Martin P. Poirier

Aging impairs heat loss, but when does it matter?

Aging is associated with an attenuated physiological ability to dissipate heat. However, it remains unclear if age-related impairments in heat dissipation only occur above a certain level of heat stress and whether this response is altered by aerobic fitness. Therefore, we examined changes in whole body evaporative heat loss (HE) as determined using whole body direct calorimetry in young (n = 10; 21 ± 1 yr), untrained middle-aged (n = 10; 48 ± 5 yr), and older (n = 10; 65 ± 3 yr) males matched for body surface area. We also studied a group of trained middle-aged males (n = 10; 49 ± 5 yr) matched for body surface area with all groups and for aerobic fitness with the young group. Participants performed intermittent aerobic exercise (30-min exercise bouts separated by 15-min rest) in the heat (40°C and 15% relative humidity) at progressively greater fixed rates of heat production equal to 300 (Ex1), 400 (Ex2), and 500 (Ex3) W. Results showed that HE was significantly lower in middle-aged untrained (Ex2: 426 ± 34; and Ex3: 497 ± 17 W) and older (Ex2: 424 ± 38; and Ex3: 485 ± 44 W) compared with young (Ex2: 472 ± 42; and Ex3: 558 ± 51 W) and middle-aged trained (474 ± 21; Ex3: 552 ± 23 W) males at the end of Ex2 and Ex3 (P < 0.05). No differences among groups were observed during recovery. We conclude that impairments in HE in older and middle-aged untrained males occur at exercise-induced heat loads of ≥400 W when performed in a hot environment. These impairments in untrained middle-aged males can be minimized through regular aerobic exercise training.

Whole-Body Heat Exchange during Heat Acclimation and Its Decay.

PURPOSE The purpose of this study was to quantify how much whole-body heat loss increases during heat acclimation and the decay in these improvements after heat acclimation. METHODS Ten males underwent a 14-d heat acclimation protocol that consisted of 90 min of cycling in the heat (40°C, 20% relative humidity) at approximately 50% of maximum oxygen consumption. Before (day 0), during (day 7), and at the end (day 14) of the heat acclimation protocol as well as 7 and 14 d after heat acclimation (days 21 and 28), whole-body heat exchange (evaporative and dry) was measured using direct calorimetry during three bouts of 30-min exercise at 300 (Ex1), 350 (Ex2), and 400 W·m (Ex3), each separated by 10 and 20 min of recovery, respectively, at 35°C and 16% relative humidity. Concurrent measurements of metabolic heat production (indirect calorimetry) allowed for the direct calculation of change in body heat content (ΔHb). RESULTS After accounting for an increase in net dry heat gain, increases in whole-body evaporative heat loss were evident for Ex2 and Ex3 on day 7 (Ex2, 4.9 ± 5.6%; Ex3, 9.0 ± 6.0%; both P ≤ 0.05) and all heat loads on day 14 (Ex1, 7.6 ± 8.3%; Ex2, 7.7 ± 5.5%; Ex3, 11.2 ± 4.6%; all P ≤ 0.05) relative to day 0 (Ex1, 494 ± 27 W; Ex2, 583 ± 21 W; Ex3, 622 ± 36 W). As a result, a lower cumulative ΔHb was measured on day 7 (-18 ± 8%, P ≤ 0.001) and day 14 (-26 ± 10%, P ≤ 0.001) compared with that measured on day 0 (1062 ± 123 kJ). Most of these improvements were retained after 2 wk of nonexposure to the heat. CONCLUSIONS This is the first study to quantify how much 14 d of heat acclimation can increase whole-body evaporative heat loss, which can improve by as much as approximately 11%.

Exploring the mechanisms underpinning sweating: the development of a specialized ventilated capsule for use with intradermal microdialysis

Many studies have aimed to identify the controllers of sweating using ventilated capsules with intradermal microdialysis. It is unclear, however, if the surface area covered by the capsule influences the observed response as a result of differences in the number of sweat glands affected by the infused pharmacological agent relative to the total glands captured by the capsule. We evaluated the area of skin perfused with agents delivered via microdialysis. Thereafter, we developed a specialized sweat capsule (1.1 cm2) and compared the sweating response with a classic capsule (2.8 cm2). In Protocol 1 (n = 6), methacholine was delivered to forearm skin in a dose‐dependent manner (1–2000 mmol L−1). The area of activated sweat glands was assessed via the modified iodine‐paper technique. In Protocol 2 (n = 6), the area of inhibited sweat glands induced by ouabain and atropine was assessed during moderate‐intensity cycling. Marked variability in the affected skin area was observed (0.9 ± 0.4 to 5.2 ± 1.1 cm2). In Protocol 3 (n = 6), we compared the attenuation in local sweat rate (LSR) induced by atropine between the new and classic capsule during moderate‐intensity cycling. Atropine attenuated sweating as assessed using the new (control: 0.87 ± 0.23 mg min−1 cm−2 vs. atropine: 0.54 ± 0.22 mg min−1 cm−2; P < 0.01) and classic (control: 0.85 ± 0.33 mg min−1 cm−2 vs. atropine: 0.60 ± 0.26 mg min−1 cm−2; P = 0.05) capsule designs. Importantly, responses did not differ between capsule designs (P = 0.23). These findings provide critical information regarding the skin surface area perfused by microdialysis and suggest that use of a larger capsule does not alter the mechanistic insight into the sweating response gained when using microdialysis.

