Pediatric Thermoregulation: Considerations in the Face of Global Climate Change

Predicted global climate change, including rising average temperatures, increasing airborne pollution, and ultraviolet radiation exposure, presents multiple environmental stressors contributing to increased morbidity and mortality. Extreme temperatures and more frequent and severe heat events will increase the risk of heat-related illness and associated complications in vulnerable populations, including infants and children. Historically, children have been viewed to possess inferior thermoregulatory capabilities, owing to lower sweat rates and higher core temperature responses compared to adults. Accumulating evidence counters this notion, with limited child–adult differences in thermoregulation evident during mild and moderate heat exposure, with increased risk of heat illness only at environmental extremes. In the context of predicted global climate change, extreme environmental temperatures will be encountered more frequently, placing children at increased risk. Thermoregulatory and overall physiological strain in high temperatures may be further exacerbated by exposure to/presence of physiological and environmental stressors including pollution, ultraviolet radiation, obesity, diabetes, associated comorbidities, and polypharmacy that are more commonly occurring at younger ages. The aim of this review is to revisit fundamental differences in child–adult thermoregulation in the face of these multifaceted climate challenges, address emerging concerns, and emphasize risk reduction strategies for the health and performance of children in the heat.

Alteration of Pressure-Induced Vasodilation in Aging and Diabetes, a Neuro-Vascular Damage

Skin is constantly subjected to pressure at different levels. Pressure-induced vasodilation (PIV) is one of the response mechanisms to low pressure that maintains the homeostasis of the skin. PIV results from the interaction of primary afferent nerves and vascular endothelium of skin vessels. Thanks to this cutaneous neuro-vascular interaction, the cutaneous blood flow increase allows the maintenance of an optimal level of oxygenation and minimizes the lack of vascularization of the skin tissue under low pressure. It seems to be associated with the cutaneous protection mechanisms to prevent pressure ulcers. In some contexts, where microangiopathy and neuropathy can occur, such as aging and diabetes, PIV is impaired, leading to a dramatic early decrease in local skin blood flow when low pressure is applied. In aging, PIV alteration is due to endothelial dysfunction, essentially from an alteration of the nitric oxide pathway. In the inflamm-aging context, oxidative stress increases leading to endothelial cell and nerve damages. An age-related sensory neuropathy will exacerbate the alteration of PIV during the aging process. In diabetes, non-controlled hyperglycaemia leads to an increase in several pathological biochemical pathways that involve oxidative stress and can affect PIV. Sorbinil, alagebrium and alpha-lipoic acid are able individually to restore PIV through a possible oxidative stress reduction. Candesartan, an angiotensin II type 1 receptor blocker, is also able to restore PIV and prevent pressure ulcer formation. The possibility of preventing pressure ulcer associated to diabetes and/or aging with the restoration of PIV seems to be a promising research path.

Separate and combined effects of KCa and KATP channel blockade with NOS inhibition on cutaneous vasodilation and sweating in older men during heat stress

Our objective in this study was to examine the separate and combined effects of potassium (K⁺) channels and nitric oxide synthase (NOS) on cutaneous vasodilation and sweating in older men during rest and exercise in the heat. In 13 habitually active men (61 ± 4 yr), cutaneous vascular conductance and local sweat rate were assessed at six dorsal forearm skin sites continuously perfused with either 1) lactated Ringer (control), 2) 10 mM N^G-nitro-l-arginine methyl ester (l-NAME, NOS inhibitor), 3) 50 mM tetraethylammonium (TEA; Ca²⁺-activated K⁺ channel blocker), 4) 5 mM glybenclamide (GLY; ATP-sensitive K⁺ channel blocker), 5) 50 mM TEA + 10 mM l-NAME, and 6) 5 mM GLY + 10 mM l-NAME via microdialysis. Participants rested in non-heat stress (25°C) and heat stress (35°C) conditions for ∼60 min each, followed by 50 min of moderate-intensity cycling (∼55% V̇o_2peak) and 30 min of recovery in the heat. During rest and exercise in the heat, l-NAME, TEA + l-NAME, and GLY + l-NAME attenuated CVC relative to control (all P ≤ 0.05), although l-NAME was not different from TEA + l-NAME or GLY + l-NAME (all P > 0.05). TEA attenuated CVC during rest, whereas GLY attenuated CVC during exercise (both P ≤ 0.05). Additionally, whereas neither l-NAME nor TEA altered sweating throughout the protocol (all P > 0.05), combined TEA + l-NAME attenuated sweating during exercise in the heat (P ≤ 0.05). We conclude that in habitually active older men blockade of K_Ca and K_ATP channels attenuates cutaneous vasodilation during rest and exercise in the heat, respectively, and these effects are NOS dependent. Furthermore, combined NOS inhibition and K_Ca channel blockade attenuates sweating during exercise in the heat.

The reliability of cutaneous low-frequency oscillations in young healthy males

OBJECTIVE

Spectral analyses of laser-Doppler flowmetry measures enable a simple and non-invasive method to investigate mechanisms regulating skin blood flow. We assessed within-day and day-to-day variability of cutaneous spectral analyses.

METHODS

Eleven young, healthy males were tested twice in three identical sessions, with 19 to 24 days between visits, for a total of six tests. Wavelet data were analyzed at rest, in response to local skin heating to 42 and 44°C, and during 5-minutes PORH. We did this for six frequency bands commonly associated with physiological functions. To assess reliability, we calculated CV and ICC scores.

