Fluctuations in blood flow to acral skin in humans: connection with heart rate and blood pressure variability

Vascular endothelium as a target tissue for short-term exposure to low-frequency noise that increases cutaneous blood flow

Harmful health effects of exposure to low-frequency noise (LFN) defined as noise with frequencies at ≤100 Hz on the circulatory system have been a concern. However, there has been no study on the effects of exposure to LFN on the circulatory system with consideration of its frequencies and decibels. In this study, the effects of short-term exposure to broad-band LFNs and their pure-tone components (pure-tone LFNs) on cutaneous blood flow in the extremities including the hands were investigated. In our fieldwork study, we first sampled some kinds of common broad-band LFNs. Our human study then showed that broad-band LFN with a narrower frequency range more strongly increased cutaneous blood flow than did broad-band LFN with a wider frequency range. Pure-tone LFNs of 70-100 Hz at ≤85 dB(Z), but not pure-tone LFNs exceeding 100 Hz, further increased levels of cutaneous blood flow. Our wavelet-transform spectrum analysis of cutaneous blood flow next revealed that the nitric oxide (NO)-dependent and -independent vascular activities of the vascular endothelium were specifically increased by exposure to pure-tone LFN. Our animal study again indicated that exposure to pure-tone LFN increased cutaneous blood flow in mice with impairments of bilateral inner ears as well as cutaneous blood flow in control mice, suggesting a limited effect of inner ear function on the LFN-mediated increase in cutaneous blood flow. The NO-dependent suppressive effect of pure-tone LFN on cutaneous blood flow was confirmed by inhibition of vascular endothelial activity through intravenous injection of an NO inhibitor in wild-type mice. Taken together, the results of this study demonstrated that the vascular endothelium is a target tissue of LFN and that NO is an effector of the LFN-mediated increase in cutaneous blood flow. Since improvement of peripheral circulation could generally promote human health, short-term exposure to LFN may be beneficial for health.

Health effects of voluntary exposure to cold water - a continuing subject of debate

This review is based on a multiple database survey on published literature to determine the effects on health following voluntary exposure to cold-water immersion (CWI) in humans. After a filtering process 104 studies were regarded relevant. Many studies demonstrated significant effects of CWI on various physiological and biochemical parameters. Although some studies were based on established winter swimmers, many were performed on subjects with no previous winter swimming experience or in subjects not involving cold-water swimming, for example, CWI as a post-exercise treatment. Clear conclusions from most studies were hampered by the fact that they were carried out in small groups, often of one gender and with differences in exposure temperature and salt composition of the water. CWI seems to reduce and/or transform body adipose tissue, as well as reduce insulin resistance and improve insulin sensitivity. This may have a protective effect against cardiovascular, obesity and other metabolic diseases and could have prophylactic health effects. Whether winter swimmers as a group are naturally healthier is unclear. Some of the studies indicate that voluntary exposure to cold water has some beneficial health effects. However, without further conclusive studies, the topic will continue to be a subject of debate.

Bright Light Decreases Peripheral Skin Temperature in Healthy Men: A Forced Desynchrony Study Under Dim and Bright Light (II)

Human thermoregulation is strictly regulated by the preoptic area of the hypothalamus, which is directly influenced by the suprachiasmatic nucleus (SCN). The main input pathway of the SCN is light. Here, thermoregulatory effects of light were assessed in humans in a forced desynchrony (FD) design. The FD experiment was performed in dim light (DL, 6 lux) and bright white light (BL, 1300 lux) in 8 men in a semi-randomized within-subject design. A 4 × 18 h FD protocol (5 h sleep, 13 h wake) was applied, with continuous core body temperature (CBT) and skin temperature measurements at the forehead, clavicles, navel, palms, foot soles and toes. Skin temperature parameters indicated sleep-wake modulations as well as internal clock variations. All distal skin temperature parameters increased during sleep, when CBT decreased. Light significantly affected temperature levels during the wake phase, with decreased temperature measured at the forehead and toes and increased navel and clavicular skin temperatures. These effects persisted when the lights were turned off for sleep. Circadian amplitude of CBT and all skin temperature parameters decreased significantly during BL exposure. Circadian proximal skin temperatures cycled in phase with CBT, while distal skin temperatures cycled in anti-phase, confirming the idea that distal skin regions reflect heat dissipation and proximal regions approximate CBT. In general, we find that increased light intensity exposure may have decreased heat loss in humans, especially at times when the circadian system promotes sleep.

Effects of Heat Acclimation Following Heat Acclimatization on Whole Body Heat Exchange in Trained Endurance Athletes

The purpose of this study was to examine the changes in metabolic heat production (Hprod), evaporative heat loss (Hevap), and dry heat loss (Hdry), following heat acclimatization (HAz) and heat acclimation (HA). Twenty-two male endurance athletes (mean ± standard deviation; age, 37 ± 12 y; body mass, 73.4 ± 8.7 kg; height, 178.7 ± 6.8 cm; and VO2max, 57.1 ± 7.2 mL·kg−1·min−1) completed three trials (baseline; post-HAz; and post-HA), which consisted of 60 min steady state exercise at 59 ± 2% velocityVO2max in the heat (ambient temperature [Tamb], 35.2 ± 0.6 °C; relative humidity [%rh] 47.5 ± 0.4%). During the trial, VO2 and RER were collected to calculate Hprod, Hevap, and Hdry. Following the baseline trial, participants completed self-directed outdoor summer training followed by a post-HAz trial. Then, five days of HA were completed over eight days in the heat (Tamb, 38.7 ± 1.1 °C; %rh, 51.2 ± 2.3%). During the HA sessions, participants exercised to maintain hyperthermia (38.50 °C and 39.75 °C) for 60 min. Then, a post-HA trial was performed. There were no differences in Hprod between the baseline (459 ± 59 W·m−2), post-HAz (460 ± 61 W·m−2), and post-HA (464 ± 55 W·m−2, p = 0.866). However, Hevap was significantly increased post-HA (385 ± 84 W·m−2) compared to post-HAz (342 ± 86 W·m−2, p = 0.043) and the baseline (332 ± 77 W·m−2, p = 0.037). Additionally, Hdry was significantly lower at post-HAz (125 ± 8 W·m−2, p = 0.013) and post-HA (121 ± 10 W·m−2, p < 0.001) compared to the baseline (128 ± 7 W·m−2). Hdry at post-HA was also lower than post-HAz (p = 0.049). Hprod did not change following HAz and HA. While Hdry was decreased following HA, the decrease in Hdry was smaller than the increases in Hevap. Adaptations in body heat exchange can occur by HA following HAz.

