Conductance catheter measurements of lumen area of stenotic coronary arteries: theory and experiment

An injection of saline solution is required for the measurement of vessel lumen area using a conductance catheter. The injection of room temperature saline to displace blood in a vessel inevitably involves mass and heat transport and electric field conductance. The objective of the present study is to understand the accuracy of conductance method based on the phenomena associated with the saline injection into a stenotic blood vessel. Computational fluid dynamics were performed to simulate flow and its relation to transport and electric field in a stenotic artery for two different sized conductance catheters (0.9 and 0.35 mm diameter) over a range of occlusions [56-84% cross-sectional area (CSA) stenosis]. The results suggest that the performance of conductance catheter is dependent on catheter size and severity of stenosis more significantly for 0.9 mm than for 0.35 mm catheter. Specifically, the time of detection of 95% of injected saline solution at the detection electrodes was shown to range from 0.67 to 3.7 s and 0.82 to 0.94 s for 0.9 mm and 0.35 mm catheter, respectively. The results also suggest that the detection electrodes of conductance catheter should be placed outside of flow recirculation region distal to the stenosis to minimize the detection time. Finally, the simulations show that the accuracy in distal CSA measurements, however, is not significantly altered by whether the position of detection electrodes is inside or outside of recirculation zone (error was within 12% regardless of detection electrodes position). The results were experimentally validated for one lesion geometry and the simulation results are within 8% of actual measurements. The simulation of conductance catheter injection method may lead to further optimization of device and method for accurate sizing of diseased coronary arteries, which has clinical relevance to percutaneous intervention.     

In vivo measurement of cortical impedance spectrum in monkeys: implications for signal propagation

To combine insights obtained from electric field potentials (LFPs) and neuronal spiking activity (MUA) we need a better understanding of the relative spatial summation of these indices of neuronal activity. Compared to MUA, the LFP has greater spatial coherence, resulting in lower spatial specificity and lower stimulus selectivity. A differential propagation of low- and high-frequency electric signals supposedly underlies this phenomenon, which could result from cortical tissue specifically attenuating higher frequencies, i.e., from a frequency-dependent impedance spectrum. Here we directly measure the cortical impedance spectrum in vivo in monkey primary visual cortex. Our results show that impedance is independent of frequency, is homogeneous and tangentially isotropic within gray matter, and can be theoretically predicted assuming a pure-resistive conductor. We propose that the spatial summation of LFP and MUA is determined by the size of these signals' generators and the nature of neural events underlying them, rather than by biophysical properties of gray matter.     

A non-invasive method for an in vivo assessment of corneal epithelium permeability through tetrapolar impedance measurements

The permeability of the cornea epithelial layer has an important role in optimal function of the cornea. To assess this property quantitatively, methods must be based on the passive electrical properties of living tissues, as they can take advantage of the fundamental role that ionic permeability plays in such properties. For such techniques, measurement of the translayer electrical resistance (TER) has been consistently used to examine the ion transport mechanisms in the corneal epithelial cells; however, this technique has been only possible in vitro. To enhance the applications of this method, in this work we present a novel sensor to perform non-invasive in vivo TER measurements. Herein, the epithelial permeability was assessed using non-invasive tetrapolar impedance measurements that were performed with four electrodes placed on the corneal surface. The geometry of these electrodes was previously optimized to maximize the sensitivity of the corneal epithelium. To evaluate the feasibility of this sensor, the permeability of a rabbit corneal epithelium was monitored by applying a solution of benzalkonium chloride (0.05% BAC). The results validate the capability of the sensor to evaluate the cornea epithelial permeability in vivo.     

Novel method for in vivo hydroxyl radical measurement by microdialysis in fetal sheep brain in utero.