RESULTS

At rest, mean CV for the wavelet data ranged from 21% to 24% and ICC scores ranged from 0.67 to 0.91. During local heating, mean CV scores ranged from 17% to 22% and mean ICC scores ranged from 0.71 to 0.95. For peak PORH, CV ranged from 14% to 23% and the ICC range was 0.88 to 0.97. For the area under the curve of the PORH, CV range was 12% to 21% and ICC range was 0.81 to 0.92.

CONCLUSIONS

These analyses indicate good-to-excellent reliability of the wavelet data in healthy young males.

Androgens drive microvascular endothelial dysfunction in women with polycystic ovary syndrome: role of the endothelin B receptor

KEY POINTS

Polycystic ovary syndrome (PCOS) is a complex syndrome with cardiovascular risk factors, including obesity and insulin resistance. PCOS is also associated with high androgens, increases the risk of cardiovascular dysfunction in women. Due to the complexity of PCOS, had it has been challenging to isolate specific causes of the cardiovascular dysfunction. Our measure of cardiovascular dysfunction (endothelial dysfunction) was most profound in lean women with PCOS. The endothelin-1-induced vasodilation in these PCOS subject, was dependent on the ET_B R but was not NO-dependent. We also demonstrated oestrogen administration improved endothelial function in lean and obese women with PCOS likely because oestrogen increased NO availability. Our studies indicate a primary role for androgens in cardiovascular dysfunction in PCOS.

ABSTRACT

Endothelin-1 (ET-1) is an indicator of endothelial injury and dysfunction and is elevated in women with androgen excess polycystic ovary syndrome (AE-PCOS). The endothelin B receptor (ET_B R) subtype mediates vasodilatation, but is blunted in women with PCOS. We hypothesized that androgen drives endothelial dysfunction in AE-PCOS women and oestradiol (EE) administration reverses these effects. We assessed microvascular endothelial function in women with (7 lean and 7 obese) and without AE-PCOS (controls, 6 lean, 7 obese). Only obese AE-PCOS women were insulin resistant (IR). We evaluated cutaneous vascular conductance (%CVC_max ) with laser Doppler flowmetry during low dose intradermal microdialysis ET-1 perfusions (1, 3, 4, 5 and 7 pmol) with either lactated Ringer solution alone, or with ET_B R (BQ-788), or nitric oxide (NO) inhibition (l-NAME). Log[ET-1]-%maxCVC dose-response curves demonstrated reduced vasodilatory responses to ET-1 in lean AE-PCOS (logED₅₀ , 0.59 ± 0.08) versus lean controls (logED₅₀ , 0.49 ± 0.09, P < 0.05), but not compared to obese AE-PCOS (logED₅₀ , 0.65 ± 0.09). ET_B R inhibition decreased ET-1-induced vasodilatation in AE-PCOS women (logED₅₀ , 0.64 ± 0. 22, P < 0.05). This was mechanistically observed at the cellular level, with ET-1-induced, DAF-FM-measurable endothelial cell NO production, which was abrogated by dihydrotestosterone in an androgen receptor-dependent manner. EE augmented the cutaneous vasodilating response to ET-1(logED₅₀ 0.29 ± 0.21, 0.47 ± 0.09, P < 0.05 for lean and obese, respectively). Androgens drive endothelial dysfunction in lean and obese AE-PCOS. We propose that the attenuated ET-1-induced vasodilatation in AE-PCOS is a consequence of androgen receptor-mediated, suppressed ET_B R-stimulated NO production, and is reversed with EE.

Cutaneous active vasodilation as a heat loss thermoeffector

Human skin is the interface between the human body and the environment. As such, human temperature regulation relies largely on cutaneous vasomotor and sudomotor adjustments to appropriately thermoregulate. In particular, changes in skin blood flow can increase or decrease the convective heat transfer from internal tissues to the periphery where it can increase or prevent heat loss to the environment. Thermoregulatory control of the cutaneous vasculature is largely due to cutaneous sympathetic nerves. Sympathetic adrenergic nerves mediate vasoconstriction of the skin, similar to other vascular beds, whereas active vasodilator nerves in nonglabrous skin respond to changes in internal and peripheral temperatures and can profoundly increase skin blood flow. Activation of these vasodilator nerves is known as cutaneous active vasodilation and has been the subject of much recent research. This research has uncovered a highly complex system that involves the activation of multiple receptors and vasodilator pathways in a synergistic and sometimes redundant manner. This complexity and redundancy has left our understanding of cutaneous active vasodilation incomplete; however, the employment of new techniques and use of new pharmacologic agents have introduced many new insights into cutaneous active vasodilation.

Local arginase inhibition does not modulate cutaneous vasodilation or sweating in young and older men during exercise