Facial temperature and pupil size as indicators of internal state in primates

Studies in human subjects have revealed that autonomic responses provide objective and biologically relevant information about cognitive and affective states. Measures of autonomic responses can also be applied to studies of non-human primates, which are neuro-anatomically and physically similar to humans. Facial temperature and pupil size are measured remotely and can be applied to physiological experiments in primates, preferably in a head-fixed condition. However, detailed guidelines for the use of these measures in non-human primates is lacking. Here, we review the neuronal circuits and methodological considerations necessary for measuring and analyzing facial temperature and pupil size in non-human primates. Previous studies have shown that the modulation of these measures primarily reflects sympathetic reactions to cognitive and emotional processes, including alertness, attention, and mental effort, over different time scales. Integrated analyses of autonomic, behavioral, and neurophysiological data in primates are promising methods that reflect multiple dimensions of emotion and could potentially provide tools for understanding the mechanisms underlying neuropsychiatric disorders and vulnerabilities characterized by cognitive and affective disturbances.

A conceptual model for changes in finger photoplethysmograph signals caused by hand posture and isothermic regulation

OBJECTIVE

To observe changes in baseline and pulsatile light absorbance (photoplethysmograph, PPG) in the finger-tip, by raising the hand above the horizontal plane in recumbent subjects. We applied current knowledge of the circulation to the finger-tip, particularly arteriovenous anastomoses (AVAs), and the physiology of the venous circulation.

APPROACH

We studied healthy young volunteers in a quiet thermoneutral environment. A finger plethysmograph on the non-dominant hand recorded transmission of red and infra-red light, and the values were converted into absorbance to allow comparisons within and between subjects. Breathing movements were recorded unobtrusively to assess any effect on absorbance and the pulse amplitude of the signals. All body movements were passive: the study arm was elevated in a trough to about 40° above the horizontal plane. The following conditions were studied, each for 15 minutes, using the last 10 minutes for analysis: recumbent, study arm elevated, study arm horizontal, and both legs elevated by 40°.

MAIN RESULTS

There was a substantial time-related effect, and considerable variation between subjects. Arm elevation reduced red light absorbance and increased the range of amplitudes of the PPG waveform: only in subjects with large absorbances, did waveform amplitude increase. The other main effect was that spontaneous, thermoregulatory decreases in absorbance were associated with decreases in waveform amplitude.

SIGNIFICANCE

Finger-tip vessels distend with blood when AVAs open. The vessels pulsate more strongly if venous collapse allows the vessels to become more compliant. The postcapillary circulation is likely to be an important source of pulsation.

The Central Role of Hypothermia and Hyperactivity in Anorexia Nervosa: A Hypothesis

Typically, the development of anorexia nervosa (AN) is attributed to psycho-social causes. Several researchers have recently challenged this view and suggested that hypothermia and hyperactivity (HyAc) are central to AN. The following hypothesis will attempt to clarify their role in AN. Anorexia nervosa patients (ANs) have significantly lower core temperatures (T_core) compared to healthy controls (HCs). This reduced temperature represents a reset T_core that needs to be maintained. However, ANs cannot maintain this T_core due primarily to a reduced basal metabolic rate (BMR); BMR usually supplies heat to sustain T_core. Therefore, to generate the requisite heat, ANs revert to the behavioral-thermoregulatory strategy of HyAc. The majority of ANs (~89%) are reportedly HyAc. Surprisingly, engagement in HyAc is not motivated by a conscious awareness of low T_core, but rather by the innocuous sensation of "cold- hands" frequently reported by ANs. That is, local hand-thermoreceptors signal the brain to initiate HyAc, which boosts perfusion of the hands and alters the sensation of "cold-discomfort" to one of "comfort." This "rewarding" consequence encourages repetition/habit formation. Simultaneously, hyperactivity increases the availability of heat to assist with the preservation of T_core. Additionally, HyAc induces the synthesis of specific brain neuromodulators that suppress food intake and further promote HyAc; this outcome helps preserve low weight and perpetuates this vicious cycle. Based on this hypothesis and supported by rodent research, external heat availability should reduce the compulsion to be HyAc to thermoregulate. A reduction in HyAc should decrease the production of brain neuromodulators that suppress appetite. If verified, hopefully, this hypothesis will assist with the development of novel treatments to aid in the resolution of this intractable condition.

Quantitative Analysis of Real-Time Infrared Thermography for the Assessment of Lumbar Sympathetic Blocks: A Preliminary Study

Lumbar sympathetic blocks (LSBs) are commonly performed to treat pain ailments in the lower limbs. LSBs involve injecting local anesthetic around the nerves. The injection is guided by fluoroscopy which is sometimes considered to be insufficiently accurate. The main aim was to analyze the plantar foot skin temperature data acquired while performing LSBs in patients with complex regional pain syndrome (CRPS) affecting the lower limbs. Forty-four LSBs for treating lower limb CRPS in 13 patients were assessed. Pain medicine physicians visualized the infrared thermography (IRT) video in real time and classified the performance depending on the observed thermal changes within the first 4 min. Thirty-two percent of the cases did not register temperature variations after lidocaine was injected, requiring the needle to be relocated. Differences between moments are indicated using the 95% confidence intervals of the differences (CI 95%), the Cohen effect size (ES) and the significance (p value). In successful cases, after injecting lidocaine, increases at minute 7 for the mean (CI 95% (1.4, 2.1 °C), p < 0.001 and ES = 0.5), at minute 5 for maximum temperature (CI 95% (2.3, 3.3 °C), p < 0.001 and ES = 0.6) and at minute 6 for SD (CI 95% (0.2, 0.3 °C), p < 0.001 and ES = 0.5) were observed. The results of our preliminary study showed that the measurement of skin temperature in real time by infrared thermography is valuable for assessing the success of lumbar sympathetic blocks.

