Showing papers in "Annals of Biomedical Engineering in 1985"
TL;DR: This work has found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure, which is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure.
Abstract: Autoregulation of cerebral blood flow is heterogeneous in several ways: regional, segmental, and temporal. We have found regional heterogeneity of the autoregulatory response during both acute reductions and increases in systemic arterial presure. Changes in blood flow are less in brain stem than in cerebrum during decreases and increases in cerebral perfusion pressure. Segmental heterogeneity of autoregulation has been demonstrated in two ways. Direct determination of segmental cerebral vascular resistance indicates that, while small cerebral vessels ( 200 μm) becomes increasingly important to the autoregulatory response at pressures above 100 mm Hg. Measurement of changes in diameter of pial vessels has shown that, during acute hypotension, autoregulation occurs predominantly in small resistance vessels (<100 μm). Finally, there is temporal heterogeneity of autoregulation. Sudden increases in arterial pressure produce transient increases in blood flow, which are not observed under steady-state conditions. In addition, the blood-brain barrier is more susceptible to hypertensive disruption after rapid, compared to step-wise, increases in arterial pressure. Thus, when investigating cerebral vascular autoregulation, regional, segmental, and temporal differences in the autoregulatory response must be taken into consideration.
118 citations
TL;DR: Subjects overwhelmingly preferred the 300 μs pulse duration regardless of waveform or source regulation, strongly preferred the symmetrical biphasic waveform, and had inconsistent preference for either regulated voltage or regulated current sources.
Abstract: Twenty-three females between the ages of 19 and 35 were studied in order to compare the effects of variations in pulse duration, waveform symmetry, and source regulation on comfort during quadriceps surface stimulation at amplitudes necessary to produce 27 Nm torque. Stimulation parameters compared were: 1) 50 and 300 μs pulse durations, 2) asymmetrical and symmetrical biphasic waveforms, and 3) current and voltage source regulation. Subjects overwhelmingly preferred the 300 μs pulse duration regardless of waveform or source regulation, strongly preferred the symmetrical biphasic waveform, and had inconsistent preference for either regulated voltage or regulated current sources.
118 citations
TL;DR: The problem of peristaltic transport of a non-Newtonian (Power law) fluid in a uniform and non-uniform tube has been investigated under zero Reynolds number and long wavelength approximation and pressure rise is found much smaller than the corresponding value in the case of uniform geometry.
Abstract: The problem of peristaltic transport of a non-Newtonian (Power law) fluid in a uniform and non-uniform tube has been investigated under zero Reynolds number and long wavelength approximation. A comparison of the results with those of Newtonian fluid model shows that the magnitude of pressure rise, under a given set of conditions, is smaller in the case of non-Newtonian fluid, when the flow behavior index n<1, at zero flow rate. Further, the pressure rise decreases as n decreases from 1, at zero flow rate, is independent of n at a certain value of flow rate, and increases if flow rate exceeds further. Also, at a given flow rate, an increase in the wavelength leads to a decrease in pressure rise and increase in the influence of non-Newtonian behavior. Pressure rise, in the case of non-uniform geometry, is found much smaller than the corresponding value in the case of uniform geometry. Finally, the analysis has been applied and compared with observed flow rates in the vas deferens in rhesus monkeys and in the small intestine.
93 citations
TL;DR: The posterior tibial nerves of 18 rabbits were intraneurally implanted with coiled wire electrodes for up to 9 weeks to evaluate their usefulness for neuromuscular electrical stimulation and showed little or no histologic demyelination or denervation in most specimens.
Abstract: The posterior tibial nerves of 18 rabbits were intraneurally implanted with coiled wire electrodes for up to 9 weeks to evaluate their usefulness for neuromuscular electrical stimulation. In one group an electrode was implanted and removed in one leg while the other leg was chronically implanted. A second group was chronically implanted without electrical stimulation in one leg and implanted with cyclical electrical stimulation applied through the electrode in the other leg. No significant changes in nerve conduction velocities between the time of implantation and up to 9 weeks post-implantation were observed in either the stimulated or the non-stimulated nerves. Little change in motor current threshold was observed beyond 10 days postimplantation. The nerves showed little or no histologic demyelination or denervation in most specimens, although in about 40% of the nerves, a bulbous formation of connective tissue was observed at electrode entry and exit sites with some demyelination in these regions. The spinal cords showed no histologic abnormalities in either group. The gastrocenemius and soleus muscles showed only occasional signs of denervation. One cat was implanted in both the posterior tibial and peroneal nerves of each leg for a 4-year period. Threshold current showed very little change during the implantation period. The nerves showed minimal focal demyelination at the electrode site and the muscles showed normal fibers.
