Role of blood as heat source or sink in human limbs during local cooling and heating

The Use of Percutaneous Thermal Sensing Microchips for Body Temperature Measurements in Horses Prior to, during and after Treadmill Exercise

Accurately measuring body temperature in horses will improve the management of horses suffering from or being at risk of developing postrace exertional heat illness. PTSM has the potential for measuring body temperature accurately, safely, rapidly, and noninvasively. This study was undertaken to investigate the relation between the core body temperature and PTSM temperatures prior to, during, and immediately after exercise. The microchips were implanted into the nuchal ligament, the right splenius, gluteal, and pectoral muscles, and these locations were then compared with the central venous temperature, which is considered to be the "gold standard" for assessing core body temperature. The changes in temperature of each implant in the horses were evaluated in each phase (prior to, during, and immediately postexercise) and combining all phases. There were strong positive correlations ranging from 0.82 to 0.94 (p < 0.001) of all the muscle sites with the central venous temperature when combining all the phases. Additionally, during the whole period, PTSM had narrow limits of agreement (LOA) with central venous temperature, which inferred that PTSM is essentially equivalent in measuring horse body temperature. Overall, the pectoral PTSM provided a valid estimation of the core body temperature.

Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update

To provide guidance to clinicians, the Wilderness Medical Society convened an expert panel to develop evidence-based guidelines for the out-of-hospital evaluation and treatment of victims of accidental hypothermia. The guidelines present the main diagnostic and therapeutic modalities and provide recommendations for the management of hypothermic patients. The panel graded the recommendations based on the quality of supporting evidence and a balance between benefits and risks/burdens according to the criteria published by the American College of Chest Physicians. The guidelines also provide suggested general approaches to the evaluation and treatment of accidental hypothermia that incorporate specific recommendations. This is the 2019 update of the Wilderness Medical Society Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2014 Update.

Cross-sectional area of the murine aorta linearly increases with increasing core body temperature

PURPOSE

The cardiovascular (CV) system plays a vital role in thermoregulation. To date, the response of core vasculature to increasing core temperature has not been adequately studied in vivo. Our objective was to non-invasively quantify the arterial response in murine models due to increases in body temperature, with a focus on core vessels of the torso and investigate whether responses were dependent on sex or age.

METHODS

Male and female, adult and aged mice were anaesthetised and underwent magnetic resonance imaging (MRI). Data were acquired from the circle of Willis (CoW), heart, infrarenal aorta and peripheral arteries at core temperatures of 35, 36, 37 and 38 °C (±0.2 °C).

RESULTS

Vessels in the CoW did not change. Ejection fraction decreased and cardiac output (CO) increased with increasing temperature in adult female mice. Cross-sectional area of the aorta increased significantly and linearly with temperature for all groups, but at a diminished rate for aged animals (p < 0.01; male and female: adult, 0.019 and 0.024 mm²/°C; aged, 0.017 and 0.011 mm²/°C). Aged male mice had a diminished response in the periphery (% increase in femoral artery area from 35 to 38 °C, male and female: adult, 67 and 65%; aged, 0.1 and 57%).

CONCLUSION

Previously unidentified increases in aortic area due to increasing core temperature are biologically important because they may affect conductive and convective heat transfer. Leveraging non-invasive methodology to quantify sex and age dependent vascular responses due to increasing core temperature could be combined with bioheat modelling in order to improve understanding of thermoregulation.

Post-warm-up muscle temperature maintenance: blood flow contribution and external heating optimisation

Purpose

Passive muscle heating has been shown to reduce the drop in post-warm-up muscle temperature (T_m) by about 25 % over 30 min, with concomitant sprint/power performance improvements. We sought to determine the role of leg blood flow in this cooling and whether optimising the heating procedure would further benefit post-warm-up T_m maintenance.

Methods

Ten male cyclists completed 15-min sprint-based warm-up followed by 30 min recovery. Vastus lateralisT_m (T_mvl) was measured at deep-, mid- and superficial-depths before and after the warm-up, and after the recovery period (POST-REC). During the recovery period, participants wore water-perfused trousers heated to 43 °C (WPT43) with either whole leg heating (WHOLE) or upper leg heating (UPPER), which was compared to heating with electrically heated trousers at 40 °C (ELEC40) and a non-heated control (CON). The blood flow cooling effect on T_mvl was studied comparing one leg with (BF) and without (NBF) blood flow.