Rapid, Large, and Synchronous Sweat and Cardiovascular Responses Upon Minor Stimuli in Healthy Subjects. Dynamics and Reproducibility

Purpose: The aim of the study was to investigate steady state levels, dynamics and reproducibility of cardiovascular variables and electrodermal activity in different skin areas in response to minor physiological and mental stimuli in healthy subjects in the thermoneutral zone, carried out in high time resolution. Methods: Thirteen healthy subjects underwent experiments on two separate days. Non-invasive electrodermal activity in five different skin areas was measured continuously using a skin conductance method, including resting supine and sitting positions, performing deep inspirations, a mental challenge and being exposed to a sudden loud sound. Blood pressure, heart rate, radial artery blood flow, and skin perfusion were measured simultaneously. Results: Electrodermal activity in the right and left palms was almost identical, with rapid and large increases within a few seconds in response to stimuli, whereas no such significant changes were seen in the face, back, and abdomen. Radial artery blood flow and palmar skin perfusion changed synchronously with electrodermal activity for each stimulus, and were correlated to changes in blood pressure and heart rate. The response patterns in each subject were very similar on the two experimental days. There was very low spontaneous electrodermal activity in the supine position, contrary to the resting sitting position. Conclusion: The electrodermal activity increased rapidly and synchronously in both palms within a few seconds as a response to minor physiological and mental stimuli, synchronous with fluctuations in radial artery blood flow, palmar skin perfusion, and cardiovascular variables. The responses are reproducible from day to day, making them a stable and constant stimuli to be used for studies in patients with hyperhidrosis.

Infrared Thermography and Vascular Disorders in Diabetic Feet

AIM

Diabetes mellitus (DM) and related foot complications constitute a growing healthcare burden. Diabetes mellitus is associated with lower-limb amputation, but diabetic foot assessment is challenging. Here, we evaluated a novel noninvasive diagnostic method-infrared thermography (IRT) -assessing its diagnostic potential compared to conventional noninvasive measurements.

METHODS

This study included patients with DM (n = 118) and healthy controls (n = 93). All participants underwent ankle brachial index and toe pressure (TP) measurements, and IRT using a standardized protocol with temperature measurement at five foot areas.

RESULTS

Compared to controls, patients with DM generally had warmer feet and exhibited a significantly greater temperature difference between feet (P < .001). Mean temperatures were highest in patients with DM with neuroischemia, followed by neuropathy. Patients with DM with angiopathy showed the lowest mean temperature-similar to controls and noncomplicated diabetics. Mean temperatures at all measurement sites were significantly higher with abnormal TP (<50 mmHg) than normal TP (≥50 mmHg) (P < .001). Infrared thermography revealed differences between angiosome areas, subclinical infections, and plantar high-pressure areas.

CONCLUSION

Infrared thermography revealed local temperature differences in high-risk diabetic feet. Normal skin surface temperature varies between individuals, but in combination with other tools, IRT might be useful in clinical screening.

CLINICALTRIALS ID

14212016.

A comparison of finger and forehead pulse oximeters in heart failure patients during maximal exercise

BACKGROUND

Pulse oximeters, clinically used to measure oxygen saturation (SpO₂), rely on adequate perfusion of the tissues over which they are placed. Heart failure (HF) patients can have impaired peripheral perfusion which may compromise the accuracy of a peripherally placed pulse oximeter. This decrease in peripheral perfusion may be especially apparent during exercise. The objective of this study was to determine if pulse oximeter accuracy is dependent on location in heart failure patients during peak exercise.

METHODS

20 participants with HF (7F, age 64.±11 yr) and 9 participants with coronary artery disease as controls (CAD: 3F, age 66±5 yr) performed a maximal exertion treadmill exercise stress test while wearing both finger and forehead pulse oximeters.

RESULTS

At peak exercise, the two pulse oximeters measurements of SpO₂ differed from each other by 3.8 ± 3.3% in the HF group (p<0.01) and 2.0 ± 1.4% in the CAD group (p = 0.065). The difference between the pulse rate from the pulse oximeters and the heart rate from the 12-lead ECG in the HF group was 12±20 BPM (p<0.01) for the finger pulse oximeter, and 2 ± 3 BPM (p = 0.162) for the forehead pulse oximeter.

CONCLUSIONS

Forehead pulse oximeters may be more reliable compared to finger pulse oximeters in obtaining SpO₂ measurements in HF patients during a treadmill maximal exercise test.

Temporal Vascular Changes in Leg with Ulcer Due to Autonomic Neuropathy

Background  Vascular dysfunction dominates the clinical picture of peripheral autonomic neuropathy in lower extremity. Patients and Methods  We have studied functional changes of leg vasculature in 30 patients with chronic ulceration due to peripheral autonomic neuropathy between clinical stages 1 and 3. They suffered from lower extremity wounds. After sympathetic skin response test, pedal arterial blood flow analysis including peak systolic velocity (PSV) and pulsatility index (PI) was made by duplex ultrasonography (DUS) in involved legs. Vascular anatomy of leg was also examined by magnetic resonance angiography. Results  The mean PSV value was found 58.32 cm/s in stage 1, 35.31 cm/s in stage 2, and 15.71 cm/s in stage 3. The mean PI value was observed 1.17 in stage 1, 1.43 in stage 2, and 1.87 in stage 3. In chronic stage 3, three patients had inadequate arterial blood supply and recurrent ulcer. Conclusions  We suggest that reduced sympathetic activity due to small fiber neuropathy causes temporal variations in leg blood flow. There was a nonlinear relationship between vascular functional changes and stages of disease with increased, intermediate, and decreased blood flow, respectively. DUS assessment of pedal arteries contributed to differentiation of clinical stages and permitted vascular evaluation in the course of peripheral autonomic neuropathy.

