Effects of increased ambient temperature on skin sympathetic nerve activity and core temperature in humans

Skin sympathetic nerve activity and nocturnal blood pressure nondipping in patients with postural orthostatic tachycardia syndrome

OBJECTIVE

Postural orthostatic tachycardia syndrome (POTS) is associated with abnormal blood pressure (BP) regulation and increased prevalence of nocturnal nondipping. We hypothesized that nocturnal nondipping of BP is associated with elevated skin sympathetic nerve activity (SKNA) in POTS.

METHOD

We used an ambulatory monitor to record SKNA and electrocardiogram from 79 participants with POTS (36 ± 11 years, 72 women), including 67 with simultaneous 24-h ambulatory BP monitoring.

RESULTS

Nocturnal nondipping of BP was present in 19 of 67 (28%) participants. The nondipping group had a higher average SKNA (aSKNA) from midnight of day 1 to 0100 h on day 2 than the dipping group ( P  = 0.016, P  = 0.030, respectively). The differences (Δ) of aSKNA and mean BP between daytime and night-time were more significant in the dipping group compared with the nondipping group (ΔaSKNA 0.160 ± 0.103 vs. 0.095 ± 0.099 μV, P  = 0.021, and Δmean BP 15.0 ± 5.2 vs. 4.9 ± 4.2 mmHg, P  < 0.001, respectively). There were positive correlations between ΔaSKNA and standing norepinephrine (NE) (r = 0.421, P  = 0.013) and the differences between standing and supine NE levels ( r  = 0.411, P  = 0.016). There were 53 (79%) patients with SBP less than 90 mmHg and 61 patients (91%) with DBP less than 60 mmHg. These hypotensive episodes were associated with aSKNA of 0.936 ± 0.081 and 0.936 ± 0.080 μV, respectively, which were both significantly lower than the nonhypotensive aSKNA (1.034 ± 0.087 μV, P  < 0.001 for both) in the same patient.

CONCLUSION

POTS patients with nocturnal nondipping have elevated nocturnal sympathetic tone and blunted reduction of SKNA between day and night. Hypotensive episodes were associated with reduced aSKNA.

Revisiting the evaluation of central versus peripheral thermoregulatory control in humans

Human thermoregulatory control is often evaluated through the relationship between thermoeffector output and core or mean body temperature. In addition to providing a general indication of whether a variable of interest alters thermoregulatory control, this relationship is often used to determine how this alteration may occur. This latter interpretation relies upon two parameters of the thermoeffector output-body temperature relationship: the onset threshold and thermosensitivity. Traditionally, changes in the onset threshold and thermosensitivity are interpreted as "central" or "peripheral" modulation of thermoregulatory control, respectively. This mini-review revisits the origins of the thermoeffector output-body temperature relationship and its use to interpret "central" or "peripheral" modulation of thermoregulatory control. Against this background, we discuss the strengths and weaknesses of this approach and highlight that "central" thermoregulatory control reflects the neural control of body temperature whereas "peripheral" thermoregulatory control reflects properties specific to the thermoeffector organs. We highlight studies that employed more direct approaches to investigate the neural control of body temperature and peripheral properties of thermoeffector organs. We conclude by encouraging future investigations interested in studying thermoregulatory control to more directly investigate the component of the thermoeffector loop under investigation.heat; human; skin blood flow; sweat; thermoregulatory.

Thermal imaging ruled out as a supplementary assessment in patients with fibromyalgia: A cross-sectional study

BACKGROUND

The diagnosis of fibromyalgia syndrome (FMS) syndrome is often complicated and relies on diagnostic criteria based mostly on the symptoms reported by patients. Implementing objective complementary tests would be desirable to better characterize this population.

OBJECTIVE

The purpose of this cross-sectional study was to compare the skin temperature at rest using thermography in women with FMS and healthy women.

