Significant practice variations in the use of cardiac catheterization procedures have been demonstrated across national borders^4,5 and across provincial borders in Canada^6,7 and state borders in the United States.⁸ Variations across small regional areas have also been identified.^9,10 Although there is some speculation that an ideal cardiac catheterization rate exists, we actually know little about what this ideal rate might be. One potential way to search for an optimal rate is to determine whether there is a population rate of cardiac catheterization beyond which the yield of high-risk CAD does not rise. Identifying such a plateau from cardiac registry data would be useful evidence toward the identification of an optimal catheterization rate.

In 2002 Alberta had a total population of 3.1 million and 17 health regions (Fig. 1). Because of very small populations in some areas, we combined data for regions 14, 15 and 17 for our analysis. The average number of people aged more than 20 years in the resulting 15 areas ranged from 26 564 to 652 536 (based on the average of populations from 1995 to 2002 [www.health.gov.ab.ca/regions/require/list.htm]). Only 2 of these areas (regions 4 and 10) have cardiac catheterization and revascularization facilities. These regions provide invasive investigation and revascularization for the entire province.

Separate statistical analyses were performed for men and women because the optimal population rate of cardiac catheterization may differ between sexes. We categorized patients by the health region in which they resided, according to the demographic information in the APPROACH database. Cardiac catheterization rates for each region were derived as the total number of catheterization procedures divided by the total population over age 20. Rates of high-risk CAD identified were calculated as the number of patients with high-risk CAD divided by the total population in the region over age 20. Cardiac catheterization rates for age categories over 20 were calculated for each region by dividing the total number of catheterization procedures within each age category by the population in that category. The age categories were 20–34, 35–49, 50–64, 65–74, and 75 and over. Rates of high-risk CAD for each region were calculated in the same manner. We then obtained indirectly standardized catheterization rates and high-risk CAD rates per 100 000 population aged over 20 by weighting the age-specific rates on the basis of the 1996 Canadian population. These adjusted rates were obtained separately for men and women.

A simple scatter plot was constructed of the annual catheterization rate versus the annual rate of high-risk CAD for each of the regions, with data points representing individual years. We then studied the relation between the catheterization rates and the high-risk CAD rates using least squares linear regression analysis, fitting straight lines between the 8 points for each health region. Because the observations from each region are not independent, and data from one region are more closely related to each other than to data from other regions, we also performed hierarchical modelling using a mixed-effects linear model, in which random effects were modelled for each region, with regional effects representing random variation around an average underlying relation.¹² In this analysis, we first modelled a random slope and intercept for each region. We then compared this model with a more parsimonious model in which regions shared a common fixed intercept (fixed at zero) and random slopes. If there was no statistical evidence (p < 0.05) of the random intercepts model providing a better fit, the random slopes model (fixed intercept at zero) was accepted as the best representation of the data. The results of this accepted model were then used to plot a single weighted line that best reflected the linear relation between catheterization rate and high-risk CAD rate for all of the regions studied. This “best fit” line, with confidence intervals, was then presented on the original scatter plot.

A different pattern emerged among women: their adjusted catheterization rate ranged from 171.8 per 100 000 adults over age 20 (region 7) to 314.0 per 100 000 (region 5). Most of the remaining health regions had rates in the range of 200–300 per 100 000, significantly lower than the rates among men.

Fig. 2 demonstrates the relation between catheterization rate and yield of high-risk anatomy as a scatter plot containing data for each health region; individual points on the plot represent data for each year. For both men and women, higher cardiac catheterization rates were associated with higher yields of high-risk CAD.

The results of the least squares linear regression analysis are presented in Fig. 3. For men, the slope of the line was positive for all of the health regions (mean 0.35, range 0.11–0.77), which indicated a general tendency across regions for the yield of high-risk CAD to increase with increasing catheterization rates. For women, the slope was positive for all but 2 of the health regions (mean 0.197, range –0.07 to 0.60).

The results of the hierarchical linear modelling are presented in Fig. 4. The final model was a mixed-effects model, in which the regions shared a common, fixed intercept at zero but had randomly varied slopes. This figure also includes an algebraic expression of the linear relation between catheterization rate and yield of high-risk CAD. For both men and women, the slopes are statistically different from zero (p < 0.001).

To statistically test for evidence of a plateau in the yield of high-risk CAD as catheterization rates increased, we formally tested for a quadratic term in our regression analysis. We found no significant coefficient, which implied no plateau effect. We therefore concluded that the yield of high-risk CAD in Alberta during the study period rose linearly with increasing cardiac catheterization rates.

In comparison with other countries, Canada is considered to have a “medium” cardiac catheterization rate, similar to that of Australia, Belgium and Germany, with lower rates reported in the Netherlands, Sweden, the United Kingdom, Hungary and Poland, and the highest rates in Brazil and the United States.^4,13 Our results are consistent with those of a number of previous studies of cardiac procedure rates in Canada that indirectly suggested that a significant number of patients with high-risk CAD are not being identified.^14,15 In a more direct comparison, Batchelor and colleagues¹⁶ determined that the rate of catheterization after myocardial infarction in the United States was more than 2.5 times that in Canada, with US physicians identifying on average 7.4 more cases of severe CAD per 100 post-MI patients than Canadian physicians.

In a previous study of APPROACH data, a large proportion (74.2%) of the patients with high-risk CAD discovered at cardiac catheterization subsequently underwent revascularization procedures; in general, this tendency was uniform across regions.¹⁷ This finding indicates that the discovery of high-risk CAD in Alberta patients usually leads to therapeutic action (i.e., revascularization), which may in turn improve long-term prognosis.

Implicit in some of the preceding discussion is that the use of catheterization is rational and truly linked to need. In reality, this may not be the case. Decisions to perform catheterization procedures are typically made by individual physicians faced with individual patients. Such a one-on-one perspective is often blind to the perspective of population-based needs that our study has attempted to address. Furthermore, this argument could be extended to the question of whether societal investment is best aimed at sick individuals (i.e., clinical interventions) as opposed to sick populations (i.e., population-based health promotion interventions). Rose¹⁸ acknowledged the role of intervention that targets both ends of this spectrum, although he pointed out that the returns may be greater for population-based interventions. In this regard, we acknowledge that our findings need to be complemented by future work assessing the economic implications of various catheterization utilization strategies.

Our study has limitations. Because the inception point of APPROACH is cardiac catheterization, and because no reliable data on regional prevalence of CAD are available, age adjustment of catheterization rates was the only method used to account for true regional differences in prevalance of severe CAD. Also, formal assessments of the appropriateness of the catheterization procedures were not conducted. However, it has been previously shown that inappropriate use of angiography does not explain geographic variations in procedure rates.¹⁹

In conclusion, we found a linear relation between cardiac catheterization rates and the yield of high-risk CAD in all of the 17 health regions in Alberta. Increasing the current rate of use of this procedure could lead to the detection of more patients with high-risk CAD who would then potentially benefit from revascularization. Because Alberta has one of the highest rates of cardiac catheterization in Canada, and because Canada is considered to have an intermediate rate of cardiac catheterization compared with rates in other countries, these results suggest that a higher utilization rate may be required, not only in Canada but in many other countries with similar or lower catheterization rates. Whether an optimal rate of catheterization has been reached in countries with higher utilization rates (e.g., the United States) has yet to be determined, perhaps through the use of analyses similar to those presented here.