Assessment of skin temperature during regional anaesthesia-What the anaesthesiologist should know

Body temperature homeostasis is accurately regulated by complex feedback-driven neuronal mechanisms, which involve a multitude of thermoregulatory pathways. Thus, core temperature is constantly maintained within a narrow range. As one of the most effective regulatory systems skin temperature is dependent on skin blood flow. Skin blood flow in turn is highly dependent on sympathetic activity. Regional anaesthesia leads to blockade not only of somatosensory and motor nerve fibres but also of sympathetic fibres. As a consequence, vasoconstrictor tonic activity is abrogated and a vasodilation leads to an increase in skin blood flow and temperature. The aim of this review was to summarize the general physiology of thermoregulation and skin temperature as well as the alterations during regional anaesthesia. The main focus was the usefulness of measuring skin temperature as an indicator of regional anaesthesia success. According to the available literature, assessment of skin temperature can indeed serve to predict success of regional anaesthesia. Hence, it is important to realize that relevant and reliable temperature increase is only seen in the most distal body parts, ie fingers and toes. More proximally, temperature changes are frequently small and inconsistent, which means that assessment of block levels is not possible by temperature measurement. Furthermore, relevant skin temperature increases will only be observed in patients, which are initially vasoconstricted. In conclusion, measurement of skin temperature represents a reliable and feasible diagnostic tool to assess and predict the success or failure of regional anaesthesia procedures, especially in patients in which sensory testing is impossible.

The identification of higher forefoot temperatures associated with peripheral arterial disease in type 2 diabetes mellitus as detected by thermography

AIMS

The purpose of this study was to investigate whether heat emitted from the feet of patients with type 2 diabetes (DM) and peripheral arterial disease (PAD) differed from those with type 2 diabetes without complications (DM).

METHODS

A non-experimental, comparative prospective study design was employed in a tertiary referral hospital. Out of 223 randomly selected participants (430 limbs) who were initially tested, 62 limbs were categorized as DM+PAD and 22 limbs as DM without PAD. Subjects with evidence of peripheral neuropathy were excluded. Participants underwent thermographic imaging. Automatic segmentation of regions of interest extracted the temperature data.

RESULTS

A significant difference in temperature in all the toes between the two groups was found (p=0.005, p=0.033, p=0.015, p=0.038 and p=0.02 for toes 1-5 respectively). The mean forefoot temperature in DM+PAD was significantly higher than that in DM (p=.019), with DM+PAD having a higher mean temperature (28.3°C) compared to DM (26.2°C). Similarly, the toes of subjects with DM+PAD were significantly warmer than those of subjects with DM only.

CONCLUSIONS

Contrary to expectations the mean toe and forefoot temperatures in DM patients with PAD is higher than in those with DM only. This unexpected result could be attributed to disruption of noradrenergic vasoconstrictor thermoregulatory mechanisms with resulting increased flow through cutaneous vessels and subsequent increased heat emissivity. These results demonstrate that thermography may have potential in detecting PAD and associated temperature differences.

Establishing Differences in Thermographic Patterns between the Various Complications in Diabetic Foot Disease

AIM

To evaluate the potential of thermography as an assessment tool for the detection of foot complications by understanding the variations in temperature that occur in type 2 diabetes mellitus (DM).

METHODS

Participants were categorized according to a medical examination, ankle brachial index, doppler waveform analysis, and 10-gram monofilament testing into five groups: healthy adult, DM with no complications, DM with peripheral neuropathy, DM with neuroischaemia, and DM with peripheral arterial disease (PAD) groups. Thermographic imaging of the toes and forefeet was performed.

RESULTS

43 neuroischaemic feet, 41 neuropathic feet, 58 PAD feet, 21 DM feet without complications, and 126 healthy feet were analyzed. The temperatures of the feet and toes were significantly higher in the complications group when compared to the healthy adult and DM healthy groups. The higher the temperatures of the foot in DM, the higher the probability that it is affected by neuropathy, neuroischaemia, or PAD.

CONCLUSIONS

Significant differences in mean temperatures exist between participants who were healthy and those with DM with no known complications when compared to participants with neuroischaemia, neuropathy, or PAD. As foot temperature rises, so does the probability of the presence of complications of neuropathy, neuroischaemia, or peripheral arterial disease.

Heat remains unaccounted for in thermal physiology and climate change research

In the aftermath of the Paris Agreement, there is a crucial need for scientists in both thermal physiology and climate change research to develop the integrated approaches necessary to evaluate the health, economic, technological, social, and cultural impacts of 1.5°C warming. Our aim was to explore the fidelity of remote temperature measurements for quantitatively identifying the continuous redistribution of heat within both the Earth and the human body. Not accounting for the regional distribution of warming and heat storage patterns can undermine the results of thermal physiology and climate change research. These concepts are discussed herein using two parallel examples: the so-called slowdown of the Earth's surface temperature warming in the period 1998-2013; and the controversial results in thermal physiology, arising from relying heavily on core temperature measurements. In total, the concept of heat is of major importance for the integrity of systems, such as the Earth and human body. At present, our understanding about the interplay of key factors modulating the heat distribution on the surface of the Earth and in the human body remains incomplete. Identifying and accounting for the interconnections among these factors will be instrumental in improving the accuracy of both climate models and health guidelines.

Use of a novel smart heating sleeping bag to improve wearers' local thermal comfort in the feet

Previous studies have revealed that wearers had low skin temperatures and cold and pain sensations in the feet, when using sleeping bags under defined comfort and limit temperatures. To improve wearers’ local thermal comfort in the feet, a novel heating sleeping bag (i.e., MAR_HT) was developed by embedding two heating pads into the traditional sleeping bag (i.e., MAR_CON) in this region. Seven female and seven male volunteers underwent two tests on different days. Each test lasted for three hours and was performed in a climate chamber with a setting temperature deduced from EN 13537 (2012) (for females: comfort temperature of −0.4 °C, and for males: the limit temperature of −6.4 °C). MAR_HT was found to be effective in maintaining the toe and feet temperatures within the thermoneutral range for both sex groups compared to the linearly decreased temperatures in MAR_CON during the 3-hour exposure. In addition, wearing MAR_HT elevated the toe blood flow significantly for most females and all males. Thermal and comfort sensations showed a large improvement in feet and a small to moderate improvement in the whole body for both sex groups in MAR_HT. It was concluded that MAR_HT is effective in improving local thermal comfort in the feet.

