Seasonal variation in hospital admissions due to acute myocardial infarction according to sex and age in Chile

Background: The incidence of acute myocardial infarction (AMI) varies according to seasonality, being higher in winter. The effect of sex on this phenomenon is not clear. Aim: To evaluate the effect of seasonality in men and women hospitalized for AMI at different ages. Material and Methods: We included all patients with a primary diagnosis of AMI admitted in public and private hospitals in Chile during 2002-2011 (codes 121-122, of the tenth international classification of diseases). We obtained data from the National Discharge databases available at the Ministry of Health website. We estimated the number of discharges per month and per seasonality (cold /template), and the Standardized Incidence Ratio (SIR) with the formula: number of observed cases/expected cases (average annual hospitalizations), stratified by sex and age (< 50 years, 50-64 years, 65-74 years, >= 75 years). We evaluated the effect of sex with binomial regressions for the different age strata. Results: We assessed 59,557 AMI hospitalizations (69% men, with and without ST elevation segment). May, June and July (austral winter) had a SIR of 1.10; 1.12 and 1.10, respectively. Women had a 20% excess of hospitalizations during cold seasons at any age. In men, the excess of hospitalizations increased from 9% in those aged < 50 years to 21% in those >= 75 years (p = 0.043). When comparing women and men, women aged < 50 years showed the higher risk of being hospitalized during cold seasons (adjusted risk ratio = 1.06; 95% confidence intervals 1.01-1.13). Conclusions: Women have a stronger seasonal pattern in AMI hospitalizations than men. While this effect increases with age in men, in women it remains constant at all ages.