An open-source machine-learning approach for obtaining high-quality quantitative wood anatomy data from E. grandis and P. radiata xylem.

Quantitative wood anatomy is a subfield in dendrochronology that requires effective open-source image analysis tools. In this research, the bioimage analysis software QuPath (v0.4.4) is introduced as a candidate for accurately quantifying the cellular properties of the xylem in an automated manner. Additionally, the potential of QuPath to detect the transition of early- to latewood tracheids over the growing season was evaluated to assess a potential application in dendroecological studies. Various algorithms in QuPath were optimized to quantify different xylem cell types in Eucalyptus grandis and the transition of early- to latewood tracheids in Pinus radiata. These algorithms were coded into cell detection scripts for automatic quantification of stem microsections and compared to a manually curated method to assess the accuracy of the cell detections. The automatic cell detection approach, using QuPath, has been validated to be reproducible with an acceptable error when assessing fibers, vessels, early- and latewood tracheids. However, further optimization for parenchyma is still required. This proposed method developed in QuPath provides a scalable and accurate approach for quantifying anatomical features in stem microsections. With minor amendments to the detection and classification algorithms, this strategy is likely to be viable in other plant species.

Interaction between beech and spruce trees in temperate forests affects water use, root water uptake pattern and canopy structure.

Beneficial and negative effects of species interactions can strongly influence water fluxes in forest ecosystems. However, little is known about how trees dynamically adjust their water use when growing with interspecific neighbours. Therefore, we investigated the interaction effects between Fagus sylvatica (European beech) and Picea abies (Norway spruce) on water-use strategies and aboveground structural characteristics. We used continuous in situ isotope spectroscopy of xylem and soil water to investigate source water dynamics and root water uptake depths. Picea abies exhibited a reduced sun-exposed crown area in equally mixed compared with spruce-dominated sites, which was further correlated to a reduction in sap flow of -14.5 ± 8.2%. Contrarily, F. sylvatica trees showed +13.3 ± 33.3% higher water fluxes in equally mixed compared with beech-dominated forest sites. Although a significantly higher crown interference by neighbouring trees was observed, no correlation of water fluxes and crown structure was found. High time-resolved xylem δ2H values showed a large plasticity of tree water use (-74.1 to -28.5‰), reflecting the δ2H dynamics of soil and especially precipitation water sources. Fagus sylvatica in equally mixed sites shifted water uptake to deeper soil layers, while uptake of fresh precipitation was faster in beech-dominated sites. Our continuous in situ water stable isotope measurements traced root water uptake dynamics at unprecedented temporal resolution, indicating highly dynamic use of water sources in response to precipitation and to neighbouring species competition. Understanding this plasticity may be highly relevant in the context of increasing water scarcity and precipitation variability under climate change.

Climate and soil effects on tree species diversity and aboveground carbon patterns in semi-arid tree savannas.

Climatic and edaphic effects are increasingly being discussed in the context of biodiversity-ecosystem functioning. Here we use data from West African semi-arid tree savannas and contrasting climatic conditions (lower vs. higher mean annual precipitation-MAP and mean annual temperature-MAT) to (1) determine how climate modulates the effects of species richness on aboveground carbon (AGC); (2) explore how species richness and AGC relate with soil variables in these contrasting climatic conditions; and (3) assess how climate and soil influence directly, and/or indirectly AGC through species richness and stand structural attributes such as tree density and size variation. We find that greater species richness is generally associated with higher AGC, but more strongly in areas with higher MAP, which also have greater stem density. There is a climate-related influence of soils on AGC, which decreases from lower to higher MAP conditions. Variance partitioning analyses and structural equation modelling show that, across all sites, MAP, relative to soils, has smaller effect on AGC, mediated by stand structural attributes whereas soil texture and fertility explain 14% of variations in AGC and influence AGC directly and indirectly via species richness and stand structural attributes. Our results highlight coordinated effects of climate and soils on AGC, which operated primarily via the mediation role of species diversity and stand structures.

Experiments and Modeling on the Stain-Controlled Time- and Temperature-Dependent Cyclic Ratchetting Plasticity of the Nickel-Based Superalloy IN100.