Finger skin temperatures in 8- to 11-year-old children: determinants including physical characteristics and seasonal variation. The Physical Activity and Nutrition in Children (PANIC) Study

PURPOSE

The fingertip skin temperature (FST) reflects skin blood flow, and FST measurement has been suggested for the investigation of vascular responses. As a limitation, the multifactorial nature and the seasonal variation in measured values have been earlier described in adults but not in children. In the present study, we identify the modifiers of FST in a population sample of Finnish children.

METHODS

FST was measured in children (age range 8-11 years, n = 432) with infrared thermometer, and its possible determinants including the subjects' physical characteristics and seasonal variables, such as daylight time and outdoor temperature, were identified.

RESULTS

In univariate regression models, FST was dependent on the sex, age and anthropometric characteristics of the children with the higher body fat content-related variables and a lower surface area-to-mass ratio as strongest single modifiers of FST. There was interaction between sex and puberty with FST. In addition, FST was directly related to daylight time and outdoor temperature although the children had stayed inside for at least 2 h before the measurements. The FST values were lowest in the winter and highest in the summer. In multivariate regression model, main determinants of FST were a higher body fat percentage (standardized regression coefficient β = 0.472; p < 0.001), male sex (β = 0.291; p < 0.001) and longer daylight time (0.226; p < 0.001).

CONCLUSIONS

Altogether, complex effects of body composition and sex with the confounding effect of seasonal variation may complicate the use of FST as a tool to study the vascular function in children.

Arterio-venous anastomoses in the human skin and their role in temperature control

Arterio-venous anastomoses (AVAs) are direct connections between small arteries and small veins. In humans they are numerous in the glabrous skin of the hands and feet. The AVAs are short vessel segments with a large inner diameter and a very thick muscular wall. They are densely innervated by adrenergic axons. When they are open, they provide a low-resistance connection between arteries and veins, shunting blood directly into the venous plexuses of the limbs. The AVAs play an important role in temperature regulation in humans in their thermoneutral zone, which for a naked resting human is about 26°C to 36°C, but lower when active and clothed. From the temperature control center in the hypothalamus, bursts of nerve impulses are sent simultaneously to all AVAs. The AVAs are all closed near the lower end and all open near the upper end of the thermoneutral zone. The small veins in the skin of the arms and legs are also contracted near the lower end of the thermoneutral zone and relax to a wider cross section as the ambient temperature rises. At the cold end of the thermoneutral range, the blood returns to the heart through the deep veins and cools the arterial blood through a countercurrent mechanism. As the ambient temperature rises, more blood is returned through the superficial venous plexuses and veins and heats the skin surface of the full length of the 4 limbs. This skin surface is responsible for a large part of the loss of heat from the body toward the upper end of the thermoneutral zone.

Detection of Site-Specific Blood Flow Variation in Humans during Running by a Wearable Laser Doppler Flowmeter

Wearable wireless physiological sensors are helpful for monitoring and maintaining human health. Blood flow contains abundant physiological information but it is hard to measure blood flow during exercise using conventional blood flowmeters because of their size, weight, and use of optic fibers. To resolve these disadvantages, we previously developed a micro integrated laser Doppler blood flowmeter using microelectromechanical systems technology. This micro blood flowmeter is wearable and capable of stable measurement signals even during movement. Therefore, we attempted to measure skin blood flow at the forehead, fingertip, and earlobe of seven young men while running as a pilot experiment to extend the utility of the micro blood flowmeter. We measured blood flow in each subject at velocities of 6, 8, and 10 km/h. We succeeded in obtaining stable measurements of blood flow, with few motion artifacts, using the micro blood flowmeter, and the pulse wave signal and motion artifacts were clearly separated by conducting frequency analysis. Furthermore, the results showed that the extent of the changes in blood flow depended on the intensity of exercise as well as previous work with an ergometer. Thus, we demonstrated the capability of this wearable blood flow sensor for measurement during exercise.

A vascular mechanistic approach to understanding Raynaud phenomenon

During exposure to cold, our bodies attempt to maintain normal core temperature by restricting heat loss through cutaneous vasoconstriction, and by increasing heat production through shivering and nonshivering thermogenesis. In selected areas of human skin (including on the fingers and toes), the vascular system has specialized structural and functional features that enable it to contribute to thermoregulation. These features include arteriovenous anastomoses, which directly connect the arterial and venous systems and bypass the nutritional capillaries supplying blood to the skin tissue. Of note, Raynaud phenomenon predominantly affects the arterial territories supplying these specialized areas of skin. Indeed, Raynaud phenomenon can be considered a disorder of vascular thermoregulatory control. This Review presents an understanding of Raynaud phenomenon in the context of vascular and thermoregulatory control mechanisms, including the role of unique thermosensitive vascular structural and functional specialization, and describes the potential role of thermogenesis in this disorder. This new approach provides remarkable insight into the disease process and builds a framework to critically appraise the existing knowledge base. This paradigm also explains the deficiencies in some current therapeutic approaches, and highlights new areas of potential relevance to the pathogenesis and treatment of Raynaud phenomenon that should be expanded and explored.

Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

In this review, we focus on significant developments in our understanding of the mechanisms that control the cutaneous vasculature in humans, with emphasis on the literature of the last half-century. To provide a background for subsequent sections, we review methods of measurement and techniques of importance in elucidating control mechanisms for studying skin blood flow. In addition, the anatomy of the skin relevant to its thermoregulatory function is outlined. The mechanisms by which sympathetic nerves mediate cutaneous active vasodilation during whole body heating and cutaneous vasoconstriction during whole body cooling are reviewed, including discussions of mechanisms involving cotransmission, NO, and other effectors. Current concepts for the mechanisms that effect local cutaneous vascular responses to local skin warming and cooling are examined, including the roles of temperature sensitive afferent neurons as well as NO and other mediators. Factors that can modulate control mechanisms of the cutaneous vasculature, such as gender, aging, and clinical conditions, are discussed, as are nonthermoregulatory reflex modifiers of thermoregulatory cutaneous vascular responses.

