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Department of Medicine, University of Colorado Health Sciences Center, Aurora, ColoDenver Health and the Rocky Mountain Poison and Drug Center, Denver, ColoColorado Prevention Center, Community Health, Aurora, Colo
Public smoking ordinances may reduce acute myocardial infarction events. Most studies assessed small communities with reported reductions as high as 40%. No reduction or smaller reductions were found in countrywide studies; less is known about the impact of statewide ordinances. We previously demonstrated identical 27% reductions in acute myocardial infarction hospitalizations in 2 Colorado communities after enactment of strict smoking ordinances. Subsequently, on July 1, 2006, a statewide ordinance went into effect. We sought to determine the impact of this legislation on acute myocardial infarction hospitalization rates.
Hospital admissions for a primary acute myocardial infarction diagnosis were examined from 2000 to 2008. Poisson regression models were fit to the monthly events from January 1, 2000, to March 31, 2008. The final model included a quadratic trend over time, harmonic terms, and a post-ordinance effect. The model was adjusted temporally for population changes, using population estimates as an offset variable.
A total of 58,399 unique acute myocardial infarctions were recorded during the study period. No significant reduction in acute myocardial infarction rates was observed post-ordinance (relative risk, 1.059; 95% confidence interval, 0.993-1.131). However, a steep decline in acute myocardial infarction rates was noted from 2000 to 2005 just before enactment. There were 11 strict, local smoking ordinances in effect within Colorado before enactment of the statewide ordinance. After excluding these communities, the findings were similar (relative risk, 1.038; 95% confidence interval, 0.971-1.11).
Although local smoking ordinances in Colorado previously suggested a reduction in acute myocardial infarction hospitalizations, no significant impact of smoke-free legislation was demonstrated at the state level, even after accounting for preexisting ordinances.
The biologic basis for secondhand smoke as a trigger for acute myocardial infarction has been documented extensively. The cascade of ischemic events may be mediated through platelet aggregation and plaque destabilization via augmentation of collagenases that degrade vulnerable intracoronary plaques.
This proposed mechanism of plaque destabilization suggests that secondhand smoke exposure may precipitously increase acute myocardial infarction risk, which could explain the abrupt reduction in acute myocardial infarction rates observed soon after smoking ordinance enactment in both Colorado and Arizona.
These findings from geographically isolated communities have led to the assumption that wider adoption of public smoking restrictions would decrease acute myocardial infarction incidence in the broader population. Comprehensive public smoking ordinances now exist in population centers worldwide. Many studies have attempted to quantify the reduction in acute myocardial infarction risk associated with the elimination of secondhand smoke in public venues. However, there have been considerable discrepancies in the magnitude of the effect of smoke-free legislation. Initial studies examining smoking ordinances and acute myocardial infarction incidence found unexpectedly large reductions in acute myocardial infarction hospitalizations in the post-ordinance period, ranging from 11% to 40%.
Statewide and countrywide smoking regulations provide the opportunity to study larger populations compared with local ordinances. Several countrywide studies have been completed, including studies in the United States, Italy, Ireland, England, Netherlands, and Scotland.
These studies report a lesser impact (no effect to a 17% reduction), suggesting that inadequate sample size could have led to random variation within smaller populations. Two small communities in Colorado previously enacted smoke-free legislation, and a RR reduction of 27% in acute myocardial infarction hospitalizations was demonstrated.
In 2006, Colorado enacted a statewide smoking ordinance. We therefore assessed the impact of this statewide ordinance on acute myocardial infarction hospitalizations overall and after accounting for preexisting local ordinances.
Materials and Methods
On July 1, 2006, the Colorado Clean Indoor Air Act was enacted. The ordinance prohibited smoking in most indoor enclosed areas open to the public, including bars, restaurants, building common areas such as elevators and hallways, and in all areas of employment that were not specifically exempted. Smoking within 15 feet of the main entrance to a building also was prohibited. Outdoor patio areas and cigar bars were excluded, but signage warning the public of possible secondhand smoke was required. The ordinance included a fine of up to $200 for a first violation within a calendar year escalating to a fine up to $500 for the third and each subsequent violation within any given calendar year.