In this paper, the time- and temperature-dependent cyclic ratchetting plasticity of the nickel-based alloy IN100 is experimentally investigated in strain-controlled experiments in the temperature range from 300 °C to 1050 °C. To this end, uniaxial material tests are performed with complex loading histories designed to activate phenomena as strain rate dependency, stress relaxation as well as the Bauschinger effect, cyclic hardening and softening, ratchetting and recovery from hardening. Plasticity models with different levels of complexity are presented that consider these phenomena, and a strategy is derived to determine the multitude of temperature-dependent material properties of the models in a step-by-step procedure based on sub-sets of experimental data of isothermal experiments. The models and the material properties are validated based on the results of non-isothermal experiments. A good description of the time- and temperature-dependent cyclic ratchetting plasticity of IN100 is obtained for isothermal as well as non-isothermal loading with models including ratchetting terms in the kinematic hardening law and the material properties obtained with the proposed strategy.

A Temperature-Dependent Viscoplasticity Model for the Hot Work Steel X38CrMoV5-3, Including Thermal and Cyclic Softening under Thermomechanical Fatigue Loading.

In this paper, a temperature-dependent viscoplasticity model is presented that describes thermal and cyclic softening of the hot work steel X38CrMoV5-3 under thermomechanical fatigue loading. The model describes the softening state of the material by evolution equations, the material properties of which can be determined on the basis of a defined experimental program. A kinetic model is employed to capture the effect of coarsening carbides and a new isotropic cyclic softening model is developed that takes history effects during thermomechanical loadings into account. The temperature-dependent material properties of the viscoplasticity model are determined on the basis of experimental data measured in isothermal and thermomechanical fatigue tests for the material X38CrMoV5-3 in the temperature range between 20 and 650 ∘C. The comparison of the model and an existing model for isotropic softening shows an improved description of the softening behavior under thermomechanical fatigue loading. A good overall description of the experimental data is possible with the presented viscoplasticity model, so that it is suited for the assessment of operating loads of hot forging tools.

Later growth onsets or reduced growth rates: What characterises legacy effects at the tree-ring level in conifers after the severe 2018 drought?

Severe drought events negatively affect tree growth and often cause legacy effects, expressed by smaller tree rings in the post-drought recovery years. While the pattern of reduced tree-ring widths is frequently described the processes underlying such legacy effects, i.e., whether it is due to shorter growth periods or lower growth rates, remains unclear and is investigated in this study. To elucidate these post-drought effects, we examined radial stem growth dynamics monitored with precision band-dendrometers on 144 Douglas fir, Norway spruce and silver fir sample trees distributed along four elevational gradients in the Black Forest (Southwest Germany) during the post-drought years 2019 and 2020. Growth onset of all investigated species occurred between 11 and 24 days significantly earlier in 2020 compared to 2019. Modelling growth onset based on chilling and forcing units and taking the study year into account explained 88-98 % of the variance in the growth onset data. The highly significant effect of the study year (p < 0.001) led to the conclusion, that other factors than the prevailing site conditions (chilling and forcing units) must have triggered the earlier growth onset in 2020. On the other hand, for Douglas fir growth rates were significantly higher in 2020 compared to 2019 (2.9 µm d-1) and marginally significantly higher for silver fir (1.3 µm d-1), underlining the explanatory power of growth rate on recovery processes in general and suggesting that Douglas fir copes better with droughts, as it recovered faster. Growth dynamics at the beginning of the year showed limited growth for earlier growth onsets, which, however, could not explain the difference between the investigated years. Our results provide evidence that legacy effects of drought events are expressed by a delayed growth onset and a reduced growth rate in the post-drought year and that Douglas fir has a superior recovery potential.

A Multi-Criteria Assessment Strategy for 3D Printed Porous Polyetheretherketone (PEEK) Patient-Specific Implants for Orbital Wall Reconstruction.

Pure orbital blowout fractures occur within the confines of the internal orbital wall. Restoration of orbital form and volume is paramount to prevent functional and esthetic impairment. The anatomical peculiarity of the orbit has encouraged surgeons to develop implants with customized features to restore its architecture. This has resulted in worldwide clinical demand for patient-specific implants (PSIs) designed to fit precisely in the patient's unique anatomy. Material extrusion or Fused filament fabrication (FFF) three-dimensional (3D) printing technology has enabled the fabrication of implant-grade polymers such as Polyetheretherketone (PEEK), paving the way for a more sophisticated generation of biomaterials. This study evaluates the FFF 3D printed PEEK orbital mesh customized implants with a metric considering the relevant design, biomechanical, and morphological parameters. The performance of the implants is studied as a function of varying thicknesses and porous design constructs through a finite element (FE) based computational model and a decision matrix based statistical approach. The maximum stress values achieved in our results predict the high durability of the implants, and the maximum deformation values were under one-tenth of a millimeter (mm) domain in all the implant profile configurations. The circular patterned implant (0.9 mm) had the best performance score. The study demonstrates that compounding multi-design computational analysis with 3D printing can be beneficial for the optimal restoration of the orbital floor.

