November 15, 2022 | Fitbit smartwatches and fitness trackers can be outfitted with an algorithm that can detect abnormal heart rhythms that have a “very high likelihood of being atrial fibrillation [AFib],” according to Steven A. Lubitz, M.D., a cardiac electrophysiologist at Massachusetts General Hospital and director in translational medicine at Novartis Institutes for BioMedical Research. At least that is suggested by findings of a recent study where the algorithm was found to have a high positive predictive value (98%).
The novel algorithm, developed by Fitbit, uses overlapping, five-minute pulse tachogram windows and requires at least 30 minutes of an irregular rhythm to generate notification that an abnormality was found, he says. While this isn’t considered a medical-grade device, its high specificity for AFib should be reassuring to consumers and doctors.
Given the growing popularity of Fitbit wearables, “validation of the algorithm is a big advance,” says Lubitz. “It opens up access to a tool that can detect this potentially dangerous cardiac arrhythmia to so many people.”
AFib is often asymptomatic yet increases the risk of a stroke. Spotting the condition in unsuspecting patients could therefore save lives by intervening with treatments such as beta blockers and cardiac ablation.
The trial was the largest-ever remote study of wearable devices and enrolled 455,699 participants, about 1% of whom were flagged by the algorithm as having an irregular heart rhythm (4% among those 65 and up). Among the 225 participants who had an irregular heart rhythm detected, atrial fibrillation was confirmed in all but four of those individuals. Results of the Fitbit Heart Study published in Circulation (DOI: 10.1161/CIRCULATIONAHA.122.060291).
Fitbit devices, like most consumer wearables, contain optical photoplethysmography (PPG) sensors to measure pulse rate. The new algorithm analyzes PPG data continuously during periods of inactivity—as determined by device accelerometers—to help identify individuals with undiagnosed AFib.
During the five-month study period, participants were encouraged to wear their device through the night to maximize periods of stationary time. Since an irregular heart rhythm takes a minimum of 30 minutes to sense, “very short episodes of atrial fibrillation may be missed,” Lubitz says. “However, we are learning that it is the longer episodes, and greater burden of atrial fibrillation, that are most strongly linked to adverse outcomes.”
Longer overnight device use is “likely to maximize the sensitivity,” he adds. “Patients, and clinicians, should be aware that irregular heart rhythm detections are likely to represent atrial fibrillation.”
For the study, irregular heart rhythms detected in 1,057 participants was confirmed by having them wear an ECG patch monitor for a week, notes Lubitz. AFib was identified in 32% of cases.
“Longer durations of monitoring are likely to detect more atrial fibrillation since the arrhythmia may self-terminate and may not have occurred during the one-week monitoring period,” he says. “In practice, careful follow-up of patients with an irregular heart rhythm detection on their device—including with confirmatory ambulatory ECG patch monitoring—is warranted.”
Participant attrition was significant, but this is typical with digital trials, reports Lubitz. More than 3,000 people who did not receive notification of an irregular heart rhythm detection withdrew from the study, and only a little over half of enrolled participants returned an end-of-study survey. Moreover, a fair number of participants who received notification of a heart rhythm abnormality contacted their own healthcare provider in lieu of a study physician.
“The large-scale remote nature of the trial is very powerful, but future efforts at enhancing participant adherence to the study protocol are warranted for digital trials,” Lubitz says. “I suspect that engaging patients’ own physician in the study protocol may enhance adherence and long-term participation in [such studies].”
Overwhelming interest in the Fitbit Heart Study “underscores the widespread interest in participation in remote digital research,” he adds. “I think many people are curious about how their wearable and mobile devices can be used to improve health outcomes and are interested in contributing data to facilitate research in this capacity. Since so many people have digital devices, trials that engage participants via digital platforms and incorporate digital endpoints... hold a lot of promise for efficient enrollment and pragmatic conduct.”
Another study that recently appeared in the Canadian Journal of Cardiology (DOI: 10.1016/j.cjca.2022.08.222) threw into question whether smartwatch health apps for detecting AFib are smart enough for the job. But, as Lubitz points out, what was being examined in this case were an Apple algorithm and electrophysiologist accuracy for diagnosing AFib using a single-lead (1L) ECG from an Apple Series 5 smartwatch among hospitalized adult patients with various rhythm abnormalities.
“The article basically notes that neither the Apple ECG algorithm nor the electrophysiologist are perfect at detecting the presence and absence of atrial fibrillation,” says Lubitz. Among the key takeaways here are that “1L-ECGs can be very hard to interpret, and therefore confirmatory medical-grade ECGs are warranted to confirm the presence of AFib.
“For certain patients, particularly those with a risk of or known cardiac rhythm abnormalities, physicians should be very cautious about diagnosing or excluding AFib using a consumer-based 1L-ECG like those used in this study,” he adds.
It is unclear whether physician training in interpretation of 1L consumer ECGs would enhance performance, Lubitz says. But improved algorithms are needed to help them better classify cardiac rhythms with such devices.