School of Health Sciences, University of Tasmania, Hobart, Australia.
School of Human Kinetics, University of Ottawa, Ottawa, Ontario, Canada.
J Occup Environ Hyg. 2020 Sep;17(9):383-389. doi: 10.1080/15459624.2020.1779932. Epub 2020 Jul 10.
Susceptibility to heat illness during physically demanding work in hot environments is greater on the second of two consecutive workdays. While it has been demonstrated that heat storage is exacerbated on the second compared to first workday in older workers (50-65 yr), the effects on heart rate variability (HRV), an established surrogate of cardiac autonomic modulation, remain unclear. This study evaluated HRV in older workers on the day following prolonged work in the heat. Electrocardiogram was recorded in nine older (53-64 yr) males at rest, during three 30-min bouts of semi-recumbent cycling at fixed rates of metabolic heat production (150, 200, 250 W/m), each separated by 15-min recovery. Experiments were conducted in hot-dry conditions (40 °C, 20% relative humidity), immediately prior to (Day 1), and on the day following (Day 2), a prolonged work simulation (∼7.5 hr) involving moderate intensity intermittent exercise in hot-dry conditions (38 °C, 34% relative humidity). Core temperature, as well as time, frequency, and nonlinear HRV indices were derived for analysis during rest, the final 5-min of exercise at the highest heat production and recovery. The change in core temperature at the end of work (mean ±SD) was significantly greater on Day 2 (1.0 °C ±0.3) relative to Day 1 (0.8 °C ±0.2; < 0.01). Heart rate, however, did not significantly differ between days 1 and 2 at rest (Day 1, 59 ±11 bpm; Day 2, 62 ±13 bpm), during exercise (Day 1, 113 ±21 bpm; Day 2, 114 ±18 bpm ) and at the end of recovery (Day 1, 75 ±16 bpm; Day 2, 76 ±12 bpm). Likewise, there were no significant differences in any HRV indices derived from time, frequency, and nonlinear domains (all > 0.05). Prolonged work in the heat did not modulate next-day heart rhythms, as reflected by HRV, despite augmented core temperature. While HRV can reflect physiological aspects of cardiac autonomic stressors, these findings indicate it does not provide a means to identify exacerbated heat strain in older workers over consecutive work shifts in the heat.
在炎热环境中进行体力要求高的工作时，第二天比连续两天中的第一天更容易出现热病。虽然已经证明在较年长的工人（50-65 岁）中，与第一天相比，第二天的热量储存会加剧，但对心率变异性（HRV）的影响（HRV 是心脏自主调节的既定替代指标）仍不清楚。这项研究评估了在炎热环境中长时间工作后的第二天老年工人的 HRV。在休息时，对九名较年长的男性（53-64 岁）进行了心电图记录，他们在三个 30 分钟的半卧位自行车运动期间以固定的代谢热产生率（150、200、250W/m）进行运动，每个运动之间间隔 15 分钟恢复期。实验在炎热干燥的条件下（40°C，20%相对湿度）进行，分别在长时间工作模拟（约 7.5 小时）之前（第 1 天）和之后（第 2 天）进行，该模拟涉及在炎热干燥的条件下（38°C，34%相对湿度）进行中等强度的间歇性运动。在休息时、在最高产热和恢复期的最后 5 分钟的运动期间，得出了核心温度以及时间、频率和非线性 HRV 指数，以供分析。工作结束时的核心温度变化（平均值±SD）在第 2 天（1.0°C±0.3）明显大于第 1 天（0.8°C±0.2； < 0.01）。然而，在休息时（第 1 天，59±11 bpm；第 2 天，62±13 bpm）、运动时（第 1 天，113±21 bpm；第 2 天，114±18 bpm）和恢复期结束时（第 1 天，75±16 bpm；第 2 天，76±12 bpm），心率在第 1 天和第 2 天之间没有显著差异。同样，在源自时间、频率和非线性域的任何 HRV 指数上均无显著差异（均 > 0.05）。尽管核心温度升高，但在炎热环境中长时间工作并没有调节第二天的心率，这反映在 HRV 上。虽然 HRV 可以反映心脏自主应激的生理方面，但这些发现表明，它不能提供一种方法来识别在炎热环境中连续工作班次中较年长工人的加剧的热应激。
