Influence of meteorological factors on acute
aortic events in a subtropical territory
Y. Law, Y.C. Chan
, S.W. Cheng
Division of Vascular and Endovascular Surgery, Department of Surgery, The University of Hong Kong, Queen Mary Hospital, Hong Kong
Received 22 September 2015; received in revised form 6 November 2015; accepted 16 November 2015
acute aortic dissection; acute aortic events; ambient temperature; humidity; pressure; ruptured aortic aneurysm; thunderstorm; weather
Summary Background/Objective: This study aims to examine the relationship between weather changes and acute aortic events in a subtropical territory.
Methods: A linear regression analysis was performed in a pan-territory epidemiological survey for a period of 10 years on the impacts of meteorological factors (ambient temperature, atmo-spheric pressure, relative humidity, amount of cloud, rainfall, number of lightning strikes, presence of typhoon, and thunderstorm warning) on the daily incidences of acute aortic dis-sections and ruptured aortic aneurysms. Meteorological variables were retrieved on a daily ba-sis from a well-established observatory, and the daily incidences of aortic dissections and rupture of aortic aneurysms were retrieved from the Clinical Data Analysis and Reporting Sys-tem.
Results: During the study period (January 2005 to December 2014), 3878 patients were identi-fied as having acute aortic dissections, and 1174 patients had ruptured aortic aneurysms. Cor-responding averaged daily incidences were 1.06 and 0.32, respectively. The incidences of aortic dissection and ruptured aortic aneurysm in a day could be predicted by ambient temper-ature in degrees Celsius using the following linear regression models: (1) incidence of aortic d i s s e c t i o n Z 1.548 0.021 temperature; (2) incidence of ruptured aortic aneurysmZ 0.564 0.010 temperature. In addition, both high atmospheric pressure and absence of thunderstorm warning are positively associated with more aortic dissections. For rupture of aortic aneurysms, high atmospheric pressure and low relative humidity were posi-tive predictors. In multiple regression analysis, however, ambient temperature was the only significant predictor for both acute aortic dissections and ruptured aortic aneurysms.
*This paper was presented in abstract form as Law Y, Chan YC, Cheng SW. Impact of Meteorological Factors on Acute Aortic Events at The Royal College of Surgeons of Edinburgh & The College of Surgeons of Hong Kong Conjoint Scientific Congress & 20thAsian Congress of Surgery, Hong Kong, September 2015.
Conflicts of interest: No conflicts of interest declared.
* Corresponding author. Division of Vascular and Endovascular Surgery, Department of Surgery, The University of Hong Kong, South Wing, 14thFloor K Block, Queen Mary Hospital, Pokfulam Road, Hong Kong.
E-mail address:firstname.lastname@example.org(Y.C. Chan). http://dx.doi.org/10.1016/j.asjsur.2015.11.002
1015-9584/Copyrightª 2015, Asian Surgical Association. Published by Elsevier Taiwan LLC. All rights reserved.
Available online atwww.sciencedirect.com
Conclusion: This is the first pan-territory study to show an attributable effect of ambient tem-perature on acute aortic events. This paper confirms that even in a subtropical country, mete-orological variables were important factors influencing acute aortic events.
Copyrightª 2015, Asian Surgical Association. Published by Elsevier Taiwan LLC. All rights reserved.
Many studies have reported an association between cold weather and incidence rate of acute cardiovascular events such as myocardial infarction1e14 or cerebrovascular accidents.15e24 Several publications have demonstrated seasonal variation in acute aortic events such as aortic dissection25e27 and rupture of aortic aneurysm.28e32
How-ever, these observatory studies were conducted almost exclusively in countries with severe winter conditions, and to date there are no such studies in countries with sub-tropical climates.
This study was based on a pan-territory survey con-ducted for a period of 10 years in Hong Kong, in a sub-tropical territory where summer and winter seasons are distinct. In the study setting, the use of household heater is not common and people are exposed to the true ambient atmospheric weather. Hong Kong is one of the most densely populated areas in Southeast Asia with 7.4 million people concentrated in 1104 km2(426 mi2), equating to an overall population density of about 6300 people/km2of land, 93.6% of whom are ethnic Han Chinese.33 There is usually little variation in meteorological factors across our small terri-tory. Hong Kong has a world-renowned reliable and accu-rate observatory, established in 1883, which maintains well-documented, meticulous daily meteorological data-set.34 Hong Kong also has in service a dedicated, reliable Pan-territory electronic registry containing data on pa-tients’ morbidity and mortality due to acute aortic events over a long period. The clinical management system (CMS) and electronic patient record (ePR), built in-house since 1991 in all the public hospitals, store essential clinical data of more than 7 million patients who attended our public hospitals.
