Atrial fibrillation doesn’t produce the steady “lub-dub, lub-dub” you’d expect from a healthy heart. Instead, the rhythm becomes completely unpredictable, with beats landing at random intervals and varying in strength. Doctors describe this pattern as “irregularly irregular,” meaning there’s no repeating cycle at all. If you could listen through a stethoscope, you’d hear something like a drummer who has lost the beat entirely, with some strikes loud, others faint, and the gaps between them constantly changing.
What a Normal Heartbeat Sounds Like
A healthy heart produces two distinct sounds per beat. The first sound (“lub”) happens when the valves between the upper and lower chambers snap shut as the lower chambers begin to squeeze. The second sound (“dub”) follows quickly when the valves leading to the lungs and body close after the squeeze finishes. These two sounds repeat in a steady, predictable rhythm, typically 60 to 100 times per minute at rest. You can think of it as a metronome: evenly spaced, consistent in volume, and reliable enough that you could tap your foot along with it.
How Afib Changes That Sound
In atrial fibrillation, the heart’s upper chambers stop contracting in an organized way. Instead of squeezing once per beat, they quiver or tremble with chaotic electrical signals. This trembling is forceful enough to vibrate the heart’s walls, valves, and surrounding structures, creating additional sound artifacts that muddy the clean lub-dub pattern.
The bigger change, though, is in the rhythm itself. Because the upper chambers are firing erratically, the lower chambers receive electrical signals at random times. Some beats come rapid-fire, barely a fraction of a second apart. Others are separated by a noticeable pause. The volume of each beat varies too: a beat that follows a long pause tends to be stronger (the heart had more time to fill with blood), while a beat that comes right on the heels of another is weaker and sometimes barely audible. Through a stethoscope, it sounds chaotic, like someone tapping on a table with no rhythm or pattern.
The heart rate during afib often runs faster than normal. Guidelines consider rates below 100 to 110 beats per minute acceptable for people with afib, but untreated episodes can push well above that range. At high rates, the beats crowd together so closely that distinguishing individual sounds becomes difficult, producing something closer to a rapid, disorganized flutter.
What Afib Feels Like From the Inside
Most people with afib don’t literally “hear” their heart, but they feel it in ways that are hard to miss. The most common descriptions include a fast, fluttering sensation in the chest, a pounding heartbeat, or the feeling that the heart is skipping beats. Some people describe it as a fish flopping in their chest. Others say it feels like their heart is racing and stumbling at the same time.
These sensations, called palpitations, can show up in the chest or even the neck. They range from vague and hard to pin down to intensely noticeable. What separates afib palpitations from other types is their randomness. A single skipped beat (from a premature contraction) feels like one sudden thud followed by a return to normal rhythm. Afib, by contrast, stays irregular for the duration of the episode, which can last minutes, hours, or indefinitely. There’s no settling back into a steady pattern until the episode ends on its own or is treated.
Some people feel nothing at all. Silent afib is common enough that many cases are discovered incidentally during a routine exam or when a wearable device flags an irregular rhythm.
How Afib Differs From Other Irregular Rhythms
Not every skipped beat or flutter is afib. Premature beats, which are extra heartbeats that nearly everyone experiences occasionally, create a distinct sensation: a single skip or thud, sometimes followed by a stronger-than-usual beat as the heart resets. The key difference is that premature beats interrupt an otherwise regular rhythm. You feel a momentary hiccup, then things go back to normal.
Afib has no “normal” baseline during an episode. The irregularity is continuous and unpredictable. If you tap your finger along with your pulse during afib, you’ll find it impossible to establish any pattern. With premature beats, you can still identify the underlying steady rhythm between the skips.
Other fast rhythms, like supraventricular tachycardia, tend to start and stop abruptly but remain regular while they’re happening. Your heart races at a fixed, rapid rate. Afib races too, but the speed constantly shifts. That combination of fast and unpredictable is what makes afib distinctive.
Checking Your Own Pulse for Afib
You can get a rough sense of your heart’s rhythm by placing two fingers on the inside of your wrist, just below the thumb, and pressing gently until you feel the pulse. Hold it for at least 15 seconds and pay attention to two things: whether the beats arrive at even intervals, and whether each beat feels equally strong.
A normal pulse feels like a steady tap, tap, tap with equal spacing. In afib, the taps come at random intervals and some feel weaker than others. You might also notice that certain beats you can hear with a stethoscope (or feel in your chest) don’t produce a pulse at your wrist at all. This happens because some heartbeats are too weak to push blood all the way to your wrist. That gap between heartbeats and detectable pulses is called a pulse deficit, and it’s a hallmark of afib.
If you notice a completely irregular pulse that doesn’t settle into any pattern over 30 seconds or more, that’s worth getting evaluated. A single skipped beat here and there is usually benign.
What Wearable Devices Can Detect
Smartwatches and fitness trackers have added another way to catch afib. These devices use two different approaches, and their accuracy varies dramatically depending on which one is doing the detecting.
Passive rhythm monitoring, the feature that runs in the background and sends you an “irregular rhythm” notification, has high specificity (essentially no false positives in one study of the Apple Watch) but low sensitivity, catching only about 21% of afib episodes in people wearing a simultaneous medical-grade heart monitor. That means if it alerts you, it’s almost certainly real, but it misses most episodes.
The on-demand ECG feature, where you place your finger on the watch and take a 30-second reading, performs far better. In the same study, it matched the medical monitor with 100% sensitivity and 99.1% specificity. The catch is that you have to actively take the reading, which means you need to feel something unusual and think to check. If you have a smartwatch with an ECG feature and you feel fluttering or skipping, taking a reading in the moment and saving it gives your doctor something concrete to review.