The mechanism of the AAD proves to be multifactorial. However, three key factors has been consistently agreed: weakened aortic wall, damaged vascular endometrium resulting of the vascular endothelial slap and the spread of endometrial damage due to hypertension .9 Previous studies had proved many cardiovascular risk factors were related to seasonal and climatic changes including external environmental factors such as temperature and UV radiation, lifestyle such as diet, obesity, exercise and smoking, and other factors such as blood pressure, serum cholesterol, glucose tolerance, coagulation, acute and chronic infections.10,11 All these risk factors were more common in winter and might also cause seasonal changes in the incidence of AAD.12,13 Many previous studies suggested a possible seasonal effect on the onset of AAD by reporting a high incidence of AAD in winter and a low incidence of AAD in summer. 5–8 In winter, the temperature was low, the sympathetic nervous system of the human body was activated, and catecholamine secretion was increased to cope with low temperature, which leaded to increasing heart rate and peripheral vascular resistance, resulting in increased blood pressure. Cold environment promoted the occurrence of high blood pressure, and by increasing blood friction against the vascular wall and surface shear stress, high blood pressure increased the risk of the occurrence and rupture of aortic dissection in patients with hypertension history.14 However, rare study had been conducted on the seasonal and climatic effect on the postoperative prognosis of AAD patients.
Non-accidental mortality has been reported to increase significantly in the transitional season in the general population. It has also been reported that mortality distribution in the transition season in Russia showed that the increase in mortality from hot to cold (autumn) was greater than that from cold to hot (spring).15 It is also observed that large diurnal temperature variation lead to increased risk of cardiac mortality in autumn.16,17 Unlike the seasonally and climatically regular pattern of the incidence of AAD, our univariate regression analysis found that among the seasonal and climatic high risk factors for postoperative in-hospital mortality of AAD, patients’ admission day in autumn were more likely to die after surgery than other seasons, multivariate regression analysis also showed the admission day in autumn was a contributor to increased in-hospital mortality, although we found no significant link between climatic factors such as average minimum temperature on the day of onset, average daily temperature difference and average AQI and in-hospital mortality. Many studies have shown that seasonal meteorological variables affect blood pressure in hypertensive and normotensive individuals.18–20 In china, it becomes cool and humid due to frequent rainfall in autumn, and atmospheric pressure (AP) fluctuates intensely.21 The drop in temperature and fluctuation in AP may lead to increased sympathetic activity, causing more severe vasoconstriction or vasospasm, higher blood pressure.22 Some studies have confirmed that the fluctuation of AP might increase the risk of rupture in abdominal aortic aneurysm.23,24 In the same way, drastic fluctuations of systolic blood pressure result from increased alteration of AP in autumn can cause greater risk of postoperative residual dissection rupture, leading to statistically significant higher in-hospital mortality rate in our study. Another reason was that meteorological risk factors including humidity, rainfall and other specific climatic data also may have provided a more detailed explanation for the seasonal and climatic impacts. 25 And other currently unknown and related risk factors might also have an impact. In addition, when used a univariate and multiple logistic regression analysis to analyze all predictors of postoperative AAD in-hospital mortality, it could be found that coronary heart disease also caused the increase of postoperative AAD in-hospital mortality, indicating that timely intervention of coronary heart disease may reduce postoperative in-hospital mortality.
When used a multiple logistic regression analysis in the second study to determine whether there were seasonal and climatic high risk factors associated with LOS. It could still be found that there were statistical difference in the LOS and seasonal high risk factor: admission day in autumn seemed to prolong LOS; but climatic high risk factors (average minimum temperature of the onset day, average daily temperature difference and average AQI) were not contributors to increase LOS. In addition, patients with hypertension might prolonged LOS.
This study had some inevitable limitations. First, data were limited to some patients underwent surgery in a single center in our hospital, our findings might not be necessarily reflected in other countries with more seasonal and climatic risk model. The second limitation was the relationship among mortality, LOS, season and climate might also be affected by other administrative and logistical unknown variables.