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At least 865 records · Page 48Linked to original sources
PubMed2004

Application of theoretical modeling to multichannel active control of cooling fan noise.

Multichannel active control has been applied to the global reduction of tonal noise from a cooling fan. In order to achieve consistent far-field attenuation of multiple harmonics of the blade passage frequency (BPF) of the fan, an analytical model has been applied to the control system in order to determine appropriate transducer configurations. The results of the modeling show that the additional global reduction possible by locating acoustically compact secondary sources coplanar with a compact primary source rapidly lessens as the number of symmetrically placed sources is increased beyond three. Furthermore, the model suggests that there are locations in the extreme near field of the sources that can be considered ideal for the minimization of far-field radiated power. Experiments carried out show that a four-channel control system is more effective than a two-channel system at achieving far-field attenuations, especially at the higher harmonics of the BPF for the fan tested. In addition, greater far-field mean-square pressure attenuations are achieved with the error microphones located along the calculated ideal regions than for nonideal placement.

Journal Article↗
PubMed1972

Taste stimuli: time course of peripheral nerve response and theoretical models.

The responses of a taste nerve in rats to sodium chloride were integrated over successive 10-millisecond intervals and averaged. The time course of the mean responses consisted of a 30-millisecond latency, a rapid rise to a maximum, and a slower decline to a sustained level. The chemoreceptor theories of Beidler and Paton failed to predict the relation between phasic response and time or concentration.

Animals↗
PubMed1977

Early chemical evolution of nucleic acids: a theoretical model.

Recent experimental work suggests a possible cyclical pathway for early prebiotic oligonucleotide formation that involves (i) dry-state (nontemplate) synthesis of random copolymers with mixed 2',5' and 3',5' bonds, (ii) passage of these oligomers into solution at low temperatures, and (iii) a preferential hydrolysis of the 2',5' bond in any short helices that have formed. This early system could have selected for complementary sequences that were largely 3',5'-linked, but may not have selected efficiently for a single enantiomer of ribose.

Cold Temperature↗
PubMed2003

Defining a role for the subthalamic nucleus within operative theoretical models of subcortical participation in language.

OBJECTIVE: To investigate the effects of bilateral, surgically induced functional inhibition of the subthalamic nucleus (STN) on general language, high level linguistic abilities, and semantic processing skills in a group of patients with Parkinson's disease. METHODS: Comprehensive linguistic profiles were obtained up to one month before and three months after bilateral implantation of electrodes in the STN during active deep brain stimulation (DBS) in five subjects with Parkinson's disease (mean age, 63.2 years). Equivalent linguistic profiles were generated over a three month period for a non-surgical control cohort of 16 subjects with Parkinson's disease (NSPD) (mean age, 64.4 years). Education and disease duration were similar in the two groups. Initial assessment and three month follow up performance profiles were compared within subjects by paired t tests. Reliability change indices (RCI), representing clinically significant alterations in performance over time, were calculated for each of the assessment scores achieved by the five STN-DBS cases and the 16 NSPD controls, relative to performance variability within a group of 16 non-neurologically impaired adults (mean age, 61.9 years). Proportions of reliable change were then compared between the STN-DBS and NSPD groups. RESULTS: Paired comparisons within the STN-DBS group showed prolonged postoperative semantic processing reaction times for a range of word types coded for meanings and meaning relatedness. Case by case analyses of reliable change across language assessments and groups revealed differences in proportions of change over time within the STN-DBS and NSPD groups in the domains of high level linguistics and semantic processing. Specifically, when compared with the NSPD group, the STN-DBS group showed a proportionally significant (p<0.05) reliable improvement in postoperative scores achieved on the word test-revised (TWT-R), as well as a reliable decline (p<0.01) in the accuracy of lexical decisions about words with many meanings and a high degree of relatedness between meanings. CONCLUSIONS: Bilateral STN-DBS affects certain aspects of linguistic functioning, supporting a potential role for the STN in the mediation of language processes.

Aged↗
PubMed2005

Theoretical model of the transition between C1 and C2 chevron structures in smectic liquid crystals.

