Bigeminy on an Apple Watch ECG: when every second beat comes early

What bigeminy is

The name comes from the Latin for "twin". In bigeminy, beats arrive in pairs: one normal beat, then one early beat, then a normal beat again, and so on.¹

• Ventricular bigeminy is the most common form. Every second beat is a PVC, an early beat from the lower chambers.
• Atrial bigeminy follows the same pattern with PACs, early beats from the upper chambers.

When every third beat is early, the pattern is called trigeminy.

A real example

This is a real recording in lead I, the lead closest to what an Apple Watch records, with every beat labeled by experts. Scroll sideways to follow it from start to end.

Before the pattern starts, the beats arrive about once a second. Then every second beat comes early, each one about 0.55 to 0.6 seconds after the normal beat before it. ECG Tracker+ marked possible bigeminy from about 7.3 to 12.4 seconds, a stretch with five early beats in a row, each paired with a normal beat. After that, the regular spacing returns.

How to recognize it

1. A repeating short-⁠long rhythm. The gaps between beats alternate: short before each early beat, longer after it. The spacing looks like a limp that repeats.
2. Every second beat looks the same way different. In ventricular bigeminy the early beats are usually wide and shaped differently from the normal ones. In atrial bigeminy they usually look like their neighbors.²
3. It keeps going. One early beat is a single PVC or PAC. Bigeminy is the same early beat repeating in a steady pattern.

How ECG Tracker+ shows it

ECG Tracker+ looks for early beats that alternate with normal beats. When the pattern continues for at least four cycles in a row, it marks possible bigeminy and shades the stretch on the waveform where it was found, so you can see where it starts and ends.

Recordings with many early beats can also change other measurements. HRV is calculated from normal beat-⁠to-⁠beat intervals, so a recording in bigeminy may have too few of them for an HRV value.

Why the app says possible

A single lead cannot always show where an early beat started, and noise or movement can imitate one. Medical experts note that consumer ECG devices cannot currently classify PACs and PVCs the way a clinical assessment does.³ That is why ECG Tracker+ calls it possible bigeminy. The shaded stretch is a cue to look closer and to show a clinician. It is not a confirmed finding.

Why your pulse can feel slow

In bigeminy, an early beat can come so soon that it produces only a weak pulse, or none that can be felt. A pulse you count at the wrist can then be much slower than the heart rate on the ECG, which counts every electrical beat.⁴⁵

What one strip cannot tell you

A 30-⁠second recording shows whether the pattern was present during those 30 seconds. It cannot show how long it lasts, how often it returns or how many early beats you have across a day. Clinicians use a Holter or patch monitor for that.⁶

Frequently asked questions

Is bigeminy dangerous?

Bigeminy is often harmless, especially in people without heart disease or symptoms, but it is worth discussing with a clinician.¹⁵ What matters is how often it happens, whether you have symptoms and the health of your heart, which one strip cannot show.

Can an Apple Watch detect bigeminy?

Apple's ECG app does not label bigeminy, and a recording with frequent early beats may come back Inconclusive. The pattern often shows on the waveform, and ECG Tracker+ marks possible bigeminy for review.

What is the difference between bigeminy and trigeminy?

In bigeminy every second beat is early. In trigeminy every third beat is early, so two normal beats come between each early beat.

Why can my pulse feel slower than my ECG heart rate?

In bigeminy some early beats make only a weak pulse, or none that can be felt. The ECG still counts every electrical beat, so it can show a higher rate than the pulse you count.