Hyperthermia and cardiovascular strain during an extreme heat exposure in young versus older adults

ABSTRACT We examined whether older individuals experience greater levels of hyperthermia and cardiovascular strain during an extreme heat exposure compared to young adults. During a 3-hour extreme heat exposure (44°C, 30% relative humidity), we compared body heat storage, core temperature (rectal, visceral) and cardiovascular (heart rate, cardiac output, mean arterial pressure, limb blood flow) responses of young adults (n = 30, 19–28 years) against those of older adults (n = 30, 55–73 years). Direct calorimetry measured whole-body evaporative and dry heat exchange. Body heat storage was calculated as the temporal summation of heat production (indirect calorimetry) and whole-body heat loss (direct calorimetry) over the exposure period. While both groups gained a similar amount of heat in the first hour, the older adults showed an attenuated increase in evaporative heat loss (p < 0.033) in the first 30-min. Thereafter, the older adults were unable to compensate for a greater rate of heat gain (11 ± 1 ; p < 0.05) with a corresponding increase in evaporative heat loss. Older adults stored more heat (358 ± 173 kJ) relative to their younger (202 ± 92 kJ; p < 0.001) counterparts at the end of the exposure leading to greater elevations in rectal (p = 0.043) and visceral (p = 0.05) temperatures, albeit not clinically significant (rise < 0.5°C). Older adults experienced a reduction in calf blood flow (p < 0.01) with heat stress, yet no differences in cardiac output, blood pressure or heart rate. We conclude, in healthy habitually active individuals, despite no clinically observable cardiovascular or temperature changes, older adults experience greater heat gain and decreased limb perfusion in response to 3-hour heat exposure.

Water Immersion in the Treatment of Exertional Hyperthermia: Physical Determinants

PURPOSE We examined the effect of differences in body surface area-to-lean body mass ratio (AD/LBM) on core temperature cooling rates during cold water immersion (CWI, 2°C) and temperate water immersion (TWI, 26°C) after exercise-induced hyperthermia. METHODS Twenty male participants were divided into two groups: high (315.6 ± 7.9 cm·kg, n = 10) and low (275.6 ± 8.6 cm·kg, n = 10) AD/LBM. On two separate occasions, participants ran on a treadmill in the heat (40.0°C, 20% relative humidity) wearing an impermeable rain suit until rectal temperature reached 40.0°C. After exercise, participants were immersed up to the nipples (arms remained out of the water) in either a CWI (2°C) or a TWI (26°C) circulated water bath until rectal temperature returned to 37.5°C. RESULTS Overall rectal cooling rates were significantly different between experimental groups (high vs low AD/LBM, P = 0.005) and between immersion conditions (CWI vs TWI, P < 0.001). Individuals with a high AD/LBM had an approximately 1.7-fold greater overall rectal cooling rate relative to those with low AD/LBM during both CWI (high: 0.27°C·min ± 0.10°C·min vs low: 0.16°C·min ± 0.10°C·min) and TWI (high: 0.10°C·min ± 0.05°C·min vs low: 0.06°C·min ± 0.02°C·min). Further, the overall rectal cooling rates during CWI were approximately 2.7-fold greater than during TWI for both the high (CWI: 0.27°C·min ± 0.10°C·min vs TWI: 0.10°C·min ± 0.05°C·min) and the low (CWI: 0.16°C·min ± 0.10°C·min vs TWI: 0.06°C·min ± 0.02°C·min) AD/LBM groups. CONCLUSION We show that individuals with a low AD/LBM have a reduced rectal cooling rate and take longer to cool than those with a high AD/LBM during both CWI and TWI. However, CWI provides the most effective cooling treatment irrespective of physical differences.

