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At least 19 recordsLinked to original sources
PubMed1984

The simultaneous measurement of thermal conductivity, thermal diffusivity, and perfusion in small volumes of tissue.

An improved technique is presented for the "in-vivo" determination of thermal conductivity, thermal diffusivity, and perfusion using a self-heated spherical thermistor probe. In the presence of flow, solution of the time-dependent, probe-tissue coupled thermal model allows the measurement of "effective" thermal conductivity and "effective" thermal diffusivity, which represent the thermal properties of the perfused tissue. Perfusion can be quantified from both "effective" thermal properties. In the presence of flow, it has been shown that the transient power responses does not follow t-1/2 as has been previously assumed. An isolated rat liver preparation has been developed validate the measurement technique. Radioactive microspheres are used to determine the true perfusion from the total collected hepatic vein flow. Experimental data demonstrates the ability to quantify perfusion in small volumes of tissue.

Animals↗
PubMed1988

Differential effects of adrenergic blockade on seasonal changes in core temperatures and thermal conductances of deer mice maintained in thermal neutral environments.

1. Resting, daytime, thermal conductances and metabolic rates of mice conditioned to winter (10:14LD) and summer (14:10LD) photoperiods were reduced by social huddling; huddling resulted in group size related elevations in core temperature during summer, but not with winter light-dark cycle exposures. 2. Core temperatures of resting, solitary winter animals were lower than those of summer; both summer and winter animals' core temperatures were further reduced by increased thermal conductance resulting from (phentolamine) alpha receptor blockade. 3. Social huddling reduction of the heat loss from phentolamine treatment was more effective for winter (10:14LD) animals. 4. While phentolamine treatment resulted in increased thermal conductance and lower core temperatures of the mice, propranalol treatment resulted in lower core temperatures and resting metabolic rates, with a resulting decrease in thermal conductance. 5. Since adrenergic blockade was less dose-effective on winter animals, we reasoned that winter animals display higher levels of endogenous adrenergic capacity than summer animals and that lower winter thermoregulatory set points provide for energy conservation with enhanced capacity for meeting cold challenge.

Animals↗
PubMed1990

The effects of microsphere injections into the left atrium on the myocardial blood supply measured by thermal conductance probes.

Former experiments with thermal conduction probes showed signs of reductions or increases of myocardial perfusion shortly after injection of microspheres into the left atrium. Because of this, 210 measurements made during experiments on 66 dogs under propionyl-promazine/pentobarbital narcosis were newly analysed to verify a possible influence of microspheres (9 microns phi) injected into the left atrium on microcirculation. Using 20 additional dogs in identically performed experiments, the myocardial perfusion was measured using thermal conductance probes, following injections of isotonic NaCl solution (8 ml each), Ringer's solution, 5% glucose, the subject's blood, and isotonic NaCl solution mixed with the surface-active substance Tween 80. These suspension media were injected both with and without unlabelled microspheres (8.6 microns phi). The results led to the following conclusions: An obligatory decrease in the blood supply as the result of a mechanical blocking of capillaries by microspheres can be ruled out. The particles, the suspension media, and a suspension temperature not sufficiently adjusted to the body temperature induce reactive negative or positive changes in the microcirculation of the myocardium. This was found in approx. 45%-75% of cases. Solutions containing particles cause, in the majority of cases, a decrease. The suspension medium with the smallest effect proved to be isotonic NaCl solution. From the results one can conclude that artefacts may arise from the application of the heat conductance probe method when the temperatures are not perfectly matched. However, the injected solution itself can often lead to various reactions in microcirculation which may last up to 5 min. (ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

Measurement of blood flow using temperature decay: effect of thermal conduction.

Within the framework of the bioheat equation, we studied the effect of conduction on the thermal washout curve for a model tissue configuration subjected to a thermal perturbation such as hyperthermia treatment. In particular, we studied the implications of the assumption made by many investigators to neglect the effect of thermal conduction while analyzing the temperature decay curve for measuring blood perfusion. The present analysis suggests that during the localized hyperthermia treatments, this assumption can lead to inaccurate values for the blood perfusion parameter. This is particularly so under non-steady state conditions when the temperature distribution is changing. In addition to the value of blood flow, the shape of the temperature decay curve depends on the temperature distribution at the start of temperature decay.

