Acute respiratory-tract infections and risk of first-time acute myocardial infarction.

Background: There is growing interest in the role of infections in the aetiology of acute myocardial infarction (AMI). We undertook a large, population-based study to explore the association between risk of AMI and recent acute respiratory-tract infection.
Methods: We used data from general practices in the UK (General Practice Research Database). Potential cases were people aged 75 years or younger, with no history of clinical risk factors, who had a first-time diagnosis of AMI between Jan 1, 1994, and Oct 31, 1996. Four controls were matched to each case on age, sex, and the practice attended. The date of the AMI in the case was defined as the index date. For both cases and controls the date of the last respiratory-tract infection before the index date was identified. We also did a case-crossover analysis of cases who had an acute respiratory-tract infection either before the index date or before an arbitrarily chosen date (1 year before AMI).
Findings: In the case-control analysis of 1922 cases and 7649 matched controls, significantly more cases than controls had an acute respiratory-tract infection in the 10 days before the index date (54 [2.8%] vs 72 [0.9%]). The odds ratios, adjusted for smoking and body-mass index, for first-time AMI in association with an acute respiratory-tract infection 1-5, 6-10, 11-15, or 16-30 days before the index date (compared with participants who had no such infection during the preceding year) were 3.6 (95% CI 2.2-5.7), 2.3 (1.3-4.2), 1.8 (1.0-3.3), and 1.0 (0.7-1.6); (test for trend p<0.01). The case-crossover analysis showed a relative risk of 2.7 (1.6-4.7) for AMI in relation to an acute respiratory-tract infection in the 10 days before the index date.
Interpretation: Our findings suggest that in people without a history of clinical risk factors for AMI, acute respiratory-tract infections are associated with an increased risk of AMI for a period of about 2 weeks. We cannot, however, completely exclude the possibility of misdiagnosis bias, if prodromal symptoms of AMI were mistaken for respiratory-tract infection.