69 citations
TL;DR: A self-tuning controller was implemented for the automated infusion of sodium nitroprusside to lower mean arterial pressure in anesthetized dogs and performed as well as the physician who devoted 100% of his attention to the task.
Abstract: A self-tuning controller was implemented for the automated infusion of sodium nitroprusside to lower mean arterial pressure in anesthetized dogs. The system incorporated a recursive least-squares parameter identifier and a modified minimum-variance controller. The onset delay was estimated on-line, the performance criterion included the cost of control, and requested step-changes were automatically translated into five successive smaller steps to reduce overshoot. The performance of the system in lowering mean arterial pressure was quantitatively compared with that of a well-trained anesthesiologist. In 10 runs in four animals, the automated system performed as well as the physician who devoted 100% of his attention to the task. Since the stability of the self-tuning controller cannot be guaranteed, such a system should be operated only in the presence of appropriate supervisory algorithms.
51 citations
TL;DR: Both metabolic and myogenic mechanisms play a role in cerebral arteriolar autoregulation under normal conditions, and the major metabolic mechanism seems to be dependent on changes in PO2 within the brain with secondary release of adenosine.
Abstract: Autoregulatory adjustments in the caliber of cerebral arterioles were studied in anesthetized cats equipped with cranial windows for the direct observation of the pial microcirculation. Increased venous pressure caused slight, but consistent, arteriolar dilation, at normal and at reduced arterial blood pressure and irrespective of whether or not intracranial pressure was kept constant or allowed to increase. Arterial hypotension caused arteriolar dilation which was inhibited partially by perfusion of the space under the cranial window with artificial CSF equilibrated with high concentrations of oxygen. This vasodilation was inhibited to a greater extent by perfusion of the space under the cranial window with fluorocarbon FC-80, equilibrated with high concentrations of oxygen. CSF or fluorocarbon equilibrated with nitrogen did not influence the vasodilation in response to arterial hypotension. The response to increased venous pressure was converted to vasoconstriction when fluorocarbon equilibrated with high concentrations of oxygen was flowing under the cranial window. The vasodilation in response to arterial hypotension was inhibited by topical application of adenosine deaminase. The results show that both metabolic and myogenic mechanisms play a role in cerebral arteriolar autoregulation. Under normal conditions, the metabolic mechanisms predominate. The presence of the myogenic mechanisms may be unmasked by preventing the operation of the metabolic mechanisms. The major metabolic mechanism seems to be dependent on changes in PO2 within the brain with secondary release of adenosine.
50 citations
TL;DR: Brain adenosine concentrations are rapidly increased within 5 sec of the onset of systemic hypotension and parallel, in a temporal fashion, the changes in pial vessel diameter and alterations in cerebral vascular resistance.
Abstract: We have investigated the role of adenosine, a purine nucleoside and potent vasodilator of cerebral pial vessels, during both acute (0-60 sec) and sustained (2-5 min) changes in cerebral perfusion pressure. Brain adenosine concentrations are rapidly increased within 5 sec of the onset of systemic hypotension and parallel, in a temporal fashion, the changes in pial vessel diameter and alterations in cerebral vascular resistance. During sustained hypotension, brain levels of adenosine are increased even within the autoregulatory range. These data are constant with the hypothesis that adenosine is an important metabolic factor in cerebral autoregulation.
50 citations
TL;DR: A microcomputer based control system has been developed to automatically induce and maintain neuromuscular block during surgery by repeatedly adjusts the infusion rate of a muscle relaxant, succinylcholine, based on the evoked, rectified, and integrated EMG.
Abstract: A microcomputer based control system has been developed to automatically induce and maintain neuromuscular block during surgery. The system repeatedly adjusts the infusion rate of a muscle relaxant, succinylcholine, based on the evoked, rectified, and integrated electromyogram (EMG). The system was tested in 12 patients for a 30-min infusion period with a setpoint of 80% depression of the evoked, rectified, and integrated thenar EMG. The mean time to reach the setpoint for 10 of the patients was 5.5 (±1.87 SD) min and the mean time for 95% recovery after infusion was stopped was 5.4 (±0.83 SD) min. Average overshoot was 9.9% of the baseline (±3.1% SD), and the average time within ±10% of the setpoint was 22.1 min (±7.26 SD). The total dose of succinylcholine for these 10 patients ranged from 1.21 to 3.77 mg/kg with a mean of 1.92 mg/kg. The other two patients were relatively insensitive to the drug and the controller was unable to bring the response to the set-point due to a ceiling placed on infusion rate by the control algorithm.
