Understanding Your Heart Rhythm: A Plain-Language Guide for Wearable ECG Users
- Channy Chan
- Jul 31
- 12 min read
Welcome to the future!
Ok, maybe we thought that we would have flying cars and (more) robots walking around, but it's safe to say that technology has taken the leaps and bounds that science fiction authors from the 1980s told us it would, even if it looks a little different from their predictions: we have the internet, artificial intelligence, and smart devices.
Look down at your wrist. Chances are, most of the people reading this have some sort of smart watch on. Just about 50 years ago, the amount of technology inside of that little watch would have taken up an entire room. One of the coolest functions of these watches is its ability to monitor your heart rhythm (electrocardiogram, also known as an ECG or EKG), and also notify you if something is happening in your heart that is out of the ordinary. But what do these alerts really mean?
You're not the only one who's confused, which goes to show that simply having access to data doesn't mean that you will necessarily know how to interpret it. Wearable smart devices are incredibly powerful tools, and it's remarkable just how much information you can get from their tiny screens. However, if you want to better understand what's going on in your heart when you see those weird lines and alerts, it's worth the time to go over just how the heart and reading heart rhythms works.
The heart as a pump
The heart essentially has one very important job: to pump blood. But "pump blood" is actually two distinct jobs happening simultaneously, which is why the heart is built the way it is.
The heart has four chambers - two on the top (atria) and two on the bottom (ventricles). Think of the atria as the receiving rooms and the ventricles as the main pumping chambers. The right side of the heart handles one circuit and the left side handles another:
The right side collects oxygen-depleted blood returning from the body and sends it to the lungs to pick up fresh oxygen.
The left side collects that oxygen-rich blood returning from the lungs and pumps it out to the rest of the body.
The atria squeeze first, filling the ventricles below them. Then the ventricles squeeze, sending blood either to the lungs (from the right ventricle) or out of the body (from the left ventricle). There are also special valves between the chambers that ensure blood flows in only one direction.
How exactly is the heart capable of doing this? Electricity.
The heart as an electrical system
The heart is a muscle, but it doesn't get signals from your brain to move like the rest of your muscles. Instead, the heart is capable of creating electricity all on its own to initiate its movements. The electrical system is complex and makes sure the heart beats in a specific way, every single time. Here is the path it takes:
The SA node - The Pacemaker
The sinoatrial node (SA node) is a tiny cluster of specialized cells tucked in the upper right atrium. The SA node is the heart's natural pacemaker, full of charged ions that create electricity. It spontaneously generates an electrical impulse roughly 60-100 times per minute in a healthy heart.
The Atria Fire
When the SA node fires, the electrical impulse spreads across both atria. As the electricity moves through the atrial muscle cells, those cells contract and the atria squeeze, pushing blood down into the ventricles.
The AV Node - The Gateway
The electrical signal then arrives at the atrioventricular node (AV node) which sits between the atria and the ventricles. The AV node performs a critical role: it deliberately slows the signal down for a fraction of a second, and this pause gives the ventricles time to fill with blood before they are told to squeeze. Think of it as a toll booth; everyone must briefly stop before proceeding.
The Bundle of His and Purkinje Fibers - The Expressway
Once the signal passes through the AV node, it enters a fast conduction highway called the Bundle of His, which splits into left and right bundle branches that travel down either side of the muscular wall separating the ventricles. From there, tiny fibers called the Purkinje fibers distribute the signal rapidly throughout the entire ventricular muscle, triggering a powerful, coordinated squeeze that propels blood out to the lungs and body.
This entire sequence from the SA node firing to ventricles squeezing takes less than a second, and it repeats, flawlessly, tens of thousands of times every day. Now that we have some information on the heart's structure and electrical system, we can start to piece together what exactly an ECG is trying to tell us.
What does the squiggly line mean?
