Effects of artificially-induced anaemia on sudomotor and cutaneous blood flow responses to heat stress

The relationship between hemoglobin and [Formula: see text]: A systematic review and meta-analysis

OBJECTIVE

There is widespread agreement about the key role of hemoglobin for oxygen transport. Both observational and interventional studies have examined the relationship between hemoglobin levels and maximal oxygen uptake ([Formula: see text]) in humans. However, there exists considerable variability in the scientific literature regarding the potential relationship between hemoglobin and [Formula: see text]. Thus, we aimed to provide a comprehensive analysis of the diverse literature and examine the relationship between hemoglobin levels (hemoglobin concentration and mass) and [Formula: see text] (absolute and relative [Formula: see text]) among both observational and interventional studies.

METHODS

A systematic search was performed on December 6th, 2021. The study procedures and reporting of findings followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Article selection and data abstraction were performed in duplicate by two independent reviewers. Primary outcomes were hemoglobin levels and [Formula: see text] values (absolute and relative). For observational studies, meta-regression models were performed to examine the relationship between hemoglobin levels and [Formula: see text] values. For interventional studies, meta-analysis models were performed to determine the change in [Formula: see text] values (standard paired difference) associated with interventions designed to modify hemoglobin levels or [Formula: see text]. Meta-regression models were then performed to determine the relationship between a change in hemoglobin levels and the change in [Formula: see text] values.

RESULTS

Data from 384 studies (226 observational studies and 158 interventional studies) were examined. For observational data, there was a positive association between absolute [Formula: see text] and hemoglobin levels (hemoglobin concentration, hemoglobin mass, and hematocrit (P<0.001 for all)). Prespecified subgroup analyses demonstrated no apparent sex-related differences among these relationships. For interventional data, there was a positive association between the change of absolute [Formula: see text] (standard paired difference) and the change in hemoglobin levels (hemoglobin concentration (P<0.0001) and hemoglobin mass (P = 0.006)).

CONCLUSION

These findings suggest that [Formula: see text] values are closely associated with hemoglobin levels among both observational and interventional studies. Although our findings suggest a lack of sex differences in these relationships, there were limited studies incorporating females or stratifying results by biological sex.

Revisiting the dermatomal recruitment of, and pressure-dependent influences on, human eccrine sweating

Herein we describe two experiments in which the recruitment and pressure-induced modifications of human eccrine sweating were investigated. In one experiment, the longstanding belief that glandular recruitment follows a gradual, caudal-to-rostral (dermatomal) recruitment pattern was re-evaluated. The onset of sweating was simultaneously determined (ventilated capsules) from four spinal (dermatomal) segments (forehead, dorsal hand, lower chest and dorsal foot) during the passive heating of supine participants (N = 8). No evidence was found to support either dermatomal or simultaneous glandular recruitment patterns. Instead, the results were more consistent with individualised (random) patterns of regional activation (P > 0.05), with significant time delays among sites. Such delays in the appearance of discharged sweat may reflect differences in neurotransmitter sensitivity, precursor sweat production or ductal reabsorption. In the second experiment, the pressure-induced hemihidrotic reflex (contralateral sudomotor enhancement) was revisited, using pressures applied over 10 cm² areas of the chest (left side: 6 N cm^-2) and left heel (3 N cm^-2) during both supine and seated postures (N = 12). Participants were passively heated and thermally clamped before pressure application. Hemihidrosis was not observed from the contralateral surfaces within the same (chest) or lower spinal segments (abdomen; both P > 0.05) during chest pressure, but a generalised enhancement followed heel pressure when supine. We suggest that previous observations of hemihidrosis possibly resulted from elevated heat storage, rather than a neural reflex. Chest pressure significantly inhibited ipsilateral sweating (forehead, hand, chest; all P < 0.05), and that influence is hypothesised to result from interactions between ascending mechanoreceptor afferents and the descending sudomotor pathways.