Publically available data on acute myocardial infarction hospitalizations were obtained from the Colorado Hospital Association, an organization that gathers information from all acute care hospitals in the State. Deidentified individual hospitalization records for a primary diagnosis of acute myocardial infarction (International Classification of Diseases, Ninth Revision = 410.xx) were obtained for the study period of January 1, 2000 to March 31, 2008. Secondary diagnoses of acute myocardial infarction were excluded to enhance diagnostic accuracy as previously described.
The statewide sum of acute myocardial infarction admission for each calendar month in the study period was computed. Information about preexisting local ordinances was obtained from the American Non-smokers' Rights Foundation.
A strict local ordinance was defined as one prohibiting smoking in both bars and restaurants and was required to have been enacted at least 1 full month before the statewide ordinance was implemented. Yearly population estimates were obtained from the US Census Bureau to calculate population acute myocardial infarction rates per 100,000 individuals.
Linear interpolation of the yearly data was used to produce monthly population estimates.
The study was exempted by the Colorado Multiple Institutional Review Board. Sociodemographic and clinical characteristics of the study population were tabulated, and differences between the pre- and post-ordinance cohort were compared using the Student t test and the chi-square test. A Poisson regression model was fit to the time series of statewide monthly acute myocardial infarction counts adjusted for monthly population estimates. Predictor variables included a harmonic to model the seasonal trend in primary acute myocardial infarction rates, a cubic model of time to adjust for secular trends, and an indicator variable for post-ordinance effect. We then evaluated the overall population-adjusted acute myocardial infarction rates from 2000 to 2008 and generated relative risk ratios. To determine whether model estimates were biased to the null effect by preexisting local smoking ordinances, analyses were performed subsequently excluding locations with a strict smoke-free ordinance. An unadjusted P value less than .05 was considered to indicate statistical significance. All analyses were performed in SAS version 8.0 (SAS Institute Inc, Cary, NC).
Demographics and hospital length of stay for the population with acute myocardial infarction are summarized in Table 1. Primary acute myocardial infarction hospitalization counts and rates during each year of the study period are shown in Table 2. At the time of the last calendar year during the study period, the state of Colorado had an overall population of 4,939,456 individuals, and a total of 58,399 acute myocardial infarction hospitalizations were recorded. Among the cohort of patients with acute myocardial infarction, 63.9% were male. The mean age at hospitalization was 66.9 years. A sequential time series constructed for acute myocardial infarction hospitalizations both before and after implementation of the statewide smoking ordinance is shown in Figure 1. The time series illustrates both the raw monthly counts and the fitted (predicted) model. Evaluation of the time series plot reveals several prominent features. First, a substantial decline over the course of the study, more prominent during 2000-2005, is apparent. Second, a clear seasonal trend in raw acute myocardial infarction counts is shown with peaks occurring in the winter months. Overall, a nonsignificant increase in trend-adjusted acute myocardial infarction rates was observed after enactment of the smoking ordinance (RR, 1.059; 95% confidence interval [CI], 0.993-1.131).
Table 1Demographic Characteristics of Patients with Acute Myocardial Infarction 2000-2008
In Colorado, a total of 11 strict local smoke free ordinances enacted before the statewide ordinance met our prespecified criteria. Accounting for these local ordinances removed a total of 5411 patients with acute myocardial infarction and 674,634 individuals from the overall census population. Removal of corresponding acute myocardial infarction counts from the numerator and population counts from the denominator did not modify the principal findings significantly (RR, 1.038; 95% CI, 0.971-1.11).