We present a study of the effect of weak anchoring on the transition between C1 and C2 chevron structures in smectic-C liquid crystals. The coexistence of C1 and C2 chevron structures within a single cell causes zigzag defects to occur and may affect the optical characteristics of the cell. By standard Euler-Lagrange minimisation of the total energy of the system, we obtain analytical expressions for the equilibrium director cone angle in the two chevron states. These in turn allow us to compare the total energies of the states and determine the globally stable chevron profile. We show that analytical predictions for the critical transition temperature, which depends on anchoring strength and pretilt angle, are in good agreement with those obtained numerically.

Journal Article↗
PubMed1980

Interpretation by theoretical model of dynamic and steady components in frog gustatory response.

The behavior of dynamic component (slow component) and steady component of the frog glossopharyngeal nerve response to CaCl2, measured in number of impulses firing per second, was examined under various conditions. 1) The magnitude of the dynamic component increased with the flow rate below about 0.3 ml/s, whereas that of the steady component was independent of the flow rate. 2) The magnitude of the dynamic component increased with a rise of temperature between 2.5 and 20 degrees C, whereas that of the steady component was practically independent of temperature. Removal of Ca2+ from the receptor membrane yielded a large dynamic component even at 2.5 degrees C. 3) The treatment of the tongue with procaine led to a large reduction in the magnitude of the dynamic component, whereas the effect of procaine on the steady component was not pronounced. 4) The magnitude of the dynamic component varied with the pH of the stimulating solution, whereas that of steady component was practically independent of a pH between 4.5 and 7.5. The above results were well interpreted by the following reaction scheme: S (stimulus + A (receptor domain) in equilibrium (SA)active in equilibrium (SA) inactive where (SA) active and (SA) inactive are active and inactive complexes.

Action Potentials↗
PubMed2007

Macromolecular transport in heart valves. II. Theoretical models.

This paper proposes a new, two-dimensional convection-diffusion model for macromolecular transport in heart valves based on horseradish peroxidase (HRP) experiments on rats presented in the first of the papers in this series (Part I; Zeng Z, Yin Y, Huang AL, Jan KM, Rumschitzki DS. Am J Physiol Heart Circ Physiol 292: H2664-H2670, 2007). Experiments require two valvular intimae, one underneath each endothelium. Tompkins et al. (Tompkins RG, Schnitzer JJ, Yarmush ML. Circ Res 64: 1213-1223, 1989) found large variations in shape and magnitude in transvalvular (125)I-labeled low-density lipoprotein (LDL) profiles from identical experiments on four squirrel monkeys. Their one-dimensional, uniform-medium diffusion-only model fit three parameters independently for each profile; data variability resulted in large parameter spreads. Our theory aims to explain their data with one parameter set. It uses measured parameters and some aortic values but fits the endothelial mass transfer coefficient (k(a)=k(v)=1.63 x 10(-8) cm/s, where subscripts a and v indicate aortic aspect and ventricular aspect, respectively) and middle layer permeability (K(p(2))=2.28 x 10(-16)cm(2)) and LDL diffusion coefficient [D(2)(LDL)=5.93 x 10(-9) cm(2)/s], using one of Tompkins et al.'s profiles, and fixes them throughout. It accurately predicts Part I's rapid localized HRP leakage spot growth rate in rat leaflets that results from the intima's much sparser structure, dictating its far larger transport parameters [K(p(1))= 1.10 x 10(-12)cm(2), D(1)(LDL/HRP)=1.02/4.09 x 10(-7)cm(2)/s] than the middle layer. This contrasts with large arteries with similarly large HRP spots, since the valve has no internal elastic lamina. The model quantitatively explains all of Tompkins et al.'s monkey profiles with these same parameters. Different numbers and locations of isolated macromolecular leaks on both aspects and different section-leak(s) distances yield all profiles.

Animals↗
PubMed1981

A theoretical model of protein, fluid, and small molecule transport in the lung.