Variations in body morphology explain sex differences in thermoeffector function during compensable heat stress

NEW FINDINGS

What is the central question of this study? Can sex-related differences in cutaneous vascular and sudomotor responses be explained primarily by variations in the ratio between body surface area and mass during compensable exercise that elicits equivalent heat-loss requirements and mean body temperature changes across participants? What is the main finding and its importance? Mass-specific surface area was a significant determinant of vasomotor and sudomotor responses in men and women, explaining 10-48% of the individual thermoeffector variance. Nonetheless, after accounting for changes in mean body temperature and morphological differences, sex explained only 5% of that inter-individual variability. It was concluded that sex differences in thermoeffector function are morphologically dependent, but not sex dependent. Sex is sometimes thought to be an independent modulator of cutaneous vasomotor and sudomotor function during heat exposure. Nevertheless, it was hypothesized that, when assessed during compensable exercise that evoked equal heat-loss requirements across participants, sex differences in those thermoeffectors would be explained by variations in the ratio between body surface area and mass (specific surface area). To evaluate that possibility, vasomotor and sudomotor functions were assessed in 60 individuals (36 men and 24 women) with widely varying (overlapping) specific surface areas (range, 232.3-292.7 and 241.2-303.1 cm²  kg^-1 , respectively). Subjects completed two trials in compensable conditions (28°C, 36% relative humidity) involving rest (20 min) and steady-state cycling (45 min) at fixed, area-specific metabolic heat-production rates (light, ∼135 W m^-2 ; moderate, ∼200 W m^-2 ). Equivalent heat-loss requirements and mean body temperature changes were evoked across participants. Forearm blood flow and vascular conductance were positively related to specific surface area during light work in men (r = 0.67 and r = 0.66, respectively; both P < 0.05) and during both exercise intensities in women (light, r = 0.57 and r = 0.69; and moderate, r = 0.64 and r = 0.68; all P < 0.05). Whole-body and local sweat rates were negatively related to that ratio (correlation coefficient range, -0.33 to -0.62; all P < 0.05) during both work rates in men and women. Those relationships accounted for 10-48% of inter-individual thermoeffector variance (P < 0.05). Furthermore, after accounting for morphological differences, sex explained no more than 5% of that variability (P < 0.05). It was concluded that, when assessed during compensable exercise, sex differences in thermoeffector function were largely determined morphologically, rather than being sex dependent.

Non-thermal modulation of sudomotor function during static exercise and the impact of intensity and muscle-mass recruitment

Aim: Static muscle activation elicits intensity-dependent, non-thermal sweating that is presumably controlled by feedforward (central command) mechanisms. However, it is currently unknown how the size of the recruited muscle mass interacts with that mechanism. To investigate the possible muscle-size dependency of that non-thermal sweating, the recruitment of two muscle groups of significantly different size was investigated in individuals within whom steady-state thermal sweating had been established and clamped. Methods: Fourteen passively heated subjects (climate chamber and water-perfusion garment) performed 60-s, static handgrip and knee-extension activations at 30% and 50% of maximal voluntary force, plus a handgrip at 40% intensity (143.4 N) and a third knee extension at the same absolute force. Local sweating from four body segments (averaged to represent whole-body sudomotor activity), three deep-body and eight skin temperatures, heart rates and perceptions of physical effort were measured continuously, and analyzed over the final 30 s of exercise. Results: In the presence of thermal clamping and low-level, steady-state sweating, static muscle activation resulted in exercise-intensity dependent changes in the whole-body sudomotor response during these handgrip and knee-extension actions (P < 0.05). However, there was no evidence of a dependency on the size of the recruited muscle mass (P > 0.05), yet both dependencies were apparent for heart rate, and partially evident for the sensations of physical effort. Conclusion: These observations represent the first evidence that exercise-related sudomotor feedforward is not influenced by the size of the activated muscle mass, but is instead primarily dictated by the intensity of the exercise itself.

Morphological dependency of cutaneous blood flow and sweating during compensable heat stress when heat-loss requirements are matched across participants

Human heat loss is thought, in part, to be morphologically related. It was therefore hypothesized that when heat-loss requirements and body temperatures were matched, that the mass-specific surface area alone could significantly explain both cutaneous vascular and sudomotor responses during compensable exercise. These thermoeffector responses were examined in 36 men with widely varying mass-specific surface areas (range, 232.3-292.7 cm(2)/kg), but of similar age, aerobic fitness, and adiposity. Subjects completed two trials under compensable conditions (28.1°C, 36.8% relative humidity), each involving rest (20 min) and steady-state cycling (45 min) at two matched metabolic heat-production rates (light, ∼135 W/m(2); moderate, ∼200 W/m(2)). Following equivalent mean body temperature changes, forearm blood flow and vascular conductance (r = 0.63 and r = 0.65) shared significant, positive associations with the mass-specific surface area during light work (P < 0.05), explaining ∼45% of the vasomotor variation. Conversely, during light and moderate work, whole body sweat rate, as well as local sweat rate and sudomotor sensitivity at three of four measured sites, revealed moderate, negative relationships with the mass-specific surface area (correlation coefficient range -0.37 to -0.73, P < 0.05). Moreover, those relationships could uniquely account for between 10 and 53% of those sweating responses (P < 0.05). Therefore, both thermoeffector responses displayed a significant morphological dependency in the presence of equivalent thermoafferent drive. Indeed, up to half of the interindividual variation in these effector responses could now be explained through morphological differences and the first principles governing heat transfer.

