Influence of the menstrual cycle on the sweating response measured by direct calorimetry in women exposed to warm environmental conditions

Summary The whole body sweating response was measured at rest in eight women during the follicular (F) and the luteal (L) phases of the menstrual cycle. Subjects were exposed for 30-min to neutral (N) environmental conditions [ambient temperature (T _a) 28°C] and then for 90-min to warm (W) environmental conditions (Ta, 35°C) in a direct calorimeter. At the end of the N exposure, tympanic temperature (T _ty) was 0.18 (SEM 0.06)°C higher in the L than in the F phase (P<0.05), whereas mean skin temperature ( ) was unchanged. During W exposure, the time to the onset of sweating as well as the concomitant increase in body heat content were similar in both phases. At the onset of sweating, the tympanic threshold temperature (T _{ty, thresh}) was higher in the L phase [37.18 (SEM 0.08)°C] than in the F phase [36.95 (SEM 0.07)°C;P<0.01]. The magnitude of the shift inT _ty, thresh [0.23 (SEM 0.07)°C] was similar to the L-F difference inT _ty observed at the end of the N exposure. The mean skin threshold temperature was not statistically different between the two phases. The slope of the relationship between sweating rate andT _ty was similar in F and L. It was concluded that the internal set point temperature of resting women exposed to warm environmental conditions shifted to a higher value during the L phase compared to the F phase of the menstrual cycle; and that the magnitude of the shift corresponded to the difference in internal temperature observed in neutral environmental conditions between the two phases.     

Passive temperature lability in the elderly

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28°C water. The exercise was conducted to elevate tympanic temperature (T _ty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling ofT _ty, and changes inT _ty relative to resting levels (ΔT _ty) at which sweating abated and shivering commenced were defined as the ΔT _ty thresholds for the cessation of sweating (T _sw) and onset of shivering (T _sh), respectively. In addition toT _ty, oxygen uptake ( ; 1 · min⁻¹), sweating rate (g · m⁻² · min⁻¹), and forehead skin blood perfusion were also measured during the trials. The mean (SD)T _sw occurred at a significantly (P <0.005) higher ΔT _ty [0.48 (0.18)°C] in the elderly than in the young adults [0.21(0.06)°C], while the Tsh occurred at significantly (P < 0.005) lower ΔT _ty in the elderly [ −0.64 (0.34)°C] than in young adults [−0.22 (0.10)°C]. Decreases in ΔT _ty below the shivering threshold were met with a significantly (P <0.01) reduced . The range of temperature lability between Ts, andT _sh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39)°C] than in the young adults [0.43 (0.12)°C], and the slope of the vasoconstrictor response in the null-zone was significantly (P <0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels ofT _ty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.     

Physical dilatation of the nostrils lowers the thermal strain of exercising humans

Increased nasal air flow during exercise was examined as a possible heat loss avenue contributing to selective brain cooling in hyperthermic humans. On 2 separate days, eight subjects [mean (SE) age, 26.4 (1.2) years] exercised on a cycle ergometer in a warm room [28 (0.2)°C; 28 (5)% relative humidity] to induce a moderate level of hyperthermia. In one session the nostrils were physically dilatated [average dilatation 1.55 (0.17) times] and in the other they were not (control). Both sessions started with a 5-min resting period; then subjects pedaled at 60 W for 5 min, 100 W for 15 min, and 150 W for 20 min. During dilatation both tympanic temperature (T _ty) and forehead skin blood flow, estimated by laser doppler velocimetry, were significantly lower than during the control exercise of 150 W. Rates of increase of (T _ty) during the 100-W exercise were the same in both conditions; however, during the 150-W exercise with dilatated nostrils (T _ty) increased at a rate significantly lower than during control [1.1 (0.3)°C·h^–1 vs 1.5 (0.4)°C·h^–1]. The change in the rate of increase of T _ty between conditions was significantly correlated to the degree of nostril dilatation (r = –0.77, P = 0.02), suggesting that the lower (T _ty) observed was due to nostril dilatation. Facial skin temperature was not significantly different between sessions. The results suggest that the nasal cavity may act as a heat exchanger in selective brain cooling of exercising humans.     

