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PubMed1976

Diffusion of dextran in concentrated solutions.

A free-diffusion method has been developed for the determination of the intradiffusion coefficient ('self-diffusion coefficient') of a polymer in highly concentrated solutions. A fraction of the polymer is labelled with a small amount of light-absorbing substituent. The diffusion of this labelled species, present in low concentration, is followed in the presence of a high concentration of unlabelled material with the aid of absorption optics in the analytical ultracentrifuge. The diffusion proceeds over a boundary at which the difference in concentration of unlabelled material is varied. The average concentration of total polymer and the concentration of the labelled material are, however, constant. From theoretical considerations it is shown that by extrapolation of the diffusion coefficient so obtained to zero concentration difference of total material, the intradiffusion coefficient of the polymer at that concentration is obtained. The procedure also permits the ordinary translational diffusion coefficient to be estimated. The method has been applied to two dextran fractions with weight-average molecular weights of 19000 and 150000, which were labelled with fluorescein groups. As expected, the intradiffusion coefficient decreases with increasing polymer concentration, the decrease being more pronounced for the high-molecular-weight material. This decrease in the diffusion rate of dextran is, however, less than the corresponding decrease in the sedimentation rate which proteins with similar hydrodynamic parameters experience in dextran solutions. This agrees with the hypothesis that flexible linear polymers move through a network as chains rather than as hydrodynamic spheres. By combining measurements of the ordinary diffusion coefficient and the intradiffusion coefficient, it is possible to calculate the thermodynamic properties (as expressed by the virial expansion) of the system. This method is of particular importance in studies on concentrated solutions of high-molecular-weight polymers.

Dextrans↗

PubMed2005

Diffusion- and perfusion-weighted MR imaging in persistent migrainous visual disturbances.

Pathological changes on diffusion-weighted MR scans had been described in hemiplegic migraine and perfusion changes had been demonstrated in typical migraine aura with radio-isotope studies and, more recently, MR perfusion imaging. However, there is relatively little knowledge of the pathophysiology of long-lasting migraine aura and its possibly variant phenotype, visual snow. Our aim was to investigate with advanced MR techniques whether patients with long-lasting visual disturbance showed regional alterations in cerebral water diffusion and perfusion. We have studied four patients using MR perfusion and MR diffusion imaging. Two patients had typical visual aura and two had a primary persistent visual disturbance (visual snow phenomenon). All patients had normal conventional structural MR imaging. MR diffusion-weighted images were acquired with a b-value of up to 1000 s/mm2. From the diffusion weighted images we generated maps of apparent diffusion coefficient (ADC), which were inspected visually and used for ADC measurements of predefined regions of interest, which included the visual, frontal, insular and temporal cortices. MR perfusion imaging was performed using a bolus tracking technique with dynamic susceptibility-weighted images. Colour coded maps of relative cerebral blood volume, mean transit time and bolus arrival time were generated, as well as time-signal intensity curves over the anterior, middle and posterior cerebral artery territories. The maps of the ADC and above perfusion parameters appeared symmetrical in all patients with no evidence of decreased water diffusion or cerebral perfusion in the occipital regions, or elsewhere. There was no statistically significant difference between the ADC measurements of the primary visual cortices and other cortical regions. Our findings suggest that regional changes in cerebral water diffusion and perfusion do not play an important part in the pathophysiology of persistent migraine aura or primary persistent visual disturbance.

Adult↗

PubMed1992

Participation of endothelium-derived nitric oxide but not prostacyclin in the gastric mucosal hyperaemia due to acid back-diffusion.