The effect of plasma osmolality and baroreceptor loading status on postexercise heat loss responses

We examined the separate and combined effects of plasma osmolality and baroreceptor loading status on postexercise heat loss responses. Nine young males completed a 45-min treadmill exercise protocol at 58 ± 2% V̇o2 peak, followed by a 60-min recovery. On separate days, participants received 0.9% NaCl (ISO), 3.0% NaCl (HYP), or no infusion (natural recovery) throughout exercise. In two additional sessions (no infusion), lower-body negative (LBNP) or positive (LBPP) pressure was applied throughout the final 45 min of recovery. Local sweat rate (LSR; ventilated capsule: chest, forearm, upper back, forehead) and skin blood flow (SkBF; laser-Doppler flowmetry: forearm, upper back) were continuously measured. During HYP, upper back LSR was attenuated from end-exercise to 10 min of recovery by ∼0.35 ± 0.10 mg·min(-1)·cm(-2) and during the last 20 min of recovery by ∼0.13 ± 0.03 mg·min(-1)·cm(-2), while chest LSR was lower by 0.18 ± 0.06 mg·min(-1)·cm(-2) at 50 min of recovery compared with natural recovery (all P < 0.05). Forearm and forehead LSRs were not affected by plasma hyperosmolality during HYP (all P > 0.28), which suggests regional differences in the osmotic modulation of postexercise LSR. Furthermore, LBPP application attenuated LSR by ∼0.07-0.28 mg·min(-1)·cm(-2) during the last 30 min of recovery at all sites except the forehead compared with natural recovery (all P < 0.05). Relative to natural recovery, forearm and upper back SkBF were elevated during LBPP, ISO, and HYP by ∼6-10% by the end of recovery (all P < 0.05). We conclude that 1) hyperosmolality attenuates postexercise sweating heterogeneously among skin regions, and 2) baroreceptor loading modulates postexercise SkBF independently of changes in plasma osmolality without regional differences.

Does intramuscular thermal feedback modulate eccrine sweating in exercising humans?

AIM

Few investigators have considered the possibility that skeletal muscles might contain thermosensitive elements capable of modifying thermoeffector responses. In this experiment, the temporal relationships between dynamic changes in deep-body and intramuscular temperatures and eccrine sweat secretion were explored during rhythmical and reproducible variations in heat production.

METHODS

Eight subjects performed semi-recumbent cycling (25 °C) at a constant load to first establish whole-body thermal and sudomotor steady states (35 min), followed by a 24-min block of sinusoidal workload variations (three, 8-min periods) and then returning to steady-state cycling (20 min). Individual oesophageal, mean skin and intramuscular (vastus lateralis) temperatures were independently cross-correlated with simultaneously measured forehead sweat rates to evaluate the possible thermal modulation of sudomotor activity.

RESULTS

Both intramuscular and oesophageal temperatures showed strong correlations with sinusoidal variations in sweating with respective maximal cross-correlation coefficients of 0.807 (±0.044) and 0.845 (±0.035), but these were not different (P = 0.40). However, the phase delay between intramuscular temperature changes and sweat secretion was significantly shorter than the delay between oesophageal temperature and sweating [25.6 s (±12.6) vs. 46.9 s (±11.3); P = 0.03].

CONCLUSION

The temporal coupling of eccrine sweating to intramuscular temperature, combined with a shorter phase delay, was consistent with the presence of thermosensitive elements within skeletal muscles that appear to participate in the modulation of thermal sweating.