Hydroxyl radical (.OH) is a reactive oxygen species produced during severe hypoxia, asphyxia, or ischemia that can cause cell death resulting in brain damage. Generation of .OH may occur in the fetal brain during asphyxia in utero. The very short half-life of .OH requires use of trapping agents such as salicylic acid or phenylalanine for detection, but their hydroxylated derivatives are either unstable, produced endogenously, or difficult to measure in the small volume of microdialysis samples. In the present study, we used terephthalic acid (TA), hydroxylation of which yields a stable and highly fluorometric isomer (excitation, 326 nm; emission, 432 nm). In vitro studies using .OH generated by the Fenton reaction showed that hydroxylated TA formed quickly (<10 s), was resistant to bleaching (<5% change in fluorescence), and permitted detection of <0.5 pmol .OH. In vivo studies were performed in fetal sheep using microdialysis probes implanted into the parasagittal cortex. The probe was perfused at 2 mul/min with artificial cerebrospinal fluid containing 5 mM TA, and samples were collected every 30 min. Fluorescence measured in 10 mul of dialysate was significantly greater than in the efflux from probes perfused without TA. High-performance liquid chromotography analysis showed that the fluorescence in dialysis samples was entirely due to hydroxylation of TA. Thus this study shows that it is possible to use TA as a trapping agent for detecting low concentrations of .OH both in vitro and in vivo and that low concentrations of .OH are present in fetal brain tissue and fluctuate with time.     

Alternative method for measurement of arterial pressure in the rabbit.

By using an electrical impedance plethysmorgraph and a pressure applicator to a rabbit's earlobe artery, it became possible to monitor the blood pressure for periods of several months. A study of the correlations of the blood pressure from carotid and ear was also made at varied blood pressures under different conditions which showed excellent correlations. This method could effectively replace the encannulation technique for monitoring rabbit's blood pressure in chronic experiments.     

Quantification of coronary microvascular resistance using angiographic images for volumetric blood flow measurement: in vivo validation

Structural coronary microcirculation abnormalities are important prognostic determinants in clinical settings. However, an assessment of microvascular resistance (MR) requires a velocity wire. A first-pass distribution analysis technique to measure volumetric blood flow has been previously validated. The aim of this study was the in vivo validation of the MR measurement technique using first-pass distribution analysis. Twelve anesthetized swine were instrumented with a transit-time ultrasound flow probe on the proximal segment of the left anterior descending coronary artery (LAD). Microspheres were injected into the LAD to create a model of microvascular dysfunction. Adenosine (400 μg·kg(-1)·min(-1)) was used to produce maximum hyperemia. A region of interest in the LAD arterial bed was drawn to generate time-density curves using angiographic images. Volumetric blood flow measurements (Q(a)) were made using a time-density curve and the assumption that blood was momentarily replaced with contrast agent during the injection. Blood flow from the flow probe (Q(p)), coronary pressure (P(a)), and right atrium pressure (P(v)) were continuously recorded. Flow probe-based normalized MR (NMR(p)) and angiography-based normalized MR (NMR(a)) were calculated using Q(p) and Q(a), respectively. In 258 measurements, Q(a) showed a strong correlation with the gold standard Q(p) (Q(a) = 0.90 Q(p) + 6.6 ml/min, r(2) = 0.91, P < 0.0001). NMR(a) correlated linearly with NMR(p) (NMR(a) = 0.90 NMR(p) + 0.02 mmHg·ml(-1)·min(-1), r(2) = 0.91, P < 0.0001). Additionally, the Bland-Altman analysis showed a close agreement between NMR(a) and NMR(p). In conclusion, a technique based on angiographic image data for quantifying NMR was validated using a swine model. This study provides a method to measure NMR without using a velocity wire, which can potentially be used to evaluate microvascular conditions during coronary arteriography.     

Continuous in vivo measurement of arterial PO2 in humans.