METHODS

Eighty-six women with FMS and 92 healthy controls volunteered to participate. The temperature of all participants was measured by infra-red thermography, registering the skin surface temperature (minimum, maximum and average) at rest in different areas: neck, upper and lower back, chest, knees and elbows. In order to analyze the differences in the skin temperature between groups, inferential analyses of the data were performed using Mann-Whitney U test.

RESULTS

The results showed no significant difference in skin temperature between groups in the neck, upper back, chest and elbows (p>0.05). The lower back and knees areas showed significant differences between groups (p<0.05), although these differences did not reach a minimum of clinically detectable change.

CONCLUSIONS

Women with fibromyalgia presented no clinically meaningful reduction or difference in skin temperature at rest when compared with a group of healthy women. The infra-red thermography is not an effective supplementary assessment tool in women with fibromyalgia.

Moderate whole body heating attenuates the exercise pressor reflex responses in older humans

Prior reports show that whole body heat stress attenuates the pressor response to exercise in young healthy subjects. The effects of moderate whole body heating (WBH; e.g., increase in internal temperature T_core of ∼0.4°C-0.5°C) or limb heating on sympathetic and cardiovascular responses to exercise in older healthy humans remain unclear. We examined the muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP), and heart rate (HR) in 14 older (62 ± 2 yr) healthy subjects during fatiguing isometric handgrip exercise and postexercise circulatory occlusion (PECO). The protocol was performed under normothermic, moderate WBH, and local limb (i.e., forearm) heating conditions during three visits. During the mild WBH stage (increase in T_core of <0.3°C), HR increased, whereas BP and MSNA decreased from baseline. Under the moderate WBH condition (increase in T_core of ∼0.4°C), BP decreased, HR increased, and MSNA was unchanged from baseline. Compared with the normothermic trial, the absolute MAP during fatiguing exercise and PECO was lower during the WBH trial. Moreover, MSNA and MAP responses (i.e., changes) to fatiguing exercise were also less than those seen during the normothermic trial. Limb heating induced a similar increase in forearm muscle temperature to that seen in the WBH trial (∼0.7°C-1.5°C). Limb heating did not alter resting MAP, HR, or MSNA. The MSNA and hemodynamic responses to exercise in the limb heating trial were not different from those in the normothermic trial. These data suggest that moderate WBH attenuates MSNA and BP responses to exercise in older healthy humans.

Thermographic skin temperature measurement compared with cold sensation in predicting the efficacy and distribution of epidural anesthesia

Due to the high rates of epidural failure (3-32%), novel techniques are required to objectively assess the successfulness of an epidural block. In this study we therefore investigated whether thermographic temperature measurements have a higher predictive value for a successful epidural block when compared to the cold sensation test as gold standard. Epidural anesthesia was induced in 61 patients undergoing elective abdominal, thoracic or orthopedic surgery. A thermographic picture was recorded at 5, 10 and 15 min following epidural anesthesia induction. After 15 min a cold sensation test was performed. Epidural anesthesia is associated with a decrease in skin temperature. Thermography predicts a successful epidural block with a sensitivity of 54% and a PPV of 92% and a specificity of 67% and a NPV of 17%. The cold sensation test shows a higher sensitivity and PPV than thermography (97 and 93%), but a lower specificity and NPV than thermography (25 and 50%). Thermographic temperature measurements can be used as an additional and objective method for the assessment of the effectiveness of an epidural block next to the cold sensation test, but have a low sensitivity and negative predictive value. The local decrease in temperature as observed in our study during epidural anesthesia is mainly attributed to a core-to-peripheral redistribution of body heat and vasodilation.

Nerve Conduction Studies as a Measure of Disease Progression: Objectivity or Illusion?

BACKGROUND

Clinical nerve conduction studies (NCS) are often used as a secondary outcome measure in therapeutic trials, but show a high degree of inter-trial variability even when technical factors known to affect the recorded responses are minimised. This raises the intriguing possibility that some of the observed variability may reflect true changes in nerve activity.

