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Biomedical subjects

A Shitzer

Publications and source records attributed to A Shitzer.

At least 19 recordsLinked to original sources
PubMed2000

Role of core temperature as a stimulus for cold acclimation during repeated immersion in 20 degrees C water.

The relative importance of skin vs. core temperature for stimulating cold acclimation (CA) was examined by 5 wk of daily 1-h water immersions (20 degrees C) in resting (RG) and exercising (EG) subjects. Rectal temperature fell (0.8 degrees C; P < 0.05) during immersion only in RG. Skin temperature fell (P < 0.05) similarly in both groups. Physiological responses during cold-air exposure (90 min, 5 degrees C) were assessed before and after CA. Body temperatures and metabolic heat production were similar in both groups with no change due to CA. Cardiac output was lower (P < 0.05) in both groups post-CA (10.4 +/- 1.2 l/min) than pre-CA (12.2 +/- 1. 0 l/min), but mean arterial pressure was unchanged (pre-CA 107 +/- 2 mmHg, post-CA 101 +/- 2 mmHg). The increase in norepinephrine was greater (P < 0.05) post-CA (954 +/- 358 pg/ml) compared with pre-CA (1,577 +/- 716 pg/ml) for RG, but CA had no effect on the increase in norepinephrine for EG (pre-CA 1,288 +/- 438 pg/ml, post-CA 1,074 +/- 279 pg/ml). Skin temperature reduction alone may be a sufficient stimulus during CA for increased vasoconstrictor response, but core temperature reduction appears necessary to enhance sympathetic activation during cold exposure.

Adaptation, Physiological↗
PubMed1998

On the thermal efficiency of cold-stressed fingers.

Thermal efficiency of cold-stressed finger-tips during cold induced vasodilatation (CIVD) is considered. The actual heat loss from the finger-tip is compared to either the minimal or the maximal heat losses. The actual heat loss is estimated by integrating the area under the time-temperature curve of the finger-tip. The minimal heat loss is estimated by extrapolating an exponential approximation of finger-tip temperature until it reaches a certain minimal value. The value used in this study is 5 degrees C, which is the pain threshold. The maximal heat loss is calculated by assuming finger-tip temperature to be maintained at its initial value throughout the cold exposure. These quantities were calculated for a series of exposures involving two environmental conditions of gloved subjects: Tdry bulb = -17.2 degrees C, Tdew point = -25.1 degrees C (cold-dry) and Tdry bulb = 0 degree C, Tdew point = -8.4 degrees C (cold-wet). Thermal efficiency was in the range of 0.40-0.85 for the minimal heat loss value (eta min) and 0.22-0.72 for the maximal heat loss value (eta max). Weak linear relationships between the two definitions of the thermal efficiencies and the total duration of the CIVD phase was indicated. The thermal efficiency based on minimal heat loss indicated an inverse relation with the total duration of the CIVD phase. This contradiction could be reconciled by the application of the common concept of "coefficient of performance". Considerable inter- and intra-subjects variability was found.

Adult↗
PubMed1998

Characterization of a three-phase response in gloved cold-stressed fingers.

Seven gloves were studied worn by eight sedentary subjects (six men and two women) exposed to cold-dry, C D, (mean dry bulb temperature Tdb = -17.2 degrees C; mean dew point temperature Tdp = -25.1 degrees C), and cold-wet, C-W, (Tdh = 0 degrees C; Tdp = -8.4 degrees C) conditions. Mean endurance times were 75 min for the C-D and 162 min for the C W conditions. A three-phase response pattern of the temperature in the fingers was characterized. Phase I comprised an initial period during which finger temperature remained close to the pre-exposed level, due to delayed vasoconstriction in the finger. Phase II involved an exponential-like decrease of finger temperature indicative of the onset of vasoconstriction in the finger. Phase III manifested periodic finger temperature changes due to cold induced vasodilatation (CIVD). Mean wave patterns for phase III indicated approximately 3.5 waves x h(-1) in the C D but only about 2 waves x h(-1) in the C-W condition. Extension of endurance time, due to CIVD, was defined as the difference in time between the actual end of the experiment and the time the finger-tip would have reached the set temperature endurance limit as extrapolated by a continued exponential drop. Three overall response patterns of fingers in the cold were characterized: type A exhibiting all 3 phases; type B1 or B2 exhibiting either phases I+ II or phases II+ III; and type C showing only phase II. Considerable inter- and intra-subject variability was found. In both test conditions the final physiological thermal states of the subjects were between comfortable and slightly uncomfortable but acceptable and thus did not correlate with the responses in the fingers.