1. The possible participation of prostacyclin and nitric oxide (NO) in the gastric mucosal hyperaemic response to acid back-diffusion through a disrupted gastric mucosal barrier was examined. The experiments were carried out on anaesthetized rats in which acid back-diffusion was elicited by gastric perfusion with dilute ethanol in 0.15 M HCl and gastric mucosal blood flow (MBF) was measured by the hydrogen gas clearance technique. 2. Indomethacin (28 mumols kg-1, s.c.), an inhibitor of the formation of cyclo-oxygenase products including prostacyclin, failed to alter mean arterial blood pressure (MAP), basal MBF and the hyperaemic response to acid back-diffusion in urethane-anaesthetized rats. 3. NG-nitro-L-arginine methyl ester (L-NAME; 13 and 43 mumols kg-1, i.v.), an inhibitor of endothelium-derived NO formation, increased MAP in a dose-dependent manner. Whilst basal MBF in urethane-anaesthetized rats was not changed, the increase in MBF caused by gastric perfusion with dilute ethanol in acid was dose-dependently depressed by L-NAME. The loss of H+ ions from the gastric lumen, an indirect measure of acid back-diffusion, was significantly enhanced by 43 mumols kg-1 L-NAME. In contrast, D-NAME (13 and 43 mumols kg-1) was without effect on MAP, basal and stimulated MBF, and acid back-diffusion. 4. Unlike in urethane-anaesthetized rats, L-NAME led to a significant reduction of basal MBF in phenobarbitone-anaesthetized rats. MAP in the phenobarbitone-anaesthetized rats was significantly higher than in urethane-anaesthetized rats, and the hypertensive effect of L-NAME under phenobarbitone anaesthesia was significantly smaller than under urethane anaesthesia.5. The rise in MBF brought about by acid back-diffusion was blocked by L-NAME administered to phenobarbitone-anaesthetized rats. Infusion of L-arginine (120 pmol kg -1 min- ', i.v.) led to a partial, but significant, reversal of the effects of L-NAME on MAP and the hyperaemia due to acid back-diffusion.6. These findings indicate that endothelium-derived NO plays an important mediator role in the gastric mucosal vasodilatation caused by back-diffusion whilst vasodilator prostanoids such as prostacyclin are not involved.

Anesthesia↗

PubMed2006

Diffusion of green fluorescent protein in three cell environments in Escherichia coli.

Surprisingly little is known about the physical environment inside a prokaryotic cell. Knowledge of the rates at which proteins and other cell components can diffuse is crucial for the understanding of a cell as a physical system. There have been numerous measurements of diffusion coefficients in eukaryotic cells by using fluorescence recovery after photobleaching (FRAP) and related techniques. Much less information is available about diffusion coefficients in prokaryotic cells, which differ from eukaryotic cells in a number of significant respects. We have used FRAP to observe the diffusion of green fluorescent protein (GFP) in cells of Escherichia coli elongated by growth in the presence of cephalexin. GFP was expressed in the cytoplasm, exported into the periplasm using the twin-arginine translocation (Tat) system, or fused to an integral plasma membrane protein (TatA). We show that TatA-GFP diffuses in the plasma membrane with a diffusion coefficient comparable to that of a typical eukaryotic membrane protein. A previous report showed a very low rate of protein diffusion in the E. coli periplasm. However, we measured a GFP diffusion coefficient only slightly smaller in the periplasm than that in the cytoplasm, showing that both cell compartments are relatively fluid environments.

Cell Membrane↗

PubMedDate not supplied

Organizational factors that influence information technology diffusion in academic health sciences centers.

OBJECTIVE: To identify the organizational factors which influence the diffusion of end user online literature searching, the computer-based patient record, and electronic mail systems in academic health sciences centers in the United States. DESIGN: A total of 1335 individuals working in informatics and library areas at 67 academic health sciences centers in the U.S. were surveyed. Multivariate techniques were used to evaluate the relationship between the set of six organizational factors and two measures of innovation diffusion. MEASUREMENTS: A Guttman-like scale was developed to measure infusion, or depth or sophistication, of each of the three innovations at each institution. Diffusion was measured by a question previously developed for another study. Six independent variables were measured via five formerly developed scales and one new one. RESULTS: The overall response rate was 41%. The set of organizational variables produced significant results in the diffusion of each of the three innovations, with individual variables influencing diffusion to varying degrees. The same set produced significant results in relation to infusion only for online searching. There was little or no correlation between infusion and diffusion for each innovation. CONCLUSION: Organizational attributes are important predictors for diffusion of information technology innovations. Individual variables differ in their effect on each innovation. The set of attributes seems less able to predict infusion. It is recommended that both infusion and diffusion be measured in future studies because there is little relation between them. It is further recommended that individuals charged with implementing information technology in the health sciences receive training in managing organizational issues.