Age-related impairments in cutaneous vascular conductance (CVC) and sweat rate (SR) during exercise may result from increased arginase activity, which can attenuate endogenous nitric oxide (NO) production. We therefore evaluated whether arginase inhibition modulates these heat-loss responses in young (n = 9, 23 ± 3 yr) and older (n = 9, 66 ± 6 yr) men during two 30-min bouts of moderate-intensity cycling (Ex1 and Ex2) in the heat (35°C). CVC and SR were measured at forearm skin sites perfused with 1) lactated Ringer's (control), 2) N^G-nitro-_L-arginine methyl ester (L-NAME; NO synthase-inhibited), or 3) N^ω-hydroxy-nor-arginine and S-(2-boronoethyl)-l-cysteine (Nor-NOHA + BEC; arginase-inhibited). In both groups, CVC was reduced at L-NAME relative to control and Nor-NOHA + BEC (both P < 0.01). Likewise, SR was attenuated with L-NAME compared with control and Nor-NOHA + BEC during each exercise bout in the young men (all P ≤ 0.05); however, no influence of treatment on SR in the older men was observed (P = 0.14). Based on these findings, we then evaluated responses in 7 older men (64 ± 7 yr) during passively induced elevations in esophageal temperature (∆T_es) equal to those in Ex1 (0.6°C) and Ex2 (0.8°C). L-NAME reduced CVC by 18 ± 20% CVC_max at a ∆T_es of 0.8°C (P = 0.03) compared with control, whereas Nor-NOHA + BEC augmented CVC by 20 ± 18% CVC_max, on average, throughout heating (both P ≤ 0.03). SR was not influenced by either treatment (P = 0.80) Thus, arginase inhibition does not modulate CVC or SR during exercise in the heat but, consistent with previous findings, does augment CVC in older men during passive heating. NEW & NOTEWORTHY In the current study, we demonstrate that local arginase inhibition does not influence forearm cutaneous vasodilatory and sweating responses in young or older men during exercise-heat stress. Consistent with previous findings, however, we observed augmented cutaneous blood flow with arginase inhibition during whole-body passive heat stress. Thus, arginase differentially affects cutaneous vasodilation depending on the mode of heat stress but does not influence sweating during exercise or passive heating.

Seven consecutive days of remote ischaemic preconditioning improves cutaneous vasodilatory capacity in young adults

KEY POINTS

Remote ischaemic preconditioning (RIPC), induced by brief bouts of ischaemia followed by reperfusion, confers vascular adaptations that protect against subsequent bouts of ischaemia; however, the effect of RIPC repeated over several days on the human microcirculation is unknown. Using skin as a model, microvascular function was assessed at a control and a NO-inhibited area of skin before 1 day after and 1 week after administering seven consecutive days of repeated RIPC on the contralateral arm. Maximal vasodilatation was increased by ∼20-50% following 7 days of repeated RIPC, and this response remained elevated 1 week after stopping RIPC; however, NO-mediated vasodilatation was not affected by the RIPC stimulus. These data indicate that repeated RIPC augments maximal vasodilatation, but the underlying mechanism for this improvement is largely independent of NO. This finding suggests a role for other endothelium-derived mediators and/or for endothelium-independent adaptations with repeated RIPC.

ABSTRACT

Remote ischaemic preconditioning (RIPC), induced by intermittent periods of ischaemia followed by reperfusion, confers cardiovascular protection from subsequent ischaemic bouts. RIPC increases conduit and resistance vessel function; however, the effect of RIPC on the microvasculature remains unclear. Using human skin as a microvascular model, we hypothesized that cutaneous vasodilatory (VD) function elicited by localized heating would be increased following repeated RIPC. Ten participants (23 ± 1 years, 6 males, 4 females) performed RIPC for seven consecutive days. Each daily RIPC session consisted of 4 repetitions of 5 min of arm blood flow occlusion interspersed by 5 min reperfusion. Before, 1 day after and 1 week after the 7 days of RIPC, two microdialysis fibres were placed in ventral forearm skin for continuous infusion of Ringer solution or 20 mM l-NAME. Red blood cell flux was measured by laser Doppler flowmetry at each fibre site during local heating (T_loc  = 39°C) and during maximal VD elicited by heating (T_loc  = 43°C) and 28 mM sodium nitroprusside infusion. Data were normalized to cutaneous vascular conductance (flux/mmHg). Seven days of RIPC did not alter the nitric oxide (NO) contribution to the VD response to local heating (P > 0.05). However, the maximal VD was augmented (Pre: 2.5 ± 0.2, Post: 3.8 ± 0.5 flux/mmHg; P < 0.05) and remained elevated 1 week post RIPC (3.3 ± 0.4 flux/mmHg; P < 0.05). Repeated RIPC improves maximal VD but does not affect NO-mediated VD in the cutaneous microvasculature. This finding suggests that other factors may explain the vasodilatory adaptations that occur following repeated RIPC.

Age-related differences in the cutaneous vascular response to exogenous angiotensin II

Angiotensin II (ANG II) is locally produced in human skin and contributes to the reflex vasoconstriction (VC) response in aged but not young skin. We hypothesized that the exogenous ANG II-mediated VC response would be greater in older adults and would be affected by inhibition of adrenoreceptor or ANG II type II receptor (AT₂R) pathways. Three microdialysis (MD) fibers were placed in the forearm skin of 11 young (26 ± 3 yr) and 11 older (68 ± 4 yr) individuals for perfusion of 1) Ringer solution (control), 2) adrenoreceptor blockade with yohimbine + propranolol, and 3) AT₂R inhibition with PD-123319. ANG II was then added to the perfusates at eight graded dose concentrations ranging from 10^-10 to 10^-3 M. Laser Doppler flux was measured at each MD site, and cutaneous vascular conductance (CVC) was calculated as CVC =  laser Doppler flux/mean arterial pressure and normalized to baseline CVC values collected before ANG II perfusion (%ΔCVC_baseline). At the control site, older adults (-34 ± 4%ΔCVC_baseline) exhibited a greater peak VC compared with young adults (-22 ± 2%ΔCVC_baseline, P < 0.05), which was attenuated with adrenoreceptor blockade. Young skin exhibited a vasodilation in response to lower ANG II doses that was inhibited with AT₂R inhibition. AT₂R inhibition also increased the VC response to higher ANG II doses such that young skin responded similarly to older skin. These results indicate that ANG II has a greater VC influence in older than young individuals. Furthermore, ANG II may be affecting multiple targets, including adrenergic and AT₂R pathways. NEW & NOTEWORTHY Intradermal perfusion of successive doses of angiotensin II (ANG II) revealed a role for ANG II type II receptors and dose-dependent, ANG II-mediated vasodilation in young but not older adults. In contrast, older adults exhibited greater vasoconstriction for a given dose of ANG II. The increased vasoconstriction in older adults was subsequently blunted with adrenoreceptor blockade, which indicates an interaction between ANG II and adrenergic signaling pathways in the cutaneous microcirculation.