This 10-year population-based longitudinal study is the first pan-territory survey to examine the influence of meteorological variables on acute aortic events (acute aortic dissection and ruptured aortic aneurysms) in a sub-tropical territory.
2.1. Collection of meteorological data
The Hong Kong Observatory has an accurate and reliable electronic meteorological database, which is readily avail-able to the public through their website.34 From this database, data on meteorological factors such as maximum ambient temperature (C), mean temperature (C), mini-mum temperature (C), atmospheric pressure (hPa), mean
relative humidity (%), mean amount of cloud (%), number of lightning strokes, typhoon signal warning (Yes or No), and thunderstorm signal warning (Yes or No) are available on a daily basis. Retrieved data were tabulated electronically onto an SPSS spreadsheet (IBM SPSS statistics, version 22.0) on a daily basis.
2.2. Collection of clinical data on acute aortic events
For the purpose of this study, we divided acute aortic events into either acute aortic dissections or ruptured aortic aneurysms. From a centralized public health care (computerized) database of Hong Kong public hospitals (the CMS and ePR, and data extracted from the Clinical Data Analysis and Reporting System), data on regional daily incidence of (1) aortic dissection, (2) aortic dissec-tion with operadissec-tion, (3) ruptured aortic aneurysm, and (4) ruptured aortic aneurysm with operation were retrieved. Search criteria were all emergency admissions with (1) primary discharge diagnosis of aortic dissection [9th
revi-sion of the International Statistical Classification of Dis-eases and Related Health Problems (ICD9) 441.0], (2) diagnosis of aortic dissection and a procedure (ICD9 38.xx and 39.xx), (3) diagnosis of ruptured aortic aneurysm (ICD9 441.1, 441.3, 441.5, 441.6), and (4) diagnosis of ruptured aortic aneurysm and a procedure (ICD9 38.xx, 39.xx, 54.xx).
The precise number of events occurring each day was then entered electronically onto an electronic spreadsheet. Thus, we have data on both the meteorological factors and the incidence of acute aortic events within every single day in the study period.
2.3. Statistical analysis
Data were analyzed on a daily basis from 00:00AMto 23:59 PM. Meteorological factors were used as predictors of acute
aortic events. The incidences of aortic dissections and ruptured aortic aneurysms were regarded as event outcomes.
A linear regression model was built using mean ambient temperature, atmospheric pressure, relative humidity, amount of cloud, number of lightning strokes, and presence or absence of typhoon signal warning or thunderstorm signal warning as independent variable and incidence of acute aortic events as dependent variables. The correlation coefficient (R) indicated the direction of correlation (posi-tive or nega(posi-tive) between the two variables; correlation of determination (R2) measured the amount of variability in one variable that was shared by other; and the F ratio
measured how much the model improved the prediction of the outcome compared with the level of inaccuracy.35 Factors that significantly predicted incidence of acute aortic events were subjected to multiple regression analysis.
In addition, we investigated the temperature parame-ters as predictors of event outcome. Temperature param-eters included maximum ambient temperature, mean ambient temperature, minimum ambient temperature, mean temperature 1 day before, mean temperature 2 days before, difference in temperature compared with 1 day before, difference in temperature compared with 2 days before, percentage change in temperature compared with 1 day before, percentage change in temperature compared with 2 days before, and diurnal variation (maximum temperature minimum temperature). Each temperature parameter was tested with linear regression to evaluate which parameter was the best predictor.
Statistical analysis was performed using SPSS version 22.0. A p value of 0.05 was considered to be statistically significant.
During the 10-year study period (January 1, 2005, to December 31, 2014), there were 5052 acute aortic events: 3878 acute aortic dissections, of which 288 underwent surgery, and 1174 ruptured aortic aneurysms, of which 631 underwent surgery (Figures 1 and 2). The averaged daily incidences of aortic dissection and ruptured aortic aneu-rysm were 1.06 and 0.32, respectively.
