even in the presence of simulated noise with a value of signal-to-noise ratio (SNR) equal to 10, estimations of the electrical conductivities within 5% of the true values were obtained. Simulations showed the existence of a strong relation between errors in the skull thickness and the EIT estimated conductivities. If the skull thickness is wrongly specified, for example overestimated by a factor of two, the conductivity determined by EIT is also overestimated by a factor of two. Simulations showed that this compensation effect also works in the inverse problem of EEG. Application of the proposed method reduces systematic errors in the dipole localization, up to an amount of 1 cm. However it proved to be ineffective to decrease the dipole strength error.

Two-dimensional finite element modelling of the neonatal head

Abstract: Electrical impedance tomography (EIT) could allow the early diagnosis of infant brain injury following birth asphyxia. The purpose of this work was to determine the effect of variations in skull, scalp or cerebrospinal fluid (CSF) resistivity, as these vary in clinical conditions and could degrade image quality. These factors were investigated using finite element models of the adult and neonatal head. The results suggest that there is a wide range over which the resistivity of the neonatal skull has little effect on the sensitivity to a central impedance change. The scalp and CSF appear to shunt current away from the brain; when their resistivity was decreased from normal values, this shunting effect increased and caused a decrease in sensitivity to a central resistance change. The resistivity of neonatal skull has not, to our knowledge, been directly measured and will anyway vary within and between individuals; this work suggests that EIT will be relatively insensitive to variations in neonatal skull impedance.

Respiratory monitoring using an air-mattress system

Abstract: This paper describes a non-invasive respiratory monitoring system using an air mattress. The air-mattress system features multiple air compartments to monitor movements of the thorax and the abdomen separately. To evaluate the performance of the air-mattress system, four subjects were selected for the study and their separate ribcage and abdominal movements were monitored simultaneously by respiratory inductive phlethysmography belts and the air-mattress system. The sensitivity and accuracy indices of the air-mattress system for the detection of hypopnoeas scored remarkably well (above 90%). In addition, it was noted that the mean error in the measurement of the respiratory rate between the two systems was very small. This paper shows that the air-mattress system can be a reliable non-invasive respiratory monitoring system to detect simple abnormalities in breathing, such as respiration rate and hypopnoeas.

A quality control procedure for force platforms

Abstract: A force platform is a technical method of quantitatively assessing balance indirectly. The use of force platforms in physiotherapy departments has become more prominent over the last few years. However, the main drawback in the use of force platforms is the lack of comprehensive calibration procedures, which casts doubt on the results obtained with these systems. Existing calibration tests are limited to testing the spatial accuracy of the force platform. This paper describes a comprehensive quality control test procedure which was developed. It is proposed that the developed quality control test procedure could be used to test all types of force platform and it includes a description of how the tests should be carried out, the frequency with which they should be carried out and the expected performance for each of the tests as recommended for the most part by the Association Francaise de Posturologie.

A method for bio-electrical impedance analysis based on a step-voltage response

Abstract: Bioimpedance analysis (BIA) has been researched broadly, since it is simple, it presents good results and the analysers are portable, allowing it to be used in field studies. This paper presents a new technique of BIA based on a step-voltage current response and bipolar electrode array. A prototype of this new kind of analyser was developed and constructed to test the technique. Bench tests were performed to calibrate the prototype and the obtained results were comparable to those of commercial analysers. Body composition tests were conducted on 67 subjects of both sexes. Besides the bioimpedance analysis, anthropometric measures, consisting of weight, height, circumference and skinfold thickness, were also obtained from the subjects to allow an estimation of the body composition from anthropometric equations established in the literature. The results point to a good correlation (Pearson coefficient, r = 0.9645) between the anthropometric estimated fat-free mass (FFM) and its analogue estimated by the new bioimpedance technique.