Acral coldness - severely reduced blood flow to fingers and toes

The term acral coldness is used to describe physiologic or pathologic situations in humans where the fingers and toes are exceptionally cold in spite of normal central body temperature. In the thermoneutral zone, the blood flow to acral skin normally shows large fluctuations between high and low values, with a frequency of about 3 cycles per minute. At an acral skin temperature of about 21°C, finger blood flow is constantly low. At lower temperatures the fingers and toes become painful. This is a normal physiologic reaction, probably because of ischemia. The characteristics of the most frequent acral vascular syndromes, Raynaud phenomenon, acrocyanosis, and chilblains, are discussed. Common to all three is pathologically low blood flow and disappearance of physiologic fluctuations even in the thermoneutral zone. Ischemic vascular diseases in acral skin are usually diagnosed from clinical observations. Measurements of fluctuating blood flow by laser or ultrasound Doppler could be useful, but should be carried out at a room temperature of 24-25°C.

Early and Late Cardiovascular and Metabolic Responses to Mixed Wine: Effect of Drink Temperature

Aim: Red wine is usually ingested as an unmixed drink. However, mixtures of wine with juices and/or sucrose (mixed wine) are becoming more and more popular and could be ingested at either cold or hot temperature. Although the temperature effects on the cardiovascular system have been described for water and tea, with greater energy expenditure (EE) and lower cardiac workload with a colder drink, little information is available on the impact of temperature of alcoholic beverages on alcoholemia and cardiometabolic parameters. The purpose of the present study was to compare the acute cardiovascular and metabolic changes in response to mixed wine ingested at a cold or at a hot temperature.

Methods: In a randomized crossover design, 14 healthy young adults (seven men and seven women) were assigned to cold or hot mixed wine ingestion. Continuous cardiovascular, metabolic, and cutaneous monitoring was performed in a comfortable sitting position during a 30-min baseline and for 120 min after ingesting 400 ml of mixed wine, with the alcohol content adjusted to provide 0.4 g ethanol/kg of body weight and drunk at either cold (3°C) or hot (55°C) temperature. Breath alcohol concentration was measured intermittently throughout the study.

Results: Overall, alcoholemia was not altered by drink temperature, with a tendency toward greater values in women compared to men. Early responses to mixed wine ingestion (0–20 min) indicated that cold drink transiently increased mean blood pressure (BP), cardiac vagal tone, and decreased skin blood flow (SkBf) whereas hot drink did not change BP, decreased vagal tone, and increased SkBf. Both cold and hot mixed wine led to increases in EE and reductions in respiratory quotient. Late responses (60–120 min) led to similar cardiovascular and metabolic changes at both drink temperatures.

Conclusion: The magnitude and/or the directional change of most of the study variables differed during the first 20 min following ingestion and may be related to drink temperature. By contrast, late changes in cardiometabolic outcomes were similar between cold and hot wine ingestion, underlying the typical effect of alcohol and sugar intake on the cardiovascular system.

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.

Acute ultraviolet radiation exposure attenuates nitric oxide-mediated vasodilation in the cutaneous microvasculature of healthy humans

5-methyltetrahydrofolate (5-MTHF) is important for nitric oxide (NO)-mediated cutaneous microvascular vasodilation. Ultraviolet B (UVB) radiation may deplete 5-MTHF, either directly or via production of reactive oxygen species (ROS), decreasing NO-mediated vasodilation. We hypothesized that 1) acute UVB exposure would attenuate NO-dependent cutaneous vasodilation, 2) local perfusion of 5-MTHF or ascorbate (ASC; anti-oxidant) would augment NO-dependent vasodilation after UVB, and 3) darker skin pigmentation would be UVB-protective. Three intradermal microdialysis fibers placed in each forearm of 21 healthy young adults (23±1 yr; 8M/13F) locally delivered lactated Ringer's (control), 5mM 5-MTHF, or 10mM ASC. One arm was UVB-exposed (300mJ/cm²), the other served as non-exposed control (CON). Following UVB exposure, a standardized local heating (42˚C) protocol induced cutaneous vasodilation. After attaining a plateau blood flow, 15mM N^G-nitro-L-arginine methyl ester (L-NAME; nitric oxide synthase inhibiter) was infused at all sites to quantify the NO contribution. Red cell flux was measured at each site by laser-Doppler flowmetry (LDF) and cutaneous vascular conductance (CVC=LDF/MAP) was expressed as a percentage of maximum (%CVC_max; 28mM sodium nitroprusside+43°C). UVB attenuated NO-mediated vasodilation compared to CON (23.1±3.6 vs 33.9±3.4 %; p=0.001). Delivery of 5-MTHF or ASC improved NO-mediated vasodilation versus lactated Ringer's in the UVB-exposed arm (MTHF: 30.1±4.8 vs 23.1±3.8 %; p=0.03; ASC: 30.9±4.3 vs 23.1±3.8 %; p=0.02). Neither treatment affected the response in the non-exposed arm (p≥0.09). Skin pigmentation (M-index) was not predictive of the UVB response (p≥0.34). These data suggest that acute UVB exposure attenuates NO-mediated vasodilation via direct and/or ROS-induced reductions in 5-MTHF, independent of skin pigmentation.