3.1. Seasonal variations of acute aortic events
The climatic patterns of Hong Kong is summarized in
Figure 3. Hong Kong has distinct seasons of a cool and dry winter from November to February, and a hot and humid summer from May to August. There was a clear seasonal
difference in the incidences of aortic dissection and ruptured aortic aneurysm. More occurrences were observed in winter months and fewer occurrences in summer months. The averaged daily incidences of aortic dissection were 1.20, 1.09, 1.09, 0.98, 0.94, 0.97, 0.94, 0.98, 1.01, 1.09, 1.11, and 1.34 in January, February, March, April, May, June, July, August, September, October, November, and December, respectively [analysis of variance (ANOVA) p < 0.001; Figure 4]. The averaged daily incidences of ruptured aortic aneurysm were 0.42, 0.32, 0.34, 0.33, 0.23, 0.25, 0.30, 0.31, 0.32, 0.29, 0.35, and 0.41 in January, February, March, April, May, June, July, August, September, October, November, and December respec-tively (ANOVA p < 0.001; Figure 5). Season wise, the averaged daily incidences of aortic dissection were 1.22, 1.01, 0.96, and 1.07 in winter, spring, summer, and autumn, respectively (ANOVA p < 0.001; Figure 6). The averaged daily incidences of ruptured aortic aneurysm were 0.38, 0.30, 0.29, and 0.32 in winter, spring, summer, and autumn, respectively (ANOVA p< 0.001;Figure 7).
3.2. Temperature as predictor of acute aortic events
The incidence of aortic dissection and ruptured aortic aneurysm in a day could be predicted by ambient temper-ature in degrees Celsius using the following linear regres-sion models:
Incidence aortic dissectionð ÞZ1:548 0:021 temperature ð1Þ (linear regression; R Z 0.102, R2 Z 0.010, F Z 38.13, p< 0.001;Figure 8)
Incidence ruptured aortic aneurysmð ÞZ0:564 0:010
(linear regression; R Z 0.098, R2 Z 0.010, F Z 35.16, p< 0.001;Figure 9)
Figure 1 Annual incidence of aortic dissection.
Both slopes were negative, indicating that the higher the temperature, the lower the incidence. We can therefore predict the number of cases of aortic dissection or ruptured aortic aneurysm in 1 day given the specific temperature of that day. For example, at 0C, the predicted daily inci-dence of aortic dissection is 1.55. For every 10C increase, the incidence drops by 0.21. At 30C, the daily incidence of aortic dissection is 0.92 (Equation1).
Similarly, at 0C, the predicted daily incidence of rupture of aortic aneurysm is 0.56. For every 10C increase, the incidence drops by 0.10. At 30C, the daily incidence of ruptured aortic aneurysm is 0.26 (Equation2).
3.3. Other meteorological factors as predictors
In a similar fashion, mean atmospheric pressure (hPa), mean relative humidity (%), mean amount of cloud (%), number of lightning strokes, presence or absence of typhoon signal warning, and presence and absence of thunderstorm signal warning were analyzed using the linear regression models.
High atmospheric pressure and absence of thunderstorm warning were positively associated with increased inci-dence of acute aortic dissection in univariant analysis (linear regression: R Z 0.093, R2 Z 0.009, F Z 31.52,
Figure 3 Climate of Hong Kong.
Figure 4 Monthly variation of mean daily incidences of aortic dissection. ANOVA Z analysis of variance; CI Z confidence interval.
p < 0.001 and R Z 0.043, R2 Z 0.002, F Z 6.69, p Z 0.010 for atmospheric pressure and thunderstorm warning, respectively). In the case of ruptured aortic an-eurysms, high atmospheric pressure and low relative hu-midity predicted increased occurrence of ruptured aortic aneurysms (linear regression: R Z 0.081, R2 Z 0.007, F Z 24.22, p < 0.001 and R Z 0.037, R2 Z 0.001,
FZ 5.08, p Z 0.024 for atmospheric pressure and relative humidity, respectively; Table 1). In multiple regression
analysis, however, ambient temperature was the only sin-gle significant predictor for both aortic dissection and ruptured aortic aneurysm (Tables 2 and 3).
3.4. Which temperature parameters best predict acute aortic events?
Among all the temperature parameters, mean ambient temperature best predicted aortic dissection, whereas
Figure 5 Monthly variation of mean daily incidences of ruptured aortic aneurysm. ANOVA Z analysis of variance; CIZ confidence interval.
Figure 6 Seasonal variation of mean daily incidences of aortic dissection. ANOVA Z analysis of variance; CIZ confidence interval.