Myogenic responses occur on a beat-to-beat basis in the resting human limb

Investigations of human myogenic responses typically use maneuvers that evoke robust changes in transmural pressure. Although this strategy has demonstrated peripheral myogenic responsiveness in the limbs, particularly in glabrous skin of the hand or foot, it has not considered the potential influence of the myogenic mechanism in beat-to-beat blood flow (BF) control during unprovoked rest. In the present study, we examined the interactions of spontaneous beat-to-beat mean arterial pressure (MAP; Finapres) with BF (Doppler ultrasound) supplying the forearm (brachial artery), lower leg (popliteal artery), and hand (ulnar artery) during 10 min of supine rest in healthy young men. Cross-correlation analyses revealed a negative association between MAP and BF, which was more prominent in the forearm than lower leg. The strongest correlation resulted when a -2-heart beat offset of MAP was applied (R=-0.53±0.04 in the forearm and -0.23±0.05 in the leg, P<0.05), suggesting an ∼2-s delay from instances of high/low MAP to low/high BF. Negatively associated episodes (high MAP/low BF and low MAP/high BF) outnumbered positively associated data (P<0.05). BF during low MAP values was greater than the steady-state average BF and vice versa. Wrist and ankle occlusion blunted the strength of correlations, homogenized the incidence of MAP and BF pairings, and reduced the magnitude of deviation from steady-state values. In contrast, these relationships were matched or accentuated for hand BF. Overall, these results suggest that myogenic responses are present and occur rapidly in human limbs during rest, overwhelm perfusion pressure gradient influences, and are primarily mediated by the distal limb circulation.

Evaluation of oscillometric and Doppler ultrasonic devices for blood pressure measurements in anesthetized and conscious dogs

Two non-invasive blood pressure (NIBP) devices (oscillometry and Doppler) were compared to invasive blood pressure using a Bland-Altman analysis, in anesthetized and conscious dogs. When considering the systolic arterial pressure only during general anesthesia, both NIBP devices slightly underestimated the systolic arterial blood pressure however the precision and the limits of agreement for the Doppler were of a greater magnitude. This indicates a worse clinical performance by the Doppler. The performance of both NIBP devices deteriorated as measured in conscious animals. In general, for the oscillometric device, determination of invasive diastolic and mean arterial pressures was better than the invasive systolic arterial pressure. Overall, the oscillometric device satisfied more of the criteria set by the American College of Veterinary Internal Medicine consensus statement. Based upon these results, the oscillometric device is more reliable than the Doppler in the determination of blood pressure in healthy medium to large breed dogs.

Is the effect of mobile phone radiofrequency waves on human skin perfusion non-thermal?

OBJECTIVE

To establish whether SkBF can be modified by exposure to the radiofrequency waves emitted by a mobile phone when the latter is held against the jaw and ear.

METHODS

Variations in SkBF and Tsk in adult volunteers were simultaneously recorded with a thermostatic laser Doppler system during a 20-minute "radiofrequency" exposure session and a 20-minute "sham" session. The skin microvessels' vasodilatory reserve was assessed with a heat challenge at the end of the protocol.

RESULTS

During the radiofrequency exposure session, SkBF increased (vs. baseline) more than during the sham exposure session. The sessions did not differ significant in terms of the Tsk time-course response. The skin microvessels' vasodilatory ability was found to be greater during radiofrequency exposure than during sham exposure.

CONCLUSIONS

Our results reveal the existence of a specific vasodilatory effect of mobile phone radiofrequency emission on skin perfusion.

Ultrathin conformal devices for precise and continuous thermal characterization of human skin

Precision thermometry of the skin can, together with other measurements, provide clinically relevant information about cardiovascular health, cognitive state, malignancy and many other important aspects of human physiology. Here, we introduce an ultrathin, compliant skin-like sensor/actuator technology that can pliably laminate onto the epidermis to provide continuous, accurate thermal characterizations that are unavailable with other methods. Examples include non-invasive spatial mapping of skin temperature with millikelvin precision, and simultaneous quantitative assessment of tissue thermal conductivity. Such devices can also be implemented in ways that reveal the time-dynamic influence of blood flow and perfusion on these properties. Experimental and theoretical studies establish the underlying principles of operation, and define engineering guidelines for device design. Evaluation of subtle variations in skin temperature associated with mental activity, physical stimulation and vasoconstriction/dilation along with accurate determination of skin hydration through measurements of thermal conductivity represent some important operational examples.

Ultrathin conformal devices for precise and continuous thermal characterization of human skin

Precision thermometry of the skin can, together with other measurements, provide clinically relevant information about cardiovascular health, cognitive state, malignancy and many other important aspects of human physiology. Here, we introduce an ultrathin, compliant skin-like sensor/actuator technology that can pliably laminate onto the epidermis to provide continuous, accurate thermal characterizations that are unavailable with other methods. Examples include non-invasive spatial mapping of skin temperature with millikelvin precision, and simultaneous quantitative assessment of tissue thermal conductivity. Such devices can also be implemented in ways that reveal the time-dynamic influence of blood flow and perfusion on these properties. Experimental and theoretical studies establish the underlying principles of operation, and define engineering guidelines for device design. Evaluation of subtle variations in skin temperature associated with mental activity, physical stimulation and vasoconstriction/dilation along with accurate determination of skin hydration through measurements of thermal conductivity represent some important operational examples.

Blood pressure rises more in pre-eclampsia than normal pregnancy when acral skin is locally cooled

OBJECTIVE

Investigate blood pressure response to acral skin vasoconstriction in healthy and pre-eclamptic pregnancies.

METHODS

Healthy women were investigated from gestational week 8 to 52 weeks postpartum and pre-eclampsia subjects at diagnosis. Finger artery ultrasound Doppler, forearm laser Doppler fluximetry and photoplethysmographic blood pressure were recorded. Hand cooling to 19 °C induced vasoconstriction.

RESULTS

Acral skin vasoconstriction increases blood pressure from 16 weeks until 12 weeks postpartum (p ≤ 0.01), with greatest responses in pre-eclampsia (p=0.047). Forearm skin perfusion is higher in pre-eclampsia (p=0.04).

