Differential and directional effects of perfusion on electrical and thermal conductivities in liver

Two different measurement probes--an electrical probe and a thermal conductivity probe--were designed, fabricated, calibrated, and used in experimental studies on a pig liver model that was designed to control perfusion rates. These probes were fabricated by photolithography and mounted in 1.5-mm diameter catheters. We measured the local impedance and thermal conductivity, respectively, of the artificially perfused liver at different flow rates and, by rotating the probes, in different directions. The results show that both the local electrical conductivity and the thermal conductivity varied location to location, that thermal conductivity increased with decreased distance to large blood vessels, and that significant directional differences exist in both electrical and thermal conductivities. Measurements at different perfusion rates demonstrated that both the local electrical and local thermal conductivities increased linearly with the square root of perfusion rate. These correlations may be of great value to many energy-based biomedical applications.

Effects of monopolar radiofrequency heating on intradiscal pressure in sheep

BACKGROUND CONTEXT

No previous study has assessed the effect of monopolar radiofrequency (RF) heating on intradiscal pressure.

PURPOSE

To determine the decrease in lumbar intradiscal pressure after monopolar RF heating.

STUDY DESIGN/SETTING

Intradiscal pressure was measured in sheep lumbar discs treated with monopolar RF heating.

METHODS

Two monopolar RF heat treatments at 90 degrees C were applied for 2 minutes each to lumbar intervertebral discs of sheep. Intradiscal pressure was measured in live sheep at 0, 7, 14, 21, and 28 days posttreatment. Pressure measurements were taken with a microtip pressure transducer. Electrodes were inserted but not activated in separate discs as a sham control. In vitro sheep spine of different age groups, loading conditions, and electrode orientations were similarly heat treated and intradiscal pressures were measured.

RESULTS

Intradiscal pressure was significantly reduced 1 week after monopolar RF heating and remained stable through the 4-week observation period. The RF electrode orientation, the age, and the type of disc loading have significant effects on the amount of initial intradiscal pressure reduction.

CONCLUSIONS

Monopolar RF heating can reduce intradiscal pressure in the lumbar spine of sheep.

The histologic effects of pulsed and continuous radiofrequency lesions at 42 degrees C to rat dorsal root ganglion and sciatic nerve

STUDY DESIGN

Experimental histologic study of the effects of radiofrequency (RF) or convective heating of the rat dorsal root ganglion or sciatic nerve to 42 degrees C.

OBJECTIVE

To determine whether treatment causes neuropathologic changes in an effort to explore the mechanisms and safety of pulsed RF pain therapy.

SUMMARY OF BACKGROUND DATA

Clinical data suggest that low temperature pulsed RF energy delivered to the DRG is a safe and effective form of therapy for low back pain. However, the mechanism by which this treatment modifies pain is unclear.

METHODS

A total of 118 Sprague-Dawley rats were divided into five groups for different RF and thermal treatments. All treatments increased tissue temperature to 42 degrees C. Treatments of the DRG included pulsed RF, continuous RF, and conductive heat. The generator output was increased until 42 degrees C was obtained in the tissue and was then maintained for 120 seconds. As a positive control, some rat sciatic nerves were treated with continuous RF lesions at 80 degrees C. Animals were killed for histologic study at 2, 7, or 21 days after treatment. Tissue was fixed in gluteraldehyde and embedded in plastic resin for detailed light microscopic neuropathologic evaluation.

RESULTS

The methods used to heat the tissue to 42 degrees C caused no significant difference in pathology. However, subclinical changes included endoneurial edema caused by alterations in the function of the blood-nerve barrier, fibroblast activation, and collagen deposition. Tissue returned to normal conditions by 7 days in nerve and 21 days in the DRG. These minor structural changes observed at the light microscopic level in normal animals do not exclude the possibility that there would be nonstructural changes in gene expression or cytokine upregulation in injured tissue. Lesions at 80 degrees C caused consistent thermal injury characterized by Wallerian degeneration of nerve fibers.

CONCLUSIONS

The data support the hypothesis that pulsed RF treatment does not rely on thermal injury of neurologic tissue to achieve its effect.

