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Background: The Threshold Limit Values (TLV) of the American Conference of Governmental and Industrial Hygienists indicate the levels of heat stress that all workers may be repeatedly exposed to without adverse health effects. In this study, we evaluated heart rate variability (HRV) during moderate‐to‐heavy work performed continuously or according to different TLV work‐rest (WR) allocations in healthy physically active older workers. Methods: Nine healthy older (58 ± 5 years) males performed three different 120‐minute conditions in accordance with TLV guidelines for moderate‐to‐heavy intensity work (360 W fixed rate of heat production) in different wet‐bulb globe temperatures (WBGT): continuous cycling at 28°C WBGT (CON), as well as intermitted work performed at WR of 3:1 in 29°C WBGT (WR3:1), and at WR of 1:1 at 30°C (WR1:1). Rectal temperature and HRV (3‐lead electrocardiogram [ECG]) were assessed throughout. Results: Coefficient of Variation, Poincaré SD2, and Shannon Entropy were decreased during the CON compared with the WR3:1 when core temperature exceeded 38°C and after 1 hour of continuous work (P <.05). Also, 4 of the 12 HRV indices studied were reduced at CON compared with WR1:1 after 2 hours of accumulated work time (P <.05). Participants worked longer before core temperature reached 38°C during the WR1:1 and the WR3:1, compared with CON (P <.05). Conclusions: Incorporating breaks during moderate‐to‐heavy work in the heat for older adults can reduce autonomic stress and prolong the work performed at safe core temperature levels. The TLV WR1:1 provides increased cardiac protection for older workers, as compared with the CON and the WR3:1. [ABSTRACT FROM AUTHOR]

Key points: We introduce a technique to test whether intrinsic fetal heart rate variability (iFHRV) exists and we show the utility of the technique by testing the hypothesis that iFHRV is affected by chronic fetal hypoxia, one of the most common adverse outcomes of human pregnancy complicated by fetal growth restriction.Using an established late gestation ovine model of fetal development under chronic hypoxic conditions, we identify iFHRV in isolated fetal hearts and show that it is markedly affected by hypoxic pregnancy.Therefore, the isolated fetal heart has intrinsic variability and carries a memory of adverse intrauterine conditions experienced during the last third of pregnancy. Fetal heart rate variability (FHRV) emerges from influences of the autonomic nervous system, fetal body and breathing movements, and from baroreflex and circadian processes. We tested whether intrinsic heart rate variability (iHRV), devoid of any external influences, exists in the fetal period and whether it is affected by chronic fetal hypoxia. Chronically catheterized ewes carrying male singleton fetuses were exposed to normoxia (n = 6) or hypoxia (10% inspired O2, n = 9) for the last third of gestation (105–138 days of gestation (dG); term ∼145 dG) in isobaric chambers. At 138 dG, isolated hearts were studied using a Langendorff preparation. We calculated basal intrinsic FHRV (iFHRV) indices reflecting iFHRV's variability, predictability, temporal symmetry, fractality and chaotic behaviour, from the systolic peaks within 15 min segments in each heart. Significance was assumed at P < 0.05. Hearts of fetuses isolated from hypoxic pregnancy showed approximately 4‐fold increases in the Grid transformation as well as the AND similarity index (sgridAND) and a 4‐fold reduction in the scale‐dependent Lyapunov exponent slope. We also detected a 2‐fold reduction in the Recurrence quantification analysis, percentage of laminarity (pL) and recurrences, maximum and average diagonal line (dlmax, dlmean) and the Multiscale time irreversibility asymmetry index. The iHRV measures dlmax, dlmean, pL and sgridAND correlated with left ventricular end‐diastolic pressure across both groups (average R2 = 0.38 ± 0.03). This is the first evidence that iHRV originates in fetal life and that chronic fetal hypoxia significantly alters it. Isolated fetal hearts from hypoxic pregnancy exhibit a time scale‐dependent higher complexity in iFHRV. Key points: We introduce a technique to test whether intrinsic fetal heart rate variability (iFHRV) exists and we show the utility of the technique by testing the hypothesis that iFHRV is affected by chronic fetal hypoxia, one of the most common adverse outcomes of human pregnancy complicated by fetal growth restriction.Using an established late gestation ovine model of fetal development under chronic hypoxic conditions, we identify iFHRV in isolated fetal hearts and show that it is markedly affected by hypoxic pregnancy.Therefore, the isolated fetal heart has intrinsic variability and carries a memory of adverse intrauterine conditions experienced during the last third of pregnancy. [ABSTRACT FROM AUTHOR]

New Findings: What is the central question of this study?Does a delay in cold water immersion treatment affect the cardiac autonomic control of exertionally heat‐strained individuals?What is the main finding and its importance?Cold water immersion is effective for treating exertionally heat‐strained individuals even when treatment is commenced with a significant delay. However, that treatment delay leads to only partial/transient restoration of cardiac autonomic control. Therefore, we recommend that exertional heatstroke patients are continuously monitored for several hours even after core temperature has returned to normal values. Immediate cold water immersion (CWI) is the gold‐standard treatment for exertional heatstroke. In the field, however, treatment is often delayed, primarily owing to a delayed paramedic response and/or inaccurate diagnosis. We examined the effect of treatment (reduction of rectal temperature to 37.5°C) delays of 5 (short), 20 (moderate) and 40 (prolonged) min on cardiac autonomic control [as assessed via heart rate variability (HRV)] in eight exertionally heat‐strained (40.0°C rectal temperature) individuals. Eleven HRV indices were computed that have been described commonly in the literature and characterize almost all known domains of the variability and complexity of the cardiopulmonary system. We found that the cardiac autonomic control (as assessed via HRV) of exertionally heat‐strained individuals was significantly affected by the amount of time it took for the CWI treatment to be applied. Six out of 11 HRV indices studied, from all variability domains, displayed strong (P ≤ 0.005) time × delay interaction effects. Moreover, the number of significantly (P ≤ 0.005) abnormal (i.e. different from the short delay) HRV indices more than doubled (seven versus 15) from the moderate delay to the prolonged delay. Finally, our results demonstrated that a CWI treatment applied with delays of 20 and, primarily, 40 min did not lead to a full restoration of cardiac autonomic control of exertionally heat‐strained individuals. In conclusion, this study supports CWI for treating exertionally heat‐strained individuals even when applied with prolonged delay, but it highlights the importance of continued cardiac monitoring of patients who have suffered exertional heatstroke for several hours after restoration of core temperature to normal. [ABSTRACT FROM AUTHOR]