Results

Warm-up exercise significantly increased T_mvl by ~3 °C at all depths. After the recovery period, BF T_mvl was lower (~0.3 °C) than NBF T_mvl at all measured depths, with no difference between WHOLE versus UPPER. WPT43 reduced the post-warm-up drop in deep-T_mvl (−0.12 °C ± 0.3 °C) compared to ELEC40 (−1.08 ± 0.4 °C) and CON (−1.3 ± 0.3 °C), whereas mid- and superficial-T_mvl even increased by 0.15 ± 0.3 and 1.1 ± 1.1 °C, respectively.

Conclusion

Thigh blood flow contributes to the post-warm-up T_mvl decline. Optimising the external heating procedure and increasing heating temperature of only 3 °C successfully maintained and even increased T_mvl, demonstrating that heating temperature is the major determinant of post-warm-up T_mvl cooling in this application.

Intramuscular Heating Characteristics of Multihour Low-Intensity Therapeutic Ultrasound

CONTEXT

The heating characteristics of a stationary device delivering sustained acoustic medicine with low-intensity therapeutic ultrasound (LITUS) are unknown.

OBJECTIVE

To measure intramuscular (IM) heating produced by a LITUS device developed for long-duration treatment of musculoskeletal injuries.

DESIGN

Controlled laboratory study.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 26 healthy volunteers (16 men, 10 women; age = 23.0 ± 2.1 years, height = 1.74 ± 0.09 m, mass = 73.48 ± 14.65 kg).

INTERVENTION(S)

Participants were assigned randomly to receive active (n = 20) or placebo (n = 6) LITUS at a frequency of 3 MHz and an energy intensity of 0.132 W/cm(2) continuously for 3 hours with a single transducer or dual transducers on the triceps surae muscle. We measured IM temperature using thermocouples inserted at 1.5- and 3-cm depths into muscle. Temperatures were recorded throughout treatment and 30 minutes posttreatment.

MAIN OUTCOME MEASURE(S)

We used 2-sample t tests to determine the heating curve of the LITUS treatment and differences in final temperatures between depth and number of transducers.

RESULTS

A mild IM temperature increase of 1 °C was reached 10 ± 5 minutes into the treatment, and a more vigorous temperature increase of 4 °C was reached 80 ± 10 minutes into the treatment. The maximal steady-state IM temperatures produced during the final 60 minutes of treatment at the 1.5-cm depth were 4.42 °C ± 0.08 °C and 3.92 °C ± 0.06 °C using 1 and 2 transducers, respectively. At the 3.0-cm depth, the maximal steady-state IM temperatures during the final 60 minutes of treatment were 3.05 °C ± 0.09 °C and 3.17 °C ± 0.05 °C using 1 and 2 transducers, respectively. We observed a difference between the temperatures measured at each depth (t78 = -2.45, P = .02), but the number of transducers used to generate heating was not different (t78 = 1.79, P = .08).

CONCLUSIONS

The LITUS device elicited tissue heating equivalent to traditional ultrasound but could be sustained for multiple hours. It is a safe and effective alternative tool for delivering therapeutic ultrasound and exploring dosimetry for desired physiologic responses.

Wilderness Medical Society practice guidelines for the out-of-hospital evaluation and treatment of accidental hypothermia: 2014 update

To provide guidance to clinicians, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines for the out-of-hospital evaluation and treatment of victims of accidental hypothermia. The guidelines present the main diagnostic and therapeutic modalities and provide recommendations for the management of hypothermic patients. The panel graded the recommendations based on the quality of supporting evidence and the balance between benefits and risks/burdens according the criteria published by the American College of Chest Physicians. The guidelines also provide suggested general approaches to the evaluation and treatment of accidental hypothermia that incorporate specific recommendations. This is an updated version of the original Wilderness Medical Society Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia published in Wilderness & Environmental Medicine 2014;25(4):425-445.

