In this study, based on 27712 patients with fatal AMI, we evaluated the relationship between daily average temperature and the risk of fatal AMI. We found that there was a statistically significant correlation between low-temperature exposure and lethal AMI. Women and people ≥ 65 years old were sensitive to cold, while high temperature had no effect on the occurrence of AMI. Cold temperatures cannot be disregarded because they considerably raise the risk of fatal AMI. The acute effect of cold exposure occurred on the same day, and the delayed effect continued for 30 days. These findings could help the local public health department develop preventive measures to lower the occurrence of fatal acute myocardial infarction.
Our study discovered that low-temperature exposure was associated with lethal AMI, whereas high-temperature exposure had no statistically significant association. Li Bai et al. reported that the 1st percentile temperature corresponds to the lowest risk temperature, and AMI increased by 29% (95% confidence interval 15–45%) [19]. In our study, the effect of very low temperature on lethal AMI was greater than that of moderate low temperature. Residents must enhance their adaptability to extremely low temperatures for local climate change since they are unable to adapt to extremely low temperatures, which may result in more AMI incidents. At the same time, moderate low temperatures also have risks. The 30-day cumulative CRR of the cold temperature (2 ℃) for the general population is 2.55 (1.37–4.75). A study in India found that [20] moderate cold temperatures are estimated to have a high attribution risk, with an IHD value of 9.7% [3.7–15.3], and moderately cold temperatures exceed the effect of extreme cold. This implies that we need not only take precautions to stay warm in extremely cold weather but even in moderate cold weather. To stay warm, we should pay close attention to the weather forecast in real time.
In Xuzhou, there is no correlation between extremely high temperatures and the risk of lethal AMI. Similarly, Xiao QianHuang et al. found that the temperature and the risk of AMI present a double peak shape when the temperature is 26 ˚. The second peak started at point C, and RR increased again, although not significantly (RR: 0.999, 95% CI: 0.986–1.012) [21]. However, Jaime Madrigano and researchers discovered that exceptionally high temperatures in the first two days were associated with an increased risk of death from AMI (RR: 1.44, 95% CI: 1.06–1.96) [22]. The risk of moderately high temperatures (90th percentile temperature) and extremely high temperatures (95th percentile temperature) increased by 18% (RR: 1.18, 95% CI: 0.95–1.47) and 36% (RR: 1.36, 95% CI: 1.06–1.73), respectively, in hospitalized patients with acute myocardial infarction in Vietnam [23]. The CRR curve in our study indicated an increasing trend at the early stage under extremely high temperatures, but no statistical significance was found. The differences between several studies could be the cause of the disparate outcomes of high temperatures on AMI. Different climates, ethnicities, genders, age distribution, economic levels, habits and activity patterns, use of air conditioners and other cooling facilities, and the diversity of study methods may all contribute to these variances.
We discovered that at cold temperatures, women are more prone to the harmful consequences of fatal AMI. Several earlier studies have confirmed our point of view [24–26]. The possible reason is that in our data, the average age of the women was higher than that of the men, and the elderly population accounted for a large proportion of women. According to certain research, women have lower thermogenic muscle mass than men, and because of the increased blood distribution in the uterus and ovary, it is more difficult for women to regulate body temperature [11]. It is also believed that estrogen enhances vasoconstriction activity under cold induction, which is associated with estrogen-dependent increased expression of cold-sensitive α (2C) -ARs [27]. There are various viewpoints, though. Men are thought to engage in more outside activities and are more susceptible to cold temperatures, according to Jane Wichmann et al. [28].
In a national study conducted in China, RenJie Chen et al. reported that in temperate monsoon and subtropical monsoon climate regions, the death risk and burden of specific subgroups aged ≥ 75 years are more prominent [10]. XiaoLe Liu et al. concluded in their research that patients over the age of 65 are more vulnerable to the adverse effects of AMI caused by low temperatures [27]. These data imply that cold exposure may be a risk factor for acute heart attacks in the elderly. The elderly's ability to perceive and self-regulate weather temperature has likely decreased, making it difficult for them to anticipate temperature changes and take timely precautions against the cold. In addition, Jing Cai and other researchers found that a decline in temperature increased the response of biomarkers of inflammation, coagulation, and vasoconstriction [29]. In a study of 12 healthy men, David G et al. found that peripheral vasoconstriction brought on by exposure to cold increased wave reflection and central systolic pressure. The study also revealed that during cold exposure, the change in central blood pressure was not visible in the measurement of brachial artery blood pressure, which may make monitoring the change in central blood pressure challenging [30]. Another study discovered that the amplitude of the pressor response generated by cold exposure in the elderly was more than double that of young adults [31]. Because the elderly have more comorbidities, the pathophysiological alterations induced by cold may further increase their risk of fatal AMI, according to the findings of the above-mentioned study.
A vast number of studies have revealed that the cold effect lasts a long time and has a clear hysteresis effect, whereas the thermal effect appears rapidly and disappears quickly. Yu MingGuo et al. analyzed death and temperature data from 12 countries, including China. When analyzing the nonlinear and delayed association between temperature and mortality, they discovered that the influence of low temperature (the hundredth percentile and the lowest mortality temperature) was delayed by approximately 2 days, continued for at least 10 days, and could even last longer. The effects of high temperature (99th percentile temperature and minimum mortality temperature) occur immediately, usually lasting only 3 or 4 days [32]. Several more studies found similar results [33–35]. According to our findings, the cold effect (1th) has the greatest impact on that day, and the entire cold effect can endure for 30 days. The extremely high temperatures (99th) had an instantaneous effect, and the thermal effect lasted only 7 days. The fourth day was affected by high temperatures (90th), and the thermal effect lasted for 27 days. Women and people 65 years of age and above experienced the cold and hot effects instantly, whereas people under 65 experienced a delay. Only the heat effects were delayed in men. The above results show that we should not only consider the occurrence of extreme temperatures but also take into consideration the lag times of cold and heat effects on different categories of people.