Blood Flow Velocity↗
PubMed1989

A technique for measuring the thermal conductivity and evaluating the "apparent conductivity" concept in biomaterials.

A simple technique for measuring thermal conductivity of biomaterials is described. The method is based on depositing a pulse of heat into the material of choice, and fitting the subsequent local temperature decay to that predicted by a theoretical model. This transient method is most suitable in situations where frequent measurements of the thermal conductivity are desired. The method was evaluated by calculating the thermal conductivity of several inert materials. The measured conductivities compared well with published values. The developed technique was also used to examine the applicability of the "apparent conductivity" index to combine both conductive and blood-convective thermal effects in living, blood perfused tissues. Using both simulated and experimental results, it was shown that the changes in the apparent conductivity are highly correlated with changes in blood flow. However, quantitative application of this index must be restricted to conditions that are similar to those which existed at the time the apparent conductivity was measured.

Agar↗
PubMedDate not supplied

Measurements of effective thermal conductivity during hyperthermia: a comparison of experimental and clinical results.

Temperature and effective thermal conductivity (including the convective effects due to tissue blood flow) profiles have been mapped both within a perfused phantom model containing a differentially perfused pseudo-tumour and in patients undergoing microwave (915 MHz) hyperthermia. These measurements demonstrate the influence of differential thermal characteristics of tumour vs. normal tissue on the temperature distributions obtained during hyperthermia. Effective thermal conductivity was measured using a self-heated thermistor probe, and temperature profiles were measured by means of conventional thermocouples and fibre-optic temperature probes. Measurements of effective thermal conductivity obtained in patients prior to microwave hyperthermia, and temperature profiles obtained once steady-state treatment conditions had been attained, show a strong relation between the effective thermal conductivity profile and the ability to obtain therapeutic temperatures without excessive heating of intervening tissues. These observations were confirmed in phantom experiments, demonstrating that this perfused phantom is a more realistic physical model than the conventional unperfused gels usually employed as physical models for hyperthermia experiments. These results demonstrate that tissue thermal clearance is an important determinant of treatment temperature fields, independent of and in addition to the SAR distribution of the particular applicator. Effective thermal conductivity measurements of the different tissues constituting the volume to be heated could be an important index in the planning and optimization of treatment strategies.

Clinical Protocols↗
PubMed1991

A convenient method of measuring the thermal conductivity of biological tissue.

The basic principle of the thermal conductivity probe is described. Thin probes were developed based on this principle, with a reproducibility of 5.3% and relative error less than 6.0%. Each measurement can be completed in 90 s and the temperature increase can be controlled within 2 degrees C. Using the probes, the thermal conductivities of pig fat, meat, liver, kidney and live and dead snake head were measured and it was found that water content plays an important role in influencing the magnitude of the thermal conductivity of biological tissues. The probe can be used over a temperature range from -40 to 150 degrees C.

Adipose Tissue↗
PubMed1988

How does pre-eclampsia influence thermal conductivity of the skin?

To evaluate the thermal conductivity characteristics in relation to pre-eclampsia, deep body temperature (DBT) was measured using the zero-heat flow method in non-pregnant healthy subjects, normal pregnant subjects, pregnant subjects with essential hypertension and in pre-eclamptics. The duration of the initial rise in peripheral DBT was significantly prolonged in the pre-eclamptics, as compared with findings in the other three groups. The results indicate that pre-eclamptics have a decrease of thermal conductivity of the skin.

Body Temperature↗
PubMed1979

Effect of inhalate thermal conductivity and high O2 in producing hypothermia.

