Measurement Of Blood Volume Changes Research Articles

A Yellow–Orange Wavelength-Based Short-Term Heart Rate Variability Measurement Scheme for Wrist-Based Wearables

Heart rate variability (HRV) is one of the important biomarkers of physical and psychological well-being. Hence, a convenient and minimally intrusive method for HRV measurement is advantageous. Although high levels of surrogacy of short-term HRV estimates obtained from the measurements of blood volume changes to traditional electrocardiographic (ECG) measurements have been reported, no detailed account on extraction of such parameters from a wrist-based optical monitor is found in the literature. In this paper, a yellow–orange wavelength-based optical scheme is incorporated into a wearable device for HRV estimation from dorsal side of the wrist. This design is pivotal in catering to a wider span of population with varied skin tones. The developed wearable in alliance with a gateway device is capable of picking up photoplethysmography from the measurement site, allowing estimation of HRV-indices within a confidence of 5% from ECG-derived parameters. The HRV measurement ecosystem is validated under the setting of three postural loads for 20 subjects, generating 60 data sets. Study results show statistically significant positive correlation and nonsignificant bias in Bland–Altman analysis, for the HRV-indices derived from either method. In most of the extracted HRV features, the observations in supine position showed minimum deviation from the reference. Estimation of short-term HRV-indices from wrist-based photoplethysmography under stationary conditions shows promising results from the study. Electrical and biological noninterference and ease of usage of the proposed design simplify stationary and ambulatory monitoring of HRV.

A computational model relating changes in cerebral blood volume to synaptic activity in neurons

Brain imaging methods, and in particular fMRI (functional Magnetic Resonance Imaging), do not detect neural activity directly, but rather changes in blood flow and oxygenation in neighboring blood vessels, this being the BOLD (Blood Oxygenation Level Dependent) effect. We have constructed a model of the steps leading from neural activity to increased blood flow in arterioles, in which astrocytes play a crucial role. Glutamate released from neuronal synapses binds to metabotropic receptors on the astrocyte processes that ensheath these synapses. This initiates a calcium wave that travels along the endfeet of astrocytes that abut the endothelial cells forming the walls of blood capillaries; this calcium wave is propagated by the extracellular diffusion of ATP (adenosine triphosphate) that acts on metabotropic purinergic receptors on the astrocytes. A further second messenger (taken to be nitric oxide) relays this signal to the smooth muscle cells forming the outer walls of arterioles, and the subsequent wave of hyperpolarization reduces calcium influx and allows relaxation of the muscle cells and hence increased blood flow. The model gives results that are in agreement with experimental measurements of blood volume changes in the arterioles in the visual cortex of optically stimulated cats.

QRS Amplitude and Volume Changes during Hemodialysis

Background: According to several studies the QRS amplitude of the ECG increases during hemodialysis. The detailed background to this phenomenon has not been defined. Two main mechanisms have been suggested: myocardial ischemia and volume changes. New noninvasive technologies make possible a comparison of QRS complex changes synchronously with myocardial ischemia and extracellular water (ECW)/blood volume (BV) changes during hemodialysis. Methods: In this study hemodialysis-related changes in body weight, biochemical blood variables, BV, ECW, ST segment and QRS complex were analyzed in 15 patients (age 36–76, time on dialysis 0–6 years) undergoing chronic hemodialysis treatment. QRS complex and ST segment changes were measured using a dynamic vectorcardiographic monitoring system. The ECG parameters measured were QRS vector difference (QRS-VD) and ST vector magnitude (ST-VM6). Bioimpedance analysis was used to detect changes in the ECW. Continuous measurement of BV changes was implemented using an on-line optical reflection method based on the reflection of infrared light by erythrocyte membranes. Blood hemoglobin (B-Hb), hematocrit (B-Hcr), plasma sodium (P-Na), chloride (P-Cl), magnesium (P-Mg), potassium (P-K), ionized calcium (P-iCa), phosphate (P-Pi), creatinine (P-Crea) and urea (S-Urea) were monitored. Results: The mean QRS-VD increase during the dialysis session was almost fourfold (372 ± 300%) from 4.16 ± 2.40 to 15.60 ± 7.0 μVs (p < 0.001). This change was due to a change in amplitude, since the duration of the QRS complex did not alter significantly. The correlation between the changes in QRS-VD and body weight from the start to the end of the dialysis session was moderate and statistically significant (r = –0.55, p < 0.05). The correlation between the changes in QRS-VD and ECW varied from r = –0.67 to –0.97, being statistically significant in all patients (p < 0.001). The correlation between BV and QRS-VD was assessed at one minute intervals during the dialysis and varied from r = –0.22 to –0.98, being significant in 14 of the 15 patients (p < 0.001). Significant ST segments alterations (ST-VM6 elevation > 100 μV) did not occur during dialysis. Laboratory parameters reflecting volume and osmotic changes during hemodialysis correlated with QRS-VD change: B-Hcr (r = 0.56, p < 0.05), B-Hb (r = 0.63, p < 0.05), P-Na (r = 0.62, p < 0.05) and S-Urea (r = –0.62, p < 0.05). Conclusions: The increase in QRS complex amplitude during hemodialysis is correlated to reduced ECW. The mechanism involved is most probably augmentation of electrical resistance of the tissues around the heart caused by loss of interstitial fluid.