Measurement of high resolution ECG QT interval during controlled euglycaemia and hypoglycaemia

Abstract: During hypoglycaemia, typically there is a change in the surface ECG characterized by a flattened and prolonged T wave, often accompanied by a fused U wave. The QT interval is a useful parameter for quantifying the ECG morphology. However, reliable measurement of QT is not straightforward, particularly for hypoglycaemic ECG morphology. The objective of this study was to compare the ability of two methods of QT measurement to distinguish between ECGs recorded during euglycaemia and hypoglycaemia. The first method involves manually setting the intersection of the isoelectric line and the T wave or, where this is not possible, the nadir between the T and U wave. The second method is semi-automatic and fits a tangent to the point of maximum gradient on the downward slope of the T wave. Two independent observers used both methods to measure the QT for high resolution ECG data recorded during a study of 17 non-diabetic subjects undergoing controlled euglycaemia and hypoglycaemia. Using the mean results of the two observers, the mean +/- SD increase in heart rate corrected QT, QTc, for ECGs recorded during euglycaemia and hypoglycaemia was 32 +/- 25 ms for the non-tangent method and 60 +/- 24 ms for the tangent method. Therefore, the tangent method provides greater distinction between ECGs recorded during euglycaemia and hypoglycaemia than the non-tangent method. A potential clinical application could be the non-invasive detection of impending hypoglycaemia at night, which would be of significant benefit to adults and young children with diabetes.

Measurement of extracellular fluid volume in the neonate using multiple frequency bio-impedance analysis.

Abstract: Bioelectrical impedance analysis may be useful for cot-side monitoring of fluid balance in the neonatal intensive care unit (NICU). However the presence of cardio-respiratory monitoring equipment, non-ideal electrode placement and inability to obtain accurate crown-heel measurements may interfere with the ability to obtain reliable impedance data in this setting. This study aimed to investigate the effects of these factors on impedance analysis and to develop a prediction equation for extracellular fluid volume in the neonate. The study found that cardio respiratory monitoring had no significant effect on multi-frequency impedance measurements. Placement of current delivering electrodes on the ventral rather than dorsal surfaces improved separation of current and voltage electrodes but did not alter impedance results. Contralateral measurements were not significantly different to ipsilateral measurements. In 24 infants, extracellular fluid volume was measured using corrected bromide space (CBS) and simultaneous impedance analysis was performed. There was good correlation between CBS and the impedance quotient FF2/Ro where F is foot length and R0 is resistance at zero frequency. The study concludes that despite many potential difficulties associated with impedance analysis in the NICU, reliable measurements of impedance can be obtained and further work to validate prediction equations for ECF is warranted.

Characteristics of laser Doppler perfusion imaging in vitro and in vivo.

Abstract: Traditional laser Doppler flowmetry (LDF) employs continuous recording of perfusion at one point with time. In order to eliminate the large spatial and temporal fluctuations that occur in the microcirculation, laser Doppler perfusion imaging (LDI) integrates flow readings over a large area. This paper describes a number of experiments to identify some of the characteristics of the LDI, its relationship to flow and no-flow conditions and to compare it with LDF. We undertook experiments to establish the effect of scanner head height, avascular skin thickness and haematocrit on LDI output. We also investigated the contribution of the biological zero signal (the signal obtained from skin when flow is arrested) to the LDI output. LDI output increased with scanner height in vitro and in vivo. Increasing avascular skin thickness reduced the LDI output although linear output characteristics with flow were maintained over the flow range studied. Increasing the haematocrit resulted in a loss of linearity of output with flow at lower velocities. The biological zero signal contributes a similar proportion of the output signal in LDF and LDI. We have presented a series of experiments that will contribute to the understanding of the characteristics of laser Doppler perfusion imaging, its comparison to laser Doppler flowmetry and its relationship to flow and no flow situations. However, our experiments were restricted to one machine, and may not necessarily be applicable to other instruments.

A resonator sensor for measurement of intraocular pressure--evaluation in an in vitro pig-eye model.

Abstract: Intraocular pressure (IOP) measurement is performed routinely at every eye clinic. High IOP, which can be a sign of glaucoma, can lead to degeneration of the retina and can cause blindness. In this ...

A simple method to check the dynamic performance of electrical impedance tomography systems

Abstract: The test concept as well as the design of a simple resistor phantom suitable for the evaluation of the properties of electrical impedance tomographic (EIT) systems is presented. Input and transfer impedance of the phantom are matched with those of the human thorax. Amplitude of the local impedance variations similar to in vivo conditions (ventilation) can be intentionally set to perform measurements on different states. The theoretical potential differences between the electrodes are calculated. The evaluation procedure is performed in terms of the local amplitude of the relative impedance change as well as the local distribution of noise. The whole procedure can be applied either to compare quantitatively the performance of different EIT data acquisition systems or to determine the amount of measurement disturbance caused by the external electrical environment in clinical settings.