Adult↗
PubMed1998

Simulation of a cold-stressed finger including the effects of wind, gloves, and cold-induced vasodilatation.

The thermal response of fingers exposed to cold weather conditions has been simulated. Energy balance equations were formulated, in a former study, for the tissue layers and the arterial, venous, and capillary blood vessels. The equations were solved by a finite difference scheme using the Thomas algorithm and the method of alternating directions. At this stage of development the model does not include any autonomic control functions. Model simulations assumed an electrical heating element to be embedded in the glove layers applied on the finger. A 1.3 W power input was calculated for maintaining finger temperatures at their pre-cold exposure level in a 0 degree C environment. Alternate assumptions of nutritional (low) and basal (high) blood flows in the finger demonstrated the dominance of this factor in maintaining finger temperatures at comfortable levels. Simulated exposures to still and windy air, at 4.17 m/s (15 km/h), indicated the profound chilling effects of wind on fingers in cold environments. Finally, the effects of variable blood flow in the finger, known as "cold-induced vasodilatation," were also investigated. Blood flow variations were assumed to be represented by periodic, symmetric triangular waves allowing for gradual opening-closing cycles of blood supply to the tip of the finger. Results of this part of the simulation were compared with measured records of bare finger temperatures. Good conformity was obtained for a plausible pattern of change in blood flow, which was assumed to be provided in its entirety to the tip of the finger alone.

Algorithms↗
PubMed1998

Numerical solution of the multidimensional freezing problem during cryosurgery.

A multidimensional, finite difference numerical scheme for the freezing process of biological tissues during cryosurgery is presented, which is a modification of an earlier numerical solution for inanimate materials. The tissues are treated as nonideal materials, freezing over a temperature range and possessing temperature-dependent thermophysical properties, blood perfusion, and metabolic heat generation. The numerical scheme is based on the application of an effective specific heat, substituting the intrinsic property, to include the latent heat effect within the phase transition temperature range. Results of the numerical solution were verified against an existing exact solution of a one-dimensional inverse Stefan problem in Cartesian coordinates. Results were further validated against experimental data available from the literature. The utility of the numerical solution for the design and application of cryodevices is demonstrated by parametric studies of the freezing processes around spherical and cylindrical cryoprobes. The parameters studied are the cryoprobe cooling power and the dimensions of the frozen region. Results are calculated for typical thermophysical properties of soft biological tissues, for angioma and for water.

Cell Survival↗
PubMed1998

A physiological strain index to evaluate heat stress.

A physiological strain index (PSI), based on rectal temperature (Tre) and heart rate (HR), capable of indicating heat strain online and analyzing existing databases, has been developed. The index rates the physiological strain on a universal scale of 0-10. It was assumed that the maximal Tre and HR rise during exposure to exercise heat stress from normothermia to hyperthermia was 3 degrees C (36.5-39.5 degrees C) and 120 beats/min (60-180 beats/min), respectively. Tre and HR were assigned the same weight functions as follows: PSI = 5(Tret - Tre0) . (39.5 - Tre0)-1 + 5(HRt - HR0) . (180 - HR0)-1, where Tret and HRt are simultaneous measurements taken at any time during the exposure and Tre0 and HR0 are the initial measurements. PSI was applied to data obtained from 100 men performing exercise in the heat (40 degrees C, 40% relative humidity; 1.34 m/s at a 2% grade) for 120 min. A separate database representing seven men wearing protective clothing and exercising in hot-dry and hot-wet environmental conditions was applied to test the validity of the present index. PSI differentiated significantly (P < 0.05) between the two climates. This index has the potential to be widely accepted and to serve universally after extending its validity to women and other age groups.