48 citations
TL;DR: A computerized system that uses feedback of end-tidal CO2 fraction (FETCO2) to adjust minute volume of a ventilator has been developed and tested and exhibited suitable dynamic response to step-changes in set-point with loop gains as large as two times and as small as one-half the optimal value.
Abstract: A computerized system that uses feedback of end-tidal CO2 fraction (FETCO2) to adjust minute volume of a ventilator has been developed and tested. The effectiveness and robustness of the controller were evaluated in five anesthetized dogs. The controller responded to step-changes in the set-point for FETCO2 by adjusting minute volume so that the FETCO2 settled to the new set-point in less than 60 sec with less than 20% overshoot. The system exhibited suitable dynamic response to step-changes in set-point with loop gains as large as two times and as small as one-half the optimal value. The breath-to-breath variation in FETCO2 values during prolonged periods of closed-loop controlled ventilation was smaller than the variation during periods of constant minute volume ventilation in three of five experiments. The controller generally maintained FETCO2 within +/- 0.1 vol% of the set-point. A disturbance to the controlled system was produced by releasing an occlusion of a branch of the pulmonary artery. The controller always responded to this disturbance in a stable manner, returning the FETCO2 to its desired value within 30 sec. Accurate control of arterial partial pressure of CO2(PaCO2) will require modifications enabling the system to determine the relationship between FETCO2 and PaCO2.
48 citations
TL;DR: It is concluded that myogenic mechanisms, at least in this size artery, are partly responsible for flow autoregulation, and that they are supplemented by metabolic mechanisms operative in the intact rat brain.
Abstract: We examined the diameter responses of isolated and pressurized posterior cerebral artery branches to various static and dynamic pressure alterations. These vessels, dissected from an anatomically identifiable location in the rat brain, developed tone when placed in a normal calcium physiological salt solution (1.6 mM Ca-PSS). Following a series of transmural pressure steps (Δp) of 25 or 50 mm Hg completed in 1–2 s and made every 5 min, they attained additional tone resulting in a mean luminal diameter of 139 μm at 100 mm Hg which was 35% less than their relaxed size measured in 1 mM EGTA-PSS. Continuous measurements of wall thickness and lumen diameter were obtained using a video electronic system in 1–2 mm long arterial segments, and autoregulatory gain factors calculated. Myogenic responses were obtained from each of 6 vessels taken from 6 WKY rats. Diameters following the step pressure changes were usually stable within 2–4 min. The data defined a myogenic regulatory pressure range from 49–145 mm Hg. Gain values averaged about 17% of that necessary for these arteries to maintain perfect flow autoregulation. Our results for myogenicity are comparable with the pressure range for blood flow autoregulation reported by others for the rat. We conclude that myogenic mechanisms, at least in this size artery, are partly responsible for flow autoregulation, and that they are supplemented by metabolic mechanisms operative in the intact rat brain.
48 citations
TL;DR: The results are consistent with the classical theory of “boundary” lubrication as modified and reflect the almost ideal molecular structure of the identified surfactants for adsorption, film cohesion and interaction of fatty-acid chains during sliding.
Abstract: Phospholipids have been identified by thin-layer chromatography in appreciable quantities in pericardial fluid taken from 12 dogs and found to include sphingomyelin and phosphatidylcholines, -ethanolamines, -inositols and -serines--the cholines predominating. The extracts, the synthetic surfactants and a mixture of synthetics simulating the extracts were all found to be good lubricants when tested by a standard method. The phosphatidylcholines were capable of reducing friction between two otherwise hydrophilic surfaces by as much as 100- to 200-fold when deposited as an oriented monolayer. A goniometer was used to measure an average contact angle of 33 degrees for a drop of saline placed upon the internal wall of the pericardium, indicating an appreciably hydrophobic surface anticipated if surfactant were directly adsorbed. The results are consistent with the classical theory of "boundary" lubrication (17) as modified (21) to reflect the almost ideal molecular structure of the identified surfactants for adsorption, film cohesion and interaction of fatty-acid chains during sliding. This model is proposed as an alternative to hydrodynamic lubrication in the pericardium and one compatible with several practical aspects such as pericardial rub and the maintenance of normal heart action after pericardectomy.
TL;DR: Percutaneous healing may be directly related to histocompatibility of the implant material, mechanical interfacial forces, and epidermal proliferative patterns, and the first two may eventually be controlled by selection of optimal implant materials and device configurations.