Academic Medical Centers↗

PubMed2003

Normal and ischemic epiphysis of the femur: diffusion MR imaging study in piglets.

PURPOSE: To evaluate normal diffusion characteristics in the femur in piglets and changes in diffusion with increasing duration of femoral head ischemia. MATERIALS AND METHODS: Normal epiphyses, physes, and metaphyses of piglets were evaluated with line-scan diffusion imaging (n = 12) and diffusion-tensor imaging (n = 4). Apparent diffusion coefficient (ADC) differences between normal proximal and distal femoral structures, epiphyseal and physeal cartilage, and epiphyseal and metaphyseal marrow were compared (Mann-Whitney test). Short-term femoral ischemia was investigated after maximal abduction of the hips for 3 hours (n = 6); ADCs before and after abduction were compared (Wilcoxon signed rank test). Prolonged ischemia was investigated with placement of a ligature around the neck of a femur (n = 7); the ADC of the femur in this condition was compared (Wilcoxon signed rank test) with that of the normal contralateral femur. Changes in ADC ratios at three durations of ischemia (Kruskal-Wallis test) were compared. RESULTS: ADC was greater in epiphyseal cartilage (mean +/- 1 SD, 1.62 x 10(-3) mm2/sec +/- 0.38) than it was in physeal cartilage (1.28 x 10(-3) mm2/sec +/- 0.31) (P <.007) and greater in epiphyseal marrow (1.26 x 10(-3) mm2/sec +/- 0.38) than it was in metaphyseal marrow (0.91 x 10(-3) mm2/sec +/- 0.35) (P <.001). There was columnar arrangement of tensors in the physis. ADC decreased 26% after 3 hours of maximal abduction. After femoral neck ligature, ADC increased a mean of 27% after 6 hours and a mean of 75% after 96 hours. CONCLUSION: Normal line-scan diffusion imaging findings indicate relative restriction of diffusion in the metaphysis and parallel orientation of tensors in the physis. Diffusion is initially restricted with decreased blood flow but increases if ischemia lasts longer.

Animals↗

PubMed1984

Analysis of lung multiple indicator data with an effective diffusivity model of capillary exchange.

In this study, we have compared an effective diffusivity model with several models previously used to describe lung microvascular exchange of small molecules (multiple indicator curves): the Crone integral extraction model, the Sangren-Sheppard model, and the Rowlett-Harris model. The parameters of each model were adjusted to achieve a best fit of multiple-tracer data from six isolated dog lung preparations perfused under zone III conditions at three flows and from five awake sheep before and after histamine infusion. The effective diffusivity model was the best at matching the data and the unique falling extraction pattern observed in the lung. When the isolated lung data was analyzed, the permeability-surface area products (PS) of three models were all significantly lower at lower flows. However, the effective diffusivity model parameter (D 1/2 S, an effective diffusivity-surface area product) was not significantly affected by flow reduction. Lung lymph flow and lymph-to-plasma protein ratios from the awake sheep indicated that PS and D 1/2 S should have increased, and except for the Sangren-Sheppard model, all models predicted significantly increased values. By use of sensitivity analysis, the parameters of the effective diffusivity model were found to be more independently determined than those of the Sangren-Sheppard model, but data collected at longer times were necessary to reliably identify the extravascular volume parameter (LS, diffusing distance-surface area product) of the effective diffusivity model. We conclude that the effective diffusivity model is a better descriptor of multiple tracer data from the lung, that its parameters are more reliably and independently determined, and that it more reliably describes the effects of flow and histamine infusion on capillary-tissue exchange.

Albumins↗

PubMed1992

Mechanism of detection of acute cerebral ischemia in rats by diffusion-weighted magnetic resonance microscopy.

