Seasonal variation in the incidence of unstable angina and acute myocardial infarction: effect of dry hot climate on the occurrence of complications following acute myocardial infarction.

The incidence of unstable angina (UA) and acute myocardial infarction (AMI), and the occurrence of complications following AMI were studied retrospectively by utilizing data of 7859 patients admitted during three consecutive winter and summer periods from 1979 to 1982. In all patients, UA was more frequent during summer than in winter: 58 out of 3819 (1.5%) versus 37 out of 4040 (0.9%) respectively (P less than 0.05). A similar difference was observed for the incidence of AMI: 183 out of 3819 (4.8%) in summer versus 157 out of 4040 (3.9%) in winter (P less than 0.05). This seasonal variation in the incidence of UA and AMI was significant only in male patients (P less than 0.05). Shock and mortality were higher during summer than in winter in all, and in male AMI patients with or without previous infarction. It is suggested that a hot dry summer is an important risk factor for the occurrence of UA and AMI, and the associated serious complications following the latter in males.