Friday April 1 2011
The condition prevents the heart from pumping blood effectively
BBC News has reported that “clean living”, such as avoiding smoking and eating more healthily, could prevent most cases of atrial fibrillation, the most common type of heart rhythm disturbance. The condition causes an abnormal heart rhythm, and is associated with an increased risk of stroke and heart attack.
The news is based on a US study of almost 15,000 people. It looked at the proportion of risk of atrial fibrillation associated with some potentially avoidable risk factors – such as high blood pressure, smoking and diabetes. The study followed 14,598 middle-aged adults for 17 years, looking at how these factors were related to their chances of developing the problem. Researchers found that about 1% of participants developed atrial fibrillation, and that around 57% of cases could be explained by having at least one elevated or borderline risk factor.
This study has provided information that will help doctors estimate the maximum effect that prevention strategies might have if they were able to control some of the important risk factors for atrial fibrillation. This study, which has strength due to its size and length of follow-up, adds further support to the importance of a healthy lifestyle in avoiding cardiovascular disease.
Where did the story come from?
The study was carried out by researchers from the University of Minnesota and Wake Forest University in the US. It was funded by the US National Heart, Lung and Blood Institute and the American Heart Association. The study was published in the peer-reviewed medical journal, Circulation.
This story was reported by the BBC. The basics of the story were reported accurately, but they did not highlight the possibility that lifestyle changes may not be able to remove these risk factors completely.
What kind of research was this?
This was a cohort study using data from the Atherosclerosis Risk in Communities (ARIC) study, a large ongoing project to look at various aspects of cardiovascular health. This particular study aimed to assess what proportion of atrial fibrillation could potentially be avoided if known modifiable risk factors for this condition could be controlled. Atrial fibrillation is an abnormal beating rhythm in the top chambers of the heart (the atria). When this occurs, it affects how well the heart can pump blood around the body. If the fibrillation persists, it increases a person’s risk of blood clots forming, which can cause stroke or heart attack.
This type of study is the best way to assess the relationship between a risk factor and a future outcome.
What did the research involve?
The study included 14,598 middle-aged adults (average age 54.2 years) without atrial fibrillation, living in the US. At the start of the study, the researchers assessed participants for known modifiable risk factors for atrial fibrillation (AF). They used this data to classify them as being at elevated risk, borderline risk, or having an optimal risk profile (i.e. least risk of AF). They then followed-up the individuals to identify who developed AF, and calculated what proportion of AF risk could be explained by having elevated or borderline risk.
The risk factors assessed in the study were high blood pressure, elevated body mass index (BMI), diabetes or impaired glucose tolerance, cigarette smoking and previous heart disease. These were assessed in a home interview at the start of the study. The participants also had their heart rhythm assessed using an ECG scan. Any individuals who reported having AF or showed evidence of AF (or the related condition atrial flutter) on ECG were excluded from the analysis.
Subsequently, the participants or their nominated contact person (if the participant could not be contacted) were contacted annually by phone to assess whether they had been hospitalised or died. They were also visited every three years for a more thorough assessment, including another ECG. Cases of AF were identified based on these ECGs, or from hospital records and death certificates.
Using standard methods, the researchers carried out analyses which looked at what proportion of AF risk was related to having borderline and elevated risk factors. These analyses took into account other factors that could affect results, including age, height, education, income, and at which study site they were enrolled. When estimating the effects of the individual risk factors, the researchers took into account the influence of these other risk factors. The analyses looked at the population overall, and also estimated risk according to gender and ethnicity (the study included 5,788 white women; 5,145 white men; 2,266 black women, and 1,399 black men).
What were the basic results?
Just over 5% of participants (5.4%) had an optimal risk profile for AF at the start of the study. This proportion varied by gender and race. About 10% of white women had optimal risk profile, as well as 2.7% of white men, 2.3% of black women and 1.6% of black men. About a quarter of participants had at least one borderline risk factor, and about two thirds had at least one elevated risk factor.
Participants were followed-up for an average of about 17 years. In this time, 1,520 people (10.4%) developed AF. The condition was most common in white men (7.45 cases per 1,000 person-years of follow-up), followed by black men (5.27 cases per 1,000 person years) and white women (4.59 cases per 1,000 person-years). It was least common in black women (3.67 cases per 1,000 person-years of follow-up).
Compared to those with at least one elevated risk factor:
- people with an optimal risk profile were at about a third of the risk of developing AF (relative hazard [RH] 0.33, 95% confidence interval [CI] 0.23 to 0.47)
- people with at least one borderline risk factor were at half the risk of developing AF (RH 0.50, 95% CI 0.44 to 0.57)
Overall, about 50% of AF cases could be explained by having at least one elevated risk factor. An additional 6.5% of AF cases could be explained by having at least one borderline risk factor.
When looking at the individual risk factors separately, elevated or borderline blood pressure appeared to be the most important factor, accounting for 24.5% of the risk for people with elevated or borderline risk factors. Being obese or overweight explained 17.9% of the risk, smoking 11.8%, and having diabetes or impaired glucose tolerance 3.9%. These estimates were broadly similar across different race groups and genders.
How did the researchers interpret the results?
The researchers concluded that “as with other forms of cardiovascular disease, more than half of the AF burden is potentially avoidable through the optimisation of cardiovascular risk factor levels”.
This study provides an estimate of the proportion of atrial fibrillation in the population that might be avoided if certain modifiable risk factors were controlled. Its strengths include its large size, collection of data in a prospective fashion, and long follow-up period. The researchers note that their estimates of the risk attributed to the assessed risk factors are similar to those obtained in a previous study from the US. There are some points to note:
- The level of risk attributable to these modifiable risk factors will vary between different populations; therefore the results of this study may not be applicable to other populations, e.g. in countries with different lifestyles or cardiovascular risk factor profiles.
- The figures from this study suggest the maximum benefit that could be achieved by ensuring that people never reached elevated risk levels in the first place. People who already have elevated risk factors (e.g. high blood pressure) may not be able to fully reduce their risk, even if they control these risk factors.
- Although some of the risk factors assessed could potentially be reduced by healthier lifestyles (e.g. avoiding smoking, reduced alcohol consumption, avoiding being overweight and obese, diet low in saturated fat and salt, increased physical activity, etc.), they may not be completely eliminated by these measures.
- Risk factors were only assessed at the start of the study, and may have changed during follow-up, which could affect results.
- Atrial fibrillation is asymptomatic and transient in some people. Some cases of AF may have been missed if individuals had not been hospitalised or had died of the condition, or the condition was not evident at the time the ECGs were taken. Most AF cases in this study (over 98%) were identified from hospital records, therefore the detected cases are likely to represent mainly the more severe cases of AF, with milder cases being missed.
- Although the study took into account a number of factors that could affect results, other factors could be having an effect.
The sort of information provided by this study gives an estimate of the maximum effect that prevention strategies might have if they ensured that the population had optimal risk factor profiles. The study supports the importance of having a healthy lifestyle for avoiding cardiovascular disease.