An ECG, simply, measures the electrical activity created by each heartbeat. If you go to a clinic, stickers called electrodes are placed on the chest which detect the tiny electrical currents generated as the heart's cells depolarize (fire) and repolarize (reset). The resulting signals are drawn as a continuous waveform over time.
Clinical ECGs utilize 12 leads, so the stickers placed on your chest are taking snapshots of your heart's electrical activity from different angles. A wearable smart device uses just a single-lead configuration, meaning the picture is of just one angle. This makes the 12-lead ECG much more detailed and capable of detecting an issue by looking at the heart's electrical conduction from many vantage points. Keep this distinction in mind when looking at what your smart device is telling you.
Breaking down the PQRST complex
The P Wave - The Top Chambers Wake Up
The P wave is a small, smooth, rounded bump that appears at the beginning of each heartbeat cycle. It represents the moment the SA node fires and the signal spreads across the upper chambers, causing them to contract.
In a healthy heart, P waves are will always be present before every "beat" with a consistent size and shape. When P waves are absent or chaotic, that tells us something important about where the electrical signal is (or isn't) coming from, but more on that in the arrhythmia section.
The PR interval - The Gateway Pause
The PR interval is the flat line between the end of the P wave and the start of the next deflection. It represents the signal traveling from the atria through the AV node, including that deliberate pause at the gateway.
A normal PR interval is between 0.12 and 0.20 seconds. If it is too long, the AV node is conducting slower than normal (more on that later). If it is very short, the signal may be bypassing the AV node entirely.
The QRS complex - The Big Squeeze
The QRS complex is the most prominent feature of the ECG waveform. It is a sharp, tall deflection almost like a spike. It represents the electrical activation of the ventricles: that massive, coordinated contraction that sends blood surging out to the lungs and body.
The Q wave is a small initial downward deflection. The R wave is the tall upward spike. The S wave is the downward deflection after the spike. Depending on the lead and the individual, not all three components are always visible - but the term "QRS complex" is used regardless. A normal QRS complex is less than 0.12 seconds wide and is also consistent in size and shape from beat to beat.
A wide or unusually shaped QRS suggests that the ventricles are not conducting normally. The electrical signal may be taking an abnormal route through ventricular tissue rather than the fast-conducting Purkinje fibers.
The ST segment - The Ventricles Hold
The ST segment is the flat line between the end of the QRS complex and the beginning of the T wave. It represents the brief period when the ventricular muscle cells are all contracted and holding before they begin to recover.
In a normal ECG, the ST segment sits flat at the baseline, almost exactly like the PR interval. ST segment changes (elevation or depression) are clinically very significant and can be a hallmark sign of a myocardial infarction, or the beginning of a heart attack. Wearable smart devices typically are not sensitive enough to detect these changes, but it's worth knowing what it represents, clinically.
The T wave - Recharging
The T wave is a broader, rounded deflection that follows the QRS complex. It represents ventricular repolarization, or the process of the ventricular muscle cells resetting their electrical charge in preparation for the next beat.
T waves are normally upright (positive deflection) in most leads and are smooth and asymmetric in shape. Abnormal T waves that are flipped (negative deflection), peaked, or flattened can be an abnormal finding related to electrolyte imbalances or damage to the heart muscle.
What a Normal Beat Looks Like
Putting it all together, a single normal heartbeat on an ECG looks like this: a small bump (P wave), a flat line, a tall sharp spike (QRS complex), another flat line (ST segment), and a rounded recovery wave (T wave). Then the baseline returns to flat, and the whole sequence repeats.
The distance between two consecutive R wave peaks (called the R-R interval) reflects the time between heartbeats - and therefore the heart rate. A consistent, regular spacing between beats tells you the rhythm is regular.
Variations of normal - sinus rhythms
"Sinus rhythm" means the heartbeat is originating from the right place: the SA node. Normal sinus rhythm (NSR) is, for all intents and purposes, exactly what a healthy heart should be doing and what you should be seeing in terms of its electrical activity on an ECG, clinical or smart device. The heart rate should be 60-100