Figure 7 Seasonal variation of mean daily incidences of ruptured aortic aneurysm. ANOVA Z analysis of variance; CIZ confidence interval.
minimum temperature of the day best predicted rupture of aortic aneurysm, of which R2values and F ratios were the highest (Tables 4 and 5).
To our knowledge, this is the first study to evaluate the effects of common meteorological parameters on the occurrence of acute aortic events in a subtropical territory. In particular, through the univariate analysis, it was found that temperature, pressure, and thunderstorm warning parameters were statistically associated with acute aortic dissection, and that temperature, pressure, and relative humidity parameters were statistically associated with aortic aneurysm rupture. Results of multivariate analysis indicated that only the ambient temperature was associ-ated with these acute aortic events. This finding was
rational after careful consideration of the climate pattern. Cold winter months in our territory were associated with high atmospheric pressure and lack of rainfall and thun-derstorm, whereas hot summer months are associated with low atmospheric pressure and high humidity (Figure 3). Pearson correlation indicated that the ambient tempera-ture was negatively correlated with atmospheric pressure (R Z 0.836, p < 0.001), and positively correlated with both relative humidity (R Z 0.235, p < 0.001) and total rainfall (RZ 0.131, p < 0.001). By multiple linear regres-sion analysis, these covariate factors were excluded, leaving only ambient temperature as the single predictor of acute aortic event. The territory of Hong Kong is small with
Figure 8 Incidence of aortic dissection in different temperature.
Figure 9 Incidence of ruptured aortic aneurysm in different temperature.
Table 1 Meteorological factors to predict incidence of aortic dissection or ruptured aortic aneurysm by liner regression. Factor Aortic dissection (R, p) Ruptured aortic aneurysm (R, p) Mean atmospheric pressure (hPa) 0.093,<0.001 0.081,<0.001 Relative humidity (%) 0.031, 0.065 0.037, 0.024 Amount of cloud (%) 0.000, 0.997 0.009, 0.572 Rainfall (mm) 0.002, 0.910 0.009, 0.587 Number of lightning strokes 0.018, 0.301 0.021, 0.220 Typhoon 8 or above (Yes or No) 0.004, 0.798 0.008, 0.630 Thunderstorm warning (Yes or No) 0.043, 0.010 0.016, 0.338
Significant p values were bold.
Table 2 Multiple regression analysis for aortic dissection.
Factors B SE B b p
Constant 3.323 5.461 0.543 Mean temperature (C) 0.017 0.006 0.081 0.007 Mean pressure (hPa) 0.005 0.005 0.028 0.372 Thunderstorm 0.016 0.047 0.006 0.740
RZ 0.103, R2Z 0.011, F Z 12.973. Significant p values were bold.
Table 3 Multiple regression analysis for ruptured aortic aneurysm.
Factors B SE B b p
Constant 2.030 3.008 <0.500 Mean temperature (C) 0.011 0.003 0.106 0.001 Mean pressure (hPa) 0.001 0.003 0.016 0.638 Humidity (%) 0.001 0.001 0.019 0.312
RZ 0.099, R2Z 0.010, F Z 12.058. Significant p values were bold.
a total area of 1054 km2, yet it is heavily populated with a population of 7.4 million, 98% of whom are Han Chinese. Climate wise, Hong Kong has four seasons: a cool and dry winter and a hot and humid summer. In the winter, the temperatures hover between 15C and 19C and the coldest months are December and January with temperatures sometimes dropping below 10C. Atmospheric pressure is usually high in winter months. The summer weather is hot, humid, and unstable, with thunderstorms and showers, and temperatures exceeding 30C. Atmospheric pressure is usually low with occasional attacks of low-pressure tropical cyclone or even typhoon.34
To date, the etiology of an acute aortic dissection and aneurysm rupture is poorly understood. It was thought that the moment of dissection could be precipitated by a sudden change of higher-than-normal blood pressures, and that changes in atmospheric pressure and tempera-ture may contribute to this.36 The interaction between
climate, temperature, and blood pressure is complex,37 and the rise in diastolic blood pressure secondary to the exposure to cold is reported to be a well-known physio-logical response caused by sympathetic reactivity.38 In a small prospective cross-seasonal study involving 25 healthy elderly and 21 young volunteers in England, Goodwin et al39found that the ambulatory daytime blood pressures in the elderly individuals were higher in the winter than in the summer and higher in both seasons than in the young people. The seasonally related differences were particularly associated with lower outdoor and in-door temperatures, lower body temperature, and higher activity levels in the elderly group in the winter. Exposure
to cold temperature leads to vasoconstriction and tachy-cardia, both of which contributed to increased blood pressure and cardiac load.40Short- and long-term expo-sure to the cold weather tends to increase central aortic blood pressure and cardiac workload, especially in un-treated hypertensive middle-aged men.41 Guinea et al42
reported a temperature-induced mechanical stress in rabbits at the plaqueevessel interface, which could be enough to promote plaque rupture.42 Cold-induced
hy-pertension and plaqueevessel interface stress may be the cause of acute aortic events in winter.
