Table of contents

Accelerometry offers a practical and low cost method of objectively monitoring human movements, and has particular applicability to the monitoring of free-living subjects. Accelerometers have been used to monitor a range of different movements, including gait, sit-to-stand transfers, postural sway and falls. They have also been used to measure physical activity levels and to identify and classify movements performed by subjects. This paper reviews the use of accelerometer-based systems in each of these areas. The scope and applicability of such systems in unsupervised monitoring of human movement are considered. The different systems and monitoring techniques can be integrated to provide a more comprehensive system that is suitable for measuring a range of different parameters in an unsupervised monitoring context with free-living subjects. An integrated approach is described in which a single, waist-mounted accelerometry system is used to monitor a range of different parameters of human movement in an unsupervised setting.

Hypertension is a condition of persistently elevated blood pressure, associated with increased cardiovascular risk. Non-invasive BP measurement using Korotkoff sounds is the most common method of screening for the condition. The possibility of inaccurate readings leading to a false diagnosis of hypertension (pseudo-hypertension) is of concern. Stiffened arteries in the elderly have been proposed as being the primary cause of pseudo-hypertension. Non-invasive detection of pseudo-hypertension remains problematic. This paper reviews clinical literature on pseudo hypertension and approaches to measuring the compressive stiffness of arteries, as well as biomechanical literature regarding models of arterial stiffness and the origin of Korotkoff sounds. Models of the latter show the importance of the relationship between transmural pressure and cross-sectional area (P_t/Csa curve) of the brachial artery as it closes under the influence of the pressure cuff. The review concludes that future research on pseudo-hypertension should include development of new instrumentation to measure the P_t/Csa curve of the brachial artery in vivo using non-invasive techniques suitable for application to an elderly population.

Cerebral electrical impedance is useful for the detection of cerebral edema following hypoxia in newborn infants. Thus it may be useful for determining neurological outcome or monitoring treatment. Hypothermia is a promising new therapy currently undergoing trials, but will alter impedance measurements. This study aimed to define the relationship between temperature and both cerebral and whole body electrical impedance, and to derive correction factors for adjustment of impedance measurements during hypothermia. In eight anaesthetized 1–2 day old piglets rectal, tympanic and scalp temperatures were monitored continuously. Following baseline readings at a rectal temperature of 39 °C, piglets were cooled to 32 °C. Four piglets were re-warmed. Cerebral and whole body impedance were measured at each 0.5 °C as rectal temperature decreased. There was a strong linear relationship between both cerebral and whole body impedance and each of the temperatures measured. There was no difference in the relationship between impedance and rectal, tympanic or scalp temperatures. The relationship for impedance and rectal temperature was the same during cooling and re-warming. Using the correction factors derived it will be possible to accurately monitor cerebral and whole body fluid distribution during hypothermic treatment.

Measuring PCO₂ (partial pressure of carbon dioxide) in an organ can enable early detection of ischemia. However, there are few clinical applicable solutions for measuring PCO₂. Based upon the requirement for clinical applications, a conductivity based PCO₂ sensor is proposed. A conductivity based PCO₂ sensor measures conductance in an aqueous solution separated from the measured object by a gas-permeable membrane. A bridge design with two cavities is favored for such a sensor. A planar and a cylindrical macro prototype based upon the bridge design were studied. The design criteria were based on the contribution from the electrode polarization, stray capacitances, contact area with the sample and design ability to miniaturize the sensor. The cylindrical sensor is favored because of its large contact area and advantages for miniaturization. Further investigation has to be done to confirm the functionality of such a design in a miniaturized form and its clinical performance.