Temporal and thermal variations in site-specific thermoregulatory sudomotor thresholds: precursor versus discharged sweat production

Temporal and thermal differences between the initiation of precursor, eccrine sweat and its surface discharge were investigated during passive heating. Sudomotor activity was evaluated using electrodermal (precursor) and ventilated sweat capsule measurements (dorsal fingers, dorsal hand, forehead, forearm). Passive heating significantly elevated auditory canal (0.5 degrees C) and mean body temperatures (0.9 degrees C). At each site, the precursor sudomotor thresholds occurred at a lower mean body temperature (P < .05), with an average elevation of 0.35 degrees C (SD 0.04). However, discharged thresholds were delayed until this temperature had risen 0.53 degrees C (SD 0.04), producing significant phase delays across sites (mean: 4.1 min [SD 0.5]; P < .05). It is concluded that precise sudomotor threshold determinations require methods that respond to sweat accumulating within the secretory coil, and not discharged secretions, reinforcing the importance of electrodermal techniques.

Whole-body fluid distribution in humans during dehydration and recovery, before and after humid-heat acclimation induced using controlled hyperthermia

AIM

This experiment was designed to test the hypothesis that the plasma volume is not selectively defended during exercise- and heat-induced dehydration following humid-heat acclimation.

METHODS

Eight physically active males were heat acclimated (39.8 °C, relative humidity 59.2%) using 17 days of controlled hyperthermia (core temperature: 38.5 °C). Inter-compartmental fluid losses and movements were tracked (radioisotopes and Evans blue dye) during progressive dehydration (cycling) in these same conditions and also during a resting recovery without fluid replacement (28 °C), before (day 1), during (day 8) and after heat acclimation (day 22).

RESULTS

On days 8 and 22, there were significant increases in total body water, interstitial fluid and plasma volume (P < 0.05), but the intracellular compartments did not change (P > 0.05). The baseline plasma volume remained expanded throughout: 43.4 [±2.6 (day 1)], 49.1 [±2.4 (day 8); P < 0.05] and 48.9 mL kg(-1) [±3.0 (day 22); P < 0.05]. During progressive dehydration, plasma reductions of 9.0% (±0.9: day 1), 12.4% (±1.6: day 8) and 13.6% (±1.2: day 22) were observed, with day 8 and 22 losses significantly exceeding day 1 (P < 0.05). During recovery, plasma volume restoration commenced, with the intracellular fluid contribution becoming more pronounced as acclimation progressed.

CONCLUSION

It is concluded that the plasma volume was not defended more vigorously following humid-heat acclimation. Indeed, a greater fluid loss may well underlie the mechanisms for enhancing plasma volume recovery when heat acclimation is induced using the controlled-hyperthermia technique.

Sudomotor responses from glabrous and non-glabrous skin during cognitive and painful stimulations following passive heating

AIM

It is widely accepted that thermal and psychological sweating are independently controlled and respectively restricted to non-glabrous (hairy) and glabrous skin. These assumptions were evaluated in six experiments conducted across eight body segments, in which 38 glabrous and non-glabrous skin surfaces were investigated.

METHODS

Sweating was measured in 30 passively heated individuals using ventilated sweat capsules, with passive heating used to first establish steady-state sweating, averaging 0.30 mg cm(-2) min(-1) (±0.03) across all sites, prior to the application of cognitive and painful stimuli.

RESULTS

These non-thermal (psychological) stimulations significantly increased sweat secretion at more than 70% of the sites investigated [cognitive: 28 of 38 sites (P < 0.05); pain: 23 of 32 sites (P < 0.05)], eliciting peak sweat rates averaging 0.51 mg cm(-2) min(-1) (±0.05) and 0.47 mg cm(-2) min(-1) (±0.4 respectively) across all sites. Furthermore, non-thermal sweating was evident from both the glabrous and non-glabrous surfaces and occurred without mean body or local skin temperatures changes (P > 0.05). Indeed, neither thermal nor psychological sweating was restricted to discrete skin surfaces, and there were no consistent sudomotor differences between these two skin classifications. Finally, at no site was thermal sweating inhibited during a non-thermal stimulation.

CONCLUSION

These generalized sudomotor responses challenge the hypotheses that glabrous skin sweating is driven by psychological stimuli, and that thermal sweating is a phenomenon restricted to the non-glabrous skin surfaces.

Psychological sweating from glabrous and nonglabrous skin surfaces under thermoneutral conditions

Recent experiments revealed psychological sweating to be a ubiquitous phenomenon in passively heated individuals. Since heating potentiates sweating, and since most research into psychological sweating was not conducted in this thermal state, these observations required thermoneutral verification. Thermoneutral subjects performed mental arithmetic (at 26(o) C) with psychological sweating evaluated from nine sites (ventilated capsules, skin conductance). Discharged sweating was evident from three glabrous sites (P < .05). However, significant sweating was evident from two nonglabrous surfaces (P < .05), and skin conductance increased at the volar and dorsal finger surfaces (P < .05). Each of these changes occurred while core and skin temperatures remained stable (P > .05). These thermoneutral observations further refute the proposition that psychological sweating in humans is restricted to the glabrous skin surfaces.