CONCLUSION

Acral skin vasoconstriction raises blood pressure in pregnancy, particularly in pre-eclampsia. Pregnancy accentuates important functional differences within skin.

Complexity analysis of the microcirculatory-blood-flow response following acupuncture stimulation

Beat-to-beat cardiovascular variability analysis provides important information on the circulatory regulatory activities. Changes in the arterial pulse transmission or the opening condition of arteriolar openings might change the fluctuation pattern of the MBF supply, and thus change the complexity property therein. We performed complexity analysis of beat-to-beat laser Doppler flowmetry (LDF) signals to study the microcirculatory-blood-flow (MBF) response at the needled site (Hegu acupoint) following acupuncture stimulation (AS). LDF signals were measured in male healthy volunteers (n=29). Each experiment involved recording a 20-minute baseline-data sequence and two sets of effects data recorded 0-20 and 50-70min after stopping AS. Approximate-entropy (ApEn) analysis, which quantifies the unpredictability of fluctuations in a time series, was performed on each 20-minute beat-to-beat LDF data sequence. The present findings indicate that AS can not only improve the local blood supply but may also increase ApEn values and decrease MBF variability parameters. This was the first attempt to apply complexity analysis to LDF signals in order to elucidate microcirculatory responses following AS. The observed results are probably attributable to the contradictory effects on the MBF supply induced by AS, which might interfere with the microcirculatory regulatory activities so as to increase the complexity of LDF signals. The present findings could help to identify the mechanism underlying the effects of AS, might aid the development of an index for monitoring the induced microcirculatory regulatory responses, and thus provide an evidence-based connection between AS and modern physiology.

Assessing the effects of acupuncture by comparing needling the hegu acupoint and needling nearby nonacupoints by spectral analysis of microcirculatory laser Doppler signals

We aimed to assess the effects of acupuncture by analyzing the frequency content of skin blood-flow signals simultaneously recorded at the Hegu acupoint and two nearby nonacupoints following acupuncture stimulation (AS). Laser Doppler flowmetry (LDF) signals were measured in male healthy volunteers in two groups of experiments: needling the Hegu acupoint (n = 13) and needling a nearby nonacupoint (control experiment; n = 10). Each experiment involved recording a 20 min baseline-data sequence and two sets of effects data recorded 0–20 and 50–70 min after stopping AS. Wavelet transform with Morlet mother wavelet was applied to the measured LDF signals. Needling the Hegu acupoint significantly increased the blood flow, significantly decreased the relative energy contribution at 0.02–0.06 Hz and significantly increased the relative energy contribution at 0.4–1.6 Hz at Hegu, but induced no significant changes at the nonacupoints. Also, needling a nearby nonacupoint had no effect in any band at any site. This is the first time that spectral analysis has been used to investigate the microcirculatory blood-flow responses induced by AS, and has revealed possible differences in sympathetic nerve activities between needling the Hegu acupoint and its nearby nonacupoint. One possible weakness of the present design is that different De-Qi feelings following AS could lead to nonblind experimental setup, which may bias the comparison between needling Hegu and its nearby nonacupoint. Our results suggest that the described noninvasive method can be used to evaluate sympathetic control of peripheral vascular activity, which might be useful for studying the therapeutic effects of AS.

Core body temperature and the thermoneutral zone: a longitudinal study of normal human pregnancy

AIM

Using a longitudinal study design, we investigated changes in maternal core temperature and ambient temperatures before and after a localized cooling procedure to the right hand.

METHODS

Fifteen pregnant women participated. The experiments were sequentially performed for 21 month periods on each subject: from the 8th week of gestation to 1 year after delivery on seven separate occasions (gestational weeks 8, 16, 26, 36 and 12, 24, 52 weeks post-partum). The experiments were conducted in a climactic chamber, allowing ambient temperature adjustment to each subjects' thermoneutral zone determined using Doppler ultrasound of the digital artery.

RESULTS

Maternal core temperature decreased from 37.1 °C (week 8) towards term, reaching a nadir (36.4 °C) at 12 weeks post-partum (P < 0.001). The ambient temperature required to reach the thermoneutral zone changed significantly from 26.5 °C (week 8) falling to its lowest point: 23.0 °C (week 36) (P < 0.001), then stabilized from 24 weeks post-partum.

CONCLUSION

Maternal core temperature is highest in the first trimester but falls during pregnancy to a nadir 3 months post-partum. The ambient temperature required to reach the thermoneutral zone was 4 °C lower at 36 weeks of gestation compared with early pregnancy and late post-partum. Human temperature regulation is altered in pregnancy and for at least 3 months post-partum.

Blood flow in the brachial artery increases after intense cycling exercise

During cycling blood flow is redistributed from physically inactive tissues to working leg muscles. It is unknown how long this situation persists after very intense exercise or whether it differs between intense exhausting and non-exhausting exercise. It is also not known to what extent the redistribution differs between different types of non-active tissues. Therefore nine healthy young men cycled first for 2 min at 328 W (non-exhausting exercise, mean). Blood velocity in thigh and arm (ultrasound-doppler), perfusion of forearm skin (non-acral skin) and finger tip (acral skin, with arterio-venous anastomoses) were measured for 30 min after exercise (laser-doppler). To be able to study vascular resistance and central circulation, blood pressure (Finometer), heart rate (ECG), and stroke volume (ultrasound-doppler) were measured. Thereafter the subjects cycled at the same power to exhaustion (4 min), and the measurements were repeated. After both exercises mean blood pressure was unchanged (< or = 80 mm Hg) despite increased cardiac output (> or = + 30% vs. pre-exercise). Blood velocity in the brachial artery was higher during the whole recovery period than at rest (p< or =0.02; no differences between exercises). Blood perfusion of non-acral skin was unchanged from pre-exercise level after 2 min of non-exhausting exercise, but it was twice as high after 4 min cycling to exhaustion as at rest (p=0.02). Blood perfusion of acral skin rose after both exercises and did not differ between exhausting and non-exhausting exercise. In conclusion, arm blood flow increases above the pre-exercise level in the recovery period after short-lasting, strenuous exercise.

