Measuring fetal and maternal temperature differentials: a probe for clinical use during labour

The healthy fetus maintains a higher temperature than that of its mother during gestation and labour. This results from the thermal balance between the heat generated by the fetus and the heat loss to its maternal surroundings. The heat loss can be by heat exchange via blood flowing in the umbilical cord and placenta, and via conduction through the fetal skin and amniotic fluid to the maternal wall. The temperature difference between the fetal and maternal tissue may reflect the metabolic state of the fetus and the magnitude and changing patterns of placental blood flow during labour. Physiological changes, such as those induced by epidural analgesia, and fetal infection have been shown to exhibit an increase in the absolute temperature. An intrauterine probe, previously used for non-invasive ECG detection, has been equipped with temperature sensors that measure fetal and maternal skin temperature in utero. Laboratory tests to characterize the performance of the probe reveal that absolute and differential temperatures can be resolved to around 0.01° C with a thermal time constant of approximately 9 s. Ideally the probe body should have infinite thermal insulation or thermal shunting across the probe will occur reducing the measured temperature difference. In this initial probe design, a high thermal isolation between sensors has been achieved but is not perfect, resulting in around 85% of the actual temperature difference across the probe being registered. Average feto-maternal differences of 0.2° C have been measured in a clinical investigation.     

Thermal bradycardia during radiofrequency irradiation

The present study was performed to determine if any heart rate or blood pressure changes occur during intermittent exposure to radiofrequency radiation (RFR), and to determine if parasympathetic blockade due to atropine has any effect on these changes or on thermal responses. Anesthetized rats were exposed to 2.8 GHz pulsed RFR at an average power level of 60 mW/cm2 (average specific absorption rate, 14 W/kg). During an initial exposure period to raise colonic temperature to 39.5 degrees C, heart rate decreased significantly. This thermal bradycardia is similar to that reported by other investigators during environmental heat exposure. Intermittent exposure to radiation, which was designed to result in 1 degree C colonic temperature changes, did not significantly affect heart rate or mean arterial blood pressure, before or after atropine administration. The time courses of these 1 degree C temperature changes were not altered significantly by atropine. Following administration of atropine, the thermal bradycardia during the initial heating period was still evident. Thus, factors other than vagal activity are responsible for the phenomenon. It is possible that the bradycardia is a consequence of a general reduction in metabolism, which occurs also during environmental heat exposure.     

基于侧柏蒸腾量分析Granier经验公式的测量精度

为了解Granier热扩散法测定树木蒸腾耗水量的精度,2010年5-6月采用热扩散探针测定盆栽侧柏的液流量,并以整树盆栽称重法作为对照.结果表明:热扩散法测定侧柏南、北两个方位的树干液流速率与称重法测定的蒸腾速率均具有良好的线性关系(R2 ＞0.825).侧柏南、北两个方位的树干平均日液流量分别比平均日蒸腾量低10.6％和15.1％,但未达到显著水平.尽管昼夜温差较大会造成侧柏日液流量小于日蒸腾量,但采用Granier热扩散法测定侧柏的蒸腾量在日尺度上具有较高的可靠性.     

A self-heated thermistor technique to measure effective thermal properties from the tissue surface

A microcomputer based instrument to measure effective thermal conductivity and diffusivity at the surface of a tissue has been developed. Self-heated spherical thermistors, partially embedded in an insulator, are used to simultaneously heat tissue and measure the resulting temperature rise. The temperature increase of the thermistor for a given applied power is a function of the combined thermal properties of the insulator, the thermistor, and the tissue. Once the probe is calibrated, the instrument accurately measures the thermal properties of tissue. Conductivity measurements are accurate to 2 percent and diffusivity measurements are accurate to 4 percent. A simplified bioheat equation is used which assumes the effective tissue thermal conductivity is a linear function of perfusion. Since tissue blood flow strongly affects heat transfer, the surface thermistor probe is quite sensitive to perfusion.     