Effects of repeated carbon dioxide-rich water bathing on core temperature,cutaneous blood flow and thermal sensation

We examined the effects of repeated artificial CO₂ (1,000 ppm) bathing on tympanic temperature (T _ty), cutaneous blood flow, and thermal sensation in six healthy males. Each subject was immersed in CO₂-rich water at a temperature of 34°C up to the level of the diaphragm for 20 min. The CO₂-rich water was prepared using a multi-layered composite hollow-fiber membrane. The CO₂ bathing was performed consecutively for 5 days. As a control study, subjects bathed in fresh water at 34°C under the same conditions. T _ty was significantly lowered during CO₂ bathing (P<0.05). Cutaneous blood flow in the immersed skin (right forearm) was significantly increased during CO₂ bathing compared with that during fresh-water bathing (P<0.05), whereas cutaneous blood flow in the non-immersed skin (chest) was not different between CO₂ and fresh-water bathing. Subjects reported a "warm" sensation during the CO₂ bathing, whereas they reported a "neutral" sensation during the fresh-water bathing. The effects of the repeated CO₂ bathing were not obvious for core temperature and cutaneous blood flow, but the thermal sensation score during the CO₂ bathing was reduced sequentially by repeated CO₂ bathing (P<0.05). These thermal effects of CO₂ bathing could be ascribed largely to the direct action of CO₂ on vascular smooth muscles and to the activity of thermoreceptors in the skin. Serial CO₂ bathing may influence the activity of thermoreceptors in the skin. Electronic Publication     

Nasal mucosal vasodilatation in response to passive hyperthermia in humans

The present study was conducted to measure nasal mucosal blood flow (NMBF) during body warming. Five subjects [mean (SD) 24 (2) years], wearing only shorts and a thick felt hat with ear flaps, were immersed to the neck in a bath at 40 (0.5)°C. Tympanic (T _ty), esophageal (T _es), mean unweighted skin (T _sk), nose skin and ear pinna skin were recorded at 1-min intervals. NMBF on the lower septal wall was estimated using a laser Doppler flow meter. At rest T _ty and T _es were both 36.5°C. T _ty dropped significantly below T _es during body warming, despite impeded heat loss from the head due to the felt hat. T _ty increased to 37.3°C and T _es increased to 37.5°C during the immersion. During the immersion all skin temperatures were steady or increasing, ruling out the possibility of a contamination of T _ty from (T _sk), Body warming significantly (P = 0.001) increased NMBF by approximately three times from resting values at the end of immersion. During the period of increasing core temperatures NMBF was significantly correlated to T _ty (r = 0.93, P = 0.0001) and T _es (r = 0.97, P = 0.0001), suggesting the blood flow change in this tissue was a thermo-regulatory response. The increased NMBF during hyperthermia supports the hypothesis of respiratory cooling involvement in selective brain cooling of humans.     

Thermal changes observed before and after J.-L. Etienne's journey to the North Pole

Summary The thermoregulatory responses of a French doctor, Jean-Louis Etienne, were examined in a standard cold test before and after his journey to the North Pole, to investigate whether general and/or local cold adaptation had occurred. The two tests were carried out in a climatic chamber for 2 h at rest (dry bulb temperature, 1° C; relative humidity, 40%; wind speed, 0.8 m · s^–1). After his journey, Etienne showed a general hypothermic-hypometabolic adaptation, i.e. a decrease in rectal temperature (T _re) and metabolic heat production (M), and an increased local skin temperature of the extremities. Between the two tests, a change occurred in the relationship between tympanic temperature (T _ty) and M. During the post-journey cold test, T _ty [as representative of the central nervous system (CNS) temperature] increased while the decrease in T _re was accelerated, probably due to a redistribution of blood volume towards the CNS. Such a mechanism would protect the central core with special reference to the CNS.     

Influence of bright light exposure for several hours during the daytime on cutaneous vasodilatation and local sweating induced by an exercise heat load

The aim of the present study was to investigate the effect of exposure to differing light intensities for several hours during the daytime on the cutaneous vasodilatation and local forearm sweat rate induced by exercise. Seven healthy female subjects were exposed to bright light of 6000 lux (bright) or dim light of 100 lux (dim) during the daytime between 0900 hours to 1330 hours, followed by exposure to 150 lux until the test was over at 1600 hours. They spent their time in neutral conditions (29°C, 40% relative humidity) from 0900 hours to 1500 hours, and then exercised on a cycle ergometer for 30 min at 50% maximal physical work capacity. Average tympanic temparatures were significantly lower in bright than in dim from 1133 hours to 1430 hours. The onset of cutaneous vasodilatation and local forearm sweating occurred at significantly lower tympanic temperature (T _ty) during exercise after bright than after dim. After exercise, the cessation of forearm sweating and the rapid change of skin blood flow occurred at significantly lower T _ty after bright than after dim. It was concluded that exposure to bright light over several hours during the daytime could reduce T _ty and shift the threshold T _ty for cutaneous vasodilatation and forearm sweating to a lower level.     

