AFib irregularity (CV%) on Apple Watch ECG
When your Apple Watch detects atrial fibrillation (AFib), ECG+ goes one step further: it measures how irregular your rhythm is during that recording. That's the irregularity score. Here's what it measures and how to read it.
ECG+ measures beat-to-beat timing during AFib and expresses how irregular it is as a CV percentage.
What it measures
In AFib, the heart doesn't follow a regular beat-to-beat pattern. ECG+ analyzes the time between each heartbeat and calculates how much those intervals vary, expressed as a CV percentage: short for Coefficient of Variation. Put simply: a low CV means your beat timing was relatively consistent during the recording; a high CV means the timing was more erratic, with bigger swings between beats.
What the levels mean
These thresholds are grounded in published research. Clinical studies on AFib RR intervals (Tateno & Glass, 2001) found CV during AFib typically falls between 15% and 32%, with a median around 24%. The levels below reflect that distribution, most people in AFib land in the Moderate range, which is exactly where the research says they should.
- Low (CV below 15%): your AFib rhythm was on the calmer, more regular end during this recording. Reassuring: the electrical activity wasn't highly chaotic.
- Moderate (CV 15–30%): a typical level of irregularity for AFib. Most people fall here; not a cause for alarm on its own.
- High (CV above 30%): more pronounced beat-to-beat variation. Worth mentioning to your doctor, especially if it appears consistently.
Should you be worried?
Not based on a single reading. The irregularity score is a snapshot of one 30-second window: it can vary between recordings depending on activity, stress, medication and how the AFib episode is progressing. A single High result doesn't mean something has worsened. What matters more is the pattern over time: if you consistently see High irregularity across many recordings, that's worth discussing with your doctor. You can follow that pattern on the AFib irregularity over time chart.
What to watch for
Pay more attention to how you feel than to the number. If you notice palpitations, breathlessness, chest discomfort or unusual fatigue during an AFib episode, contact your healthcare provider, regardless of what the irregularity score shows. And share your ECG+ recordings with your doctor: the trend across recordings is far more useful to them than any single score.
This information is for general education only and isn't a substitute for professional medical advice, diagnosis or treatment. Always consult your healthcare provider with any concerns about your heart rhythm.
Frequently asked questions
What is the AFib irregularity score?
When AFib is detected, ECG+ measures how much the time between heartbeats varies and expresses it as a CV percentage (coefficient of variation). A low CV means relatively consistent beat timing; a high CV means more erratic timing.
What is a normal AFib irregularity level?
Research on AFib RR intervals (Tateno & Glass, 2001) found CV typically falls between 15% and 32%, with a median around 24%. Most people in AFib land in the Moderate range (15–30%), which is exactly where the evidence says they should.
Should I worry about a high irregularity score?
Not based on one reading. The score is a 30-second snapshot that varies with activity, stress and medication. A pattern of High results across many recordings is what's worth discussing with your doctor.