Effect of temperature cycling on the production of aflatoxin by Asperfillus parasiticus.

Aspergillus parasiticus (NRRL 2999) was grown under cycling temperature conditions on rice and nutmeat substrates. Under conditions of diurnal and nocturnal time-temperature sequencing, total heat input is an important factor of toxin production. When expressed in degree hours per day, thermal input becomes more definitive and provides a finite number, which can be related to observable changes in the culture such as sporulation and toxin biosynthesis. Three well-defined levels of response were observed in relation to heat input: no growth was detected at thermal inputs of less than 208 degree hours/day; mycelial growth as well as copious amounts of an orange pigment were observed at thermal inputs between 208 and 270 degree hours/day; sporulation and aflatoxin biosynthesis occurred above 270 degree hours/day. Between the optimum and minimum thermal input, cycling temperatures significantly reduced the period of the trophophase over cultures receiving equal heat input at a constant rate. Cycling temperatures at the low and high extremes of the temperature range had little or no effect upon the growth pattern of the culture. Regardless of how temperature was manipulated, these responses were consistent with the heat input received by the culture. A. parasiticus did not compete well when mixed with natural fungal isolates from nutmeats and was easily overgrown by the wild isolates even at relatively high thermal input and when present in superior numbers. This factor and heat input generally below that required for toxin biogenesis at harvest time appear to be two significant factors that limit occurrence of aflatoxin on nut crops of the Willamette Valley. These factors are likely to have significance for other crops grown and harvested under similar circumstances.     

Intermittent drying of bioproducts--an overview

Unlike the conventional practice of supplying energy for batch drying processes at a constant rate, newly developed intermittent drying processes employ time-varying heat input tailored to match the drying kinetics of the material being dried. The energy required may be supplied by combining different modes of heat transfer (e.g. convection coupled with conduction or radiation or dielectric heating simultaneously or in a pre-selected sequence) in a time-varying fashion so as to provide optimal drying kinetics as well as quality of the bioproduct. This is especially important for drying of heat-sensitive materials (such as foods, pharmaceutical, neutraceutical substances, herbs, spices and herbal medicines). Intermittent heat supply is beneficial only for materials which dry primarily in the falling rate period where internal diffusion of heat and moisture controls the overall drying rate. Periods when little or no heat is supplied for drying allow the tempering period needed for the moisture and heat to diffuse within the material. As the moisture content increases at the surface of the biomaterial during the tempering period, the rate of drying is higher when heat input is resumed. It is possible to control the heat input such that the surface temperature of the product does not exceed a pre-determined value beyond which thermal damage of the material may occur. This process results in reduction in the use of thermal energy as well as the mass of air used in convective drying. Thus, the thermal efficiency of such a process is higher. The quality of the product, as such color and ascorbic acid content, is also typically superior to that obtained with a continuous supply of heat. However, in some cases, there will be a nominal increase in drying time. In the case of microwave-assisted and heat pump drying, for example, the capital cost of the drying system can also be reduced by drying in the intermittent mode.

This paper provides an overview of the basic process, selected results from experiments and mathematical models for a variety of biomaterials dried in a wide assortment of dryers. It begins with a classification of intermittent drying processes that may be applied e.g. time-varying temperature, air flow rate, operating pressure as well as heat input by different modes and in different temporal variations. The beneficial effects of improving the quality of dried bioproducts by different intermittent processes are also included and discussed.     