We did not observe a significant decrease in acute myocardial infarction hospitalization rates in Colorado after enactment of a comprehensive statewide smoking ordinance. Even after removal of geographic regions where preexisting smoking ordinances were under enforcement, no statistically significant reduction in acute myocardial infarction hospitalizations was detectable. This contrasts with a number of prior studies, including 2 local smoking ordinance studies in Pueblo and Greeley, Colorado,
and adds to a growing literature that the cardioprotective effect of smoking bans may be less than initially suggested.
A number of explanations for these findings warrant our consideration: First, the ability to discern an impact of smoke-free legislation may be more difficult when evaluating data at a state level compared with assessing geographically isolated communities where ordinance strength and enforcement and the ability to limit confounding variables such as population growth and health care delivery systems changes are more readily accounted for. Second, we accounted for secular trends in acute myocardial infarction incidence, which is essential when using an observational, pre-post time series design. By adjusting for known temporal reductions in acute myocardial infarction incidence in the United States,
a measurable impact of smoke-free legislation may not be discernible. Third, robust acute myocardial infarction reductions seen in smaller community studies may be attenuated because larger sample sizes are used for analysis of the cardiovascular effects of smoke-free policy.
In support of this paradigm, one national study used Medicare Provider Analysis and Review files and national death records; a nonsignificant reduction in acute myocardial infarction-related (RR, −4.1; 95% CI, −9.4 to 1.3) and all-cause (RR, −0.7, 95% CI, −2 to 0.6) mortality was observed 1 year after smoking ordinance enactment.
In this study, researchers evaluated all possible pairs of ordinance and nonordinance hospitals and recorded the change in acute myocardial infarction incidence post-ordinance. They found that RR reductions of 10% or greater were common, but that RR increases of 10% or greater were equally as common; taken in aggregate, the mean was near zero. Another study examined 74 cities geographically distributed across the United States that were affected by smoke-free legislation. Individual cities showed wide variation in acute myocardial infarction incidence after ordinance enactment, with risk ratios ranging from −36% to +54%; however, the mean risk ratio for the 74 cities was 0.97 (95% CI, 0.96-1.02).
These analyses support the hypothesis that small study populations may be more likely to find dramatic changes in acute myocardial infarction incidence, whereas increasing the study sample size attenuates the magnitude of the reduction. Also, review of the studies in aggregate reveals data asymmetry that suggests the potential for publication bias or heterogeneity not entirely explained by a random-effects meta-analysis.
may explain why small sample size studies have tended to report large decreases in acute myocardial infarction incidence, whereas relatively few small sample studies have shown no effect.
Overall, a review of published research shows that acute myocardial infarction RR reduction appears inversely related to sample size. For example, small studies in Bowling Green, Ohio, and Helena, Montana, found dramatic RR reductions (39% and 40%, respectively) but also had few acute myocardial infarction counts (58 acute myocardial infarctions in Bowling Green, 64 acute myocardial infarctions in Helena) and relatively small study populations (30,052 and 68,140, respectively).
Studies in Greeley and Pueblo, Colorado, and Graubünden, Switzerland, found less dramatic RR reductions (27%, 27%, and 22%, respectively), corresponding to somewhat larger study populations (∼86,000, 147,751, and 188,000, respectively).
Given international heterogeneity in population characteristics and healthcare delivery systems, statewide studies of acute myocardial infarction rates localized in the United States provide larger study populations that approximate smaller nations, yet provide samples larger than individual cities. These studies seem to mirror the pattern of lower RR reductions than documented in citywide smoking ordinance studies. These include Delaware (4.7% decrease, population ∼800,000), Massachusetts (7.4% reduction, population ∼3.3 million), and New York (8% reduction, population ∼19 million).
Our study adds to the available statewide ordinance data; however, in the Colorado population of approximately 5 million, no statistically significant decrease in acute myocardial infarction incidence was demonstrated. These data support the hypothesis that small sample size smoking ordinance studies are more prone to find prominent RR reductions in acute myocardial infarction incidence. Studies performed in Piedmont, Italy and the entire country of Italy, offer insight into the variable effects of a nationwide smoking ordinance as a function of population denominator. Increasing sample size from Piedmont (4.3 million) to the entire nation (58 million) attenuated the risk reduction found from 11% to 4%.