Abstract: Percutaneous devices permanently protrude through a surgically created defect in the skin. Usually they provide a connection for intracorporeal implants or organs with external devices. The skin penetration area presents unique medical problems. The interfacing tissue usually does not heal and seal to the implant but remains a focus of constant acute or chronic inflammation and eventually breaks down because of infection. This pathophysiological phenomenon has been studied previously with qualitative light microscopical methods. A large number of empirical studies have attempted to improve the implant-epidermal seal with various implant materials and designs. To allow systematic studies of the effect of biomaterials on implant-epidermal interface phenomena, quantitative histological parameters were evaluated. Test implants were made from Dacron velour and placed in dogs, goats, and rabbits for various preselected periods to determine time- and species-related histopathological variations. Results showed that the degree of connective tissue "maturity" within the pores of the implant appears to be related to the concentration of giant cells and polymorphonuclear granulocytes (histocompatibility). The process of epidermal proliferation around porous percutaneous implants appears to follow certain fixed patterns under different conditions that are accompanied by expelling forces, resulting in an outward movement of the implant until it is completely extruded. The presence of microhematomas throughout the implantation period indicates that mechanical forces disrupt interfacial tissue bridges. The basic histological processes are qualitatively the same in the three animal species studied. However, there are quantitative differences with regard to epidermal migration rate and connective tissue maturation within the implant pores, which may explain the different failure modes and times observed among species. The study indicated that percutaneous healing may be directly related to histocompatibility of the implant material, mechanical interfacial forces, and epidermal proliferative patterns. The first two may eventually be controlled by selection of optimal implant materials and device configurations. The control of epidermal migration, however, will be the key to prolonging percutaneous implant life.
TL;DR: Sensitivity analysis showed that epicardial compliance plays its major role during isovolumic contraction and relaxation; resistance plays a role throughout the cardiac cycle but is more important in diastole than in systole, whereas intramyocardial compliance play a role in syStole and in early diastoles.
Abstract: This study was performed to elucidate the effects of cardiac contraction on coronary pressure-flow relations On the basis of the waterfall mechanism, a lumped model of the coronary arterial system is presented consisting of a proximal (epicardial) compliance, a coronary resistance, and an intramyocardial compliance A “back”-pressure, assumed to be proportional (constant k) to left ventricular pressure, impedes flow From steady-state measurements of circumflex coronary artery flow and inflow pressure, together with left ventricular pressure, the values of the three model parameters and the constant k have been estimated In the control condition proximal compliance is found to be 17×10−12 m4s2kg−1, intramyocardial compliance 110×10−12m4s2kg−1, and resistance 75×109kgm−4s−1 The proportionality constant k is close to unity Effects of changes in left ventricular pressure and inflow pressure and the effect of vasoactive drugs on the parameters are also investigated Changes in coronary resistance are always opposite to changes in intramyocardial compliance Sensitivity analysis showed that epicardial compliance plays its major role during isovolumic contraction and relaxation; resistance plays a role throughout the cardiac cycle but is more important in diastole than in systole, whereas intramyocardial compliance plays a role in systole and in early diastole
TL;DR: The relationship between pulse rate (PR) and pulse width (PW) for a constant level of electrocutaneous stimulation was ascertained using the method of comparative judgments in this article.
Abstract: The relationship between pulse rate (PR) and pulse width (PW) for a constant level of electrocutaneous stimulation was ascertained using the method of comparative judgments. Twelve volunteer subjects were asked to adjust the PW of Comparison Stimulus (S2) until its intensity matched that of a Standard Stimulus (S1) for which the PW was 200 microseconds and PR was 10 or 20 pulses per sec (pps). As expected, the experimental results indicate that the PW of a constant-current amplitude pulse train should decrease as its PR increases if a constant level of tactile stimulation intensity is desired. However, PW and PR were not linear-inversely related (p less than 0.005). Rather, their relationship was best described by a logarithmic equation: log PW = a + b log PR, where PW is in microseconds, a is 2.82, b is -0.412, and PR is between 1 and 100 pps. Utilization of this relationship during electrical stimulation of the skin sense will decouple the intensity component of the tactile sensation from its frequency component, thereby enhancing the potential comfort and clarity of this sensory communication interface.
TL;DR: A computer based adaptive control algorithm for simultaneous infusions of both an inotropic agent and a vasoactive agent to maintain the blood pressures at desired levels is presented.