Thermodynamic constraints on neural dimensions,firing rates,brain temperature and size

There have been suggestions that heat caused by cerebral metabolic activity may constrain mammalian brain evolution, architecture, and function. This article investigates physical limits on brain wiring and corresponding changes in brain temperature that are imposed by thermodynamics of heat balance determined mainly by Na⁺/K⁺-ATPase, cerebral blood flow, and heat conduction. It is found that even moderate firing rates cause significant intracellular Na⁺ build-up, and the ATP consumption rate associated with pumping out these ions grows nonlinearly with frequency. Surprisingly, the power dissipated by the Na⁺/K⁺ pump depends biphasically on frequency, which can lead to the biphasic dependence of brain temperature on frequency as well. Both the total power of sodium pumps and brain temperature diverge for very small fiber diameters, indicating that too thin fibers are not beneficial for thermal balance. For very small brains blood flow is not a sufficient cooling mechanism deep in the brain. The theoretical lower bound on fiber diameter above which brain temperature is in the operational regime is strongly frequency dependent but finite due to synaptic depression. For normal neurophysiological conditions this bound is at least an order of magnitude smaller than average values of empirical fiber diameters, suggesting that neuroanatomy of the mammalian brains operates in the thermodynamically safe regime. Analytical formulas presented can be used to estimate average firing rates in mammals, and relate their changes to changes in brain temperature, which can have important practical applications. In general, activity in larger brains is found to be slower than in smaller brains.     

Calorimetric study of the glass transition of denatured collagen

Temperature dependence of heat capacity of native and denatured collagen samples with different content of bound water (6 divided by 27%) has been studied by DSC method in the temperature range from -50 to 150 degrees C. Heat capacity of denatured samples demonstrates a jump of 0.50 J/g.grad. at temperature Tg, which depends on humidity of the sample. It has been shown that Tg value also depends on the heating rate and thermal history. Annealing at the temperature below Tg produces an additional maximum in the temperature dependence on heat capacity. The magnitude of this maximum, as well as the Tg value increase with the annealing time. It is concluded that these properties of heat capacity reflect glass transition in the denatured collagen.     

Chorioretinal thermal behavior

Chorioretinal thermal response to intense light exposure is calculated for light sources with a wide variety of spatial and temporal characteristics. Transient temperature distributions are computed by means of an alternating directions implicit method for solving cylindrically symmetric heat conduction problems in biological media. Chorioretinal thermal distributions are discussed in terms of a maximum temperature damage criterion for ocular tissue.     

Steady-state analysis of self-heated thermistors using finite elements

The purpose of this work is to validate, using numerical, finite element methods, the thermal assumptions made in the analytical analysis of a coupled thermistor probe-tissue model upon which a thermal conductivity measurement scheme has been based. Analytic, closed form temperature profiles generated by the self-heated thermistors can be found if three simplifying assumptions are made: the thermistor is spherical; heat is generated in all regions of the bead; and heat is generated uniformly in the bead. This analytic solution is used to derive a linear relationship between tissue thermal conductivity and the ratio of thermistor temperature rise over electrical power required to maintain that temperature rise. This derived, linear relationship is used to determine thermal conductivity from the observed experimental data. However, in reality, the thermistor bead is a prolate spheroid surrounded by a passive shell, and the heating pattern in the bead is highly nonuniform. In the physical system, the exact relationship between the tissue thermal conductivity and parameters measured by the thermistor is not known. The finite element method was used to calculate the steady-state temperature profiles generated by thermistor beads with realistic geometry and heating patterns. The results of the finite element analysis show that the empirical, linear relationship remains valid when all three simplified assumptions are significantly relaxed.     

Parametric studies on the three-layer microcirculatory model for surface tissue energy exchange

The new three-layer microvascular mathematical model for surface tissue heat transfer developed in, which is based on detailed vascular casts and tissue temperature measurements in the rabbit thigh, is used to investigate the thermal characteristics of surface tissue under a wide variety of physiological conditions. Studies are carried out to examine the effects of vascular configuration, arterial blood supply rate, distribution of capillary perfusion, cutaneous blood circulation and metabolic heat production on the average tissue temperature profile, the local arterial-venous blood temperature difference in the thermally significant countercurrent vessels, and surface heat flux.     

Modeling of irreversible thermal protein denaturation at varying temperature. II. The complete kinetic model of Lumry and Eyring.

