Smoking Status and Risk for Recurrent Coronary Events after Myocardial Infarction

Background Smoking cessation decreases the risk for myocardial infarction among people without previous coronary disease, but its effect on risk reduction after a first myocardial infarction has not been well studied. Contribution The researchers retrospectively observed relative risk for recurrent coronary events in patients discharged from the hospital after an incident myocardial infarction. Compared to nonsmokers, active smokers were 1.5 times more likely to have reinfarction. Among persons who had stopped smoking, relative risk decreased gradually to the nonsmokers' level over 36 months. Implications Physicians should emphasize the proven benefits of smoking cessation to encourage patients to quit after a first myocardial infarction. The Editors Smoking is a risk factor for coronary artery disease (1). Among persons without coronary disease who quit smoking, the risk for incident coronary disease decreases and approaches that of nonsmokers within 2 to 3 years after cessation (2, 3). Some studies (4-17) show that quitting smoking after myocardial infarction also seems to reduce the risk for adverse outcomes, including death. However, these studies of persons with established disease generally focused on men 65 years of age or younger, had limited information about possible confounding factors, did not evaluate the time course of the potential benefits of smoking cessation, and did not compare results to those in nonsmokers. In contrast to these studies showing a benefit of smoking cessation after myocardial infarction, some evidence suggests that after infarction, smokers may not have an elevated risk for recurrent events compared with never-smokers, a finding termed the smoker's paradox (18-24). We examined the association between smoking status and risk for recurrent coronary events and explored the time course of potential benefits of cessation in a large population-based inception cohort of persons who survived to hospital discharge after an incident myocardial infarction. Methods Setting and Participants The study setting was Group Health Cooperative (GHC), a health maintenance organization with more than 500 000 enrollees, based in western Washington State. The organization's model of health care delivery is structured so that most routine care for enrollees, including care after myocardial infarction, is provided by personal primary care physicians. Group Health Cooperative employs more than 300 primary care physicians and contracts with more than 500 other physicians. Eligible patients included all GHC enrollees who survived to hospital discharge after a first myocardial infarction during the period from July 1986 (women) or July 1989 (men) through December 1996. Patients with incident myocardial infarction were identified by International Classification of Diseases, 9th revision, codes from the computerized discharge abstracts of the two GHC hospitals, bills for out-of-plan services provided by non-GHC physicians and health care facilities, and Washington State death records. A trained records abstractor, assisted by study physicians, validated the incident infarctions by reviewing available inpatient medical records. Previous work (25) has confirmed the accuracy and completeness of the methods used for ascertainment of myocardial infarction. Persons were excluded if they were younger than 30 or older than 79 years of age at the time of the incident infarction, had previously had a myocardial infarction, had the incident infarction as a result of surgery or another procedure, died before hospital discharge, or had no information available in the medical record after the incident infarction. A total of 50 patients (1.8% of the total otherwise eligible patients) were discharged alive from the hospital but had no information in the chart after the incident infarction. Persons were also excluded if they had made fewer than four visits to GHC or had been enrolled in GHC for less than 1 year before the incident infarction. Among persons otherwise eligible for inclusion, 9.3% were excluded because of inadequate duration of enrollment in GHC. Of the 2677 patients who were initially eligible, 58 were excluded because of indeterminate smoking status, leaving 2619 persons (97.8% of the original cohort) for analysis. The Human Subjects Review Committees of GHC approved the study. Smoking Status We identified smoking status on the basis of ambulatory care and inpatient records. The GHC ambulatory medical record is an important resource because it serves as the primary method of communication among GHC physicians. It includes not only notes from ambulatory care visits but also discharge summaries of hospitalizations, results of laboratory and diagnostic tests, consultant reports, responses to annual GHC health questionnaires, and updated problem lists. Data collection included a review of the entire GHC ambulatory medical record for the period before the first infarction, the period of hospitalization associated with the incident infarction, and the period after the infarction up to a predetermined date in 1996, 1997, or 1998. Patients were classified as nonsmokers (persons with no history of smoking in the medical record), former smokers (persons who had smoked but had quit before the incident myocardial infarction), quitters (persons who were smoking at the time of the incident myocardial infarction and quit after the infarction), and active smokers (persons who were smoking at the time of the incident myocardial infarction and continued to smoke during follow-up). Only those persons who quit smoking after their incident infarction and maintained cessation for the duration of follow-up were classified as quitters. Information about the quit date, number of cigarettes smoked per day, and duration of smoking over time was also recorded. Patients were closely monitored after the incident infarction; 97.6% had a clinic visit within 3 months after hospital discharge, and the median number of visits (physician, nurse, or emergency department visits or hospitalizations) during the first year after the incident infarction was 9 (25th, 75th percentiles: 6, 14). Clinical and Medication Covariates Trained research assistants reviewed the ambulatory care and inpatient medical records to collect information about potential risk factors for coronary disease, such as use of health services; marital status; alcohol consumption; body weight; height; and medical conditions such as hypertension, diabetes, and congestive heart failure. Congestive heart failure was determined to have occurred on the basis of the notes of the primary care physician and consultants and the results of diagnostic tests. Hypertension was defined as pharmacologically treated hypertension; diabetes was defined as diabetes pharmacologically treated with oral hypoglycemic agents or insulin. In addition, laboratory values for random serum glucose, total cholesterol, and high-density lipoprotein cholesterol were recorded. Medication use was assessed at the time of hospital discharge after the incident infarction by using information from the discharge summary supplemented with information from the first outpatient visit and the GHC computerized pharmacy database. The status of the clinical and medication covariates was determined at the time of hospital discharge. Laboratory information was taken from the period before the incident infarction. End Point The outcome of interest was recurrent coronary events, defined as nonfatal myocardial infarction or coronary death. Information on these end points was obtained from the GHC ambulatory medical record, available inpatient records, and the results of matches between Washington State death records and GHC enrollment records. In patients with established coronary disease, if the underlying cause of death was heart disease, heart failure, or arrhythmia, the death was classified as a coronary death. Statistical Analysis Follow-up time began at the date of discharge from the hospital after the incident infarction, and it extended to the date of the first recurrent infarction, coronary death, or censoring. Persons were censored at the date of death from causes other than heart disease, the date of unenrollment from GHC, or the end of the assigned follow-up period. Cox proportional-hazards regression analysis with a time-dependent smoking status variable was used to model the association of smoking status and recurrent coronary events after adjustment for potential confounding factors (26). A patient who quit smoking during hospitalization for the incident myocardial infarction was classified as a quitter. For those who quit smoking after discharge, smoking status was modeled with person-years accrued initially as if the patient were an active smoker and then after cessation as if the patient were a quitter. To investigate the risk for recurrent coronary events in relation to the duration of smoking cessation, we divided follow-up time into four mutually exclusive categories of duration of cessation (0 to <6 months, 6 to <18 months, 18 to <36 months, and 36 months). Because the underlying time scale used in the regression analysis was time since hospital discharge after the first myocardial infarction, the Cox model adjusts for time since infarction, enabling an assessment of the relationship of duration of cessation independent of time since infarction. In addition, because most patients who quit smoking did so very soon after the incident infarction, time since hospital discharge and time since cessation of smoking were similar for many quitters. To help discern whether the change seen among quitters in the hazard ratio over time was due to smoking cessation rather than to any other characteristic of smokers that changed over time after discharge, we also evaluated whether the coronary event hazard ratio among active smokers varied with time since infarction. We calculated the unadjusted rates of recurrent coronar