The study was renewed in 1995 to determine whether treatment with antidepressant, anticonvulsant, and antiarrhythmic drug therapies having the potential for proarrhythmia increased the risk of primary cardiac arrest. The study was a population-based case-control study nested within a cohort of patients who received medical care at a large pre-paid Health Care Plan in Seattle, Washington. Cases were patients who had a primary cardiac arrest between 1977 to 1994. Controls were a stratified random sample of patients, frequency-matched to cases by age, gender, calendar-year, and known heart disease. Treatment with drugs was assessed through a computerized pharmacy database. Ambulatory-care medical records were reviewed to assess clinical characteristics, including the indication for therapy, the severity of heart disease, co-existing morbidity, and other risk factors. For both antidepressant and anticonvulsant drugs, analyses were stratified by known heart disease, because the risk of treatment might be particularly large among patients with known heart disease. For antiarrhythmic drugs, analyses were restricted by a single, current indication for the therapy--maintenance of sinus rhythm among patients with chronic atrial fibrillation; and, by the availability of a prior echocardiogram, in order to control for the type and severity of underlying heart disease. After adjustment for potential confounders, the investigators estimated the relative safety of: 1) drugs within the same therapeutic class; and, 2) the dosage schedule for specific drugs. In addition, they determined if concurrent treatment with other drugs that altered cardiac conduction or morbidity that altered drug disposition influenced the risk among patients treated with a drug therapy.