OBJECTIVES

Our aim was determine how much variability these factors might produce, and how this might affect the results of commonly used neuropathy rating scales.

METHODS

A standardised protocol was repeated over forty consecutive trials by the same operators in two healthy subjects. The protocol included recordings that shared either a stimulating or a recording electrode position, such that changes due to electrode position could be excluded, and hand temperature was closely controlled.

RESULTS

Despite controlling for inter-operator differences, electrode position, and hand temperature, the variability in sensory nerve action potential (SNAP) amplitude was extremely high (Range 23 μV, CoV = 10.7-18.8). This variability was greater than the change in amplitude needed to move a subject from point 0 to point 4 on the CMT neuropathy rating scale. Neither temperature or electrode position accounted for all of this variability, suggesting that additional as yet unidentified factors are responsible.

CONCLUSION

Even under closely controlled conditions and sophisticated laboratory methods, test-to-test variability can be significant. The factors responsible for this variability may be difficult to control, limiting the utility of single nerve recordings as a trial outcome measure.

Augmented supraorbital skin sympathetic nerve activity responses to symptom trigger events in rosacea patients

Facial flushing in rosacea is often induced by trigger events. However, trigger causation mechanisms are currently unclear. This study tested the central hypothesis that rosacea causes sympathetic and axon reflex-mediated alterations resulting in trigger-induced symptomatology. Twenty rosacea patients and age/sex-matched controls participated in one or a combination of symptom triggering stressors. In protocol 1, forehead skin sympathetic nerve activity (SSNA; supraorbital microneurography) was measured during sympathoexcitatory mental (2-min serial subtraction of novel numbers) and physical (2-min isometric handgrip) stress. In protocol 2, forehead skin blood flow (laser-Doppler flowmetry) and transepithelial water loss/sweat rate (capacitance hygrometry) were measured during sympathoexcitatory heat stress (whole body heating by perfusing 50°C water through a tube-lined suit). In protocol 3, cheek, forehead, forearm, and palm skin blood flow were measured during nonpainful local heating to induce axon reflex vasodilation. Heart rate (HR) and mean arterial pressure (MAP) were recorded via finger photoplethysmography to calculate cutaneous vascular conductance (CVC; flux·100/MAP). Higher patient transepithelial water loss was observed (rosacea 0.20 ± 0.02 vs. control 0.10 ± 0.01 mg·cm(-2)·min(-1), P < 0.05). HR and MAP changes were not different between groups during sympathoexcitatory stressors or local heating. SSNA during early mental (32 ± 9 and 9 ± 4% increase) and physical (25 ± 4 and 5 ± 1% increase, rosacea and controls, respectively) stress was augmented in rosacea (both P < 0.05). Heat stress induced more rapid sweating and cutaneous vasodilation onset in rosacea compared with controls. No axon reflex vasodilation differences were observed between groups. These data indicate that rosacea affects SSNA and that hyperresponsiveness to trigger events appears to have a sympathetic component.

Cardiovascular responses to heat stress in chronic heart failure

Clinical reports have suggested that patients with heart diseases may be particularly vulnerable to heat injury. This review examines the effects of heat stress on cardiovascular and autonomic functions in patients with chronic heart failure (CHF). Laboratory investigations have shown that cutaneous vasodilator responses to heating are impaired in patients, whereas activation of skin sympathetic nerve activation is not attenuated in CHF as compared to controls. Attenuated cutaneous vasodilation may increase the risk of a heat related illness when CHF subjects are exposed to hyperthermic conditions.

