The muscles of a healthy heart contract and relax regularly, pumping blood into the heart's ventricles. In A-fib, however, the atria of the heart (or the upper chambers) beat irregularly, which prevents them from effectively moving blood into the ventricles.
If left untreated, the condition can cause blood clots, stroke, and a variety of cardiovascular complications, including heart failure. Approximately 15 to 20 percent of the people who have strokes also have A-fib.
A new, large-scale Swedish study - presented at EuroPrevent 2017, the annual congress of the European Association of Preventive Cardiology - investigates the link between body surface area (BSA) and the risk of A-fib in more than 1.5 million women.
Annika Rosengren, professor of internal medicine at the Sahlgrenska Academy of the University of Gothenburg in Sweden and co-author of the study, explains that the new research builds on a previous study that she also co-authored.
"Our research has previously shown that a large body size at age 20, and weight gain from age 20 to midlife, both independently increase the risk of atrial fibrillation in men. In this study, we investigated the impact of body size on atrial fibrillation risk in women."
Women with the highest BSA 2.61 times more likely to have A-fib
The study looked at 1,522,358 women who had their first pregnancy at an average age of 28 years.
The researchers accessed clinical data on these women's height, age, pregnancy, history of diabetes, high blood pressure, and smoking status from the Swedish Medical Birth Registry. The participants were followed clinically for more than 30 years, and data on A-fib-related hospital admissions were accessed from the Swedish Inpatient Registry.
During the follow-up period, more than 7,000 women were hospitalized with A-fib. The average age at the time of hospitalization was 49 years.
The team calculated the women's BSA in square meters (m2) using a standard formula that considers weight and height. The researchers divided the women into four groups based on their BSA: those with a BSA of between 0.97 and 1.61 m2, those with a BSA between 1.61 and 1.71 m2, a third group with a BSA of 1.71 to 1.82 m2, and finally, those with a BSA of 1.82 to 3.02 m2.
In ascending order of BSA, women in the second, third, and fourth groups had a 1.16, 1.55, and 2.61 times higher risk of A-fib compared with the women in the first and lowest BSA group.
"We found that bigger women have a greater risk of atrial fibrillation. There was a stepwise elevation in risk with increasing body size. The group with the highest body surface area had nearly three times the risk as those with the lowest body surface area."
Prof. Annika Rosengren
How weight, height, and age affect A-fib risk
Rosengren goes on to differentiate between overweight or obese people and people with a larger body surface. Although BSA is influenced by weight and height, it is not necessarily overweight people who are at a risk of A-fib, Rosengren explains, but people with larger bodies who also have a larger atrium.
"Atrial fibrillation is the result of obesity-related metabolic changes, but there is also a second cause," Prof. Rosengren says. "People with a bigger atrium have a higher risk of atrial fibrillation."
"Generally, it is better to be tall because you have less risk of stroke and heart attack, and better survival," Prof. Rosengren continues. "Taller people are often better educated, have higher socioeconomic status, and may have received better nutrition at a young age and in the womb. But in this case being tall is less desirable because it alters the structure of the heart in a way that may be conducive to atrial fibrillation."
However, Prof. Rosengren points out that tall people could also reduce the risk of A-fib by avoiding a build-up of excessive weight.
Prof. Rosengren also notes that age plays a significant role. The absolute risk of A-fib in younger women was revealed to be quite low - namely, lower than 0.5 percent - and this was independent of their weight or BSA. "In general young women need not worry about their risk of atrial fibrillation, whatever their body size," she says, but older men and women might be at a higher risk of A-fib if their BSA is also high.