Vasomotor response of the human face: laser-Doppler measurements during mild hypo- and hyperthermia

The skin of the face is reputed not to vasoconstrict in response to cold stress because the face skin temperature remains steady during hypothermia. The purpose of the present work was to measure the vasomotor response of the human face to whole-body hypothermia, and to compare it with hyperthermia. Six male subjects were immersed in cold and in warm water to obtain the two conditions. Skin blood flow, evaporation, and skin temperature (T_sk) were recorded in three loci of the face, the forehead, the infra orbital area, and the cheek. Tympanic (T_ty) and oesophageal (T_oes) temperatures were also recorded during the different thermal states. Normothermic measurements served as control. Blood flow was recorded with a laser-Doppler flowmeter, evaporation measured with an evaporimeter. Face T_sk remained stable between normo-, hypo-, and hyperthermia. Facial blood flow, however, did not follow the same pattern. The facial blood flow remained at minimal vasoconstricted level when the subjects' condition was changed from normo- to hypothermia. When the condition changed from hypo- to hyperthermia a 3 to 9–fold increase in the blood flow was recorded. From these results it was concluded that a vasoconstriction seems to be the general vasomotor state in the face during normothermia.     

Thermoregulatory responses of circum-pubertal children

Passive temperature lability of nine circumpubertal children [11.4 (1.2) years] was compared to that of nine young adult males [26.6 (5.2) years]. Each subject completed a 20-min period of exercise, followed immediately by post-exercise immersion in water at 28°C. The aim of the exercise protocol was to induce a steady rate of sweating (E _SW) while the postexercise immersion period induced cooling of the core region (tympanic temperature:T _ty). TheT _ty values (relative to rest, ΔT _ty) at which sweating abated and at which shivering commenced were defined as the thresholds for the cessation of sweating and onset of shivering, respectively. While there was no significant difference between the ΔT _ty sweating thresholds, the onset of shivering, as reflected in the oxygen uptake response, occurred at significantly higher (P < 0.05) ΔT _ty values in the children [mean (SD): −0.07 (0.07)°C] than in the adults [−0.22 (0.10)°C]. The slope of theE _SW/ΔT _ty relationship was found to be significantly lower in the children (z = −5.64;P < 0.05), while the slopes of the /ΔT _ty relationship were not significantly different (z = −0.84;P > 0.05). Skin blood perfusion was measured at the forehead (SkBP), and the slope of the SkBP/ΔT _ty relationship across the nullzone was significantly less in the children than in the adults (z = −2.13;P < 0.05) with the greatest reduction in perfusion occurring prior to the offset of sweating in the children. The subjective ratings of thermal comfort indicated that the children were more sensitive to changes in core temperature than the adults. It is concluded that maturation plays an important role in modifying thermoregulatory responses to deviations in core temperature. These results suggest that there may be differences in thermoregulatory “strategies” which are maturationally related.     

Effects of body and head positions on bilateral difference in tympanic temperatures

Summary We have examined the nonparallel changes in tampanic membrane temperatures (T _ty) from the two ears in response to various changes in body and head positions. Upon assuming a lateral recumbent position, the T _ty on the lower side increased while that on the upper side decreased. Pressure application over a wide area of the lateral chest only caused inconsistent and obscure asymmetric changes in T _ty. A lateral flexion of the head with the subject sitting upright and a rotation of the head to the side in a supine position induced an increase in the T _ty on the lower side compared to that on the upper side. The temperature and blood flow of the forehead often decreased on the lower side and increased on the upper side, although such responses were not always concomitant with the asymmetric changes in T _ty. A dorsal flexion of the head with the subject in a reclining position caused a slight increase in the T _ty, whereas raising the head upright induced a slight decrease in them. Two additional experiments were carried out with single photon emission computed tomography using ^99mTc-hexamethylpropyleneamine oxime as tracer, and a slight, relative decrease in counts was noted in the right hemisphere during rotation of the head to the right. These results would strongly suggest that unilateral increases and decreases in T _ty could have been caused by one-sided decreases and increases, respectively, in blood flow to the brain and/or the tympanic membrane, induced by a vasomotor reflex involving vestibular stimulation.     