Voltage clamp method for the use of electrical admittance plethysmography in human body segments.

ELECTRICAL IMPEDANCE (NYBOER, 1970; KUBICEK et aL, 1970) or admittance plethysmography (ITO et al., 1976; YAMAKOSHI et aL, 1978) may still be one of the simplest and most convenient methods for the non-invasive measurement of blood volume change and/or blood flow, requiring only the attachment of electrodes on the body segment concerned. There have therefore been many studies on the non-invasive measurement of cardiac output (CO) (or stroke volume, SV) and regional blood flow (LBF), in conjunction with the venous occlusion technique, in human limbs particularly using the impedance method (GEDOES and BAKER, 1989). In contrast, we have previously used the adamittance plethysmography techifique, which bears an inverse relation to that of impedance. This method is more improved with respect to simplicity and practicability than the impedance method, as the variable to be measured is only the first derivative of admittance pulsations (dY/dt) for SV determination (ITo et aL, 1976; BUKHA~ et al., 1981) and only the admittauce change (6Y) following the venous occlusion for LBF measurement (YAMAKOSm et al., 1978; 1979; t980; SHIMAZU et al., 1982). However, the highly accurate measurement of the bio-admittanee signal has been much more difficult to obtain with electrical circuitry than the measurement of the bio-'mlpedance signal obtained by the relatively easy technique of the eomtant current ~areuit, because an analogue IC divider was conventionally used to perform the reciprocal conversion of the bio-impedanee signal CtTO et aL, 1976; YAMAKOSI-a et aL, 1979; BUKHAm et al., 1981). In this work, we therefore propose a voltage clamp method to obtain the bio-admittance signal with fairly high accuracy, but with very simple circuitry without such an analogue divider. The outline, main electric circuits and measurement accuracy of a device designed.by this method are presented, together with an application to CO and LBF measurements.

A blood protein monitor for the continuous measurement of blood volume changes during hemodialysis

Hemodialysis treatment comprises the removal of at times considerable amounts of fluid from the circulating blood volume of over-hydrated patients. Subsequently, there is movement of fluid from the interstitial tissue and the intracellular compartment into the vascular bed. While the net fluid removal from the patient is accurately assessed by most dialysis machines, the extent and rate of blood volume reduction cannot be monitored yet. Within the last years, blood volume monitoring has gained increased interest since hypotension, one of the major and most frequent complications of hemodialysis, is believed to be related to the variable tolerance of patients to blood volume reduction. There have been several suggestions for monitoring blood volume changes during hemodialysis. All methods operate on the continuous measurement of an intrinsic property of blood, such as mass density [1, 2], optical density [3], viscosity [4], electrical conductivity [5] or hemoglobin concentration [6]. However, some of these methods proved to have major drawbacks when it comes to the application in clinical practice. Recently, we have developed a sound speed sensor which allows continuous measurement of protein concentration of whole blood [7]. The purpose of our report is the introduction of a new type of sensor which is designed for the routine application of monitoring blood volume changes during hemodialysis treatment.

Elastic Properties of Blood Vessels Determined By Photoelectric Plethysmography

The dynamic elastic properties and vascular reactions of human blood vessels in vivo are studied by means of the photoplethysmographic method. The measurement of the mechanical properties of the blood vessel walls is based on measurement of blood volume changes resulting from changes in external pressure. The blood volume changes are mon itored by measuring the opacity changes in vascular bed, in this case, the distal phalanx of the finger. The elastic properties thus measured reflect some average property of a whole vascular bed. The technique is simple, noninvasive, and does not cause any damage or discomfort to the subject. In healthy young subjects, blood vessel elasticity was found to be constant in the range between the systolic and diastolic pressure. However the volume-pressure curves showed a hysteresis loop, i.e., the curves obtained during the increase of intravascular pressure differed from those obtained when pressure was decreased. The elasticity of the vascular walls of patients suffering from peripheral vascular diseases was found to be significantly different; their volume- pressure curves were nonlinear and had a much wider hysteresis loop (indicating a lower critical opening pressure), and their vascular reactions to changes in pressure were different. Possible uses of the method for diagnosing and evaluating arterio sclerotic and other vascular diseases and for studying vascular changes and reactions under conditions such as shock, heart failure, external abnormal pressures, and various drugs, are discussed.