Diversity-biomass relationship across forest layers: implications for niche complementarity and selection effects.

Forest stratification plays a crucial role in light interception and plant photosynthetic activities. However, despite the increased number of studies on biodiversity-ecosystem function, we still lack information on how stratification in tropical forests modulates biodiversity effects. Moreover, there is less investigation and argument on the role of species and functional traits in forest layers. Here, we analysed from a perspective of forest layer (sub-canopy, canopy and emergent species layers), the relationship between diversity and aboveground biomass (AGB), focusing on functional diversity and dominance, and underlying mechanisms such as niche complementarity and selection. The sub-canopy layer had the highest species richness and diversity, while the emergent layer had the highest AGB. Species richness-AGB relationship was positive for each forest layer, but stronger for sub-canopy layer than for canopy and emergent layers. Total AGB was strongly correlated with functional diversity, leaf and wood traits of species in the sub-canopy and canopy layers. This suggests that sub-canopy and canopy species are major drivers of stand diversity-AGB relationship, and that resource filtering by canopy or emergent trees may not reduce the strength of diversity-AGB relationship in the sub-canopy layer. We argue that complementary resource use by sub-canopy species that supports niche complementarity, is a key mechanism driving AGB in natural forests. Selection effects are most evident in emergent species and niche complementarity effects for sub-canopy and canopy species, supporting arguments that AGB is affected by sub-canopy species' efficient use of limited resources despite competition from emergent species.

Potential supply of floral resources to managed honey bees in natural mistbelt forests.

Honey bees play a vital role in the pollination of flowers in many agricultural systems, while providing honey through well managed beekeeping activities. Managed honey bees rely on the provision of pollen and nectar for their survival and productivity. Using data from field plot inventories in natural mistbelt forests, we (1) assessed the diversity and relative importance of honey bee plants, (2) explored the temporal availability of honey bee forage (nectar and pollen resources), and (3) elucidated how plant diversity (bee plant richness and overall plant richness) influenced the amount of forage available (production). A forage value index was defined on the basis of species-specific nectar and pollen values, and expected flowering period. Up to 50% of the overall woody plant richness were found to be honey bee plant species, with varying flowering period. As expected, bee plant richness increased with overall plant richness. Interestingly, bee plants' flowering period was spread widely over a year, although the highest potential of forage supply was observed during the last quarter. We also found that only few honey bee plant species contributed 90 percent of the available forage. Surprisingly, overall plant richness did not significantly influence the bee forage value. Rather, bee plant species richness showed significant and greater effect. The results of this study suggest that mistbelt forests can contribute to increase the spatial and temporal availability of diverse floral resources for managed honey bees. Conservation efforts must be specifically oriented towards honey bee plant species in mistbelt forests to preserve and enhance their potential to help maintain honey bee colonies. The implications for forest management, beekeeping activities and pollination-based agriculture were discussed.

Nitrite and S-Nitrosohemoglobin Exchange Across the Human Cerebral and Femoral Circulation: Relationship to Basal and Exercise Blood Flow Responses to Hypoxia.