Autonomic reflexes enable the cardiovascular system to respond to gravitational displacement of blood during changes in posture. Spontaneous oscillations present in the cerebral and systemic circulation of healthy subjects have demonstrated a regulatory role. This study assessed the dynamic responses of the cerebral and systemic circulation upon standing up and the posture dependence of spontaneous oscillations. In ten young healthy volunteers, blood pressure and cerebral haemodynamics were continuously monitored non-invasively using the Portapres® and near-infrared spectroscopy (NIRS), respectively. Oscillatory changes in the cerebral NIRS signals and the diastolic blood pressure (DBP) signal have been identified by the fast Fourier analysis. Blood pressure increased during standing and returned to basal level when volunteers sat on a chair. The mean value of cerebral tissue oxygen index (TOI) as measured by NIRS did not demonstrate any significant changes. Oscillatory changes in DBP, oxyhaemoglobin concentration [O₂Hb] and TOI showed a significant increase when subjects were standing. Investigation of the low frequency component (∼0.1 Hz) of these fluctuations revealed posture dependence associated with activation of autonomic reflexes. Systemic and cerebral changes appeared to preserve adequate blood flow and cerebral perfusion during standing in healthy volunteers. Oscillatory changes in [O₂Hb] and TOI, which may be related to the degree of cerebral sympathetic stimulation, are posture dependent in healthy subjects.

Easily applicable and inexpensive water-specific techniques to evaluate local oedema, swollen tissue problems and fluid retention in humans are not available. In the present investigation a recently constructed non-invasive device for a local measurement of changes in tissue water in human skin and subcutaneous fat (SSF) was validated. The instrument transmits an ultra high-frequency electromagnetic (EM) wave of 300 MHz into a coaxial line and further into an open-ended coaxial probe which is in contact with the skin. Due to the dimensions of the applied probe the penetration of the EM field extends to subcutaneous fat. A major part of the EM energy is absorbed by tissue water while the rest is reflected back into a coaxial line. From the information of the reflected wave an electrical parameter, directly proportional to tissue water content, called a dielectric constant of SSF, was calculated. For system validation, the decrease of water content in SSF measured with the dielectric technique in the volar forearm of seven patients during haemodialysis treatment was compared with the decrease of the circumference of the forearm and the amount of fluid removed. Statistically highly significant correlations were obtained between the decreasing dielectric constant (i.e. water content) of the SSF and the fluid removed during haemodialysis treatment (r = −0.99, p < 0.01) and between the decreasing dielectric constant and the circumference of the arm (r = 0.97, p < 0.05). The sensitivity of the dielectric method was four-fold compared with the circumferential measurement. The repeatability 3.0% was not dependent on the phase of haemodialysis. The new device allows an easy and non-invasive measurement technique to assess changes of tissue water in SSF.

Continuous measurement of cardiac output (CCO) is useful in assessing the cardiovascular status of patients during cardiac surgery and in intensive care. Recently, a CCO system (truCCOMS, Aortech, UK), capable of detecting rapid changes in cardiac output (CO) was introduced. The method is based on the energy required to maintain an integral heat-transfer device at constant temperature above the ambient value. The aim of this study was to assess the performance of this CCO system in vitro under in steady as well as pulsatile flow conditions representative of those in the pulmonary artery. In order to determine the sensitivity of the system to changes in vessel cross-sectional area and therefore local flow velocity, the catheter was deployed in a linear-tapered tube. Steady and pulsatile flows were generated, and the electrical power at various locations along the tapered tube was recorded. The results show significant differences in the performance under the two different flow conditions. In steady flow, the CO was highly dependent on the local velocity whereas in pulsatile flow, CO varied much less with local velocity. The sensitivity expressed as a percentage increase in CO per 100% increase in velocity at a CO of 5 l min⁻¹ was 87% in steady flow and 24% in pulsatile flow. Experiments carried out with three fluids with different viscosity show that the errors in determining CO in the tapered tube were also dependent on the Reynolds number and flow regime. The mean errors ranged from about 50% at 2 l min⁻¹ to less than 10% at 8 l min⁻¹. The correlation between the predicted and actual CO was generally good. In conclusion, the pulmonary artery catheter is not recommended in situations where blood flow is expected to be steady or of low pulsatility. It may, however, be suitable under normal pulsatile flow conditions in the pulmonary artery.

Changes in cerebral blood flow (CBF) can be assessed directly with xenon clearance (XeC) or indirectly by measuring changes in middle cerebral artery blood velocity (Vmca) with transcranial Doppler (TCD). The aim of this study was to compare the changes in CBF and Vmca following caffeine ingestion.

