SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Theoretical models can help critical care nurses deliver planned and systematic care to families in a time-efficient manner. This article describes three family theoretical models: structural-functional, family systems, and family stress.
Theoretical considerations of metal complex formation in aqueous solutions were used to develop a computer program (CHELATE) to calculate all equilibrium species (free metal ions, metal complexes, etc.) in any user-defined system, such as xylem fluid. Mass-balance equations were established to describe each free metal ion and each free ligand concentration as a function of solution pH, total metal or total ligand, hydrogen-association constants, and the stability constants of known metal complexes. A default data base can be altered by the user to define any desired system covered by the stored equilibrium data. The program can currently handle nine metal ions, 35 ligands, and 500 complex species. The validity of the program was confirmed by using experimental test systems in which free-metal ion activity measurements were made with ion-selective electrodes.Program CHELATE was used to calculate the distribution of six metals in 0- to 1-hour exudate from soybean (Glycine max L. Merr.) and tomato (Lycopersicon esculentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. The results indicated that Fe is bound by citric acid, and Cu is bound by several amino acids in the normal-Zn exudate. Most of the Cu in soybean exudate is bound to asparagine and histidine. In tomato, Cu is bound to histidine, glutamine, and asparagine. Zinc, Mn, Ca, and Mg are bound primarily by citric acid and malic acid in both species; the per cent bound for these metals is lower than that for Fe and Cu. Zinc phytotoxicity caused equilibrium concentration shifts and resulted in the formation of several additional metal complexes not found in the normal-Zn exudate.
This paper is a report on the state of the art of light dosimetry in photomedicine and photobiology. The basic quantity of interest is the radiant energy fluence rate, which can either be measured using a suitable probe, or calculated theoretically from measured optical constants. First, theoretical models used to analyse experimental transmission and reflection data are briefly discussed. It is shown that a two-flux model derived from the transport equation in the diffusion approximation resembles the Kubelka-Munk and other heuristic models. This illustrates the limitations of these models and suggests their abandonment in favour of transport theory. For theoretical energy fluence rate calculations at least three optical constants are needed, namely the absorption coefficient, the scattering coefficient and the average cosine of the scattering angle. These three constants have been measured for very few tissues. In principle only two of the three constants can be measured directly on thin samples, independent of a theoretical model. The energy fluence rate can be measured quantitatively with a miniature fibre optic probe with isotropic response. Such measurements allow indirect determination of the three optical constants. It appears that we are just beginning to understand the distribution of light energy fluence rate in tissues. Tasks for the near future are comparison of methods to measure optical constants, quantitative checks of calculated and measured energy fluence rates in model tissues and optical phantoms and further development of theoretical models. Particular attention is required for boundary conditions, with and without refractive index matching.
STUDY DESIGN: In this investigation, a new concept of a miniature, bone-attached, medical robotic system for spinal operations is presented. As part of the design parameters of the robot, the forces and moments applied by the physician during insertion of Kirschner wires to soft tissues and drilling in hard tissues were examined. A theoretical model for the expected error of the robotic system due to the applied force has been derived and verified experimentally. The results of a clinical experiment that was carried out on a cadaver support the theoretical model derived and the miniature, bone-attached, robotic concept. OBJECTIVES: 1) Examining the concept of attaching a miniature robotic system to the spinous process of the operated vertebra. 2) Measuring the forces applied by the physician during insertion of Kirschner wires to soft tissues and drilling in hard tissues. 3) Evaluating the expected error of the robot due to mechanical and anatomic deflection caused by the forces applied by the physician during operation. 4) Testing and verifying the theoretical background by a clinical experiment. SUMMARY OF BACKGROUND DATA: Spinal operations are reported in the literature to have a relatively low success rate (70%-90%). This low success rate is affected by misunderstanding of the disease and its indications, resulting in bad selection of patients. From the technical point of view, the low success rate is greatly affected by the physician's lack of experience and the complexity of the spinal anatomy. The development of a miniature bone-attached robotic system for spinal operations could improve the success rate of spinal operations, introduce new percutaneous procedures, and shorten recovery and hospitalization time. Moreover, it will reduce the use of fluoroscopic exposure during operation; consequently, it will decrease considerably exposure to radiation during spinal operations. METHODS: Forces and moments applied by the physician during operation were measured by a 6-DOF miniature sensor. The measurements were taken during K-wire insertion both to soft and to hard tissues of a sheep and a human cadaver. A theoretical model of the expected location error of a K-wire, inserted to selected vertebralanatomies by the robotic system, was derived and verified experimentally. RESULTS: The theoretical model agreed with the experimental results, meaning that the combination of the spinous process and the robotic structure is rigid enough to guide a K-wire accurately. The forces and moments were measured and analyzed, and the total expected error due to the forces and moments was calculated. The clinical experiments supported the theoretical model and proved the system's feasibility. CONCLUSIONS: The given results support the theoretical model developed. Moreover, a miniature robotic guiding system can be attached to the spinous process of a given vertebra. The deflection and system error resulting from the forces and moments acting during operation are within the allowable errors.