BACKGROUND AND PURPOSE: The aim of this study was to measure apparent diffusion coefficients in rat brain tissue exposed to ouabain, glutamate, and N-methyl-D-aspartate and to compare them with apparent diffusion coefficients found in acute cerebral ischemia. METHODS: The apparent diffusion coefficient was measured using magnetic resonance microscopy in four groups of Sprague-Dawley rats after occlusion of the right middle cerebral artery and ipsilateral common carotid artery (n = 7), after ouabain exposure (n = 6), during glutamate exposure (n = 7), or during N-methyl-D-aspartate exposure (n = 3). Ouabain, glutamate, and N-methyl-D-aspartate were applied via an intracerebrally implanted microdialysis membrane. RESULTS: Three hours after the induction of focal cerebral ischemia, a 33% reduction in the apparent diffusion coefficient was observed in the right dorsolateral corpus striatum and olfactory cortex. After ouabain exposure, reductions in the apparent diffusion coefficient were observed within a 1,500-microns radius of the microdialysis membrane. Quantitative analysis revealed that apparent diffusion coefficient values in ischemic and ouabain-exposed tissue fell within the same range. Glutamate and N-methyl-D-aspartate reduced the brain tissue apparent diffusion coefficient by 35% and 40%, respectively. CONCLUSIONS: On the basis of these findings, we conclude that ischemia-induced apparent diffusion coefficient reductions are likely caused by a shift of extracellular to intracellular water.

Acute Disease↗

PubMed1998

Diffusion MR imaging during acute subarachnoid hemorrhage in rats.

BACKGROUND AND PURPOSE: We analyzed the temporal and spatial pattern of water diffusion changes during acute subarachnoid hemorrhage (SAH) in rat brain to identify factors contributing to the acute pathophysiology of SAH. METHODS: Subarachnoid hemorrhage was remotely induced via perforation of the circle of Willis with an endovascular suture during MR imaging. A fast echo-planar imaging technique was used to acquire 60 maps of the apparent diffusion coefficient (ADC) beginning 1 min before and continuing for 11 min after induction of SAH. A high-resolution spin-echo diffusion sequence was used to follow diffusion changes over 6 h after SAH. Sham-operated control (n=3), nonheparinized (n=6), and heparinized (n=5) groups were studied. RESULTS: Sham-operated control animals did not show ADC changes over time. In both SAH groups, however, a sharp decline of ADC within 2 min of SAH was consistently observed in the ipsilateral somatosensory cortex. These decreases in diffusion then spread within minutes over the ipsilateral hemisphere. Similar ADC decreases on the contralateral side started with a further time delay of 1 to 3 min. From 30 min onward, the extent of the diffusion abnormality decreased progressively in the nonheparinized animals. No recovery was observed in heparinized rats. CONCLUSIONS: MR diffusion imaging allows new insight into the pathophysiology of acute SAH: The spatial and temporal pattern of diffusion changes suggests the initial occurrence of acute vasospasm and subsequently "spreading depolarization" of brain tissue. Persistent hemorrhage in heparinized animals was reflected by early decline of ADC values throughout the entire brain.

Acute Disease↗

PubMed1993

Properties controlling the diffusion and release of water-soluble solutes from poly(ethylene oxide) hydrogels. 1. Polymer composition.

This study examines the state of water-association with poly(ethylene oxide), as evidenced by diffusivity, in a series of crosslinked polyurethanes made from poly(ethylene glycols) of a range of molecular weights. As a subsidiary underpinning exercise the correlation of diffusivity with water content at relatively high levels of swelling (> 45%) using a variety of semi-empirical equations was analyzed. Three water-soluble compounds with similar molecular weights and which exhibit minimal interaction with the polymer, as shown by their partition coefficients, were chosen for this part of the research programme. These were proxyphylline, morphine hydrochloride and caffeine. The best statistical correlations of the data were obtained for plots of: (a) diffusivity against weight percent water; and (b) log diffusivity against the reciprocal of the weight percent of water in the hydrogels. Proxyphylline results for the high levels of swelling compositions were augmented with data from lower swelling compositions and a clear break in the slope of diffusivity against percentage of water in the swollen hydrogel was obtained. This indicated a change in the nature of the diffusion at this point. The probability of this transition point corresponding to a change for diffusion through water bound as trihydrate to diffusion in free water is discussed.

