title: Use of time-dependent measures to estimate benefits of beta-blockers after myocardial infarction.
abstract: OBJECTIVE
  To estimate the reduction in all-cause mortality conferred by beta-blockers in a population-based cohort of elderly survivors of myocardial infarction during the year following hospital discharge.
  
  
  METHODS
  A dynamic retrospective cohort was assembled from persons aged 66 years and over surviving myocardial infarction in Quebec between 1990 and 1993. Information on hospitalizations was linked to medication and physician claims, demographic characteristics and vital status. Subjects prescribed beta-blockers at hospital discharge had fewer comorbid medical conditions, less pre-existing cardiovascular disease and less severe infarcts. To control for these differences, analyzes were restricted to subjects receiving at least one beta-blocker and mortality was compared between periods with and without beta-blocker exposure using Cox proportional hazard models.
  
  
  RESULTS
  Among 14,547 survivors of myocardial infarction, 41% were dispensed at least one beta-blocker. Among those subjects, the risk of dying during periods of beta-blocker use was reduced 40% (hazard ratio = 0.6; 95% CI: 0.5, 0.7).
  
  
  CONCLUSIONS
  Confounding by indication threatens the validity of observational studies of intended effects of medications. For elderly survivors of myocardial infarction, the estimated benefit of beta-blockers from observational studies is greater than the estimate from randomized trials. Greater benefits do not seem to be an artifact arising from systematically prescribing beta-blockers to subjects with better prognosis. Reducing confounding by indication can enhance the validity of observational studies of medications and widen research applications of administrative health databases. While the actual benefits of medications are never truly known these studies can provide a credible range that brackets the truth.
authors.affiliation: Department of Health Care and Epidemiology, University of British Columbia, Canada. alevy@cheos.ubc.ca
authors.forename: Adrian R
authors.initials: AR
authors.lastname: Levy
authors.forename: Robyn M
authors.initials: RM
authors.lastname: Tamblyn
authors.forename: Michal
authors.initials: M
authors.lastname: Abrahamowicz
authors.forename: Peter J
authors.initials: PJ
authors.lastname: McLeod
authors.forename: David
authors.initials: D
authors.lastname: Fitchett
mesh_terms: D000319:Adrenergic beta-Antagonists / Q000627:therapeutic use*; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009203:Myocardial Infarction / Q000188:drug therapy* / Q000401:mortality; D012189:Retrospective Studies; D012737:Sex Factors; D016019:Survival Analysis; D013997:Time Factors
chemical_list: D000319:Adrenergic beta-Antagonists