Effect of localized microclimate heating on peripheral skin temperatures and manual dexterity during cold exposure

Reduced dexterity is a major problem in cold weather, with a need for a countermeasure that increases hand (T_hand) and finger (T_fing) temperatures and improves dexterity. The purpose of this study was to determine whether electric heat (set point, 42°C) applied to the forearm (ARM, 82 W), face (FACE, 9.2 W), or combination of both (COMB, 91.2 W), either at the beginning of cold exposure (COLD; 0.5°C, 120 min; 2 clo insulation, seated, bare-handed) or after T_fing fell to 10.5°C [delayed trials (D)], improves T_hand, T_fing, dexterity, and finger key pinch strength (S_fing). Volunteers ( n = 8; 26 ± 9 yr) completed 7 experimental trials in COLD: ARM, ARM-D, FACE, FACE-D, COMB, COMB-D, and no heating (CON). Temperatures were measured before (BASE) and throughout COLD. Tests of dexterity [Purdue Pegboard assembly (PP) and magazine loading (MAGLOAD)] and S_fing were measured at BASE and after 45 and 90 min of COLD. Data presented are at minute 90. T_hand was warmer ( P < 0.001) during ARM (18.0 ± 2.6°C) and COMB (18.9 ± 2.0°C) versus CON (15.3 ± 1.5°C) and FACE (15.8 ± 1.5°C) for heating that was initiated at the beginning of COLD. T_fing was higher ( P < 0.04) during COMB (12.7 ± 5.1°C) versus CON (9.7 ± 2.1°C) and FACE (8.9 ± 2.2°C). The change from BASE for PP (no. of pieces) was less ( P < 0.005) in COMB (-4.5 ± 3.3) and ARM (-5.0 ± 6.0) versus CON (-13.0 ± 7.3) and FACE (-10.0 ± 8.3), and for MAGLOAD, it tended ( P = 0.06) to be less in COMB (-8.9 ± 6.2 cartridges) versus CON (-14.8 ± 3.7 cartridges). There was no change in S_fing from BASE (10.5 kg) to minute 90 in ARM or COMB (0.7 ± 1.4 and -0.2 ± 1.7 kg, respectively) but a decrease ( P < 0.01) in CON and FACE (-2.1 ± 2.0 and -1.6 ± 1.9 kg, respectively). There were no differences in T_hand, T_fing, dexterity, and S_fing at minute 90 when comparing heating that was initiated at the beginning of COLD versus delayed heating. In conclusion, heating using either COMB or ARM, compared with CON and FACE, improved T_hand and T_fing and reduced the decline in dexterity by 20%-50% and S_fing by 90%. Furthermore, delayed heating had no deleterious effect on T_hand, T_fing, dexterity, and S_fing compared with heating that started at the beginning of cold exposure. NEW & NOTEWORTHY The present study demonstrated that, during sedentary cold air exposure, localized heating that was applied from the beginning of cold exposure on the forearm increases hand and finger temperatures and finger strength, leading to subsequent improvements in manual dexterity. In addition, localized heating that was delayed until finger temperatures cooled significantly also caused higher peripheral temperatures, leading to better strength and manual dexterity, compared with no heating.

[Effects of Frankincense and Myrrh essential oil on transdermal absorption in vitro of Chuanxiong and penetration mechanism of skin blood flow]

The aim of this paper was to explore the effects of Frankincense and Myrrh essential oil on transdermal absorption in vitro of Chuanxiong, and to investigate the possible penetration mechanism of their essential oil from the perspective of skin blood perfusion changes. Transdermal tests were performed in vitro with excised mice skin by improved Franz diffusion cells. The cumulative penetration amounts of ferulic acid in Chuanxiong were determined by HPLC to investigate the effects of Frankincense and Myrrh essential oil on transdermal permeation properties of Chuanxiong. Simultaneously, the skin blood flows were determined by laser flow doppler. The results showed that the cumulative penetration amount of ferulic acid in Chuanxiong was (8.13±0.76) μg•cm⁻² in 24 h, and was (48.91±4.87), (57.80±2.86), (63.34±4.56), (54.17±4.40), (62.52±7.79) μg•cm⁻² respectively in Azone group, Frankincense essential oil group, Myrrh essential oil, frankincense and myrrh singly extracted essential oil mixture group, and frankincense and myrrh mixed extraction essential oil group. The enhancement ratios of each essential oil groups were 7.68, 8.26, 7.26, 8.28, which were slightly greater than 6.55 in Azone group. In addition, as compared with the conditions before treatment, there were significant differences and obvious increasing trend in blood flow of rats in Frankincense essential oil group, Myrrh essential oil group, frankincense and myrrh singly extracted essential oil mixture group, and frankincense and myrrh mixed extraction essential oil group when were dosed at 10, 20, 30, 10 min respectively, indicating that the skin blood flows were increased under the effects of Frankincense and Myrrh essential oil to a certain extent. Thus, Frankincense and Myrrh essential oil had certain effect on promoting permeability of Chuanxiong both before and after drug combination, and may promote the elimination of drugs from epidermis to dermal capillaries through increase of skin blood flow, thus enhancing the transdermal permeation amounts of drugs.