Aminophylline↗

PubMed2004

Formulation of photon diffusion from spherical bioluminescent sources in an infinite homogeneous medium.

BACKGROUND: The bioluminescent enzyme firefly luciferase (Luc) or variants of green fluorescent protein (GFP) in transformed cells can be effectively used to reveal molecular and cellular features of neoplasia in vivo. Tumor cell growth and regression in response to various therapies can be evaluated by using bioluminescent imaging. In bioluminescent imaging, light propagates in highly scattering tissue, and the diffusion approximation is sufficiently accurate to predict the imaging signal around the biological tissue. The numerical solutions to the diffusion equation take large amounts of computational time, and the studies for its analytic solutions have attracted more attention in biomedical engineering applications. METHODS: Biological tissue is a turbid medium that both scatters and absorbs photons. An accurate model for the propagation of photons through tissue can be adopted from transport theory, and its diffusion approximation is applied to predict the imaging signal around the biological tissue. The solution to the diffusion equation is formulated by the convolution between its Green's function and source term. The formulation of photon diffusion from spherical bioluminescent sources in an infinite homogeneous medium can be obtained to accelerate the forward simulation of bioluminescent phenomena. RESULTS: The closed form solutions have been derived for the time-dependent diffusion equation and the steady-state diffusion equation with solid and hollow spherical sources in a homogeneous medium, respectively. Meanwhile, the relationship between solutions with a solid sphere source and ones with a surface sphere source is obtained. CONCLUSION: We have formulated solutions for the diffusion equation with solid and hollow spherical sources in an infinite homogeneous medium. These solutions have been verified by Monte Carlo simulation for use in biomedical optical imaging studies. The closed form solution is highly accurate and more computationally efficient in biomedical engineering applications. By using our analytic solutions for spherical sources, we can better predict bioluminescent signals and better understand both the potential for, and the limitations of, bioluminescent tomography in an idealized case. The formulas are particularly valuable for furthering the development of bioluminescent tomography.

Computer Simulation↗

PubMed1999

One-dimensional diffusion on microtubules of particles coated with cytoplasmic dynein and immunoglobulins.

We characterized and compared the diffusion of beads coated with proteins such as cytoplasmic dynein, alpha-casein, and some immunoglobulins on microtubules. Such weak binding interactions could be common and convenient for concentrating proteins at the surface of cytoplasmic structures such as microtubules. In studying the motile behavior of anionic latex beads coated with limiting dilutions of cytoplasmic dynein, we observed that in addition to active movement, 20-50% of the beads moved back and forth in a random manner. The random movement was inhibited by depletion of ATP or addition of ADP or AMP-PNP. Mean-square-displacement analysis showed that the movement is a one-dimensional diffusion along the microtubule axis with a diffusion coefficient of 2.16 x 10(-10) cm2/sec. Histogram analysis of off-axis movements suggested that approximately 60% of the diffusing beads followed the path of a single microtubule protofilament. Beads coated with proteins such as alpha-casein or a monoclonal immunoglobulin were also observed to diffuse on microtubules with a similar diffusion coefficient to cytoplasmic dynein. However, alpha-casein or immunoglobulin-bead diffusion was not ATP dependent and did not follow the paths of single protofilaments. Thus, although the environment of the microtubule surface can trap a variety of different protein-coated beads, cytoplasmic dynein's interaction is unusual in its ATP dependence and tracking on a single protofilament, which is consistent with its specific interaction with microtubules. Diffusive interactions could concentrate associating proteins and still allow for freedom of movement.

Adenosine Diphosphate↗

PubMed1983

Acid back-diffusion and mucosal H+ handling in the rat stomach under normal and stress-induced conditions.