Thermoregulatory changes in the cold induced by physical training in humans

It has previously been demonstrated that metabolic heat production (M˙) during cold exposure at rest was related to maximal oxygen uptake (O_2max). Consequently, an increase in O_2max could allow an increase M˙ in the cold. The aim of the present study was therefore to test this hypothesis. Eight male volunteers undertook interval training (periods of 25% V˙O_2max of 30-s duration and 110% V˙O_2max of 60-s duration until exhaustion, five times a week over 8 weeks) to increase V˙O_2max. Both before and after this physical training, they were subjected to a 10^°, 5^° and 1^°C 2-h cold air test in a climatic chamber. During the cold exposure, rectal temperature (T _re), tympanic temperature (T _ty), mean skin temperature ( ) and M˙ were measured as well as the time to onset of shivering (t) and body temperatures ( ) at t. The results showed that physical training involved an increase in O_2max (14%–15%, P < 0.05). During the cold exposure, T _re was higher after training both at 10^°,5^° and 1^°C (P < 0.05) whereas were not significantly changed. However, an increase in the sensitivity of the thermoregulatory system was attested by a decreased t at higher These slight physiological changes found after training were not related to the increases in V˙O_2max. In conclusion, this study demonstrated that interval training induced slight thermoregulatory changes unrelated to changes in V˙O_2max and it suggested that M˙ during cold exposure could be related mainly to the level of V˙O_2max observed before training, since increases in V˙O_2max did not modify M˙.     

Effects of selective brain cooling on mechanisms of respiratory heat loss

Experiments (n=36) in three conscious goats were performed at 35 °C air temperature and low (LH) or high (HH) humidity. Prior to the experiments the animals received carotid loops and an arteriovenous shunt, which made it possible to increase the temperature of the blood flowing to head and trunk (series A), or to increase the temperature of the trunk at constant carotid blood and hypothalamic temperature (T _hyp), respectively (series B). Owing to the smaller cooling power of the inspired air in HH, the slope of respiratory evaporative heat loss versus aorta blood temperature (T _aor) was reduced in series A and B. In series A the slopes of respiratory minute volume ( _E) and respiratory frequency (RF) versus T _aor were larger in HH than in LH. The effects were caused by a reduction of selective brain cooling in HH, which resulted in higher levels of T _hyp. This is concluded from the results of series B, in which T _hyp was equal in LH and HH, and the slopes of _E and RF over T _aor showed no differences. Thus, selective brain cooling contributes to counteract the deterioration of the gain of the respiratory heat loss mechanism, which occurs during exposure to humid air.     

Core temperature thresholds for hyperpnea during passive hyperthermia in humans

Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5)°C for 25 min. Tympanic (T _ty), esophageal (T _es), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V _I at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO₂ at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V _I briefly increased to 18.6 (3.0)l·min^–1 returned to about the pre-immersion value,, and significantly increased to 19.3 (3.0) l·min^–1 by the end of immersion. VO₂ increased significantly from the pre-immersion value of 0.27 l·min^–1 to 0.67 l·min^–1 by the first 0.5 min of immersion, but then returned to its pre-immersion value. T _ty increased to 38.7 (0.2)°C and T _es increased to 39.0 (0.2)°C by the end of immersion. Core temperature thresholds for increases in V _I were evident at 38.1°C when expressed against T _ty and at 38.5°C when expressed against T _es. The results indicated that during body warming core temperature thresholds for V _I are reached and subsequently a hyperpnea was evident, despite VO₂ remaining at a resting value. This hyperpnea is seen as a thermoregulatory response likely to participate in selective brain cooling.     

Hand and finger skin temperatures in convective and contact cold exposure

The present study aimed at investigating the spatial variability of skin temperature (T _sk) measured at various points on the hand during convective and cold contact exposure. A group of 8 subjects participated in a study of convective cooling of the hand (60 min) and 20 subjects to contact cooling of the finger pad (5 min). Experiments were carried out in a small climatic chamber into which the hand was inserted. For convective cold exposure,T _sk was measured at seven points on the palmar surface of the fingers of the left hand, one on the palmar surface and one on the dorsal surface of the hand. The air temperature inside the mini-chamber was 0, 4, 10 and 16°C. With the contact cold exposure, the subjects touched at constant pressures an aluminium cube cooled to temperatures of –7, 0 and 7°C in the same mini-chamber. ContactT _sk was measured on the finger pad of the index finger of the left hand. TheT _sk of the proximal phalanx of the index finger (on both palm and back sides), and of the middle phalanx of the little finger was also measured. The variation ofT _sk between the proximal and the distal phalanx of the index finger was between 1.5 to 10°C during the convective cold exposure to an air temperature of 0°C. Considerable gradients persisted between the hand and fingers (from 2 to 17°C at 0°C air temperature) and between the phalanges of the finger (from 0.5 to 11.4°C at 0°C air temperature). The onset of cold induced vasodilatation (CIVD) on different fingers varied from about 5 to 15 min and it did not always appear in every finger. For contact cold exposure, whenT _sk on the contact skin cooled down to nearly 0°C, the temperature at the area close to the contact skin could still be 30°C. Some cases of CIVD were observed in the contact skin area, but not on other measuring points of the same finger. These results indicated that local thermal stimuli were the main determinents of CIVD. Representative hand skin temperature may require five or more measuring points. Our results strongly emphasised a need to consider the large spatial and individual variations in the prediction and modelling of extremity cooling.     