In this technical note we study the prediction of ocular chromatic aberration which provides Le Grand's complete theoretical eye model. Theoretical computations of the longitudinal chromatic aberration are lower than experimental results. However, we show that experimental results of the chromatic difference of position (CDP) agree with theoretical predictions. The fit of the CDP for this model is practically the same as the one obtained for the Thibos reduced eye.
Histamine N-methyltransferase (HNMT) catalyzes the N-methylation of histamine in mammals. The experimentally determined HNMT three-dimensional (3D) structure is not available. However, there is a common genetic polymorphism for human HNMT (Thr105Ile) that reduces enzymatic activity and is a risk factor for asthma. To obtain insights into mechanisms responsible for the effects of that polymorphism on enzymatic activity and thermal stability, we predicted the 3D structure of HNMT using the threading method and molecular dynamics simulations in water. Herein, we report a theoretical 3D model of human HNMT which reveals that polymorphic residue Thr105Ile is located in the turn between a beta strand and an alpha helix on the protein surface away from the active site of HNMT. Ile105 energetically destabilizes folded HNMT because of its low Chou-Fasman score for forming a turn conformation and the exposure of its hydrophobic side chain to aqueous solution. It thus promotes the formation of misfolded proteins that are prone to the clearance by proteasomes. This information explains, for the first time, how genetic polymorphisms can cause enhanced protein degradation and why the thermal stability of allozyme Ile105 is lower than that of Thr105. It also supports the hypothesis that the experimental observation of a significantly lower level of HNMT enzymatic activity for allozyme Ile105 than that with Thr105 is due to a decreased concentration of allozyme Ile105, but not an alternation of the active-site topology of HNMT caused by the difference at residue 105.
AIMS: Using a lumped parameter theoretical model of bladder outlet function, we previously explored the relationship between the tube law of the urethra and the pressure-flow characteristics during micturition [Mijailovich et al., 2004]. To validate this theoretical model, we constructed a mechanical analog of the male lower urinary tract that incorporated elements simulating all relevant parameters of the theoretical model. In addition, we determined the effect of alterations in compliance of the flow-controlling zone on these relationships. METHODS: In the mechanical analog, the bladder neck and urethra were represented by a thin-walled conduit made of latex rubber and the prostate with a fluid filled cuff of variable compliance encircling the conduit. We measured in the same system steady-state pressure-flow (p-Q) and pressure-area (p-A) relationships of the flow-controlling zone. The effects of bladder outlet obstruction and prostatic compliance on these relationships were simulated by varying cuff pressure and capacitative coupling of the cuff, respectively. RESULTS: We demonstrated two previously described flow regimes-critical for low Q, and subcritical for higher Q. In the critical flow regime, the cross-sectional area of the collapsible conduit downstream of the cuff became narrow up to a site where the area suddenly expanded (elastic jump). Pressure losses across the cuff region decreased with increasing Q as the elastic jump approached the cuff, and the jump vanished when Q became subcritical. By altering prostatic cuff compliance and cuff pressure, we showed that an increase in opening pressures was associated with a steeper p-Q relationship. CONCLUSIONS: Using this mechanical analog, the measurements of p-A and p-Q relationships under various experimental conduit conditions validated our previous theoretical model of the male lower urinary tract. Both the experimental measurements and our previous theoretical model predictions indicate that an increase in opening pressure is associated with an increase in the slope of the p-Q relationship which becomes steeper with decreased cuff compliance. These data are in accordance with urodynamic findings in patients with benign prostatic hypertrophy (BPH) and suggest that a reduction in prostatic compliance exacerbates the severity of obstruction.