Abstract: Control of arterial blood pressure has been successfully achieved by infusing a single vasoactive drug. However, in clinical practice, blood pressures are frequently controlled using multiple drug infusions. This paper presents a computer based adaptive control algorithm for simultaneous infusions of both an inotropic agent and a vasoactive agent to maintain the blood pressures at desired levels. We present the dynamics of the system with a bilinear two-input, two-output model. A least-squares parameter estimation algorithm has been employed using an output error method. Results of computer simulations of an electrical analog model of the heart and circulatory system are presented.
TL;DR: It is postulated that the effectiveness of bonding between the mineral and organic phases could be an important influence on the behavior of bone with respect to its mechanical properties, metabolic activity, and aging effects associated with these factors.
Abstract: It is postulated that the effectiveness of bonding between the mineral and organic phases could be an important influence on the behavior of bone with respect to its mechanical properties, metabolic activity, and aging effects associated with these factors. Changes in bonding effectiveness might also be related to the etiology of osteoporosis. If this hypothesis is correct, it would be of interest to determine the amount of debonding present in bone. An analysis that employs both macromechanical and micromechanical composite theory is performed to show how this quantity could be calculated. The approach taken is first to determine the elastic moduli of a characteristic volume from bulk elastic properties of bone and the mineral crystallite orientation distribution. Voigt and Reuss type averages are used to obtain upper and lower bounds. Modifications of the Halpin-Tsai equations that apply to chopped fiber composites are then used to calculate the amount of debonding between the phases in the characteristic volume. All of the parameters employed in the theory are measurable using established techniques. To apply the theory quantitatively the following information must be known: 1) the density and elastic moduli of the bone (and its phases), and 2) the mineral orientation distribution.
TL;DR: Calculations indicate that large pH excursions occur around electrodes pulsed at current densities used for neural stimulation, and these transient pH changes extend only a few micrometers from the electrode surface.
Abstract: A mathematical description of pH excursions produced in interstitial fluid by a spherical stimulation electrode is presented. The pH is calculated as a function of current density, electrode radius, distance, time, and pulsing regimen for an electrode driven by biphasic current pulses. Calculations indicate that large pH excursions occur around electrodes pulsed at current densities used for neural stimulation. For an electrode with a radius of about 1 micron these transient pH changes extend only a few micrometers from the electrode surface. The practical importance of these pH changes remains to be determined.
TL;DR: Results show that cerebral autoregulation is a function of the perfusion pressure gradient and cannot be accounted for predominantly by myogenic mechanisms.
Abstract: The cerebral blood flow (CBF) response to changes in perfusion pressure mediated through decreases in arterial pressure, increases in cerebrospinal fluid (CSF) pressure and increases in jugular venous pressure was studied in anesthetized dogs. A preparation was developed in which each of the three relevant pressures could be controlled and manipulated independently of each other. In this preparation, the superior vena cava and femoral vein were cannulated and drained into a reservoir. Blood was pumped from the reservoir into the right atrium. With this system, mean arterial pressure and jugular venous pressure could be independently controlled. CSF pressure (measured in the lateral ventricle) could be manipulated via a cisternal puncture. Total and regional CBF responses to alterations in perfusion pressure were studied with the radiolabelled microsphere technique. Each hemisphere was sectioned into 13 regions: spinal cord, cerebellum, medulla, pons, midbrain, diencephalon, caudate, hippocampus, parahippocampal gyrus, and occipital, temporal, parietal and frontal lobes. Despite 30 mm Hg reductions in arterial pressure or increases in jugular venous pressure or CSF pressure, little change in CBF was observed provided the perfusion pressure (arterial pressure minus jugular venous pressure or CSF pressure depending on which pressure was of greater magnitude) was greater than the lower limit for cerebral autoregulation (approximately 60 mm Hg). However, when the perfusion pressure was reduced by any of the three different methods to levels less than 60 mm Hg (average of 48 mm Hg), a comparable reduction (25–35%) in both total and regional CBF was obtained. Thus comparable changes in the perfusion pressure gradient established by decreasing arterial pressure, increasing jugular venous pressure and increasing CSF pressure resulted in similar total and regional blood flow responses. Independent alterations of arterial and CSF pressures, and jugular venous pressure produce opposite changes in vascular transmural pressure yet result in similar CBF responses. These results show that cerebral autoregulation is a function of the perfusion pressure gradient and cannot be accounted for predominantly by myogenic mechanisms.