Heart rate and blood pressure variability in normal subjects compared with data from beat-to-beat models developed from de Boer's model of the cardiovascular system.

Abstract: The objective of this study was to assess the ability of de Boer's model of the cardiovascular system to reproduce the heart rate and blood pressure variability observed in a range of normal subjects, and to make modifications to improve its performance. ECG, blood pressure and chest wall movement were recorded from 12 normal human subjects during controlled breathing. For each beat, systolic pressure, diastolic pressure, arterial time constant and RR interval were extracted. RR interval and systolic pressure spectral power in low and high frequency bands and the baroreflex sensitivity index, α, were then determined. For each subject, mean values were input to the model and the beat-to-beat output compared with the actual data for that subject. Finally, the effects of reducing the influence of baroreflex on peripheral vascular resistance and of providing separate sympathetic and vagal baroreflex sensitivities were assessed. Simulations resulted in data which were qualitatively similar to those of each subject's recording. With the modifications, the log ratio of simulated to real data improved from 7.2 to 1.5 (p = 0.003) for low frequency RR, from 0.27 to 0.55 (p = 0.011) for high frequency RR and from 8.5 to 0.9 (p = 0.003) for low frequency systolic pressure. We conclude that de Boer's model reproduces many of the characteristics of heart rate and blood pressure variability, and our modifications to baroreflex sensitivities and the feedback effect on peripheral resistance resulted in significant improvements.

3D simulation of EIT for monitoring impedance variations within the human head.

Abstract: A preliminary analysis is presented concerning the use of EIT for detecting impedance inhomogeneities within the human brain. The work to date is centred around the monitoring of two distinct impedance variations: those associated with the application of a carotid clamp during surgery and changes caused by the redistribution of blood flow during auditory stimuli. Using the commercially available Ansoft Maxwell package, a 3D finite element model of the human head has been developed to solve the forward problem. The model is hemispherical in shape and comprises regions of brain, cerebrospinal fluid, skull and skin and includes 16 scalp electrodes each of area 1 cm2 . Results from simulations using the model suggest that an EIT system, incorporating diametric current excitation, would require a voltage measurement sensitivity of 100-120 dB in order to detect the impedance variations in the above cases.

Variations in breathing patterns increase low frequency contents in HRV spectra.

Abstract: This paper shows that variations in breathing patterns broaden heart rate variability (HRV) spectral bands and increase the power amplitude of low-frequency bands Because of these influences, spectral markers for HRV signals, such as the quotient between spectral power at different frequency bands, should be compared only under controlled breathing conditions or after considering the effect of variations in breathing patterns

Right ventricular volume measurement: can conductance do it better?

Abstract: The measurement of right ventricular volume will be reviewed with special reference to the conductance catheter technique. The historical development of the intracavitary impedance technique will be described along with the theory of the multielectrode conductance method. The major potential advantage of this technique is its ability to measure dynamic volume change during the cardiac cycle. This enables a real time beat to beat assessment of ventricular volume in addition to providing continuous recordings during loading manoeuvres performed on the ventricle. However, the conductance catheter technique is based on the assumption that the electric field produced by the catheter is homogeneous and parallel to the long axis of the ventricle, and the current, created by the excitation electrodes of the catheter, is contained within the ventricular cavity. The measurement of these two calibration factors (known as parallel conductance volume (Vc) and dimensionless slope factor (α)), along with the effects of changes in blood resistivity and the orientation of the catheter on the measurement of absolute volume, will be described. Furthermore, some of the clinical applications of the technique in adults and children with heart disease will be outlined.

Time-frequency analysis of heart rate variability using short-time fourier analysis.

Abstract: This study was done to introduce new parameters derived by time frequency analysis of heart rate variability data. Four simulation experiments were carried out to compare the short-time Fourier transform (STFT) analysis method to the traditional overall spectral analysis method. Sinusoidal signals were generated with identical total power in the high-frequency band, but varying time-frequency and time-amplitude information. The STFT method was also applied to heart rate variability data from the stages of normal human sleep. Data analysis included computation of the power in the high-frequency band by overall spectral analysis. The instability coefficients (ICs) of the frequency and power in the high-frequency band were derived by STFT analysis. The ICs derived by the STFT method were able to describe time-frequency and time-amplitude variations in sinusoidal signals which contained identical total power in a specified frequency range. The ICs of the frequency and power were able to differentiate variations in vagal activity between the stages of human sleep and waking. The ICs represent new parameters derived by the STFT method, and allow the detection and quantification of short-lasting time-frequency and time-amplitude variations that remain obscured by overall spectral analysis.