The risk of adverse cardiovascular health effects attributable to cold temperatures seems to depend on the average annual temperature: for example, the Eurowinter Group found that, in cold periods, coronary event rates increased more in populations living in warm climates than in populations living in cold climates.1This observation was similar to winter mortality resulting from cerebrovascular disease, respiratory disease, and all other causes.43It was suggested that people in colder regions took more precau-tionary measures to outdoor temperature and they had the infrastructure, clothing, and knowledge to deal with cold weather. Thus, it is necessary to promote ways of keeping warm on cold days in subtropical warm regions like Hong Kong, where use of household heater is rare and residents lack the relevant knowledge. Whether promoting public awareness lessens the disease burden of acute aortic events could be a useful study topic in future. By contrast, cardiologists and vascular surgeons should be vigilant in combating acute aortic events in winter months as our study showed a clear surge in incidence rate.
Table 4 Temperature parameter best predicting the incidence of aortic dissection by liner regression.
Temperature parameter R R2 F p
Maximum temperature (C) 0.098 0.010 35.08 <0.001
Mean temperature (C) 0.102 0.010 38.13 <0.001
Minimum temperature (C) 0.102 0.010 38.07 <0.001
Mean temperature 1 d before (C) 0.101 0.010 37.41 <0.001 Difference in temperature compared to 1 d before (C) 0.003 0.000 0.04 0.850 Percentage change in temperature compared to 1 d before (%) 0.002 0.000 0.01 0.921 Mean temperature 2 d before (C) 0.097 0.009 34.72 <0.001 Difference in temperature compared with 2 d before (C) 0.011 0.000 0.41 0.523 Percentage change in temperature compared to 2 d before (%) 0.005 0.000 0.08 0.772 Diurnal variation in temperature (C) 0.001 0.000 0.00 0.965
Table 5 Temperature parameter best predicting the incidence of ruptured aortic aneurysm by liner regression.
Temperature parameter R R2 F p
Maximum temperature (C) 0.096 0.009 33.81 <0.001
Mean temperature (C) 0.098 0.010 35.15 <0.001
Minimum temperature (C) 0.101 0.010 37.72 <0.001
Mean temperature 1 d before (C) 0.096 0.009 33.83 <0.001 Difference in temperature compared to 1 d before (C) 0.006 0.000 0.15 0.702 Percentage change in temperature compared to 1 d before (%) 0.004 0.000 0.06 0.808 Mean temperature 2 d before (C) 0.074 0.006 20.35 <0.001 Difference in temperature compared with 2 d before (C) 0.054 0.003 10.64 0.001 Percentage change in temperature compared to 2 d before (%) 0.045 0.002 7.44 0.006 Diurnal variation in temperature (C) 0.005 0.000 0.10 0.748
There are of course limitations to our study. A pro-spective interpretation of retropro-spectively collected clinical data may have flaws. This study was based on hospital ad-missions and on the presumed accurate electronic data entry of the diagnosis. Patients who died of their acute aortic events may not have full autopsy examination to pinpoint the precise cause of death, or there may be misdiagnosis. Other acute aortic events leading to out-of-hospital deaths may not have been included.
This 10-year population-based longitudinal study is the first pan-territory survey to show an attributable effect of meteorological variables on acute aortic events. Although other health care variables also played an important part in triggering acute aortic events, this study highlighted the need for greater awareness in aortic disease prevention, particularly in advising the elderly population to keep warm during the cold season.
No financial gains or external funds received. The authors are grateful to the public electronic information obtained from the Hong Kong Observatory (http://www.hko.gov.hk/ cis/climat_e.htm) with their written approval.
1.Barnett AG, Dobson AJ, McElduff P, et al. Cold periods and coronary events: an analysis of populations worldwide. J Epi-demiol Community Health. 2005;59:551e557.