Physicians' acceptance of telemedicine constitutes a prerequisite for its diffusion on a national scale. Based upon the Theory of Interpersonal Behavior, this study was aimed at assessing the predictors of physicians' intention to use telemedicine in their clinical practice. All of the physicians involved in the RQTE, the extended provincial telemedicine network of Quebec (Canada) were mailed a questionnaire to identify the psychosocial determinants of their intention to adopt telemedicine. Confirmatory factor analysis (CFA) was performed to assess the measurement model and structural equation modelling (SEM) was applied to test the theoretical model. The adapted theoretical model explained 81% (P<0.001) of variance in physicians' intention to use telehealth. The main predictors of intentions were a composite normative factor, comprising personal as well as social norms (beta=1.08; P<0.001) and self identity (beta=-0.33; P<0.001). Thus, physicians who perceived professional and social responsibilities regarding adoption of telehealth in their clinical practice had stronger intention to use this technology. However, it is likely that personal identity had a suppression effect in the regression equation, indicating that physicians' intention to use telemedicine was better predicted if their self-perception as telemedicine users was considered. These results have several implications at the theoretical and practical levels that are discussed in this paper.
OBJECTIVE: To introduce the theoretical models used in literature that describe the relation between prosthetic inertial loading and amputee gait and to derive specific predictions from these models: to systematically review experimental studies on the relation between prosthetic inertial loading and energetics and kinematics of lower-limb prosthetic gait; and to compare the review outcomes with predictions derived from theoretical models. DATA SOURCES: Studies selected from Medline and from examining references in the selected Medline publications. STUDY SELECTION: Theoretical models were selected that are used in the present literature to predict the effects of prosthetic mass and mass distribution on kinematics and energetics of prosthetic gait. Experimental studies were selected that investigated the effects of prosthetic mass or center of mass location on the economy, self-selected walking speed, stride length, or stride frequency of lower limb amputee patients. DATA EXTRACTION: The design and methodologic quality was assessed using a checklist of nine criteria. Data on economy, self-selected walking speed, stride frequency, and stride length were extracted from the studies selected. DATA SYNTHESIS: The predictions of the theoretical models suggest that inertial loading of the present lightweight prostheses need not be decreased and sometimes may need to be increased to improve the gait of amputee patients. The methodologic quality of most of the experimental studies was limited. Review of the experimental studies suggests that the inertial loading of the present lightweight prostheses need not be further reduced. The discrepancy between theoretical models and experimental findings may be related to both the poor methodologic quality of the experiments as well as to the limited predictive value of the existing models.
The authors examine the roles of labor migration and commodity trade in the inter-regional equalization of the real wage rate in the United States. The emphasis is on North-South patterns. Theoretical models are first outlined and then tested using 1960 and 1970 census data and multi-regional input-output data for 1963. It is found that "the pattern of trade-embodied labor flow closely resembled the pattern of direct labor migration." It is also noted that "the South's hierarchical profile of relative plentifulness among age, education, and race-specific categories of work force tended to persist and remain fairly stable between 1960 and 1970.... One plausible hypothesis is that interregional commodity trade is accomplishing the bulk of the requisite adjustment in bringing about the real wage equalization."
The presence of random measurement error in indicators of theoretical constructs biases observed estimates of relations among those constructs. Correcting for this bias is particularly important when random measurement error is substantial, or is substantially different for indicators of distinct constructs included in a theoretical model. Validity assessment in the case of thematic apperceptive measures of the achievement motive (TAT n Achievement) has been vulnerable to interpretive errors because these indicators of the achievement motive are typically much less reliable than indicators of other constructs to which the motive may be related, and no correction has been made for the bias introduced by such differential measurement error. Here we illustrate a causal modeling approach to validity assessment for TAT n Achievement that incorporates explicit true-score measurement models of theoretical constructs. We confirm the hypothesis that the achievement motive construct is positively related to work satisfaction in a representative sample of adult males in the United States, taken in 1976 (N = 413). Evidence for the discriminant validity of story content versus story length, an issue raised in the literature on the TAT, is also presented in this nomological network.