TL;DR: The effectiveness of intraaortic balloon pumping was investigated by using a lumped parameter model of the cardiovascular/assist device system and the results suggest that when SV is not being monitored, optimal inflation timing can be controlled from the measurements of TTI or pulmonary venous pressure.
Abstract: The effectiveness of intraaortic balloon pumping was investigated by using a lumped parameter model of the cardiovascular/assist device system. The model consists of a time-varying elastance left ventricular simulation, a 2-element windkessel arterial simulation, and an RC venous return and pulmonary simulation. The four major hemodynamic variables, stroke volume (SV), aortic mean diastolic pressure (MDP), tension time index (TTI), and aortic end diastolic pressure (EDP), were divided into two categories related to system energy supply and demand: “external” and “internal” variables. The effects of balloon pumping on these variables can be described by closed-form equations that yield an optimal solution. The model prediction suggests that, in the ideal case, optimization of balloon pumping calls for instantaneous inflation of the balloon to maximum volume at end systole and instantaneous complete deflation at end diastole. For finite inflation/deflation rates, the optimal time for the start of inflation is end systole. Deflation timing, however, involves a tradeoff between maximizing the external variables and minimizing the internal variables. These predictions were tested using a nonlinear digital computer model. The results also suggest that when SV is not being monitored, optimal inflation timing can be controlled from the measurements of TTI or pulmonary venous pressure; optimal deflation timing can be controlled by a weighted combination of MDP and EDP.
TL;DR: Cerebral vessels of many mammalian species exhibit stretch-dependent or myogenic tone which would be expected to contribute to autoregulation and this tone may be contingent upon the redistribution of strategic substances within the cell membrane.
Abstract: Cerebral vessels of many mammalian species exhibit stretch-dependent or myogenic tone which would be expected to contribute to autoregulation. This tone is dependent on extracellular calcium and probably involves mechanisms other than those associated with agonist and potassium-induced tone. This tone may be contingent upon the redistribution of strategic substances within the cell membrane.
TL;DR: A membrane electrical mechanism for myogenic autoregulation of cerebral arteries is suggested, which may involve changes in the electrical potential across the plasma membrane.
Abstract: Autoregulation of cerebral blood flow is accomplished through integration of metabolic, neurogenic and myogenic mechanisms. Myogenic mechanisms involve activation of cerebral arterial muscle cells as transmural pressure increases, providing a means through which vessel caliber can be regulated to maintain blood flow constant. The cellular mechanisms involved in this myogenic response may involve changes in the electrical potential across the plasma membrane. When isolated cat middle cerebral arteries are cannulated and prepared in a manner allowing manipulation of transmural pressure, the muscle cell membrane depolarizes as pressure increases. The degree of membrane depolarization in response to an elevated pressure is dependent upon extracellular Ca2+ ([Ca]0), increasing as [Ca]0 is elevated and markedly decreasing as [Ca]0 is reduced to low levels. When these arterial preparations are maintained at a physiological pressure of around 100 mm Hg, spontaneous action potentials can be recorded which increase in frequency upon further elevation in pressure. Vessels exhibiting such electrical activity can be observed to decrease in diameter as pressure is increased. Such finding suggest a membrane electrical mechanism for myogenic autoregulation of cerebral arteries.
TL;DR: How important physical parameters may be determined for both time-varying magnetic fields and biological or cellular experiments using electromagnetic fields is described.
Abstract: For biological or cellular experiments using electromagnetic fields, it is essential that the parameters defining the field be carefully specified if the results are to be meaningful and are to be compared with the same experiment conducted in a different laboratory. The interaction of living systems with electric and magnetic fields can come only through forces exerted on the charges on the system. If the charges are stationary, the only origin of the force is the electric field. This electric field may be established by charge distributions, as in "capacitive plate" experiments, or by time-varying magnetic fields. A geometry commonly used to produce time-varying magnetic fields consists of a pair of coaxial coils, each of equal radius and separated by a distance often equal to the radius. The electric field induced by a varying current in such a pair of coils varies both in space and in time. The electric field strength is zero on the axis of symmetry, and increases to a maximum near the radius of the coils. The strength is proportional to the time rate of change of the current in the coil, which depends not only on the amplitude and shape of the voltage pulse applied to the coil but also on the resistance and inductance of the coil. The purpose of this article is to describe how these important physical parameters may be determined for both geometries.
TL;DR: The transducer system provides a simple atraumatic means of periodically screening and/or monitoring cardiovascular changes in large populations of subjects and would provide a means of tracking changes induced by therapeutic programs.
