Quantification of atrial fibrillation risks

Atrial fibrillation (AFib) is a common heart rhythm disorder (arrhythmia) characterized by rapid and irregular beating of the atria, the two upper chambers of the heart which pump blood to the ventricles, the two lower chambers of the heart. The palpitations (rapid but shallow beating) of atria disrupt the normal flow of blood in heart increasing the risk of blood clots and stroke, as well as stressing the heart muscles that can lead to heart failure and other heart-related complications. AFib can be asymptomatic or present with symptoms such as palpitations, shortness of breath, fatigue, and dizziness. Risk factors for developing AFib include age, high blood pressure, heart disease, obesity, and diabetes. Management typically involves medications to control heart rate and rhythm, anticoagulants to prevent stroke, and lifestyle modifications. In some cases, procedures like electrical cardioversion or catheter ablation may be necessary to restore normal heart rhythm. Early detection and treatment are crucial to reduce the risk of complications associated with AFib. 

There are now a host of consumer devices that can accurately detect AFib, most notably the Apple Watch. More specifically, the heart’s electrical signals are altered by AFib producing an abnormal ECG signature (QH). In late 2018 at the unveiling of the Apple Watch Series 4, Apple announced that the new watch would have an ECG (EKG) monitor, and that this monitor along with an AFib-detecting algorithm had been cleared by the FDA based on a study between Apple and Stanford. In this joint study, researchers found that the Apple Watch ECG monitor could detect AFib with nearly perfect accuracy approaching 99% (QH). In addition, there are a number of stand-alone consumer ECG monitors ranging from bulky medical-grade chest monitors to easily portable lightweight devices similar to the Apple Watch (QH). 

It is not fully appreciated how widespread atrial fibrillation is. AFib is estimated to affect 17.9 million people in Europe by 2060 and 15.9 million in the US by 2050. A previous study in 2018, calculated the overall lifetime risk to be from 31 to 37%. Despite improvements in mortality rates among individuals with AFib, the condition still poses significant risks of stroke, heart failure, and myocardial infarction. While stroke is commonly considered the main danger after AFib diagnosis, other complications are significant and need to be quantified to improve understanding and communication of the AFib burden.

To this end an investigation was undertaken to examine changes in the lifetime risks of atrial fibrillation (AFib) and its complications over time. A population-based cohort study comprised of 3,574,903 Danish individuals aged 45 years or older without AF examined the lifetime risks of atrial fibrillation (AFib) and its complications from 2000 to 2022. The primary aim was to estimate these risks and analyze temporal trends between two periods: 2000-2010 and 2011-2022. The subjects were followed until the earliest occurrence of AF, death, emigration, or the end of the study period. The primary outcome was the incidence of AFib (hospital diagnosis), and then for individuals diagnosed with AFib, the study further assessed the incidence of complications including heart failure, stroke, myocardial infarction, and systemic embolism.

The first main result was that the lifetime risk of developing AFib increased significantly from 24.2% in the period 2000-2010 to 30.9% in the period 2011-2022, representing an absolute increase of 6.7% (see Figure 1). Men were found to have a higher lifetime risk of AF compared to women, with risks of 33.6% and 28.2%, respectively. These numbers are for the index age of 45, i.e. probability of developing AFib after the age of 45. AFib is age-dependent, and typically appears after 60, and is especially common in those 75 or older.

For individuals diagnosed with AFib, the study further examined the risks of subsequent complications, including heart failure, stroke, myocardial infarction, and systemic embolism. Heart failure emerged as the most common complication, with a lifetime risk of 42.9% in the period 2000-2010 and 42.1% in 2011-2022, indicating no significant change over time. In contrast, the lifetime risk of any stroke decreased slightly from 22.4% to 19.9%, and the risk of ischemic stroke dropped more significantly from 16.1% to 10.8%. Similarly, the lifetime risk of myocardial infarction decreased from 13.7% to 9.8%, while the risk of systemic embolism remained low and stable at around 1.8%.

In summary, there were two surprises from the results. First is that the risk of developing AFib increased substantially from 2000-2010 to 2011-2022 from 24.2% to 30.9%. The latter number is still smaller than previous estimate cited above (i.e. 31-37%). Multiple factors are likely to contribute to this increase including more frequent diagnosis. 

The other surprise is that heart failure is a bigger risk complication of AFib than stroke. The lifetime risk of heart failure in subjects diagnosed with AFib is 2 out of 5 compared to 1 out of 5 for stroke. The heart failure numbers have not changed much from 2000-2010 to 2011-2020, while the stroke risk has decreased modestly with ischemic stroke accounting for most of the drop. Thus, the study highlights the need for enhanced preventive strategies for heart failure in patients with AFib, as current guidelines primarily focus on stroke prevention.

A final consideration is that the relative risk of stroke or heart failure is roughly 3 to 5 times higher in people with AFib compared to those without. Thus, given the significant frequency of AFib in the population, one can reduce heart failure or stroke risk in the general public by identifying and treating those with AFib. 

Figure 1. "Cumulative incidence of complications after atrial fibrillation." The index age is a reference point for analysis and refers to the approximate age of a group of participants at the start of the study. In the text above, the numbers are with respect to the 45 year-old index age with the plot showing the cumulative incidence of heart failure (blue), myocardial infarction (yellow), stroke (purple), and systemic embolism (orange) at ages from 45 to 95 (Vinter et al. BMJ, 2024).