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/m²), 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; p < 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 p > 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.

在连续两个工作日中的第二个工作日中，在炎热的环境中进行身体苛刻的工作时，容易患上热病。尽管已经证明，与年长的工人（50-65岁）相比，第二天的蓄热会比第一天的工作加剧，但是对心律变异性（HRV）（一种确定的心脏自主神经调节指标）的影响尚不清楚。这项研究评估了长时间在高温下工作的老工人的HRV。在固定的代谢热产生速率（150、200、250 W / m ^2）的三下半卧式循环中，进行了三轮30分钟的半卧式循环，记录了9位年龄较大（53-64岁）男性在休息时的心电图。），每次间隔15分钟恢复。在紧接（第1天）之前和第二天（第2天）的热干条件下（40°C，相对湿度20％）进行了长时间的工作模拟（〜7.5 hr），涉及中等强度的间歇性在干热条件下锻炼（38°C，相对湿度34％）。得出了休息时的核心温度以及时间，频率和非线性HRV指数，以进行分析，这是在最高热量产生和恢复的情况下进行的最后5分钟运动。与第1天（0.8°C±0.2; p）相比，第2天（1.0°C±0.3），工作结束时核心温度的变化（平均值±SD）大得多。 <0.01）。但是，运动期间（第1天，113±21 bpm；第2天，114±），在休息的第1天和第2天（第1天，59±11 bpm；第2天，62±13 bpm）之间，心率无显着差异。 18 bpm）和恢复结束时（第1天，75±16 bpm；第2天，76±12 bpm）。同样，从时间，频率和非线性域得出的任何HRV指数也无显着差异（所有p  > 0.05）。尽管核心温度升高，但长时间的高温工作并不能调节第二天的心律，正如HRV所反映的那样。尽管HRV可以反映出心脏自主神经应激源的生理方面，但这些发现表明，HRV没有提供一种方法来识别在连续的工作热班中老工人加剧的热应激。