Sweat secretion from the torso during passively-induced and exercise-related hyperthermia

Thermal sweating from the human torso accounts for about half of the whole-body sweat secretion, yet its intra-segmental distribution has not been thoroughly examined. Therefore, the aim of the current study was to provide a detailed description of the distribution of eccrine sweating within the torso during passively-induced (water-perfusion garment: 40 degrees C) and progressively increasing, exercise-related thermal strain (36 degrees C, 60% relative humidity). Sudomotor function was measured in ten males using ventilated sweat capsules (3.16 cm(2)) attached to twelve sites on the ventral (four), lateral (three) and dorsal (four) torso, and upper shoulder surfaces. Sweating increased asymptotically in all sites, with the final core temperature averaging 39.7 degrees C (+/-0.1) and heart rates being 181 b min(-1) (+/-2). During exercise, the mean torso sweat rate averaged 1.35 mgcm(-2)min(-1), with sweating from the lateral torso surfaces generally being the lowest. Each of the between-site comparisons with the lateral torso differed significantly (P < 0.05), except for comparisons with the chest (P = 0.051) and shoulder (P > 0.05). The intra-segmental differences between the lateral torso and the chest, abdomen, upper- and lower-back areas were significantly accentuated during exercise. From these data, it is evident that the torso is another region that does not have a uniform distribution of thermally-induced sweating. Thus, it is no longer acceptable for researchers, modellers, sweating manikins engineers or clothing manufacturers to assume that the sweat rates for all local sites within any body segment are equivalent.

Charming the mosquito: do malaria symptoms increase the attractiveness of the host for the vector?

The two week period that stretches between the absorption of gametocytes from a malaria patient by a mosquito and the subsequent injection of sporozoites in another human host is a most fragile segment of the malaria cycle. There is non-randomness in the blood meal choice of anopheline vectors. Manipulations of the host by the parasite to satisfy the preferences of the vector might have contributed to the robustness of the malaria cycle. Indeed, although gametocytes do not directly cause symptoms, the acute and chronic clinical manifestations caused by asexual forms seem to be organized around their transmission: fever patterns and behavior modification, anemia and thrombocytopenia converge towards making malarious individuals a preferred and safer source of blood for the vector than non-malarious individuals. Malaria symptoms offer the vector its favorite cues: increased skin temperature, increased lactates, sweating, and CO(2) expiration. In addition, this tempting menu seems easier to absorb because of thrombocytopenia and reduced blood viscosity during anemia, and because behavior modification reduces mosquito avoidance and the risk of dying. The attractive cues may be characteristic symptoms of malaria but also slight infraclinical changes that mostly go unnoticed by the doctor but not by the vector. The manifestations of malaria are often seen as adaptive for the host, this article argues that on average they may also be adaptive for malaria parasites. Examining malaria as the extended phenotype of Plasmodium leads to new research questions.

Human sudomotor responses to heating and cooling upper-body skin surfaces: cutaneous thermal sensitivity

The influence of local skin temperature (Tskl) on the control of local and whole-body sweating was evaluated in eight healthy males. A water-perfusion garment (37 degrees C) and a climatic chamber (36.45 +/- 0.78 degrees C; [+/- SD]; relative humidity 60.3 +/- 1.6%) were used to raise and clamp skin and core temperatures. Warm and cool stimuli were applied to four upper-body skin regions (face, arm, forearm, hand) using perfusion patches (249.0 +/- 0.2 cm2). Heating elevated, while cooling suppressed sweat rate (msw) locally, and at other skin surfaces. However, the tendency for Tskl manipulations to induce localized sweat responses was no more powerful than it was at stimulating sweating in non-treated regions (P > 0.05). Accordingly, neither thermal stimulus produced significantly greater local sudomotor influences than were elicited contralaterally (P > 0.05). No statistical support was found for the notion of inter-regional differences in upper-body cutaneous thermal sensitivity for sudomotor control, and, regardless of the stimulation site, whole-body sudomotor responses to localized thermal treatments were equivalent (P > 0.05).