The effect of an increase in inhalate thermal conductivity and the fraction of inspiratory O2 (FIO2) on the rate of cooling and rewarming using a surface-inhalate heat exchange method was evaluated. Male New Zealand White rabbits were divided into three groups: those ventilated with air, those with 20% O2 + 80% He, and those with 100% O2. All animals were cooled to an esophageal temperature of 22.5 degrees C (or for 180 min maximum). Following a 15-min exposure to room air, the animals were connected to the humidifying and warming system. He-O2 had the highest thermal conductivity and the animals ventilated with it had the fastest cooling rate. One hundred percent O2 and room air had similar thermal conductivities, but the animals ventilated with 100% O2 had significantly lower cooling rates. These data indicate that, while maintaining a constant surface heart exchange, the rate of heat exchange across the lung can be modified by altering the thermal conductivity of the inhalate gas mixture. Total heat exchange can also be modified by hyperoxemia-induced hemodynamic changes.

Acid-Base Equilibrium↗
PubMedDate not supplied

Thermal conductivity and diffusivity of neuroblastoma tumor.

In the application of hyperthermia to cancer management, it would be useful to know the temperature/tim profile of heated tissues, including the tumor and surrounding normal structures. To obtain this information, knowledge of thermal conductivity and diffusivity of the tissues is required. The thermal conductivity of neuroblastoma was determined by a transient technique to be 89% of the thermal conductivity of water at 25 degrees, 37 degrees, and 44 degrees C. From the latter measurements, the thermal diffusivity of neuroblastoma cells was estimated as 93% of the thermal diffusivity for water. Further, in this study of neuroblastoma cells, the water content was measured as 87.4 g/100 ml of cells, a rather high value not atypical of tumor cells. From literature values of density, specific heat, and thermal conductivity, values for the thermal diffusivity of a variety of normal tissues were estimated. The thermal diffusivity values of normal tissues and neuroblastoma cells exhibit an excellent correlation with water content.

Animals↗
PubMed1980

Thermal diffusivity, specific heat, and thermal conductivity of A-150 plastic.

Some thermal properties of A-150 tissue-equivalent plastic have been determined. The results are: thermal diffusivity, 2.72 x 10(-3) cm2s-1 +/- 0.4%; specific heat, 1.72 J g-1 K-1 +/- 1.3%; and thermal conductivity, 5.3 x 10(-3) WK-1 cm-1 +/- 1.4%. The significance of the measurements for the design of a calorimeter core calibration heater is briefly described.

Calcium Fluoride↗
PubMedDate not supplied

Continuous quantitative local cerebral blood flow measurement. Calibration of thermal conductivity measurements by the hydrogen clearance method.

The capability of a miniaturized probe to measure local cerebral blood flow in a continuous and quantitative manner is described. The incorporation of thermal conductivity measurements using the isothermal principle with the hydrogen clearance method allows calibration of the thermal conductivity component in absolute terms. Evaluation of this system in 14 cats showed a linear relationship between both measurement methods. The major limitation of this combination probe system is the need for routine intermittent recalibration in order that changes of tissues thermal conductivity induced by physiologic alterations during the experimental procedure may be recognized and resolved.

Animals↗
PubMed1991

In vivo thermal conductivity of the human forearm tissues.

The effective thermal conductivities of the skin + subcutaneous (keff skin + fat) and muscle (keff muscle) tissues of the human forearm at thermal steady state during immersion in water at temperatures (Tw) ranging from 15 to 36 degrees C were determined. Tissue temperature (Tt) was continuously monitored by a calibrated multicouple probe during a 3-h immersion of the resting forearm. Tt was measured every 5 mm from the longitudinal axis of the forearm (determined from computed-tomography scanning) to the skin surface. Skin temperature (Tsk), heat loss (Hsk), and blood flow (Q) of the forearm, as well as rectal temperature (Tre) and arterial blood temperature at the brachial artery (Tbla), were measured during the experiments. When the keff values were calculated from the finite-element (FE) solution of the bioheat equation, keff skin + fat ranged from 0.28 +/- 0.03 to 0.73 +/- 0.14 W.degrees C-1.m-1 and keff muscle varied between 0.56 +/- 0.05 and 1.91 +/- 0.19 W.degrees C-1.m-1 from 15 to 36 degrees C. The values of keff skin + fat and keff muscle, calculated from the FE solution for Tw less than or equal to 30 degrees C, were not different from the average in vitro values obtained from the literature. The keff values of the forearm tissues were linearly related (r = 0.80, P less than 0.001) to Q for Tw greater than or equal to 30 degrees C. It was found that the muscle tissue could account for 92 +/- 1% of the total forearm insulation during immersion in water between 15 and 36 degrees C.