The purposes of this research were to derive a mathematical model of blood-intestinal transport for the lung and to study the ability of this model to describe the results of previous lymph-collection and multiple-indicator experiments on the lungs of unanesthetized sheep. We used a three-pore model of the microvascular barrier to describe lymph flow, lymph-to-plasma ratios (L/P) of eight endogenous proteins, and the microvascular permeability-surface area (PST) of the lungs to [14C]urea in sheep experiments under base-line conditions and after acute elevation of the left atrial pressure. The results indicate that endothelial pathways consisting of a small pore (28 A), intermediate pore (180 A), and a large pore (1,000 A) can describe experimental L/P values and PST. The description of lymph flow required either than interstitial fluid pressure increase with left atrial pressure or that postcapillary venous resistance decrease relative to precapillary values. We concluded that multiple-pore theory is a useful approach to the description of lung blood-interstitial transport.

Animals↗
PubMed1996

Oxygen respiratory gas analysis by sine-wave measurement: a theoretical model.

A sinusoidal forcing function inert-gas-exchange model (C. E. W. Hahn, A. M. S. Black, S. A. Barton, and I. Scott. J. Appl. Physiol. 75: 1863-1876, 1993) is modified by replacing the inspired inert gas with oxygen, which then behaves mathematically in the gas phase as if it were an inert gas. A simple perturbation theory is developed that relates the ratios of the amplitudes of the inspired, end-expired, and mixed-expired oxygen sine-wave oscillations to the airways' dead space volume and lung alveolar volume. These relationships are independent of oxygen consumption, the gas-exchange ratio, and the mean fractional inspired (FIO2) and expired oxygen partial pressures. The model also predicts that blood flow shunt fraction (Qs/QT) is directly related to the oxygen sine-wave amplitude perturbations transmitted to end-expired air and arterial and mixed-venous blood through two simple equations. When the mean FIO2 is sufficiently high for arterial hemoglobin to be fully saturated, oxygen behaves mathematically in the blood like a low-solubility inert gas, and the amplitudes of the arterial and end-expired sine-wave perturbations are directly related to Qs/QT. This relationship is independent of the mean arterial and mixed-venous oxygen partial pressures and is also free from mixed-venous perturbation effects at high forcing frequencies. When arterial blood is not fully saturated, the theory predicts that QS/QT is directly related to the ratio of the amplitudes of the induced-saturation sinusoids in arterial and mixed-venous blood. The model therefore predicts that 1) on-line calculation of airway dead space and end-expired lung volume can be made by the addition of an oxygen sine-wave perturbation component to the mean FIO2; and (2) QS/QT can be measured from the resultant oxygen perturbation sine-wave amplitudes in the expired gas and in arterial and mixed-venous blood and is independent of the mean blood oxygen partial pressure and oxyhemoglobin saturation values. These calculations can be updated at the sine-wave forcing period, typically 2-4 min.

Air Pressure↗
PubMed1999

Muscle O(2) consumption by NIRS: a theoretical model.

In the past, the measurement of O(2) consumption ((2)) by the muscle could be carried out noninvasively by near-infrared spectroscopy from oxyhemoglobin and/or deoxyhemoglobin measurements only at rest or during steady isometric contractions. In the present study, a mathematical model is developed allowing calculation, together with steady-state levels of (2), of the kinetics of (2) readjustment in the muscle from the onset of ischemic but aerobic constant-load isotonic exercises. The model, which is based on the known sequence of exoergonic metabolic pathways involved in muscle energetics, allows simultaneous fitting of batched data obtained during exercises performed at different workloads. A Monte Carlo simulation has been carried out to test the quality of the model and to define the most appropriate experimental approach to obtain the best results. The use of a series of experimental protocols obtained at different levels of mechanical power, rather than repetitions of the same load, appears to be the most suitable procedure.

Adenosine Triphosphate↗
PubMed2001

A theoretical model for oxygen transport in skeletal muscle under conditions of high oxygen demand.