BACKGROUND: The mechanisms underlying red blood cell (RBC)-mediated hypoxic vasodilation remain controversial, with separate roles for nitrite () and S-nitrosohemoglobin (SNO-Hb) widely contested given their ability to transduce nitric oxide bioactivity within the microcirculation. To establish their relative contribution in vivo, we quantified arterial-venous concentration gradients across the human cerebral and femoral circulation at rest and during exercise, an ideal model system characterized by physiological extremes of O2 tension and blood flow. METHODS: Ten healthy participants (5 men, 5 women) aged 24±4 (mean±SD) years old were randomly assigned to a normoxic (21% O2) and hypoxic (10% O2) trial with measurements performed at rest and after 30 minutes of cycling at 70% of maximal power output in hypoxia and equivalent relative and absolute intensities in normoxia. Blood was sampled simultaneously from the brachial artery and internal jugular and femoral veins with plasma and RBC nitric oxide metabolites measured by tri-iodide reductive chemiluminescence. Blood flow was determined by transcranial Doppler ultrasound (cerebral blood flow) and constant infusion thermodilution (femoral blood flow) with net exchange calculated via the Fick principle. RESULTS: Hypoxia was associated with a mild increase in both cerebral blood flow and femoral blood flow (P<0.05 versus normoxia) with further, more pronounced increases observed in femoral blood flow during exercise (P<0.05 versus rest) in proportion to the reduction in RBC oxygenation (r=0.680-0.769, P<0.001). Plasma gradients reflecting consumption (arterial>venous; P<0.05) were accompanied by RBC iron nitrosylhemoglobin formation (venous>arterial; P<0.05) at rest in normoxia, during hypoxia (P<0.05 versus normoxia), and especially during exercise (P<0.05 versus rest), with the most pronounced gradients observed across the bioenergetically more active, hypoxemic, and acidotic femoral circulation (P<0.05 versus cerebral). In contrast, we failed to observe any gradients consistent with RBC SNO-Hb consumption and corresponding delivery of plasma S-nitrosothiols (P>0.05). CONCLUSIONS: These findings suggest that hypoxia and, to a far greater extent, exercise independently promote arterial-venous delivery gradients of intravascular nitric oxide, with deoxyhemoglobin-mediated reduction identified as the dominant mechanism underlying hypoxic vasodilation.

Tree species diversity promotes aboveground carbon storage through functional diversity and functional dominance.

The relationship between biodiversity and ecosystem function has increasingly been debated as the cornerstone of the processes behind ecosystem services delivery. Experimental and natural field-based studies have come up with nonconsistent patterns of biodiversity-ecosystem function, supporting either niche complementarity or selection effects hypothesis. Here, we used aboveground carbon (AGC) storage as proxy for ecosystem function in a South African mistbelt forest, and analyzed its relationship with species diversity, through functional diversity and functional dominance. We hypothesized that (1) diversity influences AGC through functional diversity and functional dominance effects; and (2) effects of diversity on AGC would be greater for functional dominance than for functional diversity. Community weight mean (CWM) of functional traits (wood density, specific leaf area, and maximum plant height) were calculated to assess functional dominance (selection effects). As for functional diversity (complementarity effects), multitrait functional diversity indices were computed. The first hypothesis was tested using structural equation modeling. For the second hypothesis, effects of environmental variables such as slope and altitude were tested first, and separate linear mixed-effects models were fitted afterward for functional diversity, functional dominance, and both. Results showed that AGC varied significantly along the slope gradient, with lower values at steeper sites. Species diversity (richness) had positive relationship with AGC, even when slope effects were considered. As predicted, diversity effects on AGC were mediated through functional diversity and functional dominance, suggesting that both the niche complementarity and the selection effects are not exclusively affecting carbon storage. However, the effects were greater for functional diversity than for functional dominance. Furthermore, functional dominance effects were strongly transmitted by CWM of maximum plant height, reflecting the importance of forest vertical stratification for diversity-carbon relationship. We therefore argue for stronger complementary effects that would be induced also by complementary light-use efficiency of tree and species growing in the understory layer.

Tree component biomass expansion factors and root-to-shoot ratio of Lebombo ironwood: measurement uncertainty.

BACKGROUND: National and regional aboveground biomass (AGB) estimates are generally computed based on standing stem volume estimates from forest inventories and default biomass expansion factors (BEFs). AGB estimates are converted to estimates of belowground biomass (BGB) using default root-to-shoot ratios (R/S). Thus, BEFs and R/S are not estimated in ordinary forest inventories, which results in uncertainty in estimates of AGB and BGB. Here, we measured BEF and R/S values (including uncertainty) for different components of Lebombo ironwood (Androstachys johnsonii Prain) trees and assessed their dependence on tree size. RESULTS: The BEF values of tree components were unrelated or weakly related to tree size, and R/S was independent of tree size. BEF values varied from 0.02 for foliage to 1.31 Mg m-3 for whole tree; measurement uncertainty (SE) varied from 2.9% for stem BEF to 10.6% for whole-tree BEF. The belowground, aboveground, and whole-tree BEF-based biomass densities were 30 ± 2.3 (SE = 3.89%), 121 ± 7.84 (SE = 3.23%), and 151 ± 9.87 Mg ha-1(SE = 3.27%), respectively. R/S was 0.24 with an uncertainty of 3.4%. CONCLUSIONS: Based on the finding of independence or weak dependence of BEF on tree size, we concluded that, for A. johnsonii, constant component BEF values can be accurately used within the interval of harvested tree sizes.