Nineteen patients (age 48–86, recovering from an acute stroke) and ten controls (age 52–85) were each studied twice. Bilateral measurements of CBF and Vmca were made before and after ingestion of 250 mg caffeine or matched placebo. The percentage change in CBF and Vmca after caffeine was calculated. Full results (CBF and Vmca) were obtained from 14 patients and 9 controls. There was no significant difference between patients and controls, so results were combined.

Caffeine reduced CBF by 22% (95% confidence interval (CI) = 17% to 28%) and reduced Vmca by 13% (95% CI = 10% to 17%). The fall in Vmca was significantly less than that in CBF (p = 0.0016), showing that caffeine reduces mca diameter. Analysis based on Poiseuille flow in the arterioles suggests that caffeine reduced arteriole diameter by 5.9% (95% CI = 4.6% to 7.3%) and mca diameter by 4.3% (95% CI = 2.0% to 6.6%).

TCD is being used as an alternative to XeC for assessing the effect of vasoconstrictors and vasodilators on CBF. This study has demonstrated that mca diameter can be changed by the vasoactive agents, and that changes in Vmca do not necessarily reflect changes in CBF.

This computer simulation study compared the ability of left ventricular coronary vein (LV) pacemaker leads against right ventricular (RV) and right atrial (RA) leads to monitor lung edema using electrical impedance measurements. MRI images were used to construct electrical models of the thorax. Four lead configurations were tested with increases of pulmonary edema, intravascular fluids and heart dilation. The impedance changes observed at end systole with severe lung edema were 8.5%, 11.2%, 12.3% and 26.8% for the RA, RV, RV coil and LV configurations, respectively. Sensitivities in ohms per litre of lung fluid were 19.15, 19.15, 25.07 and 52.11 for the same configurations. The impedance changes for intravascular fluid overload with constant lung status were 1%, 1.3%, 9.2% and 6.4% while the sensitivities were 2, 2, 17 and 11 ohms per litre of intravascular fluid, respectively. Regional analysis of the thoracic sources of impedance revealed a high sensitivity near pacing electrodes and generator, and a low sensitivity to the right lung and all pulmonary vessels. Simulations showed that LV leads have a threefold advantage in sensitivity when monitoring lung edema in comparison to conventional RV leads. To monitor vascular and lung fluids independently, combined impedance configurations may be used. Regional sensitivities must be taken into account for proper clinical interpretation of impedance changes.

Spectral analysis of the heart rate variability is becoming a usual tool as a marker of the autonomic nervous system. The final output of the spectral analysis is a set of indices that are always estimators due to technical limitations. In this work, the bias and the uncertainty in the VLF, LF, HF and LF/HF indices due to the finite sampling frequency of the ECG are analysed. The results indicate that for low sampling frequency (125 Hz), the bias and uncertainty in the HF and LF/HF indices can blur the results of the analysis, especially if the RR time series has low variability. The HF index is overestimated and, accordingly, the LF/HF index is underestimated. Then, results from RR time series with low sampling frequency must be used with care. The uncertainty of the spectral indices is proportional to the inverse of the sampling frequency and the bias is proportional to the inverse of the square sampling frequency.