Thermometry, calorimetry, and mean body temperature during heat stress

Heat balance in humans is maintained at near constant levels through the adjustment of physiological mechanisms that attain a balance between the heat produced within the body and the heat lost to the environment. Heat balance is easily disturbed during changes in metabolic heat production due to physical activity and/or exposure to a warmer environment. Under such conditions, elevations of skin blood flow and sweating occur via a hypothalamic negative feedback loop to maintain an enhanced rate of dry and evaporative heat loss. Body heat storage and changes in core temperature are a direct result of a thermal imbalance between the rate of heat production and the rate of total heat dissipation to the surrounding environment. The derivation of the change in body heat content is of fundamental importance to the physiologist assessing the exposure of the human body to environmental conditions that result in thermal imbalance. It is generally accepted that the concurrent measurement of the total heat generated by the body and the total heat dissipated to the ambient environment is the most accurate means whereby the change in body heat content can be attained. However, in the absence of calorimetric methods, thermometry is often used to estimate the change in body heat content. This review examines heat exchange during challenges to heat balance associated with progressive elevations in environmental heat load and metabolic rate during exercise. Further, we evaluate the physiological responses associated with heat stress and discuss the thermal and nonthermal influences on the body's ability to dissipate heat from a heat balance perspective.

Wilderness Medical Society practice guidelines for the out-of-hospital evaluation and treatment of accidental hypothermia

To provide guidance to clinicians, the Wilderness Medical Society (WMS) convened an expert panel to develop evidence-based guidelines for the out-of-hospital evaluation and treatment of victims of accidental hypothermia. The guidelines present the main diagnostic and therapeutic modalities and provide recommendations for the management of hypothermic patients. The panel graded the recommendations based on the quality of supporting evidence and the balance between benefits and risks/burdens according the criteria published by the American College of Chest Physicians. The guidelines also provide suggested general approaches to the evaluation and treatment of accidental hypothermia that incorporate specific recommendations.

Heating capacity of rebound shortwave diathermy and moist hot packs at superficial depths

CONTEXT

The effectiveness of a new continuous diathermy unit, ReBound, as a heating modality is unknown.

OBJECTIVE

To compare the effects of ReBound diathermy with silicate-gel moist hot packs on tissue temperature in the human triceps surae muscle.

DESIGN

Crossover study.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

A total of 12 healthy, college-aged volunteers (4 men, 8 women; age = 22.2 ± 2.25 years, calf subcutaneous fat thickness = 7.2 ± 1.9 mm).

INTERVENTION(S)

On 2 different days, 1 of 2 modalities (ReBound diathermy, silicate-gel moist hot pack) was applied to the triceps surae muscle of each participant for 30 minutes. After 30 minutes, the modality was removed, and temperature decay was recorded for 20 minutes.

MAIN OUTCOME MEASURE(S)

Medial triceps surae intramuscular tissue temperature at a depth of 1 cm was measured using an implantable thermocouple inserted horizontally into the muscle. Measurements were taken every 5 minutes during the 30-minute treatment and every minute during the 20-minute temperature decay, for a total of 50 minutes. Treatment was analyzed through a 2 × 7 mixed-model analysis of variance with repeated measures. Temperature decay was analyzed through a 2 × 21 mixed-model analysis of variance with repeated measures.

RESULTS

During the 30-minute application, tissue temperatures at a depth of 1 cm increased more with the ReBound diathermy than with the moist hot pack (F6,66 = 7.14, P < .001). ReBound diathermy and moist hot packs increased tissue temperatures 3.69°C ± 1.50°C and 2.82°C ± 0.90°C, respectively, from baseline. Throughout the temperature decay, ReBound diathermy produced a greater rate of heat dissipation than the moist hot pack (F20,222 = 4.42, P < .001).

CONCLUSIONS

During a 30-minute treatment at a superficial depth, the ReBound diathermy increased tissue temperature to moderate levels, which were greater than the levels reached with moist hot packs.

Muscle heating with Megapulse II shortwave diathermy and ReBound diathermy

CONTEXT

A new continuous diathermy called ReBound recently has been introduced. Its effectiveness as a heating modality is unknown.