Impaired cerebral blood flow and oxygenation during exercise in type 2 diabetic patients.

Endothelial vascular function and capacity to increase cardiac output during exercise are impaired in patients with type 2 diabetes (T2DM). We tested the hypothesis that the increase in cerebral blood flow (CBF) during exercise is also blunted and, therefore, that cerebral oxygenation becomes affected and perceived exertion increased in T2DM patients. We quantified cerebrovascular besides systemic hemodynamic responses to incremental ergometer cycling exercise in eight male T2DM and seven control subjects. CBF was assessed from the Fick equation and by transcranial Doppler-determined middle cerebral artery blood flow velocity. Cerebral oxygenation and metabolism were evaluated from the arterial-to-venous differences for oxygen, glucose, and lactate. Blood pressure was comparable during exercise between the two groups. However, the partial pressure of arterial carbon dioxide was lower at higher workloads in T2DM patients and their work capacity and increase in cardiac output were only ~80% of that established in the control subjects. CBF and cerebral oxygenation were reduced during exercise in T2DM patients (P < 0.05), and they expressed a higher rating of perceived exertion (P < 0.05). In contrast, CBF increased ~20% during exercise in the control group while the brain uptake of lactate and glucose was similar in the two groups. In conclusion, these results suggest that impaired CBF and oxygenation responses to exercise in T2DM patients may relate to limited ability to increase cardiac output and to reduced vasodilatory capacity and could contribute to their high perceived exertion.

Cerebral water and ion balance remains stable when humans are exposed to acute hypoxic exercise.

BACKGROUND: Intense physical activity increases the prevalence of acute mountain sickness (AMS) that can occur within 10 h after ascent to altitudes above 1500 m and is likely related to development of cerebral edema. This study evaluated whether disturbed cerebral water and ion homeostasis can be detected when intense exercise is carried out in hypoxia and monitored the influence of muscle metabolism for changes in arterial variables. METHODS: On two separate days, in random order, 30 min cycling exercise was performed in either hypoxia (10% O2) or normoxia at an intensity that was exhaustive in the hypoxic trial (∼120 W; n=9). RESULTS: Exercise in hypoxia affected muscle metabolism, as evidenced by higher (p<0.05) leg lactate release at 7.5 min and a continuous decline in arterial pH (p<0.001) that was not observed in normoxia. Middle cerebral artery flow velocity increased (p<0.01) with exercise under both circumstances. No cerebral net exchange of Na(+) or K(+) was evident. Likewise, no significant net-exchange of water over the brain was demonstrated and the arterial and jugular venous hemoglobin concentrations were similar. CONCLUSION: Challenging exercise in hypoxia for 30 min affected muscle metabolism and increased an index of cerebral blood flow, but cerebral net water and ion homeostasis remained stable. Thus, although AMS develops within hours and may be related to exercise-induced disturbance of cerebral ion and water balance, such changes are not detectable when subjects are exposed to acute 30 min maximal exercise in hypoxia.

Systemic, cerebral and skeletal muscle ketone body and energy metabolism during acute hyper-D-ß-hydroxybutyratemia in post-absorptive healthy males.

