Wednesday, August 01, 2012

New Study Concludes that Smokefree Workplace Law Reduced Sudden Cardiac Death in the Netherlands; However, Study Fails to Include Any Comparison Group

A new study published online ahead of print in the journal Heart concludes that the smokefree workplace law enacted in the Netherlands on January 1, 2004 resulted in a 6.8% decline in out-of-hospital sudden cardiac deaths within one year of implementation.

The study methods are as follows: "Weekly incidence data on SCA were obtained from the ambulance registry of South Limburg, the Netherlands. Three time periods were distinguished: the pre-ban period (1 January 2002–1 January 2004), the first post-ban period (1 January 2004–1 July 2008) and the second post-ban period (1 July 2008–1 May 2010). Trends in absolute SCA [sudden circulatory arrest] incidence were analysed using Poisson regression, adjusted for population size, ambient temperature, air pollution and influenza rates."

The results were as follows: "A total of 2305 SCA cases were observed (mean weekly incidence 5.3±2.3 SD). The adjusted Poisson regression model showed a small but significant increase in SCA incidence during the pre-ban period (+0.20% cases per week, p=0.044). This trend changed significantly after implementation of the first ban (with −0.24% cases per week, p=0.043), translating into a 6.8% (22 cases) reduction in the number of SCA cases after 1 year of smoke-free legislation. No further decrease was seen after the second smoking ban."

The paper concludes that the observed reduction in sudden circulatory arrest was attributable to reduced secondhand smoke exposure due to the workplace smoking ban: "The most reasonable explanation for our findings is that, on a population level, the reduction in exposure to secondhand smoke was larger after introduetion of the workplace smoking ban than after the smoking ban in the hospitality sector. ... In condusion. this study adds to the evidence that smoking bans benefit public health by reducing the risk of acute cardiovascular events. Moreover, this study shows that a workplace smoking ban is effective for preventing a condition with a high medical and societal impact."

The Rest of the Story

This study commits one of the most cardinal errors in all of epidemiology: the failure to include a comparison group when studying a phenomenon which is changing rapidly over time, even in the absence of the intervention of interest.

Cardiovascular disease mortality is dropping rapidly throughout the world, during the precise time period of this study. Therefore, without examining how changes in sudden cardiac death rates in the Netherlands compare to changes occurring in other countries, it is impossible for the investigators to conclude that the observed changes are attributable to the smoking ban, rather than that they merely reflect secular changes occurring anyway due to factors such as vastly improved treatments for severe coronary artery disease and increased control of hypertension and hypercholesterolemia.

Because there is no comparison group, I do not believe that these data support a conclusion that the observed decline in heart attack admissions is attributable to the workplace smoking ban. The article examines trends in one locality in the Netherlands, but it does not examine what happened to sudden heart attack deaths anywhere else during the same time period. Without knowing what the sudden heart attack death trends were elsewhere, there is no basis to conclude that the observed decline in sudden heart attack deaths in the Netherlands was attributable to the smoking ban.

This is essentially an uncontrolled study - a study without a control group. It represents a very weak study design for this type of analysis. Even the shoddy Bowling Green and Pueblo studies employed a comparison group to evaluate whether the observed changes in heart attacks in the cities with smoking bans were also occurring in comparable cities without smoking bans.

So what did happen in other places in the world between 2003 and 2004? 

Well, in South Carolina, heart attack admissions fell by 12.5% from 2003 to 2004. This is also in marked contrast to the existing trend in heart attacks in that state. Heart attack admissions were increasing by an average of 3.0% per year during the period 2001-2003 in South Carolina. So was the 12.5% decline in heart attack admissions during the same time period in South Carolina due to the absence of a statewide smoking ban?

In Nebraska, heart attack admissions fell by 28.5% from 2003 to 2004. This is in marked contrast to the existing trend in heart attacks in the state. Heart attack admissions were increasing by an average of 2.3% per year during the period 2001-2003 in Nebraska. Was this dramatic drop in heart attack admissions in Nebraska, which occurred during exactly the same period as the decline in the Netherlands, attributable to the absence of a smoking ban in the Cornhusker state?

In fact, in all other U.S. states for which data are available, heart attack admissions fell by 5.1% from 2003 to 2004. So does the decline in heart attacks in the Netherlands represent a dramatic effect of the smoking ban, or is it simply a change that would have been expected based on the secular trends occurring in developed countries during the same time period?

The article in question does not allow us to answer that question. And that is precisely why its conclusion is unwarranted.

Now, I am of course not arguing that the absence of a smoking ban in South Carolina and Nebraska was the reason why their heart attack admissions dropped far more than in the Netherlands during the same time period. The point is that there are large year-to-year variations in heart attacks that have nothing to do with smoking bans and in order to conclude that a small decline (such as 6.8%) in sudden cardiac deaths was due to a smoking ban, you absolutely have to show that the decline would not have occurred in absence of the smoking ban. And to do that, you need to look at what is happening in other places.

One lesson here is that even if a study is published, you still need to review it critically and you should not necessarily assume that its conclusions are valid and well-supported. 

As much as we might like to believe that reducing secondhand smoke exposure prevents thousands of heart attacks in a matter of months, the evidence is simply not there to support such a conclusion. By jumping the gun and drawing conclusions prematurely, I fear that we are hurting our overall scientific credibility. In the long run, that may harm the effort to promote smoking bans far more than spreading the belief that such bans are going to immediately prevent heart attacks is going to help enact these bans.

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