OBJECTIVE

To compare the effects of the ReBound diathermy with an established deep-heating diathermy, the Megapulse II pulsed shortwave diathermy, on tissue temperature in the human triceps surae muscle.

DESIGN

Crossover study.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Participants included 12 healthy, college-aged volunteers (4 men, 8 women; age = 22.2 ± 2.25 years, calf subcutaneous fat thickness = 7.2 ± 1.9 mm).

INTERVENTION(S)

Each modality treatment was applied to the triceps surae muscle group of each participant for 30 minutes. After 30 minutes, we removed the modality and recorded temperature decay for 20 minutes.

MAIN OUTCOME MEASURE(S)

We horizontally inserted an implantable thermocouple into the medial triceps surae muscle to measure intramuscular tissue temperature at 3 cm deep. We measured temperature every 5 minutes during the 30-minute treatment and each minute during the 20-minute temperature decay.

RESULTS

Tissue temperature at a depth of 3 cm increased more with Megapulse II than with ReBound diathermy over the course of the treatment (F₆,₆₆ = 10.78, P < .001). ReBound diathermy did not produce as much intramuscular heating, leading to a slower heat dissipation rate than the Megapulse II (F₂₀,₂₂₀ = 28.84, P < .001).

CONCLUSIONS

During a 30-minute treatment, the Megapulse II was more effective than ReBound diathermy at increasing deep, intramuscular tissue temperature of the triceps surae muscle group.

Skin heat dissipation: the influence of diabetes, skin thickness, and subcutaneous fat thickness

BACKGROUND

It is well established that diabetes impairs vascular endothelial function. However, the impact of impaired endothelial function on thermal conductivity of the skin, especially in relation to a constant versus a sudden heat stress, has not been established. Further, there is some evidence that aging reduces skin dermal thickness and subcutaneous fat thickness. Since these are important determinates of heat dissipation by the skin, these parameters also need to be examined in people with diabetes.

METHODS

Ninety subjects (30 younger individuals, 30 patients with diabetes, and 30 patients age-matched to the diabetes subjects) participated in two series of experiments to determine (1) the thickness of the subcutaneous fat layer and skin thickness and the skin response to a sudden heat stress and (2) the response to a continuous heat stress on the lower back. Skin thickness and subcutaneous fat thickness were assessed by ultrasound, and skin blood flow was examined by infrared laser Doppler flow meter.

RESULTS

People with diabetes had significantly less resting blood flow, blood flow in response to a single or continuous heat load, less subcutaneous fat, and thinner skin than either age-matched controls or younger people (P < 0.05). Subjects with diabetes also had the lowest concentration of red blood cells in their skin, implying a reduction in the number of capillaries in the skin.

CONCLUSIONS

Thinning of the skin and probably a reduction in capillaries in the dermal layer contribute to a reduction in the blood flow response to heat. People with diabetes, in particular, have reduced skin heat dissipation because of less resting blood flow and thinner skin than that seen in age-matched controls.

Lumped parameter thermal model for the study of vascular reactivity in the fingertip

Vascular reactivity (VR) denotes changes in volumetric blood flow in response to arterial occlusion. Current techniques to study VR rely on monitoring blood flow parameters and serve to predict the risk of future cardiovascular complications. Because tissue temperature is directly impacted by blood flow, a simplified thermal model was developed to study the alterations in fingertip temperature during arterial occlusion and subsequent reperfusion (hyperemia). This work shows that fingertip temperature variation during VR test can be used as a cost-effective alternative to blood perfusion monitoring. The model developed introduces a function to approximate the temporal alterations in blood volume during VR tests. Parametric studies are performed to analyze the effects of blood perfusion alterations, as well as any environmental contribution to fingertip temperature. Experiments were performed on eight healthy volunteers to study the thermal effect of 3 min of arterial occlusion and subsequent reperfusion (hyperemia). Fingertip temperature and heat flux were measured at the occluded and control fingers, and the finger blood perfusion was determined using venous occlusion plethysmography (VOP). The model was able to phenomenologically reproduce the experimental measurements. Significant variability was observed in the starting fingertip temperature and heat flux measurements among subjects. Difficulty in achieving thermal equilibration was observed, which indicates the important effect of initial temperature and thermal trend (i.e., vasoconstriction, vasodilatation, and oscillations).