CONTEXT: Ketone bodies are substrates during fasting and when on a ketogenic diet not the least for the brain and implicated in the management of epileptic seizures and dementia. Moreover, D-ß-hydroxybutyrate (HOB) is suggested to reduce blood glucose and fatty acid levels. OBJECTIVES: The objectives of this study were to quantitate systemic, cerebral, and skeletal muscle HOB utilization and its effect on energy metabolism. DESIGN: Single trial. SETTING: Hospital. PARTICIPANT: Healthy post-absorptive males (n = 6). INTERVENTIONS: Subjects were studied under basal condition and three consecutive 1-hour periods with a 3-, 6-, and 12-fold increased HOB concentration via HOB infusion. MAIN OUTCOME MEASURES: Systemic, cerebral, and skeletal muscle HOB kinetics, oxidation, glucose turnover, and lipolysis via arterial, jugular, and femoral venous differences in combination with stable isotopically labeled HOB, glucose, and glycerol, infusion. RESULTS: An increase in HOB from the basal 160-450 µmol/L elicited 14 ± 2% reduction (P = .03) in glucose appearance and 37 ± 4% decrease (P = .03) in lipolytic rate while insulin and glucagon were unchanged. Endogenous HOB appearance was reduced in a dose-dependent manner with complete inhibition at the highest HOB concentration (1.7 mmol/L). Cerebral HOB uptake and subsequent oxidation was linearly related to the arterial HOB concentration. Resting skeletal muscle HOB uptake showed saturation kinetics. CONCLUSION: A small increase in the HOB concentration decreases glucose production and lipolysis in post-absorptive healthy males. Moreover, cerebral HOB uptake and oxidation rates are linearly related to the arterial HOB concentration of importance for modifying brain energy utilization, potentially of relevance for patients with epileptic seizures and dementia.

Case report: (Pre)syncopal symptoms associated with a negative internal jugular venous pressure.

A siphon is suggested to support cerebral blood flow but appears not to be established because internal jugular venous (IJV) pressure is close to zero in upright humans. Thus, in eleven young healthy males, IJV pressure was 9 ± 1 mmHg (mean ± SE) when supine and fell to 3 ± 1 mmHg when seated, and middle cerebral artery mean blood velocity (MCA Vmean; P < 0.007) and the near-infrared spectroscopy-determined frontal lobe oxygenation (ScO2; P = 0.028) also decreased. Another subject, however, developed (pre)syncopal symptoms while seated and his IJV pressure decreased to -17 mmHg. Furthermore, his MCA Vmean decreased and yet within the time of observation ScO2 was not necessarily affected. These findings support the hypothesis that a negative IJV pressure that is a prerequisite for creation of a siphon provokes venous collapse inside the dura, and thereby limits rather than supports CBF.

Glycopyrrolate does not influence the visual or motor-induced increase in regional cerebral perfusion.

Acetylcholine may contribute to the increase in regional cerebral blood flow (rCBF) during cerebral activation since glycopyrrolate, a potent inhibitor of acetylcholine, abolishes the exercise-induced increase in middle cerebral artery mean flow velocity. We tested the hypothesis that cholinergic vasodilatation is important for the increase in rCBF during cerebral activation. The subjects were 11 young healthy males at an age of 24 ± 3 years (mean ± SD). We used arterial spin labeling and blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) to evaluate rCBF with and without intravenous glycopyrrolate during a handgrip motor task and visual stimulation. Glycopyrrolate increased heart rate from 56 ± 9 to 114 ± 14 beats/min (mean ± SD; p < 0.001), mean arterial pressure from 86 ± 8 to 92 ± 12 mmHg, and cardiac output from 5.6 ± 1.4 to 8.0 ± 1.7 l/min. Glycopyrrolate had, however, no effect on the arterial spin labeling or BOLD responses to the handgrip motor task or to visual stimulation. This study indicates that during a handgrip motor task and visual stimulation, the increase in rCBF is unaffected by blockade of acetylcholine receptors by glycopyrrolate. Further studies on the effect of glycopyrrolate on middle cerebral artery diameter are needed to evaluate the influence of glycopyrrolate on mean flow velocity during intense exercise.

Acute volume expansion attenuates hyperthermia-induced reductions in cerebral perfusion during simulated hemorrhage.