A quantitative immunohistochemical study of the cellular response to crush injury in optic nerve

Computer-enhanced immunohistochemical techniques were applied to quantify the cellular response of rat optic nerve to crush injury to attain the spatial and temporal detail necessary for resolving trends in the process that may be affected by experimental therapy. Changes in the major cell populations of the optic nerve during 6 weeks following crush injury were analyzed using antibodies against beta-tubulin (TuJ1), neurofilament (8-152), glial fibrillary acidic protein (GFAP), galactocerebrosides (GalC), ED-1, and von Willebrand factor to detect the presence of axons, astrocytes, oligodendrocytes, macrophages, and endothelial cells. The markers for axons, astrocytes, oligodendrocytes, and endothelial cells decreased their activity and remained depressed in the area of direct mechanical injury throughout 6 weeks following crush injury. A spatial peak in the axonal makers TuJ1 and 8-152 at the proximal boundary of the injury at both 2 and 3 weeks is consistent with the idea that the injury zone is a barrier obstructing distal elongation of sprouted neurites. Reactive astrocytes were GFAP-positive in the nerve segments proximal and distal to the injury with marker density above 1.5 times normal at 2 and 3 weeks. GFAP appeared to be absent in the site of injury even at 6 weeks postinjury, as shown by both immunohistochemistry and Western blot analysis. ED-1-positive macrophages were the most numerous population in the injury site from 1 through 6 weeks with a peak value measured at 2 weeks. The optical density of ED-1 activity in the distal and proximal nerve segments was less than 20% the density in the injury at all measured time points.

A diffusion-reaction model of nerve regeneration

The process of peripheral nerve regeneration has been modeled using 5 populations of mathematical variables to represent the biological activities of Wallerian degeneration, fibrin matrix development, Schwann cell activity, elongating neurites, and neovascularization. The mathematical model provided simulations of nerve regeneration following transection and crush injuries that correspond with growth behaviors quantified in biological experiments. Neovascularization was spatiotemporally quantified in nerve regeneration chambers and following nerve crush injury in order to test the simulations of the mathematical model. The vasculature in both the chamber and following nerve crush responded as predicted by the model, increasing beyond normal levels to a peak only to decrease back to normal. This behavior appeared as a traveling wave in the proximal-distal direction preceeding the major thrust of neuritic outgrowth suggesting that development of the vasculature is a rate-limiting step in nerve regeneration.

The vascular response to nerve transection: neovascularization in the silicone nerve regeneration chamber

The rat sciatic nerve regeneration chamber was used to study the spatial and temporal response of the endoneurial vasculature during regeneration. Proximal and distal stumps of a transected rat sciatic nerve were placed in opposite ends of a silicone tube and allowed to regenerate for periods of 2, 3, 4 or 52 weeks after the surgery. Serial, transverse sections of nerve were studied at each time-point to quantitate the number of vessels, capillary density and the vessel luminal perimeter per nerve area. The results indicate that the vascular growth relative to the existing tissue in the chamber increases to a peak beyond normal levels and later decreases to values associated with control tissue. While this growth occurred from both the proximal and distal stumps, it appeared predominantly as a traveling wave in the proximal-distal direction preceding the major thrust of neuritic outgrowth from the proximal stump. Morphologic measurements of angiogenesis were paralleled in other animals by measurements of nerve blood flow using laser Doppler flowmetry at corresponding time-points. These data differ somewhat from previous reports of angiogenesis following nerve crush injury and are useful in formulating a general mathematical model of regeneration in the peripheral nervous system.

The vascular response to nerve crush: relationship to Wallerian degeneration and regeneration

The response of the endoneurial vasculature in rat sciatic nerve following crush injury was investigated by morphometric analysis of serial nerve transverse sections at the site of injury and in distal segments at 1, 2, 3, 6, and 9 weeks after injury. Quantitative analysis included determination of the number of vessels, vessel radius vessel perimeter, and transfascicular area. The vascular response to crush injury consisted of two phases: an early phase, which peaked at 1 week after crush, consisted of an increase in vessel size but not vessel number. The second phase, which peaked at 6 weeks after crush, consisted of an increase in the number of vessels and in their density. This two-phase response was also evident as a dual peak in the total endoneurial vessel perimeter, a measure of vascular surface area, when this variable was plotted against time. The first phase of the vascular response was temporally related to the recruitment of macrophages and the clearance of degenerating axonal and myelin tissue during the early phase of Wallerian degeneration. The second phase involved an increase in the number of blood vessels and was associated with cellular proliferation, neurite elongation, and myelination during the subsequent period of nerve regeneration.