Of note, several studies have examined a portion of a countrywide population after a countrywide smoke-free ordinance and demonstrated significant reductions in acute myocardial infarction rates. Studies in Southwest Ireland and Scotland found 12% and 17% risk reductions, respectively.
Data from the Centers for Disease Control National Environmental Public Health Tracking Network recently evaluated secular trends in 20 Network states from 2000 to 2008 using a longitudinal linear mixed effects model.
The authors documented a statistically significant overall decrease in age-adjusted acute myocardial infarction hospitalization rates, with most states showing more than a 20% decline during the period. This temporal reduction in acute myocardial infarction incidence is of a magnitude that exceeds the reduction observed in many smoking ordinance studies. Despite this, some analyses have not accounted for secular trends.
Findings of a reduction in acute myocardial infarction incidence post-ordinance were no longer statistically significant in a number of smaller studies when these trends in acute myocardial infarction rates were accounted for.
Another study compared the decline in acute myocardial infarction mortality in 6 states with smoke-free ordinances, with the average decline among 44 states unaffected by smoke-free policy. No state with a smoke-free ordinance had a significantly lower observed acute myocardial infarction mortality compared with that expected by the nationwide secular decrease in states without the ordinance.
This emerging evidence highlights the importance of accounting for secular trends in acute myocardial infarction incidence before definitive attribution to smoke-free ordinances can be made.
The current study adds to the literature on the cardiovascular impact of smoke-free policy. The evidence-base now includes reports ranging from geographically isolated communities to studies involving large population bases including entire countries. Some of the heterogeneity in outcomes may reflect differences in end points and analytic methods, sample size, and unmeasured confounding variables. In addition, changes in post-ordinance levels of secondhand smoke exposure and variable duration of follow-up may account for differential findings. Overall, available evidence suggests that the decrease in acute myocardial infarction incidence associated with reductions in secondhand smoke exposure may be substantially lower than originally estimated. Although the American Heart Association has endorsed smoke-free policy
as a means to improve the burden of ischemic heart disease, the scientific evidence to support this recommendation is inconclusive and suggests the need for further research.
A number of methodological limitations of the current study merit consideration. First, misclassification of exposure via residential ZIP code may lead to a diminution in our ability to discern small cardioprotective effects of the statewide smoke-free ordinance. Inability to control for current smoking status or confounding variables, such as changes in smoking prevalence and health policy, may have influenced our results. In addition, we evaluated only nonfatal acute myocardial infarction hospitalizations. Sudden cardiac death from ventricular arrhythmia is often the presenting manifestation of acute myocardial infarction in the community setting and is often fatal. This population could not be assessed using the current hospital-based registry. Whether out-of-hospital fatal acute myocardial infarction or sudden cardiovascular death from life-threatening arrhythmia is reduced by smoke-free policy requires further investigation. Finally, it remains possible that enactment of a smoking ordinance in a population with a high baseline smoking prevalence, such as Scotland,
might still exert a substantial positive impact on the incidence of nonfatal acute myocardial infarction hospitalization.
At the state level, a smoke-free ordinance did not seem to have a measurable impact on nonfatal acute myocardial infarction incidence despite favorable effects of smoke-free policy demonstrated within local studies. Smoke-free policy clearly provides an indoor environment devoid of environmental tobacco smoke. Moreover, environmental tobacco smoke exposure may exacerbate other health problems, such as chronic obstructive pulmonary disease and asthma. However, the present study does not definitively demonstrate a reduction in acute myocardial infarction incidence attributable to smoke-free legislation. The possibility that smoke-free policy reduces the risk of out-of-hospital fatal cardiovascular disease events warrants further investigation.
The authors thank Carsten Baumen from the Colorado Department of Public Health and Environment and Adrianna Padgett for administrative assistance.