Relationship between core temperature and skin blood flux in lower limbs during prolonged arm exercise in persons with spinal cord injury

The purposes of the present study were to examine the response of the skin blood flux (SBF) in the paralyzed lower limbs of persons with spinal cord injury (PSCI) and to clarify the relationship between the SBF and core temperature during prolonged arm exercise. Eight male PSCI with lesions from T6 to L5 and six male control subjects (CS) participated in this study. The subjects rested for 60 min and then performed arm-cranking exercise at 20 W for 30 min at 25 °C. The tympanic membrane temperature (T _ty) and SBF in the anterior thigh (SBFT) and in the posterior calf (SBFC) were continuously measured throughout the experiment. The SBFC did not change in either PSCI or CS during the experiment. The SBFT in four PSCI with high lesions (T6 to T12), remained unchanged during exercise. The SBFT in the other four PSCI with low lesions (T12 to L5, SBFT+) began to elevate markedly when the T t, exceeded a threshold temperature of 36.69 °C. The pattern of increase of SBFT in SBFT+ was similar to that in CS, although onset of the increase in SBFT was delayed and the peak of SBFT during exercise was significantly lower in comparison with the CS. We consider that these differences between the SBFT+ and CS were largely attributable to the lowerT _ty in the former group, which took a prolonged time to reach the threshold of 36.69 °C.     

Finger vasodilation correlates better with tympanic than esophageal temperature

Summary The relationship of finger blood flow (FBF) measured by venous occlusion plethysmography to tympanic temperature (T_ty) was compared with that of FBF to esophageal temperature (T_es) during exercise at 50% for 40 min at an ambient temperature of 25°C. The relationship of FBF to T_es showed an inflexion as T_es increased during exercise. The slope of the regression line showing the relationship between FBF and T_es was initially moderate, and then suddenly became steeper at the inflexion point. The relationship of FBF to T_ty, however, was linear, without an inflexion. The results suggest that finger vasodilation during moderate exercise correlates better with tympanic than esophageal temperature.     

Control of shivering and heart rate in incubating bantam hens upon sudden exposure to cold eggs

To distinguish shivering released by cooling of the brood patch from shivering released by low core temperature, incubating bantam hens were exposed to water perfused eggs. Responses to a period of egg cooling were compared to the recovery period after egg temperature had been returned to 40 ^oC, but cloacal temperature (T_b) still was low. At an ambient temperature (T_a) of 23 ^oC and exposure of the hens to between two and eight eggs cooled to 10–35 ^oC (series 1), electromyographic (EMG) activity of musculus iliotibialis increased rapidly with an occasional overshoot, and was higher during egg cooling than during recovery. This hysteresis in EMG activity and T_b was weakly correlated to egg temperature and clutch size. Heart rate (HR) showed an almost parallel increased to shivering except that a maximum HR was reached at high shivering intensities. These responses were also present at a T_a of 37 ^oC when the hens were slightly hyperthermic before exposure to eight eggs at 20 ^oC (Series 2). At the highest starting T_bs EMG activity increased linearly after a drop in T_b. Shivering in m. pectoralis showed a lower threshold T_b and lower activity than m. iliotibialis during egg cooling, and immediately ceased at the end of egg cooling. Total body thermosensitivity estimated from the recovery periods at low and high T_a was –9.7 and –6.4 W kg^-1oC^-1, respectively. It is concluded that shivering in incubating birds warming cold eggs probably is stimulated both by peripheral and central thermoreceptors. The peripheral component shows phasic properties typical for skin receptors.     