Adipose Tissue↗
PubMed1992

Reliability of extravascular lung thermal volume measurements by thermal conductivity technique in sheep.

We tested the accuracy, sensitivity, and reproducibility of a new lung water computer, based on the thermal conductivity technique, in 22 anesthetized closed-chest ventilated sheep with different treatments: 1) controls (n = 8), 2) 0.05 ml/kg of oleic acid + 100 ml/kg of lactated Ringer solution (n = 6), and 3) airway instillation of saline [3.1 +/- 1.3 (SD) g/kg, n = 8]. After 4 h, we determined the extravascular lung water gravimetrically. We found a significant overall correlation between the final extravascular lung thermal volume and the gravimetric extravascular lung mass (P < 0.001). Although the average ratio of extravascular lung thermal volume to extravascular lung mass was 0.97 +/- 0.25 ml/g for all groups, the computer overestimated extravascular lung mass in controls by 10% (17 g) and underestimated it in sheep with oleic acid by 15% (95 g) and in sheep with airway instillation by 8% (37 g). The computer also underestimated the small quantities of saline placed via the airway in the alveolar space by 75% (61 g). Reproducibility of three consecutive measurements was 4.3% (SE). We conclude that the thermal conductivity technique has an ability to detect the baseline extravascular lung mass but has a poor ability to detect an accurate increment of the extravascular lung water under poor tissue perfusion in anesthetized ventilated sheep.

Animals↗
PubMed1995

Metabolic rates and thermal conductance in four species of neotropical bats roosting in hot caves.

Data are presented on metabolic rates and thermal conductance for four species of neotropical bats, Pteronotus quadridens, Mormoops blainvillii (Mormoopidae), Monophyllus redmani and Erophylla bombifrons (Phyllostomidae). Each of these bats predominantly or exclusively roosts in hot caves (28-40 degrees C) in Puerto Rico. Basal metabolic rates (BMR) for these four species were 55%, 48%, 66%, and 66% of values expected from the Kleiber curve, respectively. Thermal conductance was 93%, 55%, 175%, and 158% of values expected from the Aschoff curve, respectively. These data indicate that, in addition to food habits, variation in BMR is highly correlated to roost microclimate. Pteronotus and Mormoops, like other insectivorous species, have low BMRs. The low BMR of the nectar/fruit eating Monophyllus and Erophylla, as well as the low termal conductance in Mormoops, is consistent with their habit of roosting in hot caves, suggesting that roost microclimate is an important selective force in the physiological adaptation of these bats.

Adaptation, Physiological↗
PubMed1994

Calculated thermal conductivities and heat flux in man.

