Detection of malaria parasites in blood by laser desorption mass spectrometry.

A novel method for the in vitro detection of the protozoan Plasmodium, the causative agent of malaria, has been developed. It comprises a protocol for cleanup of whole blood samples, followed by direct ultraviolet laser desorption (LD) time-of-flight mass spectrometry. Intense ion signals are observed from intact ferriprotoporphyrin IX (heme), sequestered by malaria parasites during their growth in human red blood cells. The LD mass spectrum of the heme is structure-specific, and the signal intensities are correlated with the sample parasitemia (number of parasites per unit volume of blood). Parasitemia levels on the order of 10 parasites/microL blood can be unambiguously detected by this method. Consideration of laser beam parameters (spot size, rastering across the sample surface) and actual sample consumption suggests that the detection limits can be further improved by at least an order of magnitude. The influence of experimental factors, such as desorbed ion polarity, laser exposure and fluence, sample size, and parasite growth stage, on the threshold for parasite detection is also addressed.

Applicability of the single equivalent point dipole model to represent a spatially distributed bio-electrical source.

Although the single equivalent point dipole model has been used to represent well-localised bio-electrical sources, in realistic situations the source is distributed. Consequently, position estimates of point dipoles determined by inverse algorithms suffer from systematic error due to the non-exact applicability of the inverse model. In realistic situations, this systematic error cannot be avoided, a limitation that is independent of the complexity of the torso model used. This study quantitatively investigates the intrinsic limitations in the assignment of a location to the equivalent dipole due to distributed electrical source. To simulate arrhythmic activity in the heart, a model of a wave of depolarisation spreading from a focal source over the surface of a spherical shell is used. The activity is represented by a sequence of concentric belt sources (obtained by slicing the shell with a sequence of parallel plane pairs), with constant dipole moment per unit length (circumferentially) directed parallel to the propagation direction. The distributed source is represented by N dipoles at equal arc lengths along the belt. The sum of the dipole potentials is calculated at predefined electrode locations. The inverse problem involves finding a single equivalent point dipole that best reproduces the electrode potentials due to the distributed source. The inverse problem is implemented by minimising the chi2 per degree of freedom. It is found that the trajectory traced by the equivalent dipole is sensitive to the location of the spherical shell relative to the fixed electrodes. It is shown that this trajectory does not coincide with the sequence of geometrical centres of the consecutive belt sources. For distributed sources within a bounded spherical medium, displaced from the sphere's centre by 40% of the sphere's radius, it is found that the error in the equivalent dipole location varies from 3 to 20% for sources with size between 5 and 50% of the sphere's radius. Finally, a method is devised to obtain the size of the distributed source during the cardiac cycle.

Requirement for nitric oxide activation of p21(ras)/extracellular regulated kinase in neuronal ischemic preconditioning.

The mechanisms underlying neuronal ischemic preconditioning, a phenomenon in which brief episodes of ischemia protect against the lethal effects of subsequent periods of prolonged ischemia, are poorly understood. Ischemia can be modeled in vitro by oxygen-glucose deprivation (OGD). We report here that OGD preconditioning induces p21(ras) (Ras) activation in an N-methyl-D-aspartate receptor- and NO-dependent, but cGMP-independent, manner. We demonstrate that Ras activity is necessary and sufficient for OGD tolerance in neurons. Pharmacological inhibition of Ras, as well as a dominant negative mutant Ras, block OGD preconditioning whereas a constitutively active form of Ras promotes neuroprotection against lethal OGD insults. In contrast, the activity of phosphatidyl inositol 3-kinase is not required for OGD preconditioning because inhibition of phosphatidyl inositol 3-kinase with a chemical inhibitor or with a dominant negative mutant does not have any effect on the development of OGD tolerance. Furthermore, using recombinant adenoviruses and pharmacological inhibitors, we show that downstream of Ras the extracellular regulated kinase cascade is required for OGD preconditioning. Our observations indicate that activation of the Ras/extracellular regulated kinase cascade by NO is a critical mechanism for the development of OGD tolerance in cortical neurons, which may also play an important role in ischemic preconditioning in vivo.

[Technical and practical aspects of T-wave alternans testing]. / T-Wellen-Alternans: vom Rauschen zum Signal, vom Signal zur Prognose.