Diurnal and menstrual cycles in body temperature are regulated differently: a 28-day ambulatory study in healthy women with thermal discomfort of cold extremities and controls

Diurnal cycle variations in body-heat loss and heat production, and their resulting core body temperature (CBT), are relatively well investigated; however, little is known about their variations across the menstrual cycle under ambulatory conditions. The main purpose of this study was to determine whether menstrual cycle variations in distal and proximal skin temperatures exhibit similar patterns to those of diurnal variations, with lower internal heat conductance when CBT is high, i.e. during the luteal phase. Furthermore, we tested these relationships in two groups of women, with and without thermal discomfort of cold extremities (TDCE). In total, 19 healthy eumenorrheic women with regular menstrual cycles (28-32 days), 9 with habitual TDCE (ages 29 ± 1.5 year; BMI 20.1 ± 0.4) and 10 controls without these symptoms (CON: aged 27 ± 0.8 year; BMI 22.7 ± 0.6; p < 0.004 different to TDCE) took part in the study. Twenty-eight days continuous ambulatory skin temperature measurements of distal (mean of hands and feet) and proximal (mean of sternum and infraclavicular regions) skin regions, thighs, and calves were carried out under real-life, ambulatory conditions (i-Buttons® skin probes, sampling rate: 2.5 min). The distal minus proximal skin temperature gradient (DPG) provided a valuable measure for heat redistribution from the core to the shell, and, hence, for internal heat conduction. Additionally, basal body temperature was measured sublingually directly after waking up in bed. Mean diurnal amplitudes in skin temperatures increased from proximal to distal skin regions and the 24-h mean values were inversely related. TDCE compared to CON showed significantly lower hand skin temperatures and DPG during daytime. However, menstrual cycle phase did not modify these diurnal patterns, indicating that menstrual and diurnal cycle variations in skin temperatures reveal additive effects. Most striking was the finding that all measured skin temperatures, together with basal body temperature, revealed a similar menstrual cycle variation (independent of BMI), with highest and lowest values during the luteal and follicular phases, respectively. These findings lead to the conclusion that in contrast to diurnal cycle, variations in CBT variation across the menstrual cycle cannot be explained by changes in internal heat conduction under ambulatory conditions. Although no measurements of metabolic heat production were carried out increased metabolic heat generation during the luteal phase seems to be the most plausible explanation for similar body temperature increases.

Cold-induced vasodilatation following traumatic median or ulnar nerve injury

PURPOSE

Peripheral nerve injury of the upper extremity frequently causes changes in the thermoregulatory system of the hands and fingers and leads to reports of cold intolerance. In this study, we aimed to measure the influence of median or ulnar nerve injury on cold-induced vasodilatation (CIVD) during prolonged cooling at low temperatures.

METHODS

We tested 12 patients with a median (n = 6) or ulnar (n = 6) injury 4 to 76 months after nerve repair. The palmar sides of both hands were cooled continuously using a cold plate at 5°C. We measured the skin temperature of the fingers using videothermography and plotted graphs of the temperature changes of the nailbed. The presence of a CIVD reaction was defined as a minimum increase in temperature of 2.5°C starting at the distal phalanx. Furthermore, we measured self-reported symptoms of cold intolerance using the Cold Intolerance Severity Scale questionnaire.

RESULTS

A CIVD reaction was absent in the affected digits of 4 patients (follow-up, 6-37 mo), whereas the CIVD reaction in the uninjured hand was present. The CIVD was present in 6 patients after 50 months' follow-up (range, 24-76 mo). Two patients had no CIVD reaction in the injured or uninjured fingers. All patients with a CIVD response had at least diminished protective sensation. Presence of the CIVD reaction did not exclude self-reported symptoms of cold intolerance.

CONCLUSIONS

After peripheral nerve injury, it is possible to recover the CIVD reaction. This might be an indication of nerve recovery. However, a positive CIVD reaction does not exclude subjective symptoms of posttraumatic cold intolerance.

TYPE OF STUDY/LEVEL OF EVIDENCE

Diagnostic III.