Application of Multiscale Entropy in Assessing Plantar Skin Blood Flow Dynamics in Diabetics with Peripheral Neuropathy

Diabetic foot ulcer (DFU) is a common complication of diabetes mellitus, while tissue ischemia caused by impaired vasodilatory response to plantar pressure is thought to be a major factor of the development of DFUs, which has been assessed using various measures of skin blood flow (SBF) in the time or frequency domain. These measures, however, are incapable of characterizing nonlinear dynamics of SBF, which is an indicator of pathologic alterations of microcirculation in the diabetic foot. This study recruited 18 type 2 diabetics with peripheral neuropathy and eight healthy controls. SBF at the first metatarsal head in response to locally applied pressure and heating was measured using laser Doppler flowmetry. A multiscale entropy algorithm was utilized to quantify the regularity degree of the SBF responses. The results showed that during reactive hyperemia and thermally induced biphasic response, the regularity degree of SBF in diabetics underwent only small changes compared to baseline and significantly differed from that in controls at multiple scales (p < 0.05). On the other hand, the transition of regularity degree of SBF in diabetics distinctively differed from that in controls (p < 0.05). These findings indicated that multiscale entropy could provide a more comprehensive assessment of impaired microvascular reactivity in the diabetic foot compared to other entropy measures based on only a single scale, which strengthens the use of plantar SBF dynamics to assess the risk for DFU.

Thermoregulatory dysfunction in multiple sclerosis

Multiple sclerosis (MS) is a progressive neurologic disorder that disrupts axonal myelin in the central nervous system. Demyelination produces alterations in saltatory conduction, slowed conduction velocity, and a predisposition to conduction block. An estimated 60-80% of MS patients experience temporary worsening of clinical signs and neurologic symptoms with heat exposure (Uhthoff's phenomenon). This heat intolerance in MS is related to the detrimental effects of increased temperature on action potential propagation in demyelinated axons, resulting in conduction slowing and/or block. Additionally, MS may produce impaired neural control of autonomic and endocrine functions. Isolating and interpreting mechanisms responsible for autonomic dysfunction due to MS can be difficult as it may involve sensory impairments, altered neural integration within the central nervous system, impaired effector responses, or combinations of all of these factors. MS lesions occur in areas of the brain responsible for the control and regulation of body temperature and thermoregulatory effector responses, resulting in impaired neural control of sudomotor pathways or neural-induced changes in eccrine sweat glands, as evidenced by observations of reduced sweating responses in MS patients. Although not comprehensive, some evidence exists concerning treatments (cooling, precooling, and pharmacologic) for the MS patient to preserve function and decrease symptom worsening during heat stress. This review focuses on four main themes influencing current understanding of thermoregulatory dysfunction in MS: (1) heat intolerance; (2) central regulation of body temperature; (3) thermoregulatory effector responses; and (4) countermeasures to improve or maintain function during thermal stress.

Thermoregulation and nausea

The major symptoms of motion sickness are well known and include facial pallor, nausea and vomiting, and sweating, but it is poorly recognized that they actually reflect severely perturbed thermoregulation. Thus, the purpose of this chapter is to present and discuss existing data related to this subject. While hypothermia during seasickness was first noted nearly 150 years ago, detailed studies of this phenomenon were conducted only during the last two decades. Our own research confirmed that motion sickness-induced hypothermia is quite broadly expressed phylogenetically as, besides humans, it could be provoked in several other animals (rats, musk shrews, and mice). Evidence from human and animal experiments indicates that the physiologic mechanisms responsible for the motion sickness-induced hypothermia include cutaneous vasodilation and sweating (leading to an increase of heat loss) and reduced thermogenesis. Together, these results suggest that motion sickness triggers a highly coordinated physiologic response aiming to reduce body temperature. The chapter is concluded by presenting hypotheses of how and why motion sickness evokes this hypothermic response.

Skin vasoconstriction as a heat conservation thermoeffector

Cold exposure stimulates heat production and conservation to protect internal temperature. Heat conservation is brought about via reductions in skin blood flow. The focus, here, is an exploration of the mechanisms, particularly in humans, leading to that cutaneous vasoconstriction. Local skin cooling has several effects: (1) reduction of tonic nitric oxide formation by inhibiting nitric oxide synthase and element(s) downstream of the enzyme, which removes tonic vasodilator effects, yielding a relative vasoconstriction; (2) translocation of intracellular alpha-2c adrenoceptors to the vascular smooth-muscle cell membrane, enhancing adrenergic vasoconstriction; (3) increased norepinephrine release from vasoconstrictor nerves; and (4) cold-induced vasodilation, seen more clearly in anastomoses-rich glabrous skin. Cold-induced vasodilation occurs in nonglabrous skin when nitric oxide synthase or sympathetic function is blocked. Reflex responses to general body cooling complement these local effects. Sympathetic excitation leads to the increased release of norepinephrine and its cotransmitter neuropeptide Y, each of which contributes significantly to the vasoconstriction. The contributions of these two transmitters vary with aging, disease and, in women, reproductive hormone status. Interaction between local and reflex mechanisms is in part through effects on baseline and in part through removal of the inhibitory effects of nitric oxide on adrenergic vasoconstriction.

