Effect Modifiers of the Association Between Air Pollution and Cardiac Readmissions Among Survivors of First Myocardial Infarction

Abstract

SM4-PD-16 Introduction: A recent study found that, among survivors of a first myocardial infarction, elevated levels of ambient air pollution are associated with increased hospital admissions for cardiac causes. Whether this association occurs uniformly in this group or is modified by other factors is unknown. We analyzed effect modification by gender, area-based socioeconomic status, and chronic comorbidities in a cohort of myocardial infarction survivors with index event in the period 1998 to 2000 and follow-up through the end of 2004. Methods: Hospitalization data were obtained from the regional Hospital Discharge Registry. Persons (aged 35+ years) admitted to the hospital for a first acute myocardial infarction were included in the cohort if they survived for at least 28 days (N = 7382). They were followed up from the 29th day until death, first readmission for a cardiac cause (new myocardial infarction, angina, dysrhythmia, or heart failure), or end of study. Vital status was ascertained and hospital readmissions identified via record linkage. Daily air pollution exposures were calculated as the means from several monitors measuring PM10 and from single monitors for ozone (urban background level, daily maximum 8-hour average, only during summer months) and particle number counts (representing ultrafine particles, estimated for 1998 to March 2001 from other pollutants and meteorologic measurements). Poisson models were applied to stratified datasets, adjusting for trend, temperature, day of week, and holidays. Relative risks were calculated for an increase of one interquartile range of the respective pollutant (PM10 22.03 μg/m3, ozone 39.11 μg/m3, ultrafines 28,350/cm3) for various lags. Results reported are for the mean of lags 0 and 1. Results: Among 5160 men and 2222 women who survived a first myocardial infarction, 1675 and 719, respectively, were subsequently hospitalized for a cardiac cause. The overall relative risk (RR) for ultrafine particles was 1.090 (95% CI, 1.005–1.181), for PM10: 1.039 (95% CI, 0.987–1.094), and for ozone: 1.089 (95% CI, 0.994–1.193). No evidence was found to indicate that area-based socioeconomic status modified these effects. However, gender was an effect modifier (eg, ultrafines, men's RR = 1.045; 95% CI, 0.952–1.148, women's RR = 1.196; 95% CI, 1.058–1.352). There was convincing evidence that diabetes modified the effects of particles (eg, ultrafines, RR = 1.260; 95% CI, 1.107–1.434) versus RR = 1.037 (95% CI, 0.947–1.136) and that chronic obstructive pulmonary disease modified the effect of ozone. Discussion and Conclusions: Various characteristics of post-infarction patients may make them more susceptible to another hospital admission for cardiac causes. These characteristics include gender and comorbidity. In particular, diabetics experience a greater effect of particulate air pollution than other myocardial infarction survivors.