Digital rewarming patterns after median and ulnar nerve injury

PURPOSE

Posttraumatic cold intolerance (CI) is a frequent and important sequel after peripheral nerve injury. In this study, it is hypothesized that altered rewarming patterns after peripheral nerve injury are related to the degree of posttraumatic CI. This hypothesis is tested by quantitatively comparing rewarming patterns of the digits in controls and in median or ulnar nerve injury patients and by investigating relationships between rewarming patterns, sensory recovery, and CI.

METHODS

Twelve median or ulnar nerve injury patients with a follow-up of 4 to 76 months after nerve repair and 13 control subjects had isolated cold stress testing of the hands. Video thermography was used to analyze and compare rewarming patterns of the injured and uninjured digits after cold stress testing. Temperature curves were analyzed by calculating the Q value as an indicator of heat transfer (temperature added during the first 10 minutes after start of active rewarming) and the maximum slope.

RESULTS

Test-retest reliability was 0.64 and 0.79, respectively, for the Q value and maximum slope. High Q values and maximum slopes were interpreted as the presence of active rewarming. Patients with return of active rewarming had better sensory recovery and lower Blond McIndoe Cold Intolerance Severity Scale (CISS) scores. Better sensory recovery was correlated with lower CISS scores.

CONCLUSIONS

Test-retest reliability of cold stress testing was good, and we found a difference in rewarming patterns between nerve injury patients and controls. The presence of active rewarming in the nerve injury patients was related to sensory recovery and fewer complaints of posttraumatic CI.

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

The effect of ambient temperature on capillary vascular malformations

BACKGROUND

Following pulsed dye laser (PDL) treatment of capillary vascular malformations (CMs), the capillaries left behind tend to be smaller and deeper. The PDL is most effective against capillaries over 50 microm, suggesting that clearance of CM could be improved by inducing capillary vasodilation of the smaller remaining capillaries. However, there are reduced perivascular nerves within CMs, implying that autonomic innervation to these capillaries may be abnormal.

OBJECTIVES

To investigate whether CM capillaries will vasodilate in response to autonomic stimulation by raising ambient temperature.

METHODS

Ten patients with untreated CMs and nine with previously laser-treated CMs were studied as ambient temperature was increased from 20 degrees C to 28 degrees C. The following measurements were taken at 2 degrees C intervals: skin blood flow (SBF); capillary diameter and depth; CM colour; and skin and core temperatures.

RESULTS

All the subjects studied demonstrated superficial capillary vasodilation and increased SBF as the ambient temperature was raised from 20 degrees C to 28 degrees C. Mean+/-SEM capillary diameter increased from 66+/-7 microm to 110+/-13 microm (P<0.001) in the untreated group, compared with an increase from 28+/-5 microm to 70+/-14 microm (P<0.001) in the treated group. Mean+/-SEM SBF increased from 427.2+/-98.2 perfusion units (PU) to 580.9+/-92.7 PU (P<0.01) in the untreated group, compared with an increase from 201.3+/-28.4 PU to 458.1+/-53.7 PU (P<0.05) in the treated group.

CONCLUSIONS

Superficial capillary vasodilation within CM is achievable by raising ambient temperature, including in those patients resistant to PDL treatment, potentially allowing further clearance of these lesions.

Adverse drug reactions in patients older than 70 years during the heat wave occurred in France in summer 2003: a study from the French PharmacoVigilance Database

OBJECTIVE

To evaluate adverse drug reactions (ADRs) in patients older than 70 years reported during the 2003 summer heat wave in France to the French Network of Pharmacovigilance Centres and to discuss the interest of such a pharmacovigilance database in the surveillance of such sanitary alerts.

METHODS

All 'serious' ADRs occurred in patients older than 70 years between 1st July and 31st August 2003, reported to the French network of Pharmacovigilance Centres and recorded in the French PharmacoVigilance Database, were analysed with respect to age, gender, type of ADR, drug(s) involved as well as imputability of heat wave.

