[A case-crossover study on the association between heat waves and daily deaths caused by acute myocardial infarction]

To explore the effects of heat wave on daily deaths caused by acute myocardial infarction (AMI) in Beijing.

A case-crossover design was used to study the impact of 5 heat waves on the daily number of AMI deaths from Jan. 1, 1999 to Jun. 30, 2000. The effect of heat wave on death in different gender or age groups was also compared. The 7th day before and after death occurred was chosen as its own bi-directional self-control. The OR value and its corresponding risk period was used to reflect the impact of heat wave on daily number of AMI deaths, lag days and duration.

There were five heat waves during the study period. The first heat wave sustained 9 days and the maximum temperature was 38.8°C with average humidity as 46.7%. The OR value for the AMI death was 1.437 (95%CI: 1.066 - 1.937). 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 AMI death was 1.846 (95%CI: 0.671 - 5.076). The third heat wave continued 7 days, with the maximum temperature of 41.5°C with average humidity of 58.5%. The OR value for the daily death counts caused by AMI was 2.427 (95%CI: 1.825 - 3.229). The fourth lasted for 3 days, with the maximum temperature of 39.6°C and average humidity as 31.9%. The OR value for the AMI deaths was 2.857 (95%CI: 1.088 - 7.506). The fifth heat wave lasted for 4 days, with the maximum temperature as 37.4°C, and average humidity as 42.0% during this period. The OR value for daily death counts caused by AMI was 1.500 (95%CI: 0.632 - 3.560). The OR value of the first heat wave for daily death counts of men and women caused by AMI were 1.153 (95%CI: 0.756 - 1.758) and 1.818 (95%CI: 1.185 - 2.790) respectively. The OR value for daily death counts of under 65 age was 1.200 (95%CI: 0.669 - 2.153), with the OR value for the older than 64 age group was 1.534 (95%CI: 1.083 - 2.173). The OR value for daily death counts of older than 64 age women was 1.818 (95%CI: 1.109 - 2.981). The OR values of the third heat wave for daily death counts of men and women caused by AMI were 2.392 (95%CI: 1.649 - 3.470) and 2.514 (95%CI: 1.613 - 3.919) respectively. The OR value for daily death counts of under 65 age group was 2.000 (95%CI: 1.149-3.482) and the OR value for the older than 64 age group was 2.623 (95%CI: 1.880 - 3.660). The OR value for daily death counts of older than 64 age group women was 2.800 (95%CI: 1.676 - 4.678).

(1) Significant increase for daily death counts of AMI in Beijing was noticed during the heat wave and usually causing hysteretic effect. The lag phase was 0 - 2 days in general. (2) The increase of maximum temperature was greater at the beginning day when the heat wave occurred in the previous day, with shorter lag time and greater risk of AMI death. (3) The risk of AMI death in women was greater than in men during the heat wave period. (4) The risk of AMI death among those older than 64 age group was greater than the under 65 year olds. (5) The impact of heat wave to the elderly female was greater.