Oxygen transport from capillaries to exercising skeletal muscle is studied by use of a Krogh-type cylinder model. The goal is to predict oxygen consumption under conditions of high demand, on the basis of a consideration of transport processes occurring at the microvascular level. Effects of the decline in oxygen content of blood flowing along capillaries, intravascular resistance to oxygen diffusion, and myoglobin-facilitated diffusion are included. Parameter values are based on human skeletal muscle. The dependence of oxygen consumption on oxygen demand, perfusion, and capillary density are examined. When demand is moderate, the tissue is well oxygenated and consumption is slightly less than demand. When demand is high, capillary oxygen content declines rapidly with axial distance and radial oxygen transport is limited by diffusion resistance within the capillary and the tissue. Under these conditions, much of the tissue is hypoxic, consumption is substantially less than demand, and consumption is strongly dependent on capillary density. Predicted consumption rates are comparable with experimentally observed maximal rates of oxygen consumption.

Algorithms↗
PubMed2006

A theoretical model of selective cooling using intracarotid cold saline infusion in the human brain.

A three-dimensional mathematical model was developed to examine the transient and steady-state temperature distribution in the human brain during selective brain cooling (SBC) by unilateral intracarotid freezing-cold saline infusion. To determine the combined effect of hemodilution and hypothermia from the cold saline infusion, data from studies investigating the effect of these two parameters on cerebral blood flow (CBF) were pooled, and an analytic expression describing the combined effect of the two factors was derived. The Pennes bioheat equation used the thermal properties of the different cranial layers and the effect of cold saline infusion on CBF to propagate the evolution of brain temperature. A healthy brain and a brain with stroke (ischemic core and penumbra) were modeled. CBF and metabolic rate data were reduced to simulate the core and penumbra. Simulations using different saline flow rates were performed. The results suggested that a flow rate of 30 ml/min is sufficient to induce moderate hypothermia within 10 min in the ipsilateral hemisphere. The brain with stroke cooled to lower temperatures than the healthy brain, mainly because the stroke limited the total intracarotid blood flow. Gray matter cooled twice as fast as white matter. The continuously falling hematocrit was the main time-limiting factor, restricting the SBC to a maximum of 3 h. The study demonstrated that SBC by intracarotid saline infusion is feasible in humans and may be the fastest method of hypothermia induction.

Blood Flow Velocity↗
PubMed2002

Time dependence of recruitment and derecruitment in the lung: a theoretical model.

Recruitment and derecruitment (R/D) of air spaces within the lung is greatly enhanced in lung injury and is thought to be responsible for exacerbating injury during mechanical ventilation. There is evidence to suggest that R/D is a time-dependent phenomenon. We have developed a computer model of the lung consisting of a parallel arrangement of airways and alveolar units. Each airway has a critical pressure (Pcrit) above which it tends to open and below which it tends to close but at a rate determined by how far pressure is from Pcrit. With an appropriate distribution of Pcrit and R/D velocity characteristics, the model able to produce realistic first and second pressure-volume curves of a lung inflated from an initially degassed state. The model also predicts that lung elastance will increase transiently after a deep inflation to a degree that increases as lung volume decreases and as the lung becomes injured. We conclude that our model captures the time-dependent mechanical behavior of the lung due to gradual R/D of lung units.

Elasticity↗
PubMed2005

Vocalisation in verbal hallucinations: case report and theoretical model.

Perception of one's speech or the speech of others reflects an intrinsic process and should not to be seen as a product of information processing of external sensory input. From this perspective, the perception of one's own or others' speech is fundamentally equivalent to the experience of verbal hallucinations. Speech perception is generated primarily in the focus of attention, but auditory and proprioreceptive input from verbal articulations play an important constraining role, ensuring that perception remains adaptive to interaction with the external world. In verbal hallucinations these constraints may be partially disrupted. Perceptual qualities of hallucinatory voices may be generated in the focus of attention alone (in ignorance of actual acoustic stimulation from the environment), whilst content and grammatical aspects of hallucinatory speech perception may continue to be constrained adequately by proprioreceptive reafferentation from the speech apparatus. Thus subvocal speech may play a role in the elaboration and maintenance of verbal hallucinations.

Afferent Pathways↗
PubMed1982

Psychosomatic factors in the process of cancerogenesis. Theoretical models and empirical results.