RESULTS

The total number of 'serious' ADRs registered in the French PharmacoVigilance Database in patients older than 70 years was similar in 2003 (a year with heat wave) and in 2002 (a year without heat wave). Sixty-eight ADRs [27 'possible' (I1), 27 'plausible' (I2) and 14 'likely' (I3)] related to heat occurred in 2003, with a maximal peak between the 28th July and the 17th August. The most frequently ADRs were metabolic (dehydratation, hydroelectrolytic disturbances) and neuropsychiatric (confusion, falls, coma). Drugs more frequently involved were diuretics, angiotensin converting enzyme inhibitors, antidepressants (mainly serotonin reuptake inhibitors), proton pump inhibitors, digoxin, benzodiazepines, oral hypoglycemics and sartans.

CONCLUSION

This study underlines the role of heat wave in the occurrence of 'serious' ADRs in elderly. However, the French PharmacoVigilance Database was not enough sensitive to be used as an efficient surveillance system during such an acute episode, like a heat wave (at least while underreporting remains so important).

Thermography imaging during static and controlled thermoregulation in complex regional pain syndrome type 1: diagnostic value and involvement of the central sympathetic system

Background

Complex Regional Pain Syndrome type 1 (CRPS1) is a clinical diagnosis based on criteria describing symptoms of the disease.

The main aim of the present study was to compare the sensitivity and specificity of calculation methods used to assess thermographic images (infrared imaging) obtained during temperature provocation. The secondary objective was to obtain information about the involvement of the sympathetic system in CRPS1.

Methods

We studied 12 patients in whom CRPS1 was diagnosed according to the criteria of Bruehl. High and low whole body cooling and warming induced and reduced sympathetic vasoconstrictor activity. The degree of vasoconstrictor activity in both hands was monitored using a videothermograph. The sensitivity and specificity of the calculation methods used to assess the thermographic images were calculated.

Results

The temperature difference between the hands in the CRPS patients increases significantly when the sympathetic system is provoked. At both the maximum and minimum vasoconstriction no significant differences were found in fingertip temperatures between both hands.

Conclusion

The majority of CRPS1 patients do not show maximal obtainable temperature differences between the involved and contralateral extremity at room temperature (static measurement). During cold and warm temperature challenges this temperature difference increases significantly. As a result a higher sensitivity and specificity could be achieved in the diagnosis of CRPS1. These findings suggest that the sympathetic efferent system is involved in CRPS1.

Heat-related morbidity in patients with orthostatic hypotension and primary autonomic failure

We investigated the effects of high-external-temperature exposure on neurogenic orthostatic hypotension (OH). Thirty-one patients with autonomic failure (AF) and pharmacologically treated OH related to probable multiple-system atrophy (n=7), Parkinson's disease (n=10), pure autonomic failure (n=7), and diffuse Lewy body disease (n=7) and 26 parkinsonians without AF were included. Prevalence and severity of clinical events were studied during the August 2003 heat wave and the 2004 summer. The prevalence of OH-related events was significantly higher in 2003 [45.1 vs. 11.5%; P=0.0052; OR=6.31 (1.35--33.53)] and 2004 [42.3 vs. 12.0%; P=0.014; OR=5.40 (1.28--22.68)] in AF than in controls. The mean severity score for clinical events was significantly higher in AF than in controls during 2003 heat wave (1.2+/-1.4 vs. 0.2+/-0.5) but similar in summer 2004 (0.7+/-1.0 vs. 0.1+/-0.3). Severe events (unusual home care, assistance, or hospitalization) were only observed in AF patients. A nonstatistically significant higher prevalence of clinical events was observed in AF patients prescribed fludrocortisone (66.7%) by comparison to heptaminol (42.9%), midodrine (45.5%), or midodrine plus fludrocortisone (28.6%). This study shows that AF patients have a poor clinical outcome when exposed to high temperatures and that heat exposure is a risk factor for OH worsening.