Correlation of volumetric flow rate and skin blood flow with cold intolerance in digital replantation

Cold intolerance is a common complication of digital replantation. The exact etiology is unclear, but it is considered to be multifactorial, including nonsurgical characteristics, vascular, and neurologic conditions. Blood flow may play a significant role in cold intolerance. This study was designed to evaluate the correlation of digital blood flow, including volumetric flow rate (VFR) and skin blood flow (SkBF), with cold intolerance in replanted fingers.A retrospective study was conducted among patients who underwent digital replantation between 2010 and 2013. Patients were selected into study cohort based on the inclusion criteria. Surgical data was collected on each patient, including age, sex, injury mechanism, amputation level, ischemia time, number of arteries repaired, and whether or not vascular crisis occurred. Patients were included as study cohort with both nerves repaired and without chronic disease. Cold intolerance was defined as a Cold Intolerance Symptom Severity (CISS) score over 30. The arterial flow velocity and caliber were measured by Color Doppler Ultrasound and the digital VFR was calculated. The SkBF was measured by Laser Speckle Imager. Both VFR and SkBF were calculated as a percentage of the contralateral fingers. Comparative study of surgical data and blood flow was performed between the patient with and without cold intolerance. Correlation between VFR and SkBF was also analyzed.A total of 93 patients met inclusion criteria for the study. Approximately, 42 patients were identified as having cold intolerance. Fingers that survived vascular crisis had a higher incidence of cold intolerance with a lower VFR and SkBF. The VFR was higher in 2-artery replantation, but the SkBF and incidence of cold intolerance did not differ significantly. No differences were found in age, sex, injury mechanism, amputation level, or ischemia time. Furthermore, no correlation was found between VFR and SkBF.Cold intolerance of digital replantation is associated with decreased SkBF and VFR in the replanted fingers, which survived vascular crisis. Further work will be focused on how vascular crisis cause the decreasing of SkBF and VFR and the increasing chance of cold intolerance.

Behavioral thermoregulation in older adults with cardiovascular co-morbidities

We tested the hypotheses that older adults with cardiovascular co-morbidities will demonstrate greater changes in body temperature and exaggerated changes in blood pressure before initiating thermal behavior. We studied twelve healthy younger adults (Younger, 25 ± 4 y) and six older adults ('At Risk', 67 ± 4 y) taking prescription medications for at least two of the following conditions: hypertension, type II diabetes, hypercholesterolemia. Subjects underwent a 90-min test in which they voluntarily moved between cool (18.1 ± 1.8°C, RH: 29 ± 5%) and warm (40.2 ± 0.3°C, RH: 20 ± 0%) rooms when they felt 'too cool' (C→W) or 'too warm' (W→C). Mean skin and intestinal temperatures and blood pressure were measured. Data were analyzed as a change from pretest baseline. Changes in mean skin temperature were not different between groups at C→W (Younger: +0.2 ± 0.8°C, 'At Risk': +0.7 ± 1.8°C, P = 0.51) or W→C (Younger: +2.7 ± 0.6°C, 'At Risk': +2.9 ± 1.9°C, P = 0.53). Changes in intestinal temperature were not different at C→W (Younger: 0.0 ± 0.1°C, 'At Risk': +0.1 ± 0.2, P = 0.11), but differed at W→C (-0.1 ± 0.2°C vs. +0.1 ± 0.3°C, P = 0.02). Systolic pressure at C→W increased (Younger: +10 ± 9 mmHg, 'At Risk': +24 ± 17 mmHg) and at W→C decreased (Younger: -4 ± 13 mmHg, 'At Risk': -23 ± 19 mmHg) to a greater extent in 'At Risk' (P ≤ 0.05). Differences were also apparent for diastolic pressure at C→W (Younger: -2 ± 4 mmHg, 'At Risk': +17 ± 23 mmHg, P < 0.01), but not at W→C (Younger Y: +4 ± 13 mmHg, 'At Risk': -1 ± 6 mmHg, P = 0.29). Despite little evidence for differential control of thermal behavior, the initiation of behavior in 'at risk' older adults is preceded by exaggerated blood pressure responses.

The Effect of Being Aerobically Active vs. Inactive on Cutaneous Vascular Conductance during Local Heat Stress in an Older Population

Objective: To test the hypothesis that long- term aerobically trained elderly individuals have a greater amount of bioavailable nitric oxide (NO) and have a larger cutaneous vasodilation during local heat stress compared to their inactive elderly counterparts.

Methods: Eight aerobically trained and 8 inactive older men (>60 years old) participated in this study. NO bioavailability in blood and intradermal dialysate were measured with an ozone based chemiluminescence NO analyzer. Cutaneous vasodilator response to local heating was obtained using laser Doppler velocimetry.

Results: Whole blood NO were similar in older- trained and inactive subjects (0.75 ± 0.56 and 0.38 ± 0.32 μM, respectively; Mann–Whitney, p = 0.153), as was intradermal dialysate NO before (7.82 ± 6.32 and 4.18 ± 1.89 μM, respectively) and after local heating (7.16 ± 6.27 and 5.88 ± 3.97 μM, respectively, p = 0.354). The cutaneous vasodilator response of the older- inactive group was smaller than the older- trained group [Group-Time interaction, F_{(24, 264)} = 12.0, p < 0.0001]. When compared to a young group the peak vasodilator response of the older- trained subjects was similar. However, the time to initial dilation was 3.1 and 2.2 times longer (p < 0.05) in older- inactive and older- trained subjects, respectively, compared to young subjects.