Abstract: A transducer system is described that permits noninvasive recordings of the pressure-vs-time arterial profile at any palpable site on the body. A thin (30 μm) piezoelectric polymer film of polyvinylidene fluoride serves as the active element and as the mechanical coupler to the skin. The system has high frequency response and a low frequency cut-off of 0.16 Hz. The active element and its support provides a good mechanical impedance match with the skin. The transducer housing resembles a thick wrist watch and is strapped in place. The complete system is designed to record as many as four simultaneous profiles on a subject. Comparisons of the noninvasive transducer records with records taken with an indwelling catheter indicate congruence of the two signals and the first and second derivatives of the two signals, both when the transducer is applied directly to the skin and when a layer of fat is interposed between the skin and the transducer. The system provides a simple atraumatic means of periodically screening and/or monitoring cardiovascular changes in large populations of subjects and would provide a means of tracking changes induced by therapeutic programs.
TL;DR: The microsphere-based value of regional cortical blood flow fell below the regression line for the juxtamedullary cortex and above for the outer cortex, consistent with the preferential distribution of microspheres to theouter cortex whereas the contrast agent distributed more uniformly throughout the cortex.
Abstract: The three-dimensional image data generated by the Dynamic Spatial Reconstructor (DSR) enables measurement of the three-dimensional distribution of blood supply in organs. We have applied this imaging technique to evaluate renal cortical blood flow distribution and compare it with distribution of radiolabeled microspheres. The DSR, a high temporal resolution volumetric roentgenographic computed tomographic scanner, was used to scan the volume containing a kidney in 0.13–0.26 s and repeating this scan 8–4 times per s for six s during a renal arteriogram. Five anesthetized dogs were studied in the prone position with the left kidney exteriorized through a flank incision. An electromagnetic flowmeter was placed around the renal artery and a needle placed retrograde into the artery for injection of a 2 cc bolus of contrast agent. During the scan the contrast agent was injected over a four s period during which radioactively labelled microspheres were injected into the left atrium. The tomographic images of approximately 10 parallel, 5 mm thick sagittal slices corresponding to the slices of the kidney used for counting microspheres in the cortical layers were displayed and analyzed. The time point chosen for analysis was the one in which peak brightness (i.e., concentration of contrast agent) was detected in the cortex. The spatial distribution of peak brightness values was compared to the number of microspheres at the same sampling locations. The microsphere-based value of regional cortical blood flow fell below the regression line for the juxtamedullary cortex and above for the outer cortex. This result is consistent with the preferential distribution of microspheres to the outer cortex whereas the contrast agent distributed more uniformly throughout the cortex.
TL;DR: The junctional source is negligible and the junctional resistance affects the magnitude of the cellular source reducing it by possibly an order of magnitude, the significance of these results to cardiac electrophysiology is that the effect of the Junctional resistance may have to be considered separate from that of the Cellular resistance.
Abstract: This paper studies the relative strength of sources associated with the cell and the junction between cells of an equivalent single cardiac fiber. It is shown that the junctional source is negligible compared to the cellular source. On the other hand, the junctional resistance affects the magnitude of the cellular source reducing it by possibly an order of magnitude. The significance of these results to cardiac electrophysiology is that the effect of the junctional resistance may have to be considered separate from that of the cellular resistance.
TL;DR: The Annals of Biomedical Engineering has initiated a book review series under its newly formed section dedicated to Professional Technical Service to bring timely and informative reviews of new books in biomedical engineering to its readers.
Abstract: The Annals of Biomedical Engineering has initiated a book review series under its newly formed section dedicated to Professional Technical Service. The book review section will bring timely and informative reviews of new books in biomedical engineering to our readers. Additional book reviewers are needed to support this effort. If you would be willing to write an occasional review in exchange for a copy of the book, please send your name, address, phone number and areas of interest to:
TL;DR: To assess the magnitude and direction of these artifacts in the measurement of surface temperature in radiant warmers designed for human infants, the artifactual deviation of measured surface temperatures from mean surface temperature was determined under a short-wavelength warmer and a long-wa wavelength radiant warmer, using a copper ball as an experimental model.
Abstract: All skin temperature probes measure, to some extent, operative temperature as well as skin temperature, and thus artifactually measure a temperature different from true skin temperature. To assess the magnitude and direction of these artifacts in the measurement of surface temperature in radiant warmers designed for human infants, the artifactual deviation of measured surface temperatures from mean surface temperature was determined under a short-wavelength warmer and a long-wavelength radiant warmer, using a copper ball as an experimental model. The measuremends were made using both a disk-shaped thermistor and a tubular thermistor. All measurements were made near the top of the hemisphere of the ball facing the heating element of the warmer. In all cases, the average artifact was negative. That is, even on the surface of the ball near the radiant heat source, the surface temperature probes recorded an artifactually low temperature. In the analogous clinical setting, a some-what larger negative artifact would be expected.