The rheological properties of the arterial wall have intimate connections with the fine structure of the wall. Alteration in fine structure due to cardiovascular disease, such as atherosclerosis, could affect the rheological characteristics of the wall. The present study was designed to investigate changes in the static rheological properties of the aorta in Kurosawa and Kusanagi-Hypercholesterolemic (KHC) rabbits aged 10–12, 22–24 and 34–36 months in relation to histological alteration of the wall due to progression of atherosclerosis with age. Circumferential wall strips were excised from the ascending, proximal descending thoracic and proximal abdominal aortas and their stress/strain relationship was recorded. Tensile force of the wall showed a slight but insignificant decrease in the KHC rabbit group aged 10–12 months compared to that in the age-matched control group in the proximal thoracic aorta and increased significantly with ageing in the KHC rabbits in these aortic regions mainly at medium and high strain ranges. Wall stress was significantly smaller in the 10–12 months old KHC rabbit group than in the age-matched control group in the proximal thoracic and proximal abdominal aortas and increased significantly with ageing in the KHC rabbit groups chiefly at medium and high strain ranges. Incremental elastic modulus determined at 50% stretching of the initial length of the wall strip was also significantly lower in the KHC rabbit group aged 10–12 months in comparison to that in the age-matched control group and increased significantly with ageing in the KHC rabbit group. The intima thickened severely with abundant foam cells in the KHC rabbits aged 10–12 months. With increasing age, collagen and elastin fibres showed signs of gradual proliferation among the foam cells. The aortic wall in KHC rabbits was viscoelastic in the relatively early stage of atherosclerosis due to abundant foam cells, and thereafter increased in stiffness gradually with fibrous proliferation and calcification. We can conclude that the static rheological properties of the atherosclerotic aortic wall changed in association with alteration in the microstructure of the wall with progression of atherosclerosis.

Baroreflex sensitivity is becoming an important clinical measurement. Nevertheless there is no recommend standard measurement protocol. This study assessed the ability of eight protocols to induce regular changes in cardiac beat-to-beat interval and systolic pressure (SP), and the effect each protocol had on baroreflex sensitivity (BRS). Twelve subjects had changes in cardiac beat-to-beat intervals and SP levels induced at 8 times a minute by following 8 different protocols, each for 3 min. These comprised breathing in a supine and standing posture, breathing through a resistance, breathing into a closed orifice (the breathing protocols), and performing handgrip exercises, being rocked, having legs raised and lowered, and being presented with mental arithmetic questions (the non-breathing protocols). Induction success of each protocol was determined by the percentage of cardiac beat-to-beat interval and SP level signals with a peak at 8 times per minute in their frequency spectra. The consistency of the induced changes was measured by a signal-to-noise ratio (SNR). BRS was calculated from the frequency spectra. The induction success was 85% for breathing and 31% for non-breathing protocols. The consistency of cardiac beat-to-beat interval changes was highest with supine breathing (SNR = 1.6 ± 0.3) and resistance breathing (SNR = 1.5 ± 0.5) protocols. The consistency of SP level changes was highest with resistance breathing (SNR = 1.0 ± 0.3) and breathing into a closed orifice (SNR = 1.0 ± 0.5) protocols. BRS values in the supine breathing protocol (24 ± 10 ms mmHg⁻¹) and the handgrip protocol (32 ± 3 ms mmHg⁻¹) were significantly greater (p < 0.05) than for standing breathing (11 ± 5 ms mmHg⁻¹), resistance breathing (17 ± 8 ms mmHg⁻¹) or breathing into a closed orifice (12 ± 5 ms mmHg⁻¹) protocols. Different protocols have different induction successes and degrees of effectiveness in inducing cardiac beat-to-beat and SP level changes. BRS is affected by the induction protocol used, highlighting the need for a standard measurement protocol.

We investigated several factors which may affect the accuracy of fetal cardiac time intervals (CTI) determined in magnetocardiographic (MCG) recordings: observer differences, the number of available recording sites and the type of sensor used in acquisition. In 253 fetal MCG recordings, acquired using different biomagnetometer devices between the 15th and 42nd weeks of gestation, P-wave, QRS complex and T-wave onsets and ends were identified in signal averaged data sets independently by different observers. Using a defined procedure for setting signal events, interobserver reliability was high. Increasing the number of registration sites led to more accurate identification of the events. The differences in wave morphology between magnetometer and gradiometer configurations led to deviations in timing whereas the differences between low and high temperature devices seemed to be primarily due to noise. Signal-to-noise ratio played an important overall role in the accurate determination of CTI and changes in signal amplitude associated with fetal maturation may largely explain the effects of gestational age on reproducibility. As fetal CTI may be of value in the identification of pathologies such as intrauterine growth retardation or fetal cardiac hypertrophy, their reliable estimation will be enhanced by strategies which take these factors into account.