The model for thermal denaturation of proteins involving consecutive reversible and irreversible steps (Lumry and Eyring model) has been analyzed. The most general case, when equilibrium in the first step is established slowly in comparison with the rate of the second step and the heat effect value for the second step is either greater than or less than zero, has been considered. The theoretical dependences of excess heat capacity on temperature have been constructed. The variation of the shape of the theoretical curves with varied values of the enthalpy change for the second step, Arrhenius equation parameters for both steps, and the scanning rate has been studied.     

Skin blood flow changes in anaesthetized humans: comparison between skin thermal clearance and finger pulse amplitude measurement

The effect of general anaesthesia on skin blood flow in the left hand, measured by a new non-invasive probe using the thermal clearance method was examined. A mercury silastic gauge was placed around the third left finger and the plethysmographic wave amplitude was recorded to measure changes in finger pulse amplitude. Heart rate (HR), mean arterial blood pressure (MABP) and skin temperature were also recorded. General anaesthesia was induced by droperidol and phenoperidine injection and propanidid infusion in eight female patients. Skin thermal clearance, plethysmographic wave amplitude, HR, MABP and skin temperature were 0.40 +/- 0.02 w X m-1 degree C-1, 9 +/- 1 mm, 98 +/- 5 beats X min-1, 12.50 +/- 0.93 kPa and 33.3 +/- 3.4 degrees C respectively. The minimal value of MABP was 9.58 +/- 1.06 kPa, whereas skin thermal clearance, plethysmographic wave amplitude, HR and skin temperature increased to 0.45 +/- 0.02 w X m-1 degree C-1, 29 +/- 3 mm, 110 +/- 4 beats X min-1 and 34.4 +/- 0.4 degrees C. Changes in skin thermal clearance correlated well with plethysmographic wave amplitude. Statistically significant changes in these two parameters occurred before significant change in HR, MABP or skin temperature. The results show that the new non-invasive probe using the thermal clearance method appears to be a useful device for measuring cutaneous microcirculation in anaesthetized humans, and responds more quickly than change in skin temperature, which is a delayed effect of skin blood flow change. Our results also show that the intensity of cutaneous vasodilatation induced by general anaesthesia did not relate to the vascular tone before anaesthesia.     

Scanning calorimetry measurements of the gas temperature in an RF-discharge fluorine-containing plasma

The neutral-gas temperature in a low-pressure (50 Pa) capacitive RF discharge in a CF₄+O₂ mixture is determined from the heating kinetics of a gallium arsenide single crystal, which is chemically inert to any radicals in a fluorine-containing plasma. Experimental methods are discussed that make it possible to confirm the absence of heat sources capable of additional heating of the calorimeter in the discharge. The features and applicability limits of the method of non-steady-state gas thermometry in a weakly ionized nonequilibrium plasma are discussed. The method proposed is compared with conventional steady-state methods based on measurements of the established temperature of a thermal probe in the discharge. Temperature scanning makes it possible to study dependences that cannot be investigated by steady-state methods, in particular, the temperature dependence of the calorimeter heating power, which is very important for diagnosing the processes of plasma-surface heat transfer.     

Application of respiratory heat exchange for the measurement of lung water.

A noninvasive method for measuring pulmonary blood flow and lung mass (called airway thermal volume), based on the measurements of lung heat exchange with environment, is described. The lungs function as a steady-state heat exchange system, having an inner heat source (pulmonary blood flow) and an external heat sink (ventilation). Sudden changes in the steady-state condition, such as caused by hyperventilation of dry air, lead to a new steady state after a few minutes. The expired air temperature difference between the initial and final steady states is proportional to pulmonary blood flow, whereas the rate at which the new steady state is achieved is proportional to airway thermal volume. The method was tested in 20 isolated dogs lungs, 9 perfused goat lungs, and 27 anesthetized sheep. The expired air temperature fall during hyperventilation was inversely proportional to the perfusion rate of the isolated lungs, and half-time of the temperature fall was proportional to the lung tissue mass. Experiments in anesthetized sheep showed that the measured airway thermal volume is close to the total mass of the excised lungs, including its residual blood (r = 0.98). Pulmonary edema and fluid instillation into the bronchial tree increased in the measured lung mass.     