Evaluation of respiratory influences on left ventricular function parameters extracted from echocardiographic acoustic quantification.

Abstract: This study was designed to assess, using the echocardiographic acoustic quantification technique, the influence of respiration on left ventricular (LV) function and its modifications connected with the ageing process, quantifying in a non-invasive way the respiratory contribution to the LV volume variability. An automated algorithm is applied to extract the beat-to-beat measurements of LV function parameters from the LV volume signal, obtained from recordings lasting a few minutes. Mean values, amount of variability and spectral content were studied in a population of 17 normal young (mean age 25±1 years) and 12 normal old (mean age 64±2 years) subjects. Mean values of the beat-to-beat measurements of LV function parameters were able to point out alterations connected with the ageing process in peak filling rate, peak atrial filling rate and peak ejection rate. Spectral analysis, applied to the extracted variability series, displayed a predominance of the high-frequency (HF) component corresponding to respiration in all LV function parameters; moreover, age related changes of HF variability were observed in peak ejection rate. The HF power spectrum component of beat-to-beat series extracted from the LV signal can provide a non-invasive assessment of the fluctuations in ventricular parameters associated with respiration.

Medical Physics and Biomedical Engineering

Abstract: B H Brown, R H Smallwood, D C Barber, P V Lawford and D R Hose Bristol: Institute of Physics Publishing (1999) 736pp, price: £35.00, ISBN: 0 7503 0368 9 This excellent text was first published in 1981 and was used by many students, trainees and professionals for many years until it went out of print. It was greatly missed from the bookshelves and it is good news that Institute of Physics Publishing have now published it as a revised, updated version. All the main subject areas of medical physics and bioengineering are covered and it provides a solid foundation for students and medical physics trainees studying physics and engineering in medicine. It is also very useful for older established professionals for revision and exploring new areas. The text is split into two parts: the background physics, electronics, anatomy and physiology (477 pages); and practical applications (241 pages). All chapters end with useful problems to solve and some, where appropriate, give relevant practical experiments. All have comprehensive references. The text is well backed up by good, understandable diagrams and equations. Some sections necessitate a mathematical approach, but this is presented in an easy to follow manner. Chapters 1-4 cover the mechanical aspects of the body and deal with bone, tissue, fluid mechanics, the senses and the biocompatibility of new technologies with tissue. Chapters 5 and 6 examine ionizing radiation, covering dose, exposure and radioisotopes, including a wide range of applications using isotopes in medicine. Chapter 7 gives a comprehensive ultrasound background and covers both the generation and interaction with biological materials. Chapter 8 covers non-ionizing electromagnetic radiation, from radiowaves to ultraviolet radiation, although lasers are not included. Electrical safety and the current electrical regulations are discussed. Chapters 9 and 10 introduce physiological signals and their measurement from the body. Electrodes, transducers and amplifiers are covered, and applications such as evoked potentials. Chapters 11, 12, and 14 introduce imaging theory, analysis and processing. Nuclear medicine, impedance, ultrasound, x-ray and MRI imaging are covered. Chapter 13 covers useful background mathematics and statistics which is meant to underpin the whole book, but it is useful information anyway. The next eight chapters give the practical applications that have been covered in the first part of the book. Chapter 15 is about the practical aspects of hearing and audiometry. Chapter 16 covers the practical measurement of the main electrophysiological signals (EMG, ECG, and EEG). Chapter 17 covers respiration measurement and looks at some of the electronics involved. Several techniques to measure respiration are discussed. Chapter 18 covers both non-invasive and invasive physiological pressure measurements. Chapter 19 deals with measuring blood flow using several contrasting techniques from ultrasound to thermal dilution. Chapter 20 introduces a range of biomechanical measurements including load cells and gait analysis. Chapter 21 covers radiotherapy and looks at treatment planning, staff hazards and simulators. The final chapter looks at the concept of safety critical systems in medicine and gives several examples, e.g. implantable cardiac devices, heart valves and haemodialysis.