The core of the authors' research program consists in the assumption that complex statistical interactions of sociological and psychological factors with pathophysiological and standard risk factors are more important for the early detection, prediction and prevention of cancer than one or just a few medical risk factors, e.g. the relationship between smoking and lung cancer. As the analysis of these relationships can only be done in interdisciplinary research, interpretation and discussion of social scientific, molecular biological and epidemiological aspects was attempted. In the first part we explicate a model of relevant propositions from the psychosomatic literature to explain the relationship between psychosocial factors of carcinogenesis. A discussion of the authors' research program contains additional variables and propositions. It appears to the authors that among the psychosocial and sociological factors of carcinogenesis, the molecular biological variables are especially relevant, as they form the intervening link between the social science variables and cancer. Discussion of the methodology and the sampling of the authors' empirical studies follows. Two prospective studies, carried out in Yugoslavia and West Germany, are described in detail. The data of these studies are used in the empirical analysis. In the next part the empirical results of different multivariate analyses are presented: The effects of the sociological and psychological variables on carcinogenesis are described; standard risk factors and biochemical-molecular biological factors are brought together, special attention being given to the interaction between these two groups of variables.

Adult↗
PubMed1968

Analysis of T-wave abnormalities associated with myocardial infarction using a theoretic model.

The physiological basis of serial T-wave abnormalities associated with myocardial infarction was studied. After coronary artery ligation in dogs, ventricular functional refractory periods (FRPs) were measured at five to eight epicardial, intramural, and endocardial sites. FRPs measured during acute ischemia shortened an average of 26 msec. FRPs measured 24 to 72 hr after coronary ligation were 24 msec longer at ischemic than at nonischemic sites. Alterations of recovery times were analyzed in terms of a repolarization model which related the form of the action potential downstroke to the form of the T wave of the body surface ECG. Observed FRPs were used to infer action potential duration and T waves were derived. Shortening FRPs in the anterolateral wall of the ventricle increased the amplitude of derived T waves in the X lead and caused T-wave inversion in the Z lead. Prolonging FRPs in the same area caused T-wave inversion in lead X and increased T amplitude in lead Z. The form of derived T waves qualitatively corresponded to serial T-wave abnormalities seen in patients with myocardial infarction.

Action Potentials↗
PubMed1991

Proarrhythmic response to sodium channel blockade. Theoretical model and numerical experiments.

BACKGROUND: The use of flecainide and encainide was terminated in the Cardiac Arrhythmia Suppression Trial because of an excess of sudden cardiac deaths in the active treatment group. Such events might arise from reentrant rhythms initiated by premature stimulation in the presence of anisotropic sodium channel availability. Drugs that bind to sodium channels increase the functional dispersion of refractoriness by slowing (a result of the drug-unbinding process) the transition from an inexcitable state to an excitable state. It is interesting that encainide and flecainide unbind slowly (15-20 seconds), whereas lidocaine and moricizine unbind rapidly (0.2-1.3 seconds). METHODS AND RESULTS: With a computer representation of a cable with Beeler-Reuter membrane properties, we found a small (6 msec) vulnerable window that occurred 338 msec after the last drive stimulus. Premature stimuli falling within the vulnerable window resulted in unidirectional block and reentrant activation. In the presence of a slowly unbinding drug, the window was delayed an additional 341 msec, and its duration was extended to 38 msec. The delay (antiarrhythmic effect) before the onset of the vulnerable window and its duration (proarrhythmic effect) were both dependent on the sodium channel availability and the recovery process. Both effects were also prolonged when sodium channel availability was reduced by membrane depolarization. Defining the proarrhythmic potential as the duration of the vulnerable window, we found that hypothetical use-dependent class I drugs have a greater proarrhythmic potential than non-use-dependent drugs. CONCLUSIONS: The antiarrhythmic and proarrhythmic properties of pure sodium channel antagonists are both dependent on sodium channel availability. Consequently, the price for increased antiarrhythmic efficacy (suppressed premature ventricular contractions) is an increased proarrhythmic vulnerability to unsuppressed premature ventricular contractions.

Anti-Arrhythmia Agents↗