Acclimatization↗
PubMed1997

Combined solution of the inverse Stefan problem for successive freezing/thawing in nonideal biological tissues.

A new combined solution of the one-dimensional inverse Stefan problem in biological tissues is presented. The tissue is assumed to be a nonideal material in which phase transition occurs over a temperature range. The solution includes the thermal effects of blood perfusion and metabolic heat generation. The analysis combines a heat balance integral solution in the frozen region and a numerical enthalpy-based solution approach in the unfrozen region. The subregion of phase transition is included in the unfrozen region. Thermal effects of blood perfusion and metabolic heat generation are assumed to be temperature dependent and present in the unfrozen region only. An arbitrary initial condition is assumed that renders the solution useful for cryosurgical applications employing repeated freezing/thawing cycles. Very good agreement is obtained between the combined and an exact solution of a similar problem with constant thermophysical properties and a uniform initial condition. The solution indicated that blood perfusion does not appreciably affect either the shape of the temperature forcing function on the cryoprobe or the location and depth of penetration of the freezing front in peripheral tissues. It does, however, have a major influence on the freezing/thawing cycle duration, which is most pronounced during the thawing stage. The cooling rate imposed at the freezing front also has a major inverse effect on the duration of the freezing/thawing.

Blood Flow Velocity↗
PubMed1996

A new cryosurgical device for controlled freezing.

A new cryosurgical device utilizing liquid nitrogen, which is a modification of an existing commercial system, was developed. In the new computer-controlled cryodevice the temperature of the cryoprobe is controlled by means of an electrical heating element. The desired temperature-forcing function is calculated to ensure a specified constant cooling rate at the freezing front. The new device facilitates real-time data processing, and, in particular, simulation of the heat transfer processes. A series of tests was performed to study the characteristics of the cryodevice and to validate the underlying assumptions. These tests were performed using organic tissue, i.e., potatoes, as an in vivo simulating medium of biological tissue. The differences between experimental data and computed results were found to be within +/-0.5 degrees C, which falls within the uncertainty range of the experimental temperature measurements. A typical control error of the new device is within +/-0.3 degrees C, prior to the formation of the freezing front, and +/-0.6 degrees C thereafter, which is of the same order of magnitude as the uncertainty range of the temperature measurements. The new device is capable of producing maximal cooling rates of 50 degrees C/min down to temperatures of -165 degrees C and a maximal heating rate of 300 degrees C/min. The maximal cooling power of the cryoprobe, due to LN2 boiling, is 80 W; the maximal electrical heating power of the cryoprobe is 160 W. Precooling of the device requires about 30 min, and it can be operated continuously for about 3 h. Initial results of experimental in vivo cryosurgery performed on rabbit hindlimbs, including histological observations and thermal analysis, are presented in the second part of this study.

Computer Simulation↗
PubMed1996

A new cryosurgical device for controlled freezing.

A new cryosurgical device was developed in this study to facilitate examination of factors affecting the outcome of cryotreatment. Special emphasis was placed on the control of the cooling rate at the freezing front. In the new computer-controlled cryosurgical device, the controlling variable is the cryoprobe temperature, which is calculated to ensure prespecified cooling rates at the freezing front. Details of the new cryodevice, results of a validation test, and the system characteristics are presented in Part I of this study. In this part of the study initial results of 13 in vivo experimental cryotreatments, including histological observations, are presented. The in vivo pilot investigations include the normal, healthy skin and the underlying skeletal muscle of the thighs in rabbits. Using low cooling rate-controlled freezing, the new cryosurgical device is demonstrated here as an effective surgical tool. An in vivo temperature measurement technique is employed based on miniature thermocouples and X-ray images. Thermal analysis of the heat transfer in the cryotreated tissue is presented, based on the temperature measurements and on numerical heat transfer simulations. Cryotreated tissue was extracted either immediately or 4 or 7 days following the procedure. The histological observations on the skeletal muscle of the 4- and 7-day postcryoinjury were not substantially different. The effective penetration depth of the cryolesion was in the range of 5-15 mm, possibly extending up to 25 mm, depending on the specific area treated and operating parameters. The cryotreatment resulted in complete destruction of cells in the skin followed by rapid replacement by epithelial cells. Histological responses to cryotreatment of skeletal muscle were similar to those resulting from a range of traumatic episodes, e.g., crush damage. It was also found that most of the blood vessels in the cryotreated region remained intact without histological evidence of extravasation of erythrocytes.