Using data compiled by the Brussels Cadaver Analysis Study on 13 unembalmed cadavers, this study examined the validity of assumptions often made concerning the role of the skin and adipose tissue layers in thermal insulation in vivo. Skin thickness was previously reported to vary from 0.35 to 2.55 mm, depending on the site of measurement and gender of the subject. Assuming a thermal conductivity of 0.70 x 10(-3) kcal/(cm.s-1.degree C-1) for the skin, heat flux across the skin would vary between site and gender in the order of 7.5 times, ranging from 0.16 to 1.20 kcal.min-1.degree C-1.m-2. Due to the negligible thermal gradient across the skin layer, however, this would be of little physiologic significance. Assumptions concerning the homogeneity of skin thickness across gender and measurement site when investigating thermal insulation of the peripheral tissues would not, therefore, influence the reported results significantly. However, it has recently been shown that the calculated lipid fraction of the adipose tissue layer varies according to a person's level of adiposity. Using a two-component model of adipose tissue, the predicted thermal conductivity (k) of the adipose tissue in the present sample was found to range from 0.50 to 0.97 x 10(-3) kcal/(cm.s-1.degree C-1), being significantly lower (P = 0.005) in the five most obese cadavers [6.66 +/- 0.45 x 10(-4) kcal/(cm.s-1.degree C-1)] than in the five most lean cadavers [8.22 +/- 0.93 x 10(-4) kcal/(cm.s-1.degree C-1)]. Adiposity level correlated significantly (r = 0.80; P < 0.01) with calculated k values, suggesting caution must be taken when assuming a single k for the adipose tissue layer when examining divergent populations (i.e., obese vs. thin).

Adipose Tissue↗
PubMed1990

Decreased thermal conductance during the luteal phase of the menstrual cycle in women.

To study the influence of the menstrual cycle on whole body thermal balance and on thermoregulatory mechanisms, metabolic heat production (M) was measured by indirect calorimetry and total heat losses (H) were measured by direct calorimetry in nine women during the follicular (F) and the luteal (L) phases of the menstrual cycle. The subjects were studied while exposed for 90 min to neutral environmental conditions (ambient temperature 28 degrees C, relative humidity 40%) in a direct calorimeter. The values of M and H were not modified by the phase of the menstrual cycle. Furthermore, in both phases the subjects were in thermal equilibrium because M was similar to H (69.7 +/- 1.8 and 72.1 +/- 1.8 W in F and 70.4 +/- 1.9 and 71.4 +/- 1.7 W in L phases, respectively). Tympanic temperature (Tty) was 0.24 +/- 0.07 degrees C higher in the L than in the F phase (P less than 0.05), whereas mean skin temperature (Tsk) was unchanged. Calculated skin thermal conductance (Ksk) was lower in the L (17.9 +/- 0.6 W.m-2.degrees C-1) than in the F phase (20.1 +/- 1.1 W.m-2.degrees C-1; P less than 0.05). Calculated skin blood flow (Fsk) was also lower in the L (0.101 +/- 0.008 l.min-1.m-2) than in the F phase (0.131 +/- 0.015 l.min-1.m-2; P less than 0.05). Differences in Tty, Ksk, and Fsk were not correlated with changes in plasma progesterone concentration. It is concluded that, during the L phase, a decreased thermal conductance in women exposed to a neutral environment allows the maintenance of a higher internal temperature.

Adult↗
PubMed1991

Finite-element solution of thermal conductivity of muscle during cold water immersion.

The in vivo or effective thermal conductivity (keff) of muscle tissue of the human forearm was determined through a finite-element (FE) model solution of the bioheat equation. Data were obtained from steady-state temperatures measured in the forearm after 3 h of immersion in water at temperatures (Tw) of 15 (n = 6), 20 (n = 5), and 30 degrees C (n = 5). Temperatures were measured every 0.5 cm from the longitudinal axis of the forearm to the skin approximately 9 cm distal from the elbow. Heat flux was measured at two sites on the skin adjacent to the temperature probe. The FE model is comprised of concentric annular compartments with boundaries defined by the location of temperature measurements. Through this approach, it was possible to include both the metabolic heat production and the convective heat transfer between blood and tissue at two levels of blood flow, one perfusing the compartment and the other passing through the compartment. Without heat exchange at the passing blood flow level, the arterial blood temperature would be assumed to have a constant value everywhere in the forearm muscles, leading to a solution of the bioheat equation that greatly underpredicts keff. The extent of convective heat exchange at the passing blood flow level is estimated to be approximately 60% of the total heat exchange between blood and tissue. Concurrent with this heat exchange is a decrease in the temperature of the arterial blood as it flows radially from the axis to the skin of the forearm, and this decrease is enhanced with a lowered Tw.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