Appropriate and inappropriate hypothalamic cold thermosensitivity in Willow Ptarmigan

Hypothalamic thermosensitivity has been investigated in conscious Willow Ptarmigan (Lagopus lagopus lagopus) provided with chronically implanted hypothalamic perfusion thermodes. The birds were exposed to either cold (T_a-10°C) or warm (T_a+ 25°C) ambient conditions while hypothalamic temperature (T_hy) was clamped for periods of 20 min at different set levels between 28°C and 43°C. The responses of the animals to hypothalamic thermal stimulation were classified by comparing them with those normally found in mammals. At T_a– 10°C hypothalamic heating inhibited ongoing shivering, causing a fall in body-core temperature (T_c) (appropriate mammalian-like response). Strong levels of hypothalamic cooling (T_hy < 34.0°C) also caused a fall in T_c due to inhibition of shivering (inappropriate mammalian-like response). However, weaker levels of hypothalamic cooling (T_hy 34–36°C), facilitated ongoing shivering, resulting in small increases in T_c (appropriate mammalian-like response). At T_a+25°C hypothalamic heating facilitated ongoing panting while weak (T_hy 38°C) levels of hypothalamic cooling inhibited ongoing panting (both mammalian-like responses). The observation of a weak mammalian-like cold hypothalamic thermosensitivity in Willow Ptarmigan indicates that these birds possess some specific cold thermosensors in the hypothalamic region. This finding suggests that hypothalamic temperature dependence in birds and mammals is fundamentally similar.     

Effect of acetazolamide on cerebral blood flow and tympanic temperature in healthy subjects and patients with subacute subarachnoid haemorrhage

The influence of the increased cerebral blood flow (CBF) induced by acetazolamide on tympanic temperature (T _ty) was examined in three healthy male volunteers and in five patients with subacute subarachnoid haemorrhage (SAH). The CBF was estimated by means of stable xenon-enhanced computed tomography before and after the administration of acetazolamide. The T _ty was recorded continuously in both ears using thermistor thermometers. In all subjects, CBF increased ranging from 11% to 108% after acetazolamide administration. In all the healthy subjects and in two patients with mild SAH, T _ty was higher than the oesophageal temperature (T _oes) and T _ty decreased bilaterally, ranging from 0.07 to 0.35°C as CBF increased. Three patients with severe SAH were febrile, their T _oes exceeding T _ty, and their T _ty rose by 0.30 to 0.53°C with increased CBF. These observations suggest that T _ty follows brain temperature which changes with an increase in CBF in euthermic subjects as well as in febrile subjects.     

In vivo quantification of hyperoxic arterial blood water T1

Normocapnic hyperoxic and hypercapnic hyperoxic gas challenges are increasingly being used in cerebrovascular reactivity (CVR) and calibrated functional MRI experiments. The longitudinal arterial blood water relaxation time (T_1a) change with hyperoxia will influence signal quantification through mechanisms relating to elevated partial pressure of plasma‐dissolved O₂ (pO₂) and increased oxygen bound to hemoglobin in arteries (Y_a) and veins (Y_v). The dependence of T_1a on Y_a and Y_v has been elegantly characterized ex vivo; however, the combined influence of pO₂, Y_a and Y_v on T_1a in vivo under normal ventilation has not been reported. Here, T_1a is calculated during hyperoxia in vivo by a heuristic approach that evaluates T₁‐dependent arterial spin labeling (ASL) signal changes to varying gas stimuli. Healthy volunteers (n = 14; age, 31.5 ± 7.2 years) were scanned using pseudo‐continuous ASL in combination with room air (RA; 21% O₂/79% N₂), hypercapnic normoxic (HN; 5% CO₂/21% O₂/74% N₂) and hypercapnic hyperoxic (HH; 5% CO₂/95% O₂) gas administration. HH T_1a was calculated by requiring that the HN and HH cerebral blood flow (CBF) change be identical. The HH protocol was then repeated in patients (n = 10; age, 61.4 ± 13.3 years) with intracranial stenosis to assess whether an HH T_1a decrease prohibited ASL from being performed in subjects with known delayed blood arrival times. Arterial blood T_1a decreased from 1.65 s at baseline to 1.49 ± 0.07 s during HH. In patients, CBF values in the affected flow territory for the HH condition were increased relative to baseline CBF values and were within the physiological range (RA CBF = 36.6 ± 8.2 mL/100 g/min; HH CBF = 45.2 ± 13.9 mL/100 g/min). It can be concluded that hyperoxic (95% O₂) 3‐T arterial blood T_1aHH = 1.49 ± 0.07 s relative to a normoxic T_1a of 1.65 s. Copyright © 2015 John Wiley & Sons, Ltd.