Apple Watch vs. Fitbit: Which Tracks Your Heart Rate More Accurately During a Dangerous Arrhythmia?
One head-to-head comparison during real arrhythmias found a consistent gap between two popular wearables, and a shared weak spot neither one solved.
Covers a single electrophysiology-lab comparison of Apple Watch and Fitbit heart rate accuracy across several induced and paced heart rhythms, plus a separate, larger comparison of device sensitivity for detecting atrial fibrillation specifically. Does not cover consumer AFib alert accuracy in everyday use, which the site's other AFib-detection pieces address.
When your heart rhythm turns dangerous, the last thing you want is a watch reading that quietly stops matching what your heart is actually doing. Comparing two popular wearables side by side during real, monitored rhythm problems shows they don't lose accuracy at the same point, though both eventually do. One tracked the true rate more closely through most of what was tested, but as the rhythm shifted from an ordinary fast beat toward a genuinely dangerous one, both devices' readings moved further from reality, together.
Two Watches, One Cardiac Rhythm Lab
People who wear one of these watches to keep an eye on a heart condition tend to assume that whichever brand they picked behaves the same way whether everything is normal or something has actually gone wrong. That assumption hasn't really been tested outside of ordinary daily wear, not in any rigorous way, at least not that I found.
In one hospital electrophysiology lab, researchers fitted patients with an Apple Watch Series 4 and a Fitbit Versa at the same time, then induced and paced a series of heart rhythms while recording the true rate from inside the heart as the reference. During an ordinary heartbeat, the Apple Watch read the rate correctly every time, compared with a smaller majority of the time for the Fitbit. Not a close call. Supraventricular tachycardia, a common fast rhythm, is where the two devices landed close to each other. But once that fast rhythm was ending, and again once the rhythm turned into a genuinely dangerous ventricular rhythm, accuracy for both devices dropped hard, and by the ventricular stage neither device was getting the rate right even half the time.
2 studies
- Apple Watch Series 4 and Fitbit Versa were worn together during electrophysiology procedures across 322 induced and paced heart rhythms in 32 patients. Apple Watch read a normal heartbeat correctly 100% of the time versus 87% for Fitbit, and the two were close during supraventricular tachycardia (89% vs 86%). Once that fast rhythm was ending, accuracy fell to 62% vs 41%, and during a ventricular rhythm it fell further, to 41% vs 16%.
- In a separate, larger comparison of five consumer devices for detecting atrial fibrillation itself, Apple Watch and Samsung Galaxy Watch matched each other at 85% sensitivity, while Withings ScanWatch and Fitbit Sense trailed at 58% and 66%.
From Reasonable to Unreliable
The pattern in the electrophysiology-lab comparison is not that one device failed and the other held up. Both devices' accuracy fell as the rhythm became more dangerous, and the gap between them stayed roughly similar in direction (Apple Watch numerically ahead) even as the absolute numbers for both got worse. Most of what's been tested about wearable heart monitoring focuses on spotting atrial fibrillation specifically, which is a different question from whether the continuous heart rate number on your wrist stays trustworthy across a wider range of rhythm problems.
A separate five-device comparison points the same direction without measuring the same thing: sensitivity for catching atrial fibrillation itself varied by device, with a roughly 27-point spread between the strongest and weakest performers in that study. Same signal, different metric. Neither comparison suggests brand alone decides whether you can trust a wearable, it's really which specific rhythm problem is happening at the time that changes the picture.
What This Comparison Doesn't Answer
One comparison. Run inside a single electrophysiology lab, on patients already undergoing a scheduled heart procedure, using two specific, several-generations-old watch models.
It says nothing about whether current hardware and updated detection algorithms close this gap, whether the pattern holds for people outside a hospital setting, or how either brand's most recent watch would compare head to head today.
This comparison used the Apple Watch Series 4 and Fitbit Versa specifically, on 32 patients undergoing an already-scheduled electrophysiology procedure. It doesn't establish how current watch generations, other brands, or people outside a hospital setting would compare.
Common questions
Was one watch just better across the board?
Apple Watch read the rate more accurately than Fitbit in every rhythm category tested in this comparison, sometimes by a wide margin, but both devices' accuracy converged toward unreliable once the rhythm became ventricular.
Would a newer Apple Watch or Fitbit model perform differently?
This comparison used the Apple Watch Series 4 and Fitbit Versa specifically. It doesn't address whether newer hardware or algorithm updates change these numbers.
What is supraventricular tachycardia?
It's a fast heart rhythm originating above the heart's ventricles, and one of the arrhythmia types both devices were tested against in this comparison.
Should I rely on my watch's heart rate reading during a suspected cardiac emergency?
This comparison found both devices' accuracy fell substantially as rhythms became more dangerous, which is worth raising with a cardiologist or care team monitoring an existing arrhythmia rather than something this comparison alone can settle for a specific device or person.