A system suitable for prolonged continuous in vivo measurement of human arterial PO2 is described. The system uses a polarographic electrode developed by Kimmich and Kreuzer, inserted in a specially made shunt between the radial artery and an antecubital vein. Nhe electrode surface is maintained in a fixed position parallel to the flow of blood; blood velocity dependency is small owing to the high flow rate achieved (more than 40 cm/s); clotting is prevented by the material used and the continuous instillation of heparin through the arterial end of the shunt. The system has been tested in vitro; it is stable (variation less than 0.5% in 24 h), linear and precise (plus or minus 0.2%) in a broad range of PO2 values (from about 10 mmHg to more than 700 mmHg); its response time is 0.4 s per 95% of deflection. It has been applied to 35 patients for periods ranging between 6 and 24 h; most of the patients were ventilated by an Engstrom respirator.     

A novel method for in vivo knee prosthesis wear measurement

Wear remains an important cause of failure in knee replacement. Of the current methods of early performance assessment or prediction, simulators have been un-physiological, single X-ray film analyses remain limited by accuracy and retrieval and survival methods have a prohibitive time scale. An accurate method is needed to allow a timely assessment of polyethylene component wear in vivo, when a new design is introduced, in order to predict likely outcome. We present a new method for measuring wear in vivo that we believe will allow this prediction of long-term wear. X-ray film pairs were taken of implanted prosthetic metal components. When the X-ray system was calibrated, projections of the appropriate Computer Aided Design (CAD) model could be matched to the shapes on the scanned X-ray films to find component positions. Interpenetration of the metal femoral component into the polyethylene component could then be established and represents our estimate of "wear". This method was used to measure in vivo prosthesis wear to an accuracy of 0.11 mm.     

An in vivo parameter identification method for arteries: numerical validation for the human abdominal aorta

A method for identifying mechanical properties of arterial tissue in vivo is proposed in this paper and it is numerically validated for the human abdominal aorta. Supplied with pressure-radius data, the method determines six parameters representing relevant mechanical properties of an artery. In order to validate the method, 22 finite element arteries are created using published data for the human abdominal aorta. With these in silico abdominal aortas, which serve as mock experiments with exactly known material properties and boundary conditions, pressure-radius data sets are generated and the mechanical properties are identified using the proposed parameter identification method. By comparing the identified and pre-defined parameters, the method is quantitatively validated. For healthy abdominal aortas, the parameters show good agreement for the material constant associated with elastin and the radius of the stress-free state over a large range of values. Slightly larger discrepancies occur for the material constants associated with collagen, and the largest relative difference is obtained for the in situ axial prestretch. For pathological abdominal aortas incorrect parameters are identified, but the identification method reveals the presence of diseased aortas. The numerical validation indicates that the proposed parameter identification method is able to identify adequate parameters for healthy abdominal aortas and reveals pathological aortas from in vivo-like data.     

Novel and quantitative DNA dot-blotting method for assessment of in vivo proliferation

Immunohistochemical assessment of 5-bromo-2-deoxyuridine (BrdU) in tissue sections is a widely used method to evaluate cell proliferation in vivo. However, this method requires time-consuming preparation of paraffin sections and laborious counting of BrdU-labeled nuclei on multiple sections. Here, we report the development of a rapid and reliable method to quantitate BrdU incorporation in intestinal and liver tissues using a dot-blot method. In vivo models of colon or liver proliferation were used to analyze the usefulness and reliability of this new method. Mice were killed after BrdU injection, and genomic DNA was isolated from the tissues, denatured, and dot-blotted onto a nitrocellulose membrane. The incorporated BrdU was detected with a BrdU monoclonal antibody, and the signal intensity was densitometrically quantified. Results were compared with BrdU index determined by conventional immunohistochemistry on the same tissue samples. The patterns of colonic BrdU incorporation during fasting and refeeding, measured by the dot-blotting method and the immunohistochemical method, were similar. The BrdU incorporation in the regenerating liver after partial hepatectomy, evaluated by these two different methods, showed a strong correlation (R(2) = 0.91, P < 0.01). In addition, the inhibition of colon proliferation by the phosphoinositol 3-kinase inhibitor wortmannin was demonstrated by this dot-blotting method. In conclusion, the dot-blotting technique described in this report provides an accurate, more efficient, and possibly more reliable method for the assessment of in vivo proliferation compared with conventional immunohistochemical determination of BrdU-labeling index.     