Stiffening of the brachial artery is implicated in pseudo-hypertension. To date, a reliable clinical predictor of the condition has not been developed. This paper describes the development of prototype instrumentation and methodology for measurement of the brachial artery transmural pressure/cross-sectional area relationship in vivo. The methodology has been validated using a model of an arm and a thin-walled rubber tube. Application of the technique to a healthy subject shows that the technique is viable and gives good reproducibility. Closure of the brachial artery with reducing transmural pressure is observed and recorded. The new technique has some important advantages over existing methodologies.

The trans-cardiac conductance (TCC) method provides on-line left ventricular (LV) volume signals by determining the electrical conductance of blood in the LV using central venous and epithoracic electrodes. Conductive structures outside the LV cause a 'parallel conductance' offset term (V^p) that is determined by bolus injections of hypertonic saline in the pulmonary artery (V^p_saline). Analysis of the increased conductance signal during passage of the bolus through the LV yields V^p_saline. Since TCC signals are picked up by epithoracic electrodes, concern has been raised that hypertonic saline remaining in the lungs might lead to overestimation. The decrease in blood conductivity induced by injection of non-ionic contrast medium during a LV angiogram may also be used to determine V^p (V^p_contrast). Since the contrast is injected directly into the LV, lung conductance should be unaltered. Thus, we compared V^p_saline with V^p_contrast in six anaesthetized sheep during different hemodynamic conditions. Linear regression showed that V^p_saline = 0.99 V^p_contrast + 2.45 ml (r² = 0.99). Bland–Altman analysis yielded a small non-significant bias (±2SD) of 1.8 (±6.8) ml. We conclude that parallel conductance for TCC can be accurately determined with the conventional hypertonic saline method.

Nasal obstruction is common and may disturb well-being during daytime and sleep. Its objective assessment by standard methods requires patient cooperation and inconvenient instrumentation. Therefore, the purpose of this study was to develop a novel, unobtrusive technique for continuous monitoring of changes in nasal conductance during natural breathing based on pressure recordings at the left and right nares. In ten volunteers, pressure swings derived independently from left and right nasal cannula were recorded at baseline, and after topical application of histamine and xylomethazoline. Nasal conductance was measured by means of epipharyngeal pressure recordings and face-mask pneumotachography as the reference standard. Decreases in nasal conductance were associated with changes in relative timing and shape of left and right nasal cannula derived pressure swings during breathing. This was reflected in decreases in the coefficient of cross-correlation between left and right nasal pressure signals. Diagnostic accuracy in detecting a fall in nasal conductance to <80% baseline by the maximal coefficient of determination from cross-correlation of left versus right nasal pressure was evaluated by receiver operator characteristics. The area under the curve was 0.88, 95% confidence interval 0.76 to 0.99, n = 40, P < 0.05 versus 0.5. We conclude that patterns of pressure swings derived from left and right nasal cannula may serve as a noninvasive means to detect changes in nasal conductance without requirement of patient cooperation.

In order to obtain power spectral information on the fetal heart rate in stages of pregnancy earlier than labor an algorithm has been developed to calculate the fetal heart rate on a beat-to-beat basis from Doppler ultrasound cardiotocographic signals. The algorithm was evaluated by comparing the calculated fetal heart rate with the heart rate determined from direct ECG signals measured with a scalp electrode. Heart rates were compared both in time and frequency domain. In the time domain the results achieved by both methods correlate well (correlation coefficient = 0.977 (p < 0.001)), in the frequency domain the results correlate even better (correlation coefficient = 0.991 (p < 0.001)). Based on these findings, it can be concluded that the developed algorithm provides a valuable tool for obtaining power spectral information on the fetal heart rate in stages of pregnancy earlier than labor.

A simple mount capable of securely holding a variety of intracranial probes to the skull was constructed from commonly available clinical consumables. Using this device the cerebral cortical blood flow of preterm lambs was measured using a laser Doppler flow probe, and cerebral pH and cortical electrical impedance were measured in newborn piglets using pH electrodes and Ag/AgCl wire electrodes. In both studies, the mount held the various probes for periods up to 6 h with no dislodgement or probe failure. The simple mount presented here can be adapted to a wide variety of intracranial probes and will hold them securely to the skull.