Conclusions: Our data support the hypothesis that the age-related reductions in cutaneous vasodilation can possibly be restored by maintaining an aerobic training regimen (at least 3 years). However, some residual effects of aging remain, specifically a delayed cutaneous vasodilator response to local heating is still present in active older adults. We found no evidence for an increase in systemic or local NO-bioavailability with an extended commitment to aerobic fitness.

Application of intermittent negative pressure on the lower extremity and its effect on macro- and microcirculation in the foot of healthy volunteers

Intermittent negative pressure (INP) applied to the lower leg and foot may increase peripheral circulation. However, it is not clear how different patterns of INP affect macro‐ and microcirculation in the foot. The aim of this study was therefore to determine the effect of different patterns of negative pressure on foot perfusion in healthy volunteers. We hypothesized that short periods with INP would elicit an increase in foot perfusion compared to no negative pressure. In 23 healthy volunteers, we continuously recorded blood flow velocity in a distal foot artery, skin blood flow, heart rate, and blood pressure during application of different patterns of negative pressure (−40 mmHg) to the lower leg. Each participant had their right leg inside an airtight chamber connected to an INP generator. After a baseline period at atmospheric pressure, we applied four different 120 sec sequences with either constant negative pressure or different INP patterns, in a randomized order. The results showed corresponding fluctuations in blood flow velocity and skin blood flow throughout the INP sequences. Blood flow velocity reached a maximum at 4 sec after the onset of negative pressure (average 44% increase above baseline, P < 0.001). Skin blood flow and skin temperature increased during all INP sequences (P < 0.001). During constant negative pressure, average blood flow velocity, skin blood flow, and skin temperature decreased (P < 0.001). In conclusion, we observed increased foot perfusion in healthy volunteers after the application of INP on the lower limb.

Orderly recruitment of thermoeffectors in resting humans

The recruitment of thermoeffectors, including thermoregulatory behavior, relative to changes in body temperature has not been quantified in humans. We tested the hypothesis that changes in skin blood flow, behavior, and sweating or metabolic rate are initiated with increasing changes in mean skin temperature (T_skin) in resting humans. While wearing a water-perfused suit, 12 healthy young adults underwent heat (Heat) and cold stress (Cold) that induced gradual changes in T_skin. Subjects controlled the temperature of their dorsal neck to their perceived thermal comfort. Thus neck skin temperature provided an index of thermoregulatory behavior. Neck skin temperature (T_skin), core temperature (T_core), metabolic rate, sweat rate, and nonglabrous skin blood flow were measured continually. Data were analyzed using segmental regression analysis, providing an index of thermoeffector activation relative to changes in T_skin. In Heat, increases in skin blood flow were observed with the smallest elevations in T_skin ( P < 0.01). Thermal behavior was initiated with an increase in T_skin of 2.4 ± 1.3°C (mean ± SD, P = 0.04), while sweating was observed with further elevations in T_skin (3.4 ± 0.5°C, P = 0.04), which coincided with increases in T_core ( P = 0.98). In Cold, reductions in skin blood flow occurred with the smallest decrease in T_skin ( P < 0.01). Thermal behavior was initiated with a T_skin decrease of 1.5 ± 1.3°C, while metabolic rate ( P = 0.10) and T_core ( P = 0.76) did not change throughout. These data indicate that autonomic and behavioral thermoeffectors are recruited in coordination with one another and likely in an orderly manner relative to the comparative physiological cost.

Muscle oxygen saturation increases during head-up tilt-induced (pre)syncope

AIM

To evaluate whether muscle vasodilatation plays a role for hypotension developed during central hypovolaemia, muscle oxygenation (S_m O₂ ) was examined during (pre)syncope induced by head-up tilt (HUT). Skin blood flow (SkBF) and oxygenation (S_skin O₂ ) were determined because evaluation of S_m O₂ may be affected by superficial tissue oxygenation. Furthermore, we evaluated cerebral oxygenation (S_c O₂ ) and middle cerebral artery mean blood flow velocity (MCAv_mean ).

METHODS

Twenty healthy male volunteers (median age 24 years; range 19-38) were subjected to passive 50° HUT for 1 h or until (pre)syncope. S_c O₂ and S_m O₂ (near-infrared spectroscopy), MCAv_mean (transcranial Doppler) along with mean arterial pressure (MAP), heart rate (HR), stroke volume (SV), cardiac output (CO) and total peripheral resistance (TPR) (Modelflow^® ) were determined.

RESULTS

(Pre)syncopal symptoms appeared in 17 subjects after 11 min (median; range 2-34) accompanied by a decrease in MAP, SV, CO and TPR, while HR remained elevated. During (pre)syncope, S_c O₂ decreased [73% (71-76; mean and 95% CI) to 68% (65-71), P < 0.0001] along with MCAv_mean [40 (37-43) to 32 (29-35) cm s^-1 , P < 0.0001]. In contrast, S_m O₂ increased [63 (56-69)% to 71% (65-78), P < 0.0001], while S_skin O₂ [64% (58-69) to 53% (47-58), P < 0.0001] and SkBF [71 (44-98) compared to a baseline of 99 (72-125) units, P = 0.020] were reduced.

CONCLUSION

We confirm that the decrease in MAP during HUT is associated with a reduction in indices of cerebral perfusion. (Pre)syncope was associated with an increase in S_m O₂ despite reduced S_skin O₂ and SkBF, supporting that muscle vasodilation plays an important role in the circulatory events leading to hypotension during HUT.