Thermal stability of nucleosomes studied in situ by flow cytometry: effect of ionic strength and n-butyrate

Heating of cells permeabilized with ethanol and resuspended in aqueous media increases accessibility of DNA to intercalating dyes such as acridine orange (AO). The curves, representing increase in binding of AO as a function of rise in temperature, indicate that the transitions are cooperative. The transitions are sensitive to ionic strength and occur at lower temperatures when cells are suspended in media of increasing ionic strength. Extraction of histones raises accessibility of DNA to intercalators at room temperature, and heating has little effect on additional binding. The results are interpreted as indicating thermal destruction of nucleosomal structure in nuclear chromatin; dissociation of DNA from core histones results in its increasing ability to intercalate AO, most likely due to increased topological freedom to undergo unwinding and elongation following binding of the intercalator. Preincubation of cells with n-butyrate, known to induce histone hyperacetylation, lowers the heat stability of nucleosomes by about 5 degrees C. On the other hand, no differences are observed between chromatin of mitotic vs interphase cells tested over a wide range of ionic strengths (0.1-0.7 N NaCl). The method appears to be useful as a probe of chromatin structure at the nucleosomal level.     

Thermal behavior of biological tissues—A general analysis

A general analysis is presented for the thermal behavior of a biological tissue. Energy transport by the circulatory system is assumed to be represented by a modified Fick's law. General boundary conditions are assumed for the two-dimensional model and solutions are obtained for rectangular, cylindrical, and spherical geometries. The effects of blood perfusion rate, metabolic rate, arterial temperature and heat exchange with the environment are considered. Results indicate a region of almost constant temperature in the deeper layers of the tissue and reaffirm the important role which blood flow plays in maintaining homeostasis.     

High-throughput PCR in silicon based microchamber array

Highly integrated hybridization assay and capillary electrophoresis have improved the throughput of DNA analysis. The shift to high throughput analysis requires a high speed DNA amplification system, and several rapid PCR systems have been developed. In these thermal cyclers, the temperature was controlled by effective methodology instead of a large heating/cooling block preventing rapid thermal cycling. In our research, high speed PCR was performed using a silicon-based microchamber array and three heat blocks. The highly integrated microchamber array was fabricated by semiconductor microfabrication techniques. The temperature of the PCR microchamber was controlled by alternating between three heat blocks of different temperature. In general, silicon has excellent thermal conductivity, and the heat capacity is small in the miniaturized sample volume. Hence, the heating/cooling rate was rapid, approximately 16 °C/s. The rapid PCR was therefore completed in 18 min for 40 cycles. The thermal cycle time was reduced to 1/10 of a commercial PCR instrument (Model 9600, PE Applied Biosystems-3 h).     

A parametric study of thermal therapy of skin tissue

A thermal therapy for cancer in skin tissue is numerically investigated using three bioheat conduction models, namely Pennes, thermal wave and dual-phase lag models. A laser is applied at the surface of the skin for cancer ablation, and the temperature and thermal damage distributions are predicted using the three bioheat models and two different modeling approaches of the laser effect. The first one is a prescribed surface heat flux, in which the tissue is assumed to be highly absorbent, while the second approach is a volumetric heat source, which is reasonable if the scattering and absorption skin effects are of similar magnitude. The finite volume method is applied to solve the governing bioheat equation. A parametric study is carried out to ascertain the effects of the thermophysical properties of the cancer on the thermal damage. The temperature distributions predicted by the three models exhibit significant differences, even though the temperature distributions are similar when the laser is turned off. The type of bioheat model has more influence on the predicted thermal damage than the type of modeling approach used for the laser. The phase lags of heat flux and temperature gradient have an important influence on the results, as well as the thermal conductivity of the cancer. In contrast, the uncertainty in the specific heat and blood perfusion rate has a minor influence on the thermal damage.