2.Bhaskaran K, Hajat S, Haines A, Herrett E, Wilkinson P, Smeeth L. Effects of ambient temperature on the incidence of myocardial infarction. Heart. 2009;95:1760e1769.
3.Danet S, Richard F, Montaye M, et al. Unhealthy effects of atmospheric temperature and pressure on the occurrence of myocardial infarction and coronary deaths. A 10-year survey: the Lille-World Health Organization MONICA project (Moni-toring trends and determinants in cardiovascular disease). Circulation. 1999;100:E1eE7.
4.Dilaveris P, Synetos A, Giannopoulos G, Gialafos E, Pantazis A, Stefanadis C. CLimate Impacts on Myocardial infarction deaths in the Athens TErritory: the CLIMATE study. Heart. 2006;92: 1747e1751.
5.Ebi KL, Exuzides KA, Lau E, Kelsh M, Barnston A. Weather changes associated with hospitalizations for cardiovascular diseases and stroke in California, 1983e1998. Int J Bio-meteorol. 2004;49:48e58.
6.Enquselassie F, Dobson AJ, Alexander HM, Steele PL. Seasons, temperature and coronary disease. Int J Epidemiol. 1993;22: 632e636.
7.Gerber Y, Jacobsen SJ, Killian JM, Weston SA, Roger VL. Sea-sonality and daily weather conditions in relation to myocardial infarction and sudden cardiac death in Olmsted County, Min-nesota, 1979 to 2002. J Am Coll Cardiol. 2006;48:287e292. 8.Goggins WB, Chan EY, Yang CY. Weather, pollution, and acute
myocardial infarction in Hong Kong and Taiwan. Int J Cardiol. 2013;168:243e249.
9. Koken PJ, Piver WT, Ye F, Elixhauser A, Olsen LM, Portier CJ. Temperature, air pollution, and hospitalization for cardiovas-cular diseases among elderly people in Denver. Environ Health Perspect. 2003;111:1312e1317.
10. Messner T, Lundberg V, Wikstro¨m B. A temperature rise is associated with an increase in the number of acute myocardial infarctions in the subarctic area. Int J Circumpolar Health. 2002;61:201e207.
11. Morabito M, Modesti PA, Cecchi L, et al. Relationships between weather and myocardial infarction: a biometeorological approach. Int J Cardiol. 2005;105:288e293.
12. Ohlson CG, Bodin L, Bryngelsson IL, Helsing M, Malmberg L. Winter weather conditions and myocardial infarctions. Scand J Soc Med. 1991;19:20e25.
13. Sharovsky R, Ce´sar LA, Ramires JA. Temperature, air pollution, and mortality from myocardial infarction in Sa˜o Paulo, Brazil. Braz J Med Biol Res. 2004;37:1651e1657.
14. Wang H, Matsumura M, Kakehashi M, Eboshida A. Effects of atmospheric temperature and pressure on the occurrence of acute myocardial infarction in Hiroshima City, Japan. Hir-oshima J Med Sci. 2006;55:45e51.
15. Chang CL, Shipley M, Marmot M, Poulter N. Lower ambient temperature was associated with an increased risk of hospi-talization for stroke and acute myocardial infarction in young women. J Clin Epidemiol. 2004;57:749e757.
16. Dawson J, Weir C, Wright F, et al. Associations between meteorological variables and acute stroke hospital admissions in the west of Scotland. Acta Neurol Scand. 2008;117:85e89. 17. Feigin VL, Nikitin YP, Bots ML, Vinogradova TE, Grobbee DE. A
population-based study of the associations of stroke occur-rence with weather parameters in Siberia, Russia (1982e92). Eur J Neurol. 2000;7:171e178.
18. Jimenez-Conde J, Ois A, Gomis M, et al. Weather as a trigger of stroke. Daily meteorological factors and incidence of stroke subtypes. Cerebrovasc Dis. 2008;26:348e354.
19. Laaidi K, Minier D, Osseby GV, et al. Seasonal variation in strokes incidence and the influence of the meteorological conditions. Rev Neurol (Paris). 2004;160:321e330 [in French]. 20. Magalha˜es R, Silva MC, Correia M, Bailey T. Are stroke occur-rence and outcome related to weather parameters? Results from a population-based study in northern portugal. Cere-brovasc Dis. 2011;32:542e551.