In this paper we present a simple theoretical model of how pulsed ultrasound is attenuated by the particles in a solid/liquid flow. The theoretical model is then used to predict the attenuation of sound, given the mass fraction, the density, and the size distribution of the solid particles. The model is verified experimentally for suspensions of 0-10% (by mass) Dolomite ((Ca,Mg)CO3) particles and water. The experimental results show that the attenuation of sound due to particles varies linearly with mass fraction, and that the proposed theoretical model can be used to predict this attenuation. In all experiments the transmitter and receiver array were clamped onto the pipe wall, thus providing a completely non-invasive and non-intrusive measurement technique.
CONTEXT: Establishing a neurobiological account of delusion formation that links cognitive processes, brain activity, and symptoms is important to furthering our understanding of psychosis. OBJECTIVE: To explore a theoretical model of delusion formation that implicates prediction error-dependent associative learning processes in a pharmacological functional magnetic resonance imaging study using the psychotomimetic drug ketamine. DESIGN: Within-subject, randomized, placebo-controlled study. SETTING: Hospital-based clinical research facility, Addenbrooke's Hospital, Cambridge, England. The work was completed within the Wellcome Trust and Medical Research Council Behavioral and Clinical Neuroscience Institute, Cambridge. PARTICIPANTS: Fifteen healthy, right-handed volunteers (8 of whom were male) with a mean +/- SD age of 29 +/- 7 years and a mean +/- SD predicted full-scale IQ of 113 +/- 4 were recruited from within the local community by advertisement. INTERVENTIONS: Subjects were given low-dose ketamine (100 ng/mL of plasma) or placebo while performing a causal associative learning task during functional magnetic resonance imaging. In a separate session outside the scanner, the dose was increased (to 200 ng/mL of plasma) and subjects underwent a structured clinical interview. MAIN OUTCOME MEASURES: Brain activation, blood plasma levels of ketamine, and scores from psychiatric ratings scales (Brief Psychiatric Ratings Scale, Present State Examination, and Clinician-Administered Dissociative States Scale). RESULTS: Low-dose ketamine perturbs error-dependent learning activity in the right frontal cortex (P = .03). High-dose ketamine produces perceptual aberrations (P = .01) and delusion-like beliefs (P = .007). Critically, subjects showing the highest degree of frontal activation with placebo show the greatest occurrence of drug-induced perceptual aberrations (P = .03) and ideas or delusions of reference (P = .04). CONCLUSIONS: These findings relate aberrant prediction error-dependent associative learning to referential ideas and delusions via a perturbation of frontal cortical function. They are consistent with a model of delusion formation positing disruptions in error-dependent learning.
Homoplasy has been a prominent issue in primate systematics and phylogeny for as long as people have been studying human evolution. In the past, homoplasy, in the form of parallel evolution, was often considered the dominant theme in primate evolution. Today, it receives blame for difficulties in phylogenetic analysis, but is essential in the study of adaptation. This paper reviews the history of study of homoplasy, methods of defining homoplasy, and methodological and biological factors that generate homoplasy. A post hoc definition of homology and homoplasy, based on patterns of character distributions and their congruence or incongruence on a cladogram, is the most consistent method of recognizing these phenomena. Defined this way, homology and homoplasy are mutually exclusive. However, when different levels of analysis are examined, it is seen that homoplasy at one level, such as adult phenotype, often exists simultaneously with homology at a different level, such as developmental process. Thus, in some cases, patterns of homoplasy may point to underlying similarities that reflect the shared heritage of a particular clade. This is an old concept that is being renewed on the strength of recent trends in developmental biology. Factors that influence homoplasy include character definition and a host of biological factors, such as developmental constraints, allometry, and adaptation. These interact with one another to provide explanations of homoplastic patterns. Because of the repetition of events, explanations of homoplastic features are often more reliable than those for homologous features, and serve as effective tests for hypotheses of evolutionary process. In some cases, particular explanations of homoplasy lead to generalizations about the likelihood of homoplasy in a type of structure. The structure may be adaptive or highly epigenetic, or it may belong to an anatomical system considered to be more prone to homoplasy than others. However, our review shows that these generalizations are usually based on theory, and contradictory expectations can be developed under different theoretical models. More rigorous empirical studies are necessary to discover what, if any, generalizations can be made about the likelihood of homoplasy in different types of characters.