An Evaluation of the Physiological Strain Experienced by Electrical Utility Workers in North America

The purpose of this study was to assess the physiological strain experienced by North American electrical utility workers during the performance of their normal work duties in heat stressed conditions. Three common job categories were monitored as they are normally performed in 32 electrical utility workers: (i) Ground Work (n = 11); (ii) Bucket Work (n = 9); and (iii) Manual Pole Work (n = 12). Worker hydration status (urine specific gravity (USG)) was measured prior to and following the work monitoring period (duration: 187 ± 104 min). Core and skin temperatures as well as heart rate were measured continuously. Physiological Strain Index (PSI) was calculated from the measurements of core temperature and heart rate. Prior to the start of the work shift, 38% of workers were euhydrated (USG < 1.020; n = 12) whereas the majority of workers were dehydrated (USG > 1.020; prevalence: 75%; p < 0.01) following work. The overall mean and peak core temperatures for all monitored workers were 37.9 ± 0.3°C and 38.3 ± 0.5°C, respectively. When responses were compared between job categories, greater mean and peak increases in core temperature were observed in Manual Pole Work relative to the other job categories (both p < 0.04). In fact, six workers performing Manual Pole Work achieved core temperatures in excess of 38.5°C, while only one other worker surpassed this threshold in Bucket Work. The high levels of thermal strain were paralleled by elevated mean and peak heart rate and PSI responses, which were greater in Manual Pole Work in comparison to the other job categories (all p ≤ 0.05). Furthermore, two workers performing Manual Pole Work achieved severely elevated core temperatures reaching or exceeding 39.5°C along with prolonged periods of near maximal heart rate responses (i.e., >90% of heart rate reserve). We report elevated levels of thermal and cardiovascular strain in electrical utility workers during work in the heat and potentially dangerous levels of hyperthermia during particularly strenuous work.

At What Level of Heat Load Are Age-Related Impairments in the Ability to Dissipate Heat Evident in Females?

Studies have reported that older females have impaired heat loss responses during work in the heat compared to young females. However, it remains unclear at what level of heat stress these differences occur. Therefore, we examined whole-body heat loss [evaporative (HE) and dry heat loss, via direct calorimetry] and changes in body heat storage (∆Hb, via direct and indirect calorimetry) in 10 young (23±4 years) and 10 older (58±5 years) females matched for body surface area and aerobic fitness (VO2peak) during three 30-min exercise bouts performed at incremental rates of metabolic heat production of 250 (Ex1), 325 (Ex2) and 400 (Ex3) W in the heat (40°C, 15% relative humidity). Exercise bouts were separated by 15 min of recovery. Since dry heat gain was similar between young and older females during exercise (p=0.52) and recovery (p=0.42), differences in whole-body heat loss were solely due to HE. Our results show that older females had a significantly lower HE at the end of Ex2 (young: 383±34 W; older: 343±39 W, p=0.04) and Ex3 (young: 437±36 W; older: 389±29 W, p=0.008), however no difference was measured at the end of Ex1 (p=0.24). Also, the magnitude of difference in the maximal level of HE achieved between the young and older females became greater with increasing heat loads (Ex1=10.2%, Ex2=11.6% and Ex3=12.4%). Furthermore, a significantly greater ∆Hb was measured for all heat loads for the older females (Ex1: 178±44 kJ; Ex2: 151±38 kJ; Ex3: 216±25 kJ, p=0.002) relative to the younger females (Ex1: 127±35 kJ; Ex2: 96±45 kJ; Ex3: 146±46 kJ). In contrast, no differences in HE or ∆Hb were observed during recovery (p>0.05). We show that older habitually active females have an impaired capacity to dissipate heat compared to young females during exercise-induced heat loads of ≥325 W when performed in the heat.

Do the Threshold Limit Values for Work in Hot Conditions Adequately Protect Workers