Animals↗
PubMed1996

Lumped-parameter tissue temperature-blood perfusion model of a cold-stressed fingertip.

A lumped-parameter model of a fingertip is presented. The semispherical model includes the effects of heat storage, heat exchange with the environment, and heat transport by blood perfusion. The thermal insulation on the surface of the fingertip is represented by the overall heat transfer coefficient that is calculated by common engineering formulas. The model is solved analytically for the simple case of constant blood perfusion rate. The general case of variable blood perfusion rates is solved by an Euler finite difference technique. At this stage, the model does not include active control mechanisms of blood perfusion. Thus the effects of cold-induced vasodilatation have to be superimposed and are modeled by symmetrical triangular waveforms because these were found to best depict the behavior of fingers exposed to cold environments. Results of this model were compared with experimental data obtained in two separate studies. One included 60-min infrared thermograms of the dorsal surface of bare hands of sedentary subjects horizontally suspended on a fish net in a 0 degree C environment. Another study, on gloved finger temperatures, involved 0 and -6.7 degrees C environments. Fingertip (nail bed) temperatures of both these studies were compared with model predictions. Blood perfusion rates were assumed and adjusted within physiologically reasonable limits. Comparison of measured and computed temperature records showed very good conformity in both cases studied.

Body Temperature Regulation↗
PubMed1995

Analysis of the inverse problem of freezing and thawing of a binary solution during cryosurgical processes.

An integral solution for a one-dimensional inverse Stefan problem is presented. Both the freezing and subsequent thawing processes are considered. The medium depicting biological tissues, is a nonideal binary solution wherein phase change occurs over a range of temperatures rather than at a single one. A constant cooling, or warming, rate is imposed at the lower temperature boundary of the freezing/thawing front. This condition is believed to be essential for maximizing cell destruction rate. The integral solution yields a temperature forcing function which is applied at the surface of the cryoprobe. An average thermal conductivity, on both sides of the freezing front, is used to improve the solution. A two-dimensional, axisymmetric finite element code is used to calculate cooling/warming rates at positions in the medium away from the axis of symmetry of the cryoprobe. It was shown that these cooling/warming rates were always lower than the prescribed rate assumed in the one-dimensional solution. Thus, similar, or even higher, cell destruction rates may be expected in the medium consistent with existing in vitro data. Certain problems associated with the control of the warming rate during the melting stage are discussed.

Animals↗
PubMed1993

Finite element analysis of the temperature field around two adjacent cryo-probes.

A finite element code was developed for the analysis of the temperature field around two adjacent cylindrical cryo-probes. The two-phase, two-dimensional Stefan problem is solved using a moving boundary approach and space-time finite elements. Solution of one-cryo-probe problem compared well with an existing analytic solution. The two-cryo-probes problem yielded reasonable results. The program simulated the nonsymmetric activation of two probes and the merging of the two freezing fronts in the case of symmetric activation.

Cryosurgery↗
PubMed1991

Control of the cryosurgical process in nonideal materials.