21. Morabito M, Crisci A, Vallorani R, Modesti PA, Gensini GF, Orlandini S. Innovative approaches helpful to enhance knowl-edge on weather-related stroke events over a wide geograph-ical area and a large population. Stroke. 2011;42:593e600. 22. Moreira E, Correia M, Magalha˜es R, Silva MC. Stroke awareness
in urban and rural populations from northern Portugal: knowledge and action are independent. Neuroepidemiology. 2011;36:265e273.
23. Ohshige K, Hori Y, Tochikubo O, Sugiyama M. Influence of weather on emergency transport events coded as stroke: population-based study in Japan. Int J Biometeorol. 2006;50: 305e311.
24. Rothwell PM, Wroe SJ, Slattery J, Warlow CP. Is stroke inci-dence related to season or temperature? The Oxfordshire Community Stroke Project. Lancet. 1996;347:934e936. 25. Manfredini R, Portaluppi F, Salmi R, et al. Seasonal variation in
the occurrence of nontraumatic rupture of thoracic aorta. Am J Emerg Med. 1999;17:672e674.
26. Upshur RE, Mamdani MM, Knight K. Are there seasonal patterns to ruptured aortic aneurysms and dissections of the aorta? Eur J Vasc Endovasc Surg. 2000;20:173e176.
27. Krdzalic A, Rifatbegovic Z, Krdzalic G, Jahic E, Adam VN, Golic D. Atmospheric pressure changes are associated with type A acute aortic dissections and spontaneous abdominal
aortic aneurysm rupture in Tuzla Canton. Med Arh. 2014;68: 156e158.
28. Sterpetti AV, Cavallari N, Allegrucci P, Agosta F, Cavallaro A. Seasonal variation in the incidence of ruptured abdominal aortic aneurysm. J R Coll Surg Edinb. 1995;40:14e15. 29. Ballaro A, Cortina-Borja M, Collin J. A seasonal variation in the
incidence of ruptured abdominal aortic aneurysms. Eur J Vasc Endovasc Surg. 1998;15:429e431.
30. Castleden WM, Mercer JC. Abdominal aortic aneurysms in Western Australia: descriptive epidemiology and patterns of rupture. Br J Surg. 1985;72:109e112.
31. Kakkos SK, Tsolakis JA, Katsafados PG, Androulakis JA. Sea-sonal variation of the abdominal aortic aneurysm rupture in southwestern Greece. Int Angiol. 1997;16:155e157.
32. Liapis C, Sechas M, Iliopoulos D, et al. Seasonal variation in the incidence of ruptured abdominal aortic aneurysm. Eur J Vasc Surg. 1992;6:416e418.
33. 2011 Population Census of Hong Kong. Available from:http:// www.census2011.gov.hk/en/census-result.html [Last accessed: December 22, 2015].
34. Hong Kong Observatory Climate Extract Webpage. Available from: http://www.hko.gov.hk/cis/climat_e.htm [Last accessed: December 22, 2015].
35. Miles J, Shevlin M. Applying Regression and Correlation: A Guide for Students and Researchers. Thousand Oaks, CA: Sage Publications; 2001.
36.Repanos C, Chadha NK. Is there a relationship between weather conditions and aortic dissection? BMC Surg. 2005;5:21. 37.Modesti PA. Season, temperature and blood pressure: a
com-plex interaction. Eur J Intern Med. 2013;24:604e607. 38.Malik M. Sympathovagal balance: a critical appraisal.
39.Goodwin J, Pearce VR, Taylor RS, Read KL, Powers SJ. Seasonal cold and circadian changes in blood pressure and physical ac-tivity in young and elderly people. Age Ageing. 2001;30: 311e317.
40.Cuspidi C, Ochoa JE, Parati G. Seasonal variations in blood pressure: a complex phenomenon. J Hypertens. 2012;30: 1315e1320.
41.Hintsala H, Kandelberg A, Herzig KH, et al. Central aortic blood pressure of hypertensive men during short-term cold exposure. Am J Hypertens. 2014;27:656e664.
42.Guinea GV, Atienza JM, Fantidis P, et al. Effect of athero-sclerosis on thermo-mechanical properties of arterial wall and its repercussion on plaque instability. Int J Cardiol. 2009;132: 444e446.
43.Cold exposure and winter mortality from ischaemic heart dis-ease, cerebrovascular disdis-ease, respiratory disdis-ease, and all causes in warm and cold regions of Europe. The Eurowinter Group. Lancet. 1997;349:1341e1346.