Does aggressive care following acute myocardial infarction reduce mortality? Analysis with instrumental variables to compare effectiveness in Canadian and United States patient populations.

BACKGROUND

Previous U.S. studies suggest that the incremental ("marginal") use of the aggressive approach to care for acute myocardial infarction (AMI) in patients differing only in their distance to hospitals offering aggressive care may be associated with small mortality benefits. We hypothesized that the marginal benefits should be larger in Canada, as the country is operating on a lower margin because the approach to care is more conservative overall.

METHODS

This retrospective study used administrative data of hospital admissions and health services for all patients admitted for a first AMI in Quebec in 1988 (n = 8,674). We used differential distances to hospitals offering aggressive care as instrumental variables when measuring mortality up to four years after AMI.

RESULTS

Of the 4,422 subjects who were > or = 65 years old, 11 percent received cardiac catheterization within 90 days after admission. In a previous study that applied similar methodology to the 1987 U.S. Medicare population, 23 percent of subjects received catheterization within 90 days. As in the U.S. study, we found that subjects living closer to hospitals offering aggressive care were more likely to receive aggressive care than subjects living further away (26 percent versus 19 percent received cardiac catheterization within 90 days; 95 percent CI: 5 percent to 9 percent). Unlike the U.S. study, we found no differences in mortality across the "close" versus "far" differential distance groups (unadjusted differences at one year: 1 percent; 95 percent CI: -1 percent to 3 percent). This absence of association held in elderly (> or = 65 years) and younger age groups. Adjusted results also showed no differences between subjects receiving aggressive versus conservative care (at one year: 4 percent; 95 percent CI: -11 percent to 20 percent).

CONCLUSIONS

Contrary to our hypothesis, but consistent with results from numerous randomized trials and observational studies, we cannot confirm that, on the margin, the aggressive approach to post-AMI care is associated with mortality benefits in Canada.