TL;DR: A simulation study was performed to evaluate different recovery procedures for computing the multipole components of the cardiac electrical activity and found that LS gave better results than SI, but with a higher initial computation time.
Abstract: A simulation study was performed to evaluate different recovery procedures for computing the multipole components of the cardiac electrical activity. A series of dipolar potential distributions was first generated on a realistic numerical model of the human torso. Then, different procedures based on surface integration (SI) and least-squares (LS) minimization were used to compute the multipole components. The parameters of a single moving dipole (SMD) computed from the estimated multipoles were compared with those of the original dipole source. For a finite and homogeneous simulation as well as recovery medium, the results showed that SI employing the potentials over all 1216 surface elements of the torso model was not affected by the various numerical approximations used to perform the integration (e.g., rms error for the SMD position, p=0.7 mm). By integrating the potentials with truncated capping surfaces at the neck and the waist, the recovery errors increased (p=2.1 mm). Sampling the potentials at 63 sites, followed by interpolation over the rest of the torso surface, severely affected the SI results for the SMD (p=6.4 mm), as compared with LS minimization using also 63 values (p=0.9 mm). With lungs and intraventricular blood masses in the simulation medium but a finite and homogeneous recovery medium, SI was less effective (p=10.8 mm) than LS (p=8.6 mm). Adequate compensation for the effects of lungs was obtained by including regions of lower electrical conductivity in the recovery medium for LS, and by a correction matrix for SI. In general, LS gave better results than SI, but with a higher initial computation time.
TL;DR: Two estimates of temperature rise produced in body tissue when a spherical electrode passes current have been calculated and bracket the expected temperature rise are bracketed.
Abstract: Two estimates of temperature rise produced in body tissue when a spherical electrode passes current have been calculated The estimates bracket the expected temperature rise Time-transient and steady-state results have been obtained The effects of heat transfer through the highly conductive metal electrode and irreversible Faradaic reactions have been considered The calculations indicate that electrodes smaller than about 2 μm in radius produce a peak temperature rise of about 1°C when driven by typical square current pulses of 25 μA intensity and 200 μs duration The results are presented in a graphic form allowing for quick estimation of the expected peak temperature rise around electrodes of a specific radius, which are driven with a pulse of known current density and duration
TL;DR: Recently developed thermal analytical methods hold promise for measuring local tissue blood flow with accuracy and precision in the kidney, and two are reviewed here.
Abstract: The kidney, with its heterogeneous regional perfusion in the two anatomically and functionally distinct vascular beds of the renal cortex and medulla, and with its non-uniform blood vessel geometries, presents a unique challenge for measuring intrarenal blood flow distribution. Determining whole organ perfusion, on the other hand, is comparatively simple for the kidney, but it provides relatively little information about the suspected dependency of renal excretory function on local perfusion rate. Among the variety of methods proposed for gauging regional renal blood flow, some depend on measuring one or more of the tissue's thermal properties. The most straightforward, but least reliable, involve measurements either of focal tissue temperature alone, or of regional tissue thermal gradients. Simply using heat as a diffusible indicator, however, is unreliable as a measure of blood flow, for many of the same reasons that using an inert gas in a dilution technique is unreliable. Recently developed thermal analytical methods, though, hold promise for measuring local tissue blood flow with accuracy and precision. Two of them are reviewed here. One depends on measurement of the effective thermal conductivity of a small mass of tissue by evaluating the steady state ratio between regional unidirectional heat flux across it and the associated temperature gradient in one vector along a segment of it through an imposed spheroidal heat field. The other depends on analyses of tissue temperature decay subsequent to a controlled pulse of heat delivered through a small inserted thermistor bead. Both techniques use bioheat transfer equations to deduce regional blood flow by differentiating between heat dissipation due to local thermal conductivity and that attributable to the effects of regional convection. Although both methods are unavoidably invasive, neither produces debilitating damage in the tissue volume in which perfusion is measured, nor increases local temperature or metabolism enough to affect blood flow itself. Both techniques quantify local blood flow in small volumes of tissue by detailed evaluation of the many properties of tissue and blood which affect heat transfer, and both allow for a virtually unlimited number of nearly continuous sequential measurements at short (nom. 1 min) time intervals.