Alternation of the amplitude of the repolarization wave (T-wave) of the body surface ECG, usually referred to as T-wave alternans (TWA), has become one of the most important noninvasive tools in cardiac risk stratification. In earlier experimental and clinical studies TWA was shown to predict an individual's susceptibility of ventricular tachyarrhythmias. Recently, a number of prospectively designed clinical studies have confirmed these results and demonstrated that a positive TWA result in high risk populations, such as post-myocardial infarction patients and individuals suffering from congestive heart failure, is a very useful prognostic marker heralding sudden cardiac death due to ventricuar tachyarrhythmias. Moreover, experiments, performed in animals as well as in computer models, have shown that T-wave alternans may also be mechanistically involved in arrhythmogenesis. Since it has now become possible to measure TWA completely noninvasively by menas of exercise testing, an exponential increase in TWA tests has been observed. However, little information on the technical aspects of TWA testing is available. Therefore, in this article, technical and practical aspects of TWA assessment and interpretation are reviewed as far as they are relevant for clinicians.

Spiral waves are stable in discrete element models of two-dimensional homogeneous excitable media.

The spontaneous breakup of a single spiral wave of excitation into a turbulent wave pattern has been observed in both discrete element models and continuous reaction-diffusion models of spatially homogeneous 2D excitable media. These results have attracted considerable interest, since spiral breakup is thought to be an important mechanism of transition from the heart rhythm disturbance ventricular tachycardia to the fatal arrhythmia ventricular fibrillation. It is not known whether this process can occur in the absence of disease-induced spatial heterogeneity of the electrical properties of the ventricular tissue. Candidate mechanisms for spiral breakup in uniform 2D media have emerged, but the physical validity of the mechanisms and their applicability to myocardium require further scrutiny. In this letter, we examine the computer simulation results obtained in two discrete element models and show that the instability of each spiral is an artifact resulting from an unphysical dependence of wave speed on wave front curvature in the medium. We conclude that spiral breakup does not occur in these two models at the specified parameter values and that great care must be exercised in the representation of a continuous excitable medium via discrete elements.

Correspondence between discrete and continuous models of excitable media: trigger waves.

We present a theoretical framework for relating continuous partial differential equation (PDE) models of excitable media to discrete cellular automata (CA) models on a randomized lattice. These relations establish a quantitative link between the CA model and the specific physical system under study. We derive expressions for the CA model's plane wave speed, critical curvature, and effective diffusion constant in terms of the model's internal parameters (the interaction radius, excitation threshold, and time step). We then equate these expressions to the corresponding quantities obtained from solution of the PDEs (for a fixed excitability). This yields a set of coupled equations with a unique solution for the required CA parameter values. Here we restrict our analysis to "trigger" wave solutions obtained in the limiting case of a two-dimensional excitable medium with no recovery processes. We tested the correspondence between our CA model and two PDE models (the FitzHugh-Nagumo medium and a medium with a "sawtooth" nonlinear reaction source) and found good agreement with the numerical solutions of the PDEs. Our results suggest that the behavior of trigger waves is actually controlled by a small number of parameters.

Concurrent and construct validity of the Pediatric Evaluation of Disability Inventory.

The purpose of this study was to determine the validity of the Development Edition (pilot version) of the Pediatric Evaluation of Disability Inventory (PEDI) in groups of disabled and nondisabled children. The PEDI is a new functional assessment instrument for the evaluation of disabled children aged 6 months to 7 years. The PEDI has been developed to identify functional status and change along three dimensions: 1) functional skill level, 2) caregiver assistance, and 3) modifications or adaptive equipment used. The PEDIs were administered as a parental-report questionnaire, and the results were compared with data obtained by the Battelle Developmental Inventory Screening Test (BDIST). The BDIST is a standardized assessment with developmental and adaptive content. Subjects were 20 children between the ages of 2 and 8 years with arthritic conditions and spina bifida and 20 nondisabled children matched for age and sex. All subjects' scores on the BDIST cognitive domain were no greater than 1.50 standard deviations below the mean for their age group. Concurrent validity was supported by moderately high Pearson product-moment correlations between BDIST and PEDI summary scores (r = .70-.80). Construct validity was supported by significant differences between the disabled and nondisabled groups' PEDI scores and by discriminant analysis identifying the PEDI scores as better group discriminators than the BDIST scores. Results validate the Developmental Edition of the PEDI and support the further development and standardization of the final version. Use of the PEDI in clinical pediatric physical therapy practice is discussed.