BACKGROUND AND OBJECTIVE: The size of laser-induced coagulated lesions produced in porcine muscle in vitro using a cylindrical diffusing fiber tip and a conductive heat source, made by covering the diffuser with a hollow steel needle, were compared to investigate the influence of charring. MATERIALS AND METHODS: Light from a Nd:YAG laser was utilized for thermotherapy. A theoretical model for calculating tissue temperature was used to predict the experimental results and to simulate in vivo treatments. RESULTS: The metal-covered tip produced carbonization and tissue vaporization that was not found with the diffuser. After 20 min of irradiation at a laser power of 7 W, the coagulated volumes with and without carbonization were found to be 13.1 cm3 (range 12.4-14.1 cm3, n = 4) and 12.2 cm3 (range 11.5-13.4 cm3, n = 4), respectively. Mathematical simulations showed that in unperfused tissue, a diffusing laser heat source produces smaller lesions than does a conductive heat source at the same power, the difference in coagulated volume becoming smaller with increased treatment time and increased power. CONCLUSION: Using cylindrical diffusers, interstitial laser-induced thermotherapy without carbonization at the fiber tip can be as efficient as treatment with carbonization.
BACKGROUND AND OBJECTIVES: The theoretical model of CO2 laser tissue interaction appeared to be too simplistic. To explain the reactions seen in skin, a more complex model was needed. We hoped to correlate the clinical-histologic patterns of CO2 laser tissue interactions. STUDY DESIGN/MATERIALS AND METHODS: The Ultrapulse CO2 laser was used on normal and pathologic skin conditions. Clinical observations were correlated with histologic examinations of biopsies. RESULTS: It was possible to demonstrate cavitation at the dermal-epidermal junction 2-3 diameters beyond the actual spot of CO2 laser contact with the skin. Dermal heat damage was seen as homogenization of collagen 1-2 diameters beyond the spot of laser contact. This flow of energy laterally at the dermal-epidermal junction and vertically down the skin follicles was both clinically beneficial and detrimental. Beneficially, superficial skin lesions separated at this junction and were easily removed. The heat coagulation of the dermis facilitated lesion removal without bleeding. The clinician had a better view of the pathology and could find focal zones of deeper pathology that could be easily re-treated. Detrimentally, this extended damage delayed wound healing and led to persistent erythema. CONCLUSION: These clinical-histologic correlations have provided a better understanding of CO2 laser tissue interactions in skin. It has been possible to take advantage of these findings to remove pathologic skin conditions more efficiently.
The method for accurate structure refinement from energy-filtered convergent-beam electron diffraction (CBED) patterns is described with emphasis on recent progress in using imaging filters and 2-D detectors. Details are given about the underlying theoretical model and the statistical analysis of experimental data. The relationship between crystal potential and charge density is also derived for crystals at thermal equilibrium. The method is applied to the refinement of Si (111) and (222) structure factors using various goodness-of-fit (GOF) criteria. Results show that the refinement method is robust and highly accurate. The importance of the experimentally measured structure factors is illustrated through the study of the charge density in MgO. With the measured structure factors, it is possible to obtain details about the charge redistribution due to crystal bonding.
Different pathways of the metal-induced isomerization of D-xylose to D-xylulose are investigated and compared in detail using energy minimization and molecular dynamics simulation. Two theoretical models are constructed for the reaction: in vacuum and in the enzyme D-xylose isomerase. The vacuum model is constructed based on the X-ray structure of the active site of D-xylose isomerase. It contains the atoms directly involved in the reaction and is studied using a semi-empirical molecular orbital method (PM3). The model in the enzyme includes the effects of the enzyme environment on the reaction using a combined quantum mechanical and molecular mechanical potential. For both models, the structures of the reactants, products, and intermediate complexes along the isomerization pathway are optimized. The effects of the position of the "catalytic Mg2+ ion" on the energies of the reactions are studied. The results indicate: 1) in vacuum, the isomerization reaction is favored when the catalytic metal cation is at site A, which is remote from the substrate; 2) in the enzyme, the catalytic metal cation, starting from site A, moves and stays at site B, which is close to the substrate; analysis of the charge redistribution of the active site during the catalytic process shows that the metal ion acts as a Lewis acid to polarize the substrate and catalyze the hydride shift; these results are consistent with previous experimental observations; and 3) Lys183 plays an important role in the isomerization reaction. The epsilon-NH3+ group of its side chain can provide a proton to the carboxide ion of the substrate to form a hydroxyl group after the hydride shift step. This role of Lys183 has not been suggested before. Based on our calculations, we believe that this is a reasonable mechanism and consistent with site-directed mutation experiments.