Spectral characteristics of skin sympathetic nerve activity in heat-stressed humans

Skin sympathetic nerve activity (SSNA) exhibits low- and high-frequency spectral components in normothermic subjects. However, spectral characteristics of SSNA in heat-stressed subjects are unknown. Because the main components of the integrated SSNA during heat stress (sudomotor/vasodilator activities) are different from those during normothermia and cooling (vasoconstrictor activity), we hypothesize that spectral characteristics of SSNA in heat-stressed subjects will be different from those in subjects subjected to normothermia or cooling. In 17 healthy subjects, SSNA, electrocardiogram, arterial blood pressure (via Finapres), respiratory activity, and skin blood flow were recorded during normothermia and heat stress. In 7 of the 17 subjects, these variables were also recorded during cooling. Spectral characteristics of integrated SSNA, R-R interval, beat-by-beat mean blood pressure, skin blood flow variability, and respiratory excursions were assessed. Heat stress and cooling significantly increased total SSNA. SSNA spectral power in the low-frequency (0.03-0.15 Hz), high-frequency (0.15-0.45 Hz), and very-high-frequency (0.45-2.5 Hz) regions was significantly elevated by heat stress and cooling. Interestingly, heat stress caused a greater relative increase of SSNA spectral power within the 0.45- to 2.5-Hz region than in the other spectral ranges; cooling did not show this effect. Differences in the SSNA spectral distribution between normothermia/cooling and heat stress may reflect different characteristics of central modulation of vasoconstrictor and sudomotor/vasodilator activities.

Thermoregulation responses of broiler chickens to humidity at different ambient temperatures. I. One week of age

Three trials were conducted to investigate the effect of RH (35, 60, and 85%) on thermoregulation of 1-wk-old broiler chickens at different temperatures (35, 30, and 25 degrees C). The response to humidity in rectal temperature and plumage temperature at the back and breast within 24 h after exposure were recorded at 5 time points (1,4,8,16, and 24 h). Humidity affected the thermoregulation of 1-wk-old broiler chickens by redistributing heat within the body at high, low, and thermoneutral temperatures. The redistribution of heat resulted in decreased rectal temperature and increased peripheral temperature, which were, respectively, beneficial and unfavorable at high and low temperatures. These results suggested that feedback effects of surface temperature on core temperature also exist in poultry, as already observed in mammals, and could be induced not only by changed ambient temperature but also by the changes in humidity at high temperature. The disturbance of thermal equilibrium could not be established solely by changes in RT, but rather core and surface temperatures had to be considered. The daily rhythms in rectal and surface temperatures were affected by humidity.

Thermoregulation responses of broiler chickens to humidity at different ambient temperatures. II. Four weeks of age

Two experiments were conducted to investigate the effect of RH (35, 60, and 85%) on thermoregulation of broiler chickens at high (35 degrees C) and mild (30 degrees C) temperatures at the age of 4 wk. The effects of humidity on rectal temperature (RT) and plumage temperature at back (PBAT) and skin temperature at breast (SBRT) were determined at 1, 4, 8, 16, and 24 h after exposure. The RT, PBAT, and SBRT were all significantly increased by high temperature (35 degrees C). Humidity had a significant influence on RT at 35 degrees C but not at 30 degrees C. The peripheral temperatures (PBAT and SBRT) were significantly affected by humidity but responded differently at high (35 degrees C) compared with mild temperature (30 degrees C). In conclusion, high humidity above 60% impaired the heat transmission from body core to the periphery at 35 degrees C but facilitated it at 30 degrees C in 4-wk-old broiler chickens. The effect of humidity on nonevaporative heat loss was depended on air temperature, as nonevaporative heat loss was suppressed by high humidity (>60% RH) at high temperature but enhanced at the mild temperature. The effect of humidity on the relationship between peripheral and core temperature depends on ambient temperature as well as on the age of the broiler chicken. The disturbance of thermal balance could not be determined only by changes in RT or peripheral temperature at a single time point but could be determined by mean body temperature within a certain time frame.