Influence of adiposity on cooling efficiency in hyperthermic individuals

This study evaluated the effect of body adiposity on core cooling rates, as measured by decreases in rectal (T (re)), esophageal (T (es)) and aural canal (T (ac)) temperatures, of individuals rendered hyperthermic by dynamic exercise in the heat. Seventeen male participants were divided into two groups; low body fat (LF, 12.9 +/- 1.9%) and high body fat (HF, 22.3 +/- 4.3%). Participants exercised at 65% of their maximal oxygen uptake at an ambient air temperature of 40 degrees C until T (re) increased to 40 degrees C or until volitional fatigue. Following exercise, participants were immersed up to the clavicles in an 8 degrees C circulated water bath until T (re) returned to 37.5 degrees C. No significant differences were found between the LF and HF in the time to reach a T (re) of 39.5 degrees C (P = 0.205), 38.5 degrees C (P = 0.343) and 37.5 degrees C (P = 0.923) during the immersion. Overall cooling rate for T (re) was also similar between groups (0.23 +/- 0.09 degrees C/min (LF) vs. 0.20 +/- 0.09 degrees C/min (HF), P = 0.647) as well as those for T (es) (P = 0.502) and T (ac) (P = 0.940). Furthermore, mean rate of non-evaporative heat loss (702 +/- 217 W/m(2) (LF) vs. 612 +/- 141 W/m(2) (HF), P = 0.239) was not different between groups. These results suggest that a difference of approximately 10% of body adiposity does not affect core cooling rates in active individuals under 25% body fat rendered hyperthermic by exercise.

Disturbance of thermal homeostasis following dynamic exercise

Recovery from dynamic exercise results in significant perturbations of thermoregulatory control. These perturbations evoke a prolonged elevation in core body temperature and a concomitant decrease in sweating, skin blood flow, and skin temperature to pre-exercise baseline values within the early stages of recovery. Cutaneous vasodilation and sweating are critical responses necessary for effective thermoregulation during heat stress in humans. The ability to modulate the rate of heat loss through adjustments in vasomotor and sudomotor activity is a fundamental mechanism of thermoregulatory homeostasis. There is a growing body of evidence in support of a possible relationship between hemodynamic changes postexercise and heat loss responses. Specifically, nonthermoregulatory factors, such as baroreceptors, associated with hemodynamic changes, influence the regulation of core body temperature during exercise recovery. The following review will examine the etiology of the post-exercise disturbance in thermal homeostasis and evaluate possible thermal and nonthermal factors associated with a prolonged hyperthermic state following exercise.

Sex differences in postexercise esophageal and muscle tissue temperature response

Factors associated with blood pressure regulation during recovery from exercise dramatically influence core temperature regulation. However, it is unknown whether sex-related differences in postexercise hemodynamics affect core and muscle temperature response. Sixteen participants (8 males, 8 females) completed an incremental isotonic test on a Kin-Com isokinetic apparatus to determine their activity-specific peak oxygen consumption during bilateral knee extensions (V̇o2sp). On a separate day, participants performed 15 min of isolated bilateral knee extensions at a moderate (60% V̇o2sp) exercise intensity followed by a 90-min recovery. Esophageal temperature (Tes), mean arterial pressure (MAP), muscle temperature at four depths in the active vastus medialis (TVM) and three depths in the inactive triceps brachii (TTB) were measured concurrently with sweat rate and cutaneous vascular conductance (CVC). Relative to the preexercise resting Tes of 36.7°C (SD 0.1), between 10 and 50-min of recovery Tes was 0.19°C (SD 0.02) higher for females than males ( P = 0.037). All measurements of TVM (0.036 > P > 0.014) and TTB (0.048 > P > 0.008) were higher for females during the initial 30 min of recovery by between 0.46°C and 0.64°C for TVM and by between 0.53°C and 0.70°C for TTB. In parallel, females showed a 5 to 7 mmHg greater reduction in MAP during recovery relative to males ( P = 0.002) and a significantly lower CVC ( P = 0.020) and sweat rate ( P = 0.034). Therefore, it is concluded that females demonstrate a greater and more prolonged elevation in postexercise esophageal temperature and active and inactive muscle temperatures, which is paralleled by a greater postexercise hypotensive response.