Hyperthermia reduces the capacity to withstand a simulated hemorrhagic challenge, but volume loading preserves this capacity. This study tested the hypotheses that acute volume expansion during hyperthermia increases cerebral perfusion and attenuates reductions in cerebral perfusion during a simulated hemorrhagic challenge induced by lower-body negative pressure (LBNP). Eight healthy young male subjects underwent a supine baseline period (pre-LBNP), followed by 15- and 30-mmHg LBNP while normothermic, hyperthermic (increased pulmonary artery blood temperature ~1.1°C), and following acute volume infusion while hyperthermic. Primary dependent variables were mean middle cerebral artery blood velocity (MCAvmean), serving as an index of cerebral perfusion; mean arterial pressure (MAP); and cardiac output (thermodilution). During baseline, hyperthermia reduced MCAvmean (P = 0.001) by 12 ± 9% relative to normothermia. Volume infusion while hyperthermic increased cardiac output by 2.8 ± 1.4 l/min (P < 0.001), but did not alter MCAvmean (P = 0.99) or MAP (P = 0.39) compared with hyperthermia alone. Relative to hyperthermia, at 30-mmHg LBNP acute volume infusion attenuated reductions (P < 0.001) in cardiac output (by 2.5 ± 0.9 l/min; P < 0.001), MAP (by 5 ± 6 mmHg; P = 0.004), and MCAvmean (by 12 ± 13%; P = 0.002). These data indicate that acute volume expansion does not reverse hyperthermia-induced reductions in cerebral perfusion pre-LBNP, but that it does attenuate reductions in cerebral perfusion during simulated hemorrhage in hyperthermic humans.

Cerebral perfusion, oxygenation and metabolism during exercise in young and elderly individuals.

We evaluated cerebral perfusion, oxygenation and metabolism in 11 young (22 ± 1 years) and nine older (66 ± 2 years) individuals at rest and during cycling exercise at low (25% W(max)), moderate (50% Wmax), high (75% W(max)) and exhaustive (100% W(max)) workloads. Mean middle cerebral artery blood velocity (MCA V(mean)), mean arterial pressure (MAP), cardiac output (CO) and partial pressure of arterial carbon dioxide (P(aCO2)) were measured. Blood samples were obtained from the right internal jugular vein and brachial artery to determine concentration differences for oxygen (O2), glucose and lactate across the brain. The molar ratio between cerebral uptake of O2 versus carbohydrate (O2-carbohydrate index; O2/[glucose + 1/2 lactate]; OCI), the cerebral metabolic rate of O2 (CMRO2) and changes in mitochondrial O2 tension ( P(mitoO2)) were calculated. 100% W(max) was ~33% lower in the older group. Exercise increased MAP and CO in both groups (P < 0.05 vs. rest), but at each intensity MAP was higher and CO lower in the older group (P < 0.05). MCA V(mean), P(aCO2) and cerebral vascular conductance index (MCA V(mean)/MAP) were lower in the older group at each exercise intensity (P < 0.05). In contrast, young and older individuals exhibited similar increases in CMRO2 (by ~30 µmol (100 g(-1)) min(-1)), and decreases in OCI (by ~1.5) and (by ~10 mmHg) during exercise at 75% W(max). Thus, despite the older group having reduced cerebral perfusion and maximal exercise capacity, cerebral oxygenation and uptake of lactate and glucose are similar during exercise in young and older individuals.

Blood flow in internal carotid and vertebral arteries during orthostatic stress.

It remains unclear whether orthostatic stress evokes regional differences in cerebral blood flow. The present study compared blood flow in the internal carotid (ICA) and vertebral arteries (VA) during orthostatic stress (60 deg head-up tilt; HUT) in six healthy young men. The ICA and VA blood flow were measured using Doppler ultrasonography. Dynamic cerebral autoregulation was also determined during supine (Supine) and HUT conditions, from the rate of regulation (RoR) in cerebrovascular conductance of the ICA and VA during acute hypotension induced by the release of bilateral thigh-cuffs. The HUT decreased ICA blood flow by -9.4 ± 1.7% (P < 0.01 versus Supine), leaving ICA conductance unchanged. In contrast, there was no significant difference in VA blood flow between Supine and HUT, and VA conductance increased (+12.9 ± 0.8%, P < 0.01). In addition, dynamic cerebral autoregulation in both the ICA and VA was attenuated during HUT, and the magnitude of the attenuation in RoR was greater in the VA [0.25 ± 0.03 s(-1) Supine versus 0.16 ± 0.02 s(-1) HUT (-33.9 ± 5.8%); P < 0.05] compared with the ICA [0.23 ± 0.02 s(-1) Supine versus 0.20 ± 0.03 s(-1) HUT (-10.6 ± 13.4%); P > 0.05]. These data indicate that orthostatic stress evokes regional differences in cerebral blood flow and possible differences in dynamic cerebral autoregulation between two main brain vascular areas in response to an acute change in blood pressure during orthostatic stress.