Effects of partial humid heat exposure during different segments of sleep on human sleep stages and body temperature

The effects of partial humid heat exposure applied at different segments of sleep on sleep stages and body temperature were examined. In the first experiment, eight male subjects slept under 26 degrees C 50% (26) and 26 degrees C for the first 3 h and 45 min followed by a 30-min transition to the conditions of 32 degrees C 80%, which was maintained for the final 3 h and 45 min (26-32). Wakefulness increased significantly over the last 4 h under 26-32 compared to 26. Mean skin temperature and clothing microclimate temperature (Tcm) were significantly higher during the last 3 h and 45 min, while rectal temperature (Tre) was higher during the last 3 h under 26-32 than in 26. In the second experiment, eight male subjects slept under 26 degrees C 50% (26) and 32 degrees C 80% for the first 3 h and 45 min followed by a 30-min transition to 26, which was then maintained for the last 3 h and 45 min (32-26). Wakefulness increased both in first and during the last 4 h, and slow wave sleep (SWS) decreased in the first 4 h under 32-26 compared to 26. Mean Tsk was significantly higher during the first 4:15 h. Tcm decreased in 32-26 compared to 26 just after the 30-min transition due to cooling effects. Tre was higher during the first 5 h under 32-26 compared to 26. These results suggest that humid heat exposure during the initial segment of sleep may be more disruptive to sleep stage distribution, Tre decline, and maintenance of Tcm than the same exposure during the later sleep segments.

Effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans

This study sought to investigate the effects of humid heat exposure in later sleep segments on sleep stages and body temperature in humans. The subjects were eight healthy males, from whom informed consent had been obtained. The experiments were carried out under three different sets of conditions: a control climate [air temperature (Ta)=26 degrees C, relative humidity (RH)=50%] (C); a humid heat climate (Ta=32 degrees C, RH=80%) (H); and a humid heat exposure in later sleep segments (C for the first 3 h 45 min, followed by a 30-min transition to H, which was then maintained for the last 3 h 45 min) (C-H). Electroencephalogram, EOG, and mental electromyogram, rectal temperature (Tre), and skin temperature (Tsk) were continuously measured. The total amount of wakefulness was significantly increased in H compared to C-H or C. Compared to C, wakefulness in C-H and H was significantly increased during later sleep segments. Tre and mean Tsk were significantly higher in H than in C-H or C. In C-H, Tsk and Tre increased to levels equal to those observed in H after Ta and RH increase. Whole body sweat loss was significantly lower in C-H and C than in H. These results suggest that humid heat exposure in the later sleep segment reduces thermal load as compared to full-night humid heat exposure. In daily life, the use of air conditioning in the initial sleep hours can protect sleep and thermoregulation.

Effect of size and pressure of surface recording electrodes on amplitude of sensory nerve action potentials

The influence of electrode size on sensory nerve action potential (SNAP) amplitude of the lateral antebrachial cutaneous nerve (LACN) and sural nerve (SN) was studied in 63 healthy volunteers. The SNAP amplitudes were measured using surface recording electrodes of three different sizes, positioned across the nerve. Mean amplitudes using a 5-mm electrode were 9.0% (SN) and 15.3% (LACN) higher than with a 20-mm electrode and 19.4% (SN) and 25.8% (LACN) higher than using a 40-mm electrode. To study the influence of pressure on surface recording electrodes, studies were performed on the LACN in 31 healthy volunteers. Light pressure of the recording electrodes on the skin gave lower amplitudes (15.3%) than did greater pressure or pressure applied between active and reference electrodes. These studies demonstrate that standardized surface recording electrode size and pressure are imperative for obtaining valid and reliable results in experimental studies or in clinical follow-up of patients undergoing nerve conduction studies.