Postexercise hypotension causes a prolonged perturbation in esophageal and active muscle temperature recovery

We examined the effect of two levels of exercise-induced hypotension on esophageal (Tes) and active and nonactive muscle temperatures during and following exercise. Seven males performed an incremental isotonic test on a Kin-Com isokinetic apparatus to determine their peak oxygen consumption during bilateral knee extensions (VO2sp). This was followed on separate days by 15-min of isolated bilateral knee extensions at moderate (60% VO2sp) (MEI) and high (80% VO2sp) (HEI) exercise intensities, followed by 90 min of recovery. Muscle temperature was measured with an intramuscular probe inserted in the left vastus medialis (Tvm) and triceps brachii (Ttb) muscles under ultrasound guidance. The deepest sensor (tip) was located approximately 10 mm from the femur and deep femoral artery and from the superior ulnar collateral artery and humerus for the Tvm and Ttb, respectively. Additional sensors were located 15 and 30 mm from the tip with an additional sensor located at 45 mm for the Tvm measurements only. Following exercise, mean arterial pressure (MAP) remained significantly below preexercise rest for the initial 60 min of recovery after MEI and for the duration of the postexercise recovery period after HEI (P< or =0.05). After HEI, significantly greater elevations from preexercise rest were recorded for Tes and all muscle temperatures paralleled a greater decrease in MAP compared with MEI (P< or =0.05). By the end of 90-min postexercise recovery, MAP, Tes, and all muscle temperatures remained significantly greater after HEI than MEI. Furthermore, a significantly shallower muscle temperature profile across Tvm, relative to preexercise rest, was observed at the end of exercise for both HEI and MEI (P< or=0.05), and for 30 min of recovery for MEI and throughout 90 min of recovery for HEI. No significant differences in muscle temperature profile were observed for Ttb. Thus we conclude that the increase in the postexercise hypotensive response, induced by exercise of increasing intensity, was paralleled by an increase in the magnitude and recovery time of the postexercise esophageal and active muscle temperatures.

A three-compartment thermometry model for the improved estimation of changes in body heat content

The aim of this study was to use whole body calorimetry to directly measure the change in body heat content (DeltaH(b)) during steady-state exercise and compare these values with those estimated using thermometry. The thermometry models tested were the traditional two-compartment model of "core" and "shell" temperatures, and a three-compartment model of "core," "muscle," and "shell" temperatures; with individual compartments within each model weighted for their relative influence upon DeltaH(b) by coefficients subject to a nonnegative and a sum-to-one constraint. Fifty-two participants performed 90 min of moderate-intensity exercise (40% of Vo(2 peak)) on a cycle ergometer in the Snellen air calorimeter, at regulated air temperatures of 24 degrees C or 30 degrees C and a relative humidity of either 30% or 60%. The "core" compartment was represented by temperatures measured in the esophagus (T(es)), rectum (T(re)), and aural canal (T(au)), while the "muscle" compartment was represented by regional muscle temperature measured in the vastus lateralis (T(vl)), triceps brachii (T(tb)), and upper trapezius (T(ut)). The "shell" compartment was represented by the weighted mean of 12 skin temperatures (T(sk)). The whole body calorimetry data were used to derive optimally fitting two- and three-compartment thermometry models. The traditional two-compartment model was found to be statistically biased, systematically underestimating DeltaH(b) by 15.5% (SD 31.3) at 24 degrees C and by 35.5% (SD 21.9) at 30 degrees C. The three-compartment model showed no such bias, yielding a more precise estimate of DeltaH(b) as evidenced by a mean estimation error of 1.1% (SD 29.5) at 24 degrees C and 5.4% (SD 30.0) at 30 degrees C with an adjusted R(2) of 0.48 and 0.51, respectively. It is concluded that a major source of error in the estimation of DeltaH(b) using the traditional two-compartment thermometry model is the lack of an expression independently representing the heat storage in muscle during exercise.