Is mean blood saturation a useful marker of tissue oxygenation?

Increasingly we are monitoring the distribution of oxygen through the microcirculation using optical techniques such as optical reflectance spectroscopy (ORS) and near-infrared spectroscopy. Mean blood oxygen saturation (SmbO2) and tissue oxygenation index measured by these two techniques, respectively, evoke a concept of the measurement of oxygen delivery to tissue. This study aims to establish whether SmbO2 is an appropriate indicator of tissue oxygenation. Spontaneous fluctuations in SmbO2 observed as changes in concentration of oxyhemoglobin ([HbO2]) and deoxyhemoglobin ([Hb]) were measured by ORS in the skin microcirculation of 30 healthy subjects (15 men, age 21–42 yr). Fourier analysis identified two distinctly different spontaneous falls in SmbO2. The first type of swing, thought to be induced by fluctuations in arterial blood volume, resulted from the effects of respiration, endothelial, sympathetic, and myogenic activity. There was no apparent change in [Hb]. In contrast, a second type of swing resulted from a fall in [HbO2] accompanied by a rise in [Hb] and was only induced by endothelial and sympathetic activity. Thus the same fall in SmbO2 can be induced by two distinct responses. A “type I” swing does not suggest an inadequacy in oxygen delivery whereas a “type II” swing may indicate a change in oxygen delivery from blood to tissue. SmbO2 alone cannot therefore be accepted as a definitive marker of tissue oxygenation.

Influence of thermal balance on cold-induced vasodilation

We examined the effect of thermal balance perturbation on cold-induced vasodilation through a dynamic A-B-A-B design applying heat ( condition A) and cold ( condition B) to the body's core, while the hand is exposed to a stable cold stimulus. Fifteen healthy adults (8 men, 7 women) volunteered. Applications of heat and cold were achieved through water immersions in two tanks maintained at 42 and 12°C water temperature, respectively, in an A-B-A-B fashion. Throughout the experiment, the participants’ right hand up to the ulnar styloid process was placed inside a temperature-controlled box set at 0°C air temperature. Results demonstrated that cold-induced vasodilation occurred only during condition B and at times when body heat content was decreasing but rectal temperature had not yet dropped to baseline levels. Following the occurrence of all cold-induced vasodilation events, rectal temperature was reduced, and the phenomenon ceased when rectal temperature fell below baseline. Heart rate variability data obtained before and during cold-induced vasodilation demonstrated a shift of autonomic interaction toward parasympathetic dominance, which, however, was attributed to a sympathetic withdrawal. Receiver operating characteristics curve analyses demonstrated that the cold-induced vasodilation onset cutoff points for rectal temperature change and finger temperature were 0.62 and 16.76°C, respectively. It is concluded that cold-induced vasodilation is a centrally originating phenomenon caused by sympathetic vasoconstrictor withdrawal. It is dependent on excess heat, and it may be triggered by excess heat with the purpose of preserving thermal balance.

Reproducibility of transcutaneous oxygen pressure measurements in persons with spinal cord injury

OBJECTIVES

To assess the reproducibility and the effects of the subjects' characteristics on the reproducibility of transcutaneous oxygen pressure (TcPO2) measurements in the sacral area in persons with spinal cord injury during loading in the supine position.

DESIGN

Test-retest study.

SETTING

Physical medicine and rehabilitation center.

PARTICIPANTS

Thirty spinal cord-injured American Spinal Injury Association grade A subjects.

MAIN OUTCOME MEASURES

Two TcPO2 monitoring sessions in the sacral area during loading in the supine position were performed at 24-hour intervals, including the measurement of absolute resting sacral and chest TcPO2 values and the calculation of regional perfusion index (RPI) and delta from rest oxygen pressure, taking into account systemic TcPO2 changes.

RESULTS

The intraclass coefficient of the sacral TcPO2 absolute resting value, RPI, and delta from rest oxygen pressure was .787 and .798, .704 and .635, .760 and .465, respectively, at 20 and 40 minutes. The only characteristic with an influence on RPI reproducibility was the subject's smoking status, whereas age, weight, time since injury, lesion level, and presence of pressure ulcer showed no influence.

CONCLUSIONS

TcPO2 measurement is a reproducible method for assessing cutaneous microcirculation during loading over 20-minute monitoring sessions, with RPI exhibiting better reproducibility than delta from rest oxygen pressure at 40 minutes.

Effect of body temperature on cold induced vasodilation

Cold-induced vasodilation (CIVD) is an acute increase in peripheral blood flow observed during cold exposures. It is hypothesized to protect against cold injuries, yet despite continuous research it remains an unexplained phenomenon. Contrary to the traditionally held view, we propose that CIVD is a thermoregulatory reflex mechanism contributing to heat loss. Ten adults (4 females; 23.8 +/- 2.0 years) randomly underwent three 130-min exposures to -20 degrees C incorporating a 10-min moderate exercise period at the 65th min, while wearing a liquid conditioning garment (LCG) and military arctic clothing. In the pre-warming condition, rectal temperature was increased by 0.5 degrees C via the LCG before the cold exposure. In the warming condition, participants regulated the LCG throughout the cold exposure to subjective comfort. In the control condition, the LCG was worn but was not operated either before or during the cold exposure. Results demonstrated that the majority of CIVD occurred during the warming condition when the thermometrically-estimated mean body temperature (T (b)) was at its highest. A thermoregulatory pattern was identified whereby CIVD occurred soon after T (b) increased past a threshold (approximately 36.65 degrees C in warming and pre-warming; approximately 36.4 degrees C in control). When CIVD occurred, T (b) was reduced and CIVD ceased when T (b) fell below the threshold. These findings were independent of extremity temperature since CIVD episodes occurred at a large range of finger temperatures (7.2-33.5 degrees C). These observations were statistically confirmed by auto-regressive integrated moving average analysis (t = 9.602, P < 0.001). We conclude that CIVD is triggered by increased T (b) supporting the hypothesis that CIVD is a thermoregulatory mechanism contributing to heat loss.