Development and validation of an automated high-throughput system for zebrafish in vivo screenings

The zebrafish is a vertebrate model compatible with the paradigms of drug discovery. The small size and transparency of zebrafish embryos make them amenable for the automation necessary in high-throughput screenings. We have developed an automated high-throughput platform for in vivo chemical screenings on zebrafish embryos that includes automated methods for embryo dispensation, compound delivery, incubation, imaging and analysis of the results. At present, two different assays to detect cardiotoxic compounds and angiogenesis inhibitors can be automatically run in the platform, showing the versatility of the system. A validation of these two assays with known positive and negative compounds, as well as a screening for the detection of unknown anti-angiogenic compounds, have been successfully carried out in the system developed. We present a totally automated platform that allows for high-throughput screenings in a vertebrate organism.     

Feasibility of an in situ measurement device for bubble size and distribution

The feasibility of in situ measurement device for bubble size and distribution was explored. A novel in situ probe measurement system, the EnviroCam™, was developed. Where possible, this probe incorporated strengths, and minimized weaknesses of historical and currently available real-time measurement methods for bubbles. The system was based on a digital, high-speed, high resolution, modular camera system, attached to a stainless steel shroud, compatible with standard Ingold ports on fermenters. Still frames and/or video were produced, capturing bubbles passing through the notch of the shroud. An LED light source was integral with the shroud. Bubbles were analyzed using customized commercially available image analysis software and standard statistical methods. Using this system, bubble sizes were measured as a function of various operating parameters (e.g., agitation rate, aeration rate) and as a function of media properties (e.g., viscosity, antifoam, cottonseed flour, and microbial/animal cell broths) to demonstrate system performance and its limitations. For selected conditions, mean bubble size changes qualitatively compared favorably with published relationships. Current instrument measurement capabilities were limited primarily to clear solutions that did not contain large numbers of overlapping bubbles.     

A selective medium for the rapid detection by an impedance technique of Pseudomonasspp. associated with poultry meat

A new medium for detecting and enumerating Pseudomonas spp. associated withpoultry meat spoilage by a rapid impedance technique was developed, after testing potentialgrowth promoters for eight Pseudomonas strains and inhibitors against eight competingstrains (Enterobacteriaceae) able to grow on the medium of Mead and Adams (1977). Four basalmedia (brain heart infusion, brucella broth, Shaedler broth and Whitley impedance broth (WIB))and a synthetic medium were evaluated. Whitley impedance broth was the best basal medium fordetecting variations in impedance in relation to Pseudomonas growth. The efficiency ofWIB was improved by adding compounds which enhanced the growth of Pseudomonas onthe synthetic medium. Among the incubation temperatures tested, 22°C proved to be the bestcompromise between growth of Pseudomonas associated with poultry meat spoilage andinhibition of competitors. Among the 15 inhibitory substances evaluated against Pseudomonas competitors, five were chosen for inclusion in the final medium : metronidazole,carbenicilline, cetrimide, cycloheximide and diamide (MCCCD medium). Preliminary resultsobtained from experiments with beef and pork meat showed that this medium could also be usedwithout diamide and at an incubation temperature of 25°C. The impedance technique usingMCCCD medium was then compared with an official method which uses the medium of Meadand Adams (1977) on 106 samples of poultry neck skin. The linear regression coefficient betweenthe two techniques was approximately r = 0·85. Impedance was able to detect 10³ Pseudomonas g⁻¹ within less than 19 h making it a promisingtechnique for predicting poultry meat spoilage.