We determined acid back-diffusion and pepsin output simultaneously in vagotomized rats after instillation of HCl into the stomach under normal and stress-induced conditions. With exposure to 6 ml of 100 mM HCl, spontaneous acid back-diffusion increased with the duration of the experiment under both conditions, and the magnitude of the acid back-diffusion was decreased significantly by stress. There was no change in the output of pepsin. While disappearance of luminal acid caused by aspirin or taurocholic acid was not altered by stress, the pepsin output in response to H+ increased significantly in the stressed rats. With exposure to various concentrations of HCl for 3 hr, disappearance of the luminal acid increased linearly with the grade of HCl under both conditions. Except for the concentration of 300 mM, the magnitude of the acid back-diffusion was triple in the normal condition, and the ratio of pepsin output/net flux of H+ was significantly increased by stress. Thus, (1) spontaneous acid back-diffusion decreased with stress, while diffusion induced by chemical barrier breakers remained the same; (2) the action of H+ diffused back into the mucosa did not always parallel the amount of diffusion determined from the loss of H+ in the lumen; (3) intramucosal H+ may be largely dissipated in normal mucosa; and (4) the initiation or aggravation of drug-induced mucosal damages by stress may be related to insufficiency of the H+ dissipating mechanisms.

Animals↗

PubMed1992

Radial diffusion coefficient mapping.

The two-dimensional mapping of the effective diffusion coefficient of water in tissues may provide a useful method of tissue characterization to complement T1 and T2 relaxation time studies for diagnostic purposes. Current diffusion techniques rely on the application of large gradient strengths and long echo times to achieve the required sensitivity. This, in turn, limits the applicability of the technique to tissues having long T2s with rapid water diffusion. In addition, the inherent directionality of these methods results in only the partial encoding of diffusion information. A modified diffusion sequence is presented, radial diffusion mapping (RAD), which provides enhanced sensitivity diffusion maps by employing gradient sensitization in three orthogonal directions. Results in both phantoms and volunteers are presented, together with an investigation of the effects of T2 on the measurement accuracy. Using RAD, a two- to five-fold improvement in sensitivity was achieved, thus significantly enhancing the dynamic range of the method and allowing more accurate in vivo diffusion measurements to be carried out.

Acetone↗

PubMed2001

Recovery of optical parameters in multiple-layered diffusive media: theory and experiments.

Diffuse photon density waves have lately been used both to characterize diffusive media and to locate and characterize hidden objects, such as tumors, in soft tissue. In practice, most biological media of medical interest consist of various layers with different optical properties, such as the fat layer in the breast or the different layers present in the skin. Also, most experimental setups consist of a multilayered system, where the medium to be characterized (i.e., the patient's organ) is usually bounded by optically diffusive plates. Incorrect modeling of interfaces may induce errors comparable to the weak signals obtained from tumors embedded deep in highly heterogeneous tissue and lead to significant reconstruction artifacts. To provide a means to analyze the data acquired in these configurations, the basic expressions for the reflection and transmission coefficients for diffusive-diffusive and diffusive-nondiffusive interfaces are presented. A comparison is made between a diffusive slab and an ordinary dielectric slab, thus establishing the limiting distance between the two interfaces of the slab for multiple reflections between them to be considered important. A rigorous formulation for multiple-layered (M-layered) diffusive media is put forward, and a method for solving any M-layered medium is shown. The theory presented is used to characterize a two-layered medium from transmission measurements, showing that the coefficients of scattering, mu'(s) , and absorption, mu(a) , are retrieved with great accuracy. Finally, we demonstrate the simultaneous retrieval of both mu;(s) and mu(a).

Diffusion↗

PubMed1993

Diffusing capacity decreases after heart transplantation.