A study of a controlled cryosurgical process is presented. This study is based on the energy equations describing the probe response and the phase change occurring in the medium. First-order nonlinear differential equations (state equations) are obtained by applying the integral-solution method. In order to obtain maximal cell destruction, it is desired to control a specific cooling rate at the solid-liquid interface. This cooling rate defines the desired trajectories of the state variables through the state equations. In order to satisfy the cooling rate condition on the freezing front, a closed-loop is designed to control the probe temperature program. A simple analysis of the system stability employed linearization at several points along the desired trajectories. Ranges of stability were obtained for a system containing a proportional-integral controller. It was demonstrated that these stability ranges depend mainly on the selected sampling time of the discrete control loop and that the phase-change process does not significantly affect the stability results. A complete study of the nonlinear equations was performed by a computer simulation program which enables the selection of the final values of the controller parameters, in order to minimize the error and to ensure stability. In addition, the simulation program gives information about the effects of the A/D and D/A converters accuracy on the performance of the control loop. An A/D converter accuracy of 12 bits was found necessary in order to reduce the oscillations in probe temperature to acceptable values. The simulation also yields a complete calculated temperature field in the tissue during the controlled process. From these simulated results it can be seen that oscillations of +/- 0.5 degrees C in the desired probe temperature do not significantly affect the desired cooling rate at the freezing front. An initial overshoot of 1.5 degrees C in the desired probe temperature was obtained both experimentally and theoretically from the simulation. When this initial overshoot occurs at the beginning of the freezing process, it causes an error in freezing front velocity and consequently in ice-front position. From the numerical simulation, it can be deduced that the cooling rate obtained at the front deviates from the desired value by approximately 1%. The probe-temperature error increases at two instants: a) during the super-cooling effect and the subsequent sudden crystallization, and b) when the probe temperature is below -80 degrees C and unstable boiling of the cooling medium causes oscillations.

Cryosurgery↗
PubMed1991

Controlled freezing of nonideal solutions with application to cryosurgical processes.

Success of a cryosurgical procedure, i.e., maximal cell destruction, requires that the cooling rate be controlled during the freezing process. Standard cryosurgical devices are not usually designed to perform the required controlled process. In this study, a new cryosurgical device was developed which facilitates the achievement of a specified cooling rate during freezing by accurately controlling the probe temperature variation with time. The new device has been experimentally tested by applying it to an aqueous solution of mashed potatoes. The temperature field in the freezing medium, whose thermal properties are similar to those of biological tissue, was measured. The cryoprobe temperature was controlled according to a desired time varying profile which was assumed to maximize necrosis. The tracking accuracy and the stability of the closed loop control system were investigated. It was found that for most of the time the tracking accuracy was excellent and the error between the measured probe temperature and the desired set point is within +/- 0.4 degrees C. However, noticeable deviations from the set point occurred due to the supercooling phenomenon or due to the instability of the liquid nitrogen boiling regime in the cryoprobe. The experimental results were compared to those obtained by a finite elements program and very good agreement was obtained. The deviation between the two data sets seems to be mainly due to errors in positioning of the thermocouple junctions in the medium.

Analog-Digital Conversion↗
PubMed1991

Quantification of conservative endurance times in thermally insulated cold-stressed digits.

The estimation of endurance times of the digits exposed to cold weather is performed by an analytical, one-dimensional cylindrical model. Blood perfusion effects are lumped into a volumetric heat-generation term. Cold-induced vasodilatation (CIVD) effects are not included in the present analysis. Endurance times, defined by a drop in cylinder tip temperature to 5 degrees C, were evaluated. Parameters included in this evaluation were 1) environmental temperatures, 2) thermal insulation applied on the cylinder, 3) length of the cylinder, and 4) diameter of the cylinder. It was found that the lower the ambient temperature, the longer the finger, and the smaller its diameter, then the shorter the endurance time for the same thermal insulation. Results of the model were compared with measured data for a subject not exhibiting CIVD response to cold stress. Conformity of results calculated for an adjusted value of the volumetric heat-generation term and measured data was very good, with a maximum deviation of less than 10% at only one particular point in time. This model facilitates the conservative estimation of lower bounds to thermally insulated fingers and toes exposed to cold stress.

Body Temperature Regulation↗
PubMed1986

Investigation of temperature fields around embedded cryoprobes.

The temperature fields around cryoprobes were investigated analytically and experimentally. Two cryoprobes were employed: a spherically shaped general purpose probe utilizing liquid nitrogen and a cylindrical "glaucoma" probe utilizing the Joule-Thomson effect in gaseous CO2. Both probes were operated by commercial cryostats. The analytical solutions included a one-dimensional integral solution for the general purpose cryoprobe, and finite element solutions for both cryoprobes. Both solutions were based on the enthalpy method. Analytical and experimental results compared reasonably well. Deviations of these results are believed to be due, mainly, to the incomplete specification of the boundary conditions on the surface of the cryoprobe.

Cryosurgery↗