[A case-crossover study between heat waves and daily death from cardiovascular and cerebrovascular disease]

To explore the impct of heat wave on the daily deaths caused by cardiovascular disease and cerebrovascular diseases in Beijing.

A case-crossover design was used to study the impact of 5 heat waves on the daily number of deaths from cardiovascular diseases and cerebrovascular disease, from Jan. 1, 1999 to Jun. 30, 2000. We also investigated the relationship between the heat wave and acute myocardial infarction deaths. The 7th day before death was chose as the indicator of self-control. The OR value of different lengths of risk period was calculated. The highest OR value and its corresponding risk period was used to reflect the impact of heat wave on daily number of cardiovascular and cerebrovascular disease deaths, lag days and their durations.

There were five heat waves during the study period. The first heat wave lasted for 9 days, with the maximum temperature as 38.8°C and average humidity as 46.7%. The OR value for the cardiovascular disease death, cerebrovascular disease death and acute myocardial infarction death were 1.384 (95%CI: 1.128 - 1.697), 1.776 (95%CI: 1.456 - 2.167) and 1.276 (95%CI: 0.905 - 1.799) respectively. The second heat wave lasted 3 days, with the maximum temperature of 36.8°C and average humidity of 61.0%. The OR value for the three causes of death were 1.385 (95%CI: 0.678 - 2.826), 1.300 (95%CI: 0.726 - 2.329) and 2.000 (95%CI: 0.684 - 5.851) respectively. The third heat wave continued for 7 days, with the maximum temperature of 41.5°C, and average humidity of 58.5%. The OR value for the daily death counts caused by cardiovascular disease, cerebrovascular diseases and acute myocardial infarction were 2.613 (95%CI: 2.116 - 3.228), 2.317 (95%CI: 1.875 - 2.863) and 3.088 (95%CI: 2.098 - 4.546) respectively. The fourth wave lasted for 3 days, with the maximum temperature as 39.6°C and average humidity as 31.9%. The OR value for the deaths caused by cardiovascular disease, cerebrovascular diseases and acute myocardial infarction were 1.333 (95%CI: 0.724 - 2.457), 2.429 (95%CI: 1.007 - 5.856) and 3.333 (95%CI: 0.917 - 12.112) respectively. The fifth heat wave lasted for 4 days. The maximum temperature was 37.4°C, and the average humidity was 42.0% during the period. The OR value for daily death counts caused by cardiovascular disease, cerebrovascular disease and acute myocardial infarction were 2.333 (95%CI: 1.187 - 4.588), 1.727 (95%CI: 0.822 - 3.630) and 1.800 (95%CI: 0.603 - 5.371) respectively.

(1) There were significant increases for daily death counts of both cardiovascular and cerebrovascular disease in Beijing during the heat wave and there appeared hysteresis effect as well. The lag phase of cardiovascular disease and cerebrovascular disease death was generally 2 - 4 days and acute myocardial infarction death usually was 0 - 2 days. (2) The rising of maximum temperature was greater at the beginning day of heat wave than the previous day, but the lag time was shorter, and the risk of death was greater, especially for the risk of deaths from cerebrovascular disease. (3) Fluctuations of daily maximum air temperature during the heat wave could increase the risk of death from cardiovascular disease.