In this study, we examined the relationship between diurnal temperature range and the outpatient visits for CVD. There was a statistically significant association between the admission of CVD and the diurnal temperature range. An increment of 1°C in DTR, changing from 12°C to 13°C, was associated with a 1.16% increase in RR of CVD in the entire study group (0.84%, 1.22%, 1.77%, and 0.82% in males, females, adults and elderly, respectively). These findings suggest that the effect of DTR on CVD is stronger in females than males and that the adult younger than 65 years old suffer more risk of CVD under the temperature change when the DTR at the median level.
Two factors motivated our investigation of the DTR as the factor affecting CVD in Dingxi. First, because it is located on the edge of the Loess Plateau with a large in-day temperature difference, and second because of its high incidence of CVD and cerebrovascular diseases. Many prior scholars have demonstrated a linear or nonlinear relationship between DTR and CVD. For example, shan zheng et al. investigated the effect of DTR on ischemic heart disease ER admissions in Beijing among elderly. In Korea, Youn-Hee et al. revealed positive linear associations between DTR and CVD ER admissions. However, in Yuxi, China, Ding et al. (2015) reported a J-shaped association for the impact of DTR with cardiovascular mortality. These results were inconsistent with our study as we found a nonlinear relationship ("M-shape" pattern) between daily DTR and daily number of CVD admission. More people become active and are exposed to ambient temperature at low DTR, particularly in aging people. This might have resulted in the first peak in RR of CVD. In contrast, the second peak of the relative risk of CVD could be attributed to the stronger effect of high DTR than the low one. The results found in Taiwan reported that acute coronary syndrome admissions increased rapidly when DTR exceeded a threshold of 9.6°C. The impact of DTR on CVD has been reported in many previous studies. Shan Zheng reported significantly acute effects of DTR on all cardiovascular and cerebrovascular disease ER admissions among males and females living in Beijing. In Hong Kong, an increase of 1°C in DTR at lag days 0-3 was associated with a 1.7% increase in mortality of cardiovascular among elderly. In the United States, K. L. et al. concluded that a 10°C decrease in average daily temperature led to a 13% increase in fatal, incident, and recurrent coronary events in San Francisco and Sacramento (California). These results are consistent with what Danet et al. found in Northern France. A recently published study conducted in Jingchang, Northwestern China, showed a positive linear correlation between DTR with systolic blood pressure and pulse pressure, but a negative linear correlation between DTR with diastolic blood pressure. These findings have led to a speculation that blood pressure is a trigger in the onset of cardiovascular diseases. In addition to blood pressure, oxygen uptake, heart rate, and cardiac workload may also increase CVD risk during exposure to daily temperature ranges
Studies in China, Korea, and Japan have pointed out that the impact of DTR on CVD varies by age and gender, with the elderly and females being more susceptible to the negative effects of DTR. In our study, when DTR was relatively low (4°C, 5th percentile in the DTR distribution), our results became in agreement with these results. Jayeun Kim et al. have pointed that adverse effects of DTR were more pronounced in those aged ≥ 65 years and varied according to geographic, longitudinal, climatic characteristics, and the scale of DTR (0.33%; 95% CI: 0.12, 0.55) for overall all-cause mortality. In China, deaths of cardiovascular among females and elderly (≥ 65 years) were more strongly associated with DTR than among males and younger people (< 65 years). The elderly appear to suffer more risk of CVD, probably due to increasing age, progressive reduction in the physiological ability to sense changes in body temperature, and reduction in the body’s compensatory mechanisms (i.e., shiver or sweat) to regulate temperature[27,34]. Regarding effect modification by gender, our results provide evidence that cardiovascular disease was greater in females than in males at small amplitude of temperature change. Epidemiologic studies provide support to this ascertain. For example, in Korea, research conducted in six cities by Lim et al. agreed that cardiovascular disease death among females associated more strongly with DTR than among males. Meanwhile, Yang et al. reported that women in Guangzhou (China) were consistently more sensitive to the adverse impact of DTR on total and cardiovascular mortality. A previous study by Basu et al. (2009) has speculated that the differences in the effect of temperature by gender were the result of location and population That is why the impact of hot temperature on coronary events was greater for men in San Paulo, while greater for women in Mexico City. Furthermore，the research conducted in Europe by Lancet explained that clothing plays a significant role in explaining sex difference and that there is a biological difference between gender in the ability to thermoregulate.
A novel finding in our study was that a relatively low DTR could significantly impact cardiovascular diseases in all subgroups than a relatively high DTR, which is in line with prior research in Guangzhou for investigation. A small variation in daily temperature was associated with a greater risk of mortality than a large variation in daily temperature. A study in shanghai reported that the effect of extremely high or low DTR on daily mortality of CVD was stronger than that of moderate DTR (approximately equal to mean DTR). With the global temperature change and lack of awareness and measures to withstand the low temperature change, people will suffer more impact of low DTR. The decreasing trend in DTR was mainly caused by the significant increase in the minimum temperature during night-time, particularly in Northeast China. When encountering the drastic temperature change, most people, especially the elderly and women, should act to weaken it, such as staying at home and using heat sources[37,45].
Another distinctive finding of our study is that high DTR showed a protective effect of cardiovascular diseases in women and elderly compared with their corresponding categories. Also, Dindxi is dominated by agricultural development, with potatoes and winter wheat representing the main food crops. Therefore, our study population is mainly farmers, and our data were obtained from the NCMS (new rural cooperative medical system). In other studies of Southwest China, it has been reported that farmers bear more risk of mortality due to high DTR than non-farmers. Ravallion and Chen attributed these results to poor living conditions caused by low annual income, low educational level, and inferior socio-economic status. Also, the agricultural work may lead to an increased high- exposure of DTR. However, in the countryside of Dingxi, more than 70% of young adults go out of the city to work, and about 25% of them primarily in outdoor construction and manufacturing, while women and the elderly stay at home and work for farming under general conditions[40,49]. This means that women and the elderly are more flexible and selective at work than men in the face of high-DTR weather. To be specific, in the days of high DTR, women and the elderly could not choose to go out for agricultural work which could weaken or eliminate the unfavorable effect of high DTR for them, but young men must be forced to engage in outdoor construction, since construction projects must be completed within a certain period. This led to more exposure for young men on high DTR days. A previous study has shown that a greater likelihood of DTR exposure or weaker intrinsic susceptibility factors might result in people's exposure to increased risk. Moreover, more smoking and drinking in young men, as mediating factors, may contribute to the risk of cardiovascular diseases.
To the best of our knowledge, this is the first study to investigate the association between DTR and CVD risk in the developing area of Dingxi, Northwestern China. Such a place is a backward area for China and a target for achieving universal healthcare. Our observed relation between DTR and CVD morbidity could guide the local authorities to improve CVD preventive strategies in the rural areas.
Our study has several strengths. The results were based on data from the NRCMI of Gansu Province, which is credible and validated, and NRCMI recorded the CVD events for farmers. Inevitably, our study has several limitations as well. First, we didn't include all the poverty-stricken areas, but selected a typical rural poor area as a representative. Second, the impact of personal characteristics was not considered in our analysis, such as medical history, personal behaviors, and living conditions due to socio-economic status. Third, it is not accurate to assume all the individuals' exposure to DTR to be similar. In addition, in-door use of heating and air-conditions would lead to measurement bias in the difference between outdoor and indoor temperature.