We evaluated the following spirometric values: forced vital capacity (FVC), first second expiratory volume (FEV1), FEV1/FVC, the lung volumes, total lung capacity (TLC), residual volume (RV), and single breath diffusing capacity for CO in 22 patients, before and after heart transplant. We found abnormal pulmonary function in 21 patients before heart transplantation. Despite postoperative increases in lung volumes in 10 patients, abnormal pulmonary function persisted in 20 patients after heart transplant. Mean values for lung volumes and flow rates did not change but diffusion for CO decreased significantly after heart transplantation. Diffusion failed to correlate with ejection fraction, pulmonary arterial pressure, pulmonary capillary wedge pressure (PCWP), and pulmonary vascular resistance; however, in a subset of patients with improved postoperative lung volumes, preoperative diffusion for CO correlated with preoperative PCWP. We conclude that pulmonary function abnormalities are common among heart transplant recipients. Diffusion abnormalities are not linearly related to indices of cardiac function measured before transplantation and diffusion abnormalities appear to be multifactorial in cause. The posttransplant decrease in diffusion appears to result from the combined effects of decreased postoperative lung volumes in some patients and relief of heart failure induced pulmonary vascular engorgement in others. Improvement in lung volumes and flow rates may occur but cannot be expected after heart transplantation, and diffusion decreases after heart transplantation. The fact that pulmonary function and lung volumes do not improve following heart transplantation implies to underlying lung disease or permanent lung alterations result from chronic heart failure.

Adult↗

PubMed2000

Relationship between impaired pulmonary diffusion and cardiopulmonary exercise capacity after heart transplantation.

STUDY OBJECTIVES: Diffusion impairment and reduced performance in cardiopulmonary exercise testing (CPX) have been found in patients after heart transplantation. The pathogenesis of these abnormalities is unclear. In particular, the contribution of pulmonary interstitial changes has not yet been verified. DESIGN: We analyzed pulmonary function tests, high-resolution CT (HRCT), echocardiography, left heart catheterization, and CPX in transplanted patients. PATIENTS: Forty long-term survivors were studied at a median of 47 months (range, 12 to 89 months) after heart transplantation. RESULTS: Diffusion was impaired in 40% (transfer factor for carbon monoxide) or 82.5% (carbon monoxide transfer coefficient) of the patients. Diffusion impairment was caused by a decreased diffusing capacity of the alveolar capillary membrane in 89% and/or by a decreased blood volume of the alveolar capillaries in 46% of cases. In five patients (12.5%), CT revealed interstitial lung changes. These patients did not have different values of diffusion capacity. Maximal oxygen uptake and ventilatory efficiency during exercise (minute ventilation/carbon dioxide output slope) were impaired in 92% and 46% of the cases, respectively. CONCLUSIONS: Our data show that the diffusion abnormalities are caused by an impaired diffusion status of the alveolar capillary membrane. Interstitial changes detectable in HRCT were found not to be involved in this process. The reduced performance in CPX in our long-term survivors is caused by pulmonary perfusion abnormalities and low tidal volume, which is due to the deconditioning of respiratory muscle, rather than by interstitial changes or diffusion abnormalities.

Adult↗

PubMed2004

Diffusion in two-component lipid membranes--a fluorescence correlation spectroscopy and monte carlo simulation study.

Using fluorescence correlation spectroscopy, calorimetry, and Monte Carlo simulations, we studied diffusion processes in two-component membranes close to the chain melting transition. The aim is to describe complex diffusion behavior in lipid systems in which gel and fluid domains coexist. Diffusion processes in gel membranes are significantly slower than in fluid membranes. Diffusion processes in mixed phase regions are therefore expected to be complex. Due to statistical fluctuations the gel-fluid domain patterns are not uniform in space and time. No models for such diffusion processes are available. In this article, which is both experimental and theoretical, we investigated the diffusion in DMPC-DSPC lipid mixtures as a function of temperature and composition. We then modeled the fluorescence correlation spectroscopy experiment using Monte Carlo simulations to analyze the diffusion process. It is shown that the simulations yield a very good description of the experimental diffusion processes, and that predicted autocorrelation profiles are superimposable with the experimental curves. We believe that this study adds to the discussion on the physical nature of rafts found in biomembranes.

1,2-Dipalmitoylphosphatidylcholine↗