Muscle temperature transients before, during, and after exercise measured using an intramuscular multisensor probe

Seven subjects (1 woman) performed an incremental isotonic test on a Kin-Com isokinetic apparatus to determine their maximal oxygen consumption during bilateral knee extensions (Vo(2 sp)). A multisensor thermal probe was inserted into the left vastus medialis (middiaphysis) under ultrasound guidance. The deepest sensor (tip) was located approximately 10 mm from the femur and deep femoral artery (T(mu 10)), with additional sensors located 15 (T(mu 25)) and 30 mm (T(mu 40)) from the tip. Esophageal temperature (T(es)) was measured as an index of core temperature. Subjects rested in an upright seated position for 60 min in an ambient condition of 22 degrees C. They then performed 15 min of isolated bilateral knee extensions (60% of Vo(2 sp)) on a Kin-Com, followed by 60 min of recovery. Resting T(es) was 36.80 degrees C, whereas T(mu 10), T(mu 25), and T(mu 40) were 36.14, 35.86, and 35.01 degrees C, respectively. Exercise resulted in a T(es) increase of 0.55 degrees C above preexercise resting, whereas muscle temperature of the exercising leg increased by 2.00, 2.37, and 3.20 degrees C for T(mu 10), T(mu 25), and T(mu 40), respectively. Postexercise T(es) showed a rapid decrease followed by a prolonged sustained elevation approximately 0.3 degrees C above resting. Muscle temperature decreased gradually over the course of recovery, with values remaining significantly elevated by 0.92, 1.05, and 1.77 degrees C for T(mu 10), T(mu 25), and T(mu 40), respectively, at end of recovery (P < 0.05). These results suggest that the transfer of residual heat from previously active musculature may contribute to the sustained elevation in postexercise T(es).

Skin temperature increase during local exposure to high-power RF levels in humans

The local temperature response of the skin on heating due to prolonged exposure to RF radiation by a surface coil was investigated in five healthy volunteers. Temperature changes induced by RF radiation were measured at the skin of the calf muscle by a fluoroptic probe. Exposure to superficial specific absorption rate (SAR) levels of 6.5, 12 and 22 W/kg resulted in skin temperature increases, the highest temperature recorded was 38.3 degrees C. Although the maximum values of each temperature curve correlated with the applied superficial SAR levels, these values did not exceed the recommended temperature limit for the extremities such as given by the Food and Drug Administration (FDA).

Influence of lower limb pneumatic compression on pulmonary artery temperature: effect on cardiac output measurements

OBJECTIVES

To characterize the decreases in pulmonary artery temperature that coincide with the inflation cycle of pneumatic calf compression stockings and to examine their effects on the thermodilution measurement of cardiac output.

DESIGN

Three-part observational study.

SETTING

University hospital surgical intensive care unit.

PATIENTS

Postoperative patients with indwelling pulmonary artery catheters.

INTERVENTION

Thermodilution cardiac output measurements with and without pneumatic calf compression.

MEASUREMENTS AND MAIN RESULTS

Phase 1 (n = 18) examined the effects of pneumatic compression on pulmonary artery temperature. There was no effect on pulmonary artery temperature (device off, 37.468+/-0.008 degrees C; device on, 37.458+/-0.014 degrees C), but the difference between the maximum and minimum pulmonary artery temperatures was increased (off, 0.031+/-0.006 degrees C; on, 0.055+/-0.012 degrees C [p < .001]). Phase 2 (n = 12) found that the mean thermodilution cardiac output with 10 mL of cold (0-5 degrees C) injectate was unchanged by pneumatic compression (off, 7.00+/-2.28 L/min; on, 6.89+/-2.22 L/min). However, when the compression devices were operating, the variability between the individual measurements was increased, as reflected by larger coefficients of variation (off, 3.19+/-1.96; on, 8.72+/-6.56 [p < .02]). Similar results were obtained during phase 3 (n = 5), when cardiac output was measured with room temperature Injectate.

CONCLUSIONS

Intermittent pneumatic calf compression increased lower limb venous return, causing acute but transient decreases in pulmonary artery blood temperature. This did not affect the accuracy of thermodilution cardiac output measurements that were made using 10 mL of either cold or room temperature injectate.