PURPOSE We evaluated core temperature responses and the change in body heat content (ΔHb) during work performed according to the ACGIH threshold limit values (TLV) for heat stress, which are designed to ensure a stable core temperature that does not exceed 38.0°C. METHODS Nine young males performed a 120-min work protocol consisting of cycling at a fixed rate of heat production (360 W). On the basis of the TLV, each protocol consisted of a different work-rest (WR) allocation performed in different wet-bulb globe temperatures (WBGT). The first was 120 min of continuous (CON) cycling at 28.0°C WBGT (CON[28.0°C]). The remaining three protocols were intermittent work bouts (15-min duration) performed at various WR and WBGT: (i) WR of 3:1 at 29.0°C (WR3:1[29.0°C]), (ii) WR of 1:1 at 30.0°C (WR1:1[30.0°C]), and (iii) WR of 1:3 at 31.5°C (WR1:3[31.5°C]) (total exercise time: 90, 60, and 30 min, respectively). The change in rectal (ΔTre) and mean body temperature (ΔTb) was evaluated with thermometry. ΔHb was determined via direct calorimetry and also used to calculate ΔTb. RESULTS Although average rectal temperature did not exceed 38.0°C, heat balance was not achieved during exercise in any work protocol (i.e., rate of ΔTre > 0°C·min; all P values ≤ 0.02). Consequently, it was projected that if work was extended to 4 h, the distribution of participant core temperatures higher and lower than 38.0°C would be statistically similar (all P values ≥ 0.10). Furthermore, ΔHb was similar between protocols (P = 0.70). However, a greater ΔTb was observed with calorimetry relative to thermometry in WR3:1[29.0°C] (P = 0.03), WR1:1[30.0°C] (P = 0.02), and WR1:3[31.5°C] (P < 0.01) but not CON[28.0°C] (P = 0.32). CONCLUSION The current study demonstrated that heat balance was not achieved and ΔTb and ΔHb were inconsistent, suggesting that the TLV may not adequately protect workers during work in hot conditions.

Heart rate variability during high heat stress: a comparison between young and older adults with and without type 2 diabetes

We examined whether older individuals with and without Type 2 diabetes (T2D) experience differences in heart rate variability (HRV) during a 3-h exposure to high heat stress compared with young adults. Young (Young; n = 22; 23 ± 3 yr) and older individuals with (T2D; n = 11; 59 ± 9 yr) and without (Older; n = 25; 63 ± 5 yr) T2D were exposed to heat stress (44°C, 30% relative humidity) for 3 h. Fifty-five HRV measures were assessed for 15 min at baseline and at minutes 82.5-97.5 (Mid) and minutes 165-180 (End) during heat stress. When compared with Young, a similar number of HRV indices were significantly different (P < 0.05) in Older (Baseline: 35; Mid: 29; End: 32) and T2D (Baseline: 31; Mid: 30; End: 27). In contrast, the number of HRV indices significantly different (P < 0.05) between Older and T2D were far fewer (Baseline: 13, Mid: 1, End: 3). Within-group analyses demonstrated a greater change in the Young groups HRV during heat stress compared with Older and T2D; the number of significantly different (P < 0.05) HRV indices between baseline and End were 42, 29, and 20, for Young, Older, and T2D, respectively. Analysis of specific HRV domains suggest that the Young group experienced greater sympathetic activity during heat stress compared with Older and T2D. In conclusion, when compared with young, older individuals with and without T2D demonstrate low HRV at baseline and less change in HRV (including an attenuated sympathetic response) during 3 h high heat stress, potentially contributing to impaired thermoregulatory function.

The physical demands of electrical utilities work in North America

ABSTRACT We assessed the physical demands associated with electrical utilities work in North America and how they influence the level of thermal and cardiovascular strain experienced. Three common job categories were monitored as they are normally performed in thirty-two electrical utility workers: (i) Ground Work (n = 11), (ii) Bucket Work (n = 9), and (iii) Manual Pole Work (n = 12). Video analysis was performed to determine the proportion of the work monitoring period (duration: 187 ± 104 min) spent at different levels of physical effort (i.e., rest as well as light, moderate and heavy effort). Core and skin temperatures as well as heart rate were measured continuously. On average, workers spent 35.9 ± 15.9, 36.8 ± 17.8, 24.7 ± 12.8, and 2.6 ± 3.3% of the work period at rest and performing work classified as light, moderate, and heavy physical effort, respectively. Moreover, a greater proportion of the work period was spent performing heavy work in Ground Work (1.6 ± 1.4%) relative to Bucket Work (0.0 ± 0.0%; P<0.01) and in Manual Pole Climbing (5.5 ± 3.6%) in comparison to both other work job (both P≤0.03). Furthermore, the proportion of time spent during work classified as heavy physical effort was positively correlated to the mean (r = 0.51, P<0.01) and peak (r = 0.42, P = 0.02) core temperatures achieved during the work period as well as the mean heart rate response (presented as a percentage of heart rate reserve; r = 0.40, P = 0.03). Finally, mean and peak core temperatures and mean heart rate responses increased from the first to the second half of the work shift; however, no differences in the proportion of the work spent at the different intensity classifications were observed. We show that Manual Pole Work is associated with greater levels of physical effort compared to Ground or Bucket Work. Moreover, we suggest that the proportion of time spent performing work classified as heavy physical exertion is related to the level of thermal and cardiovascular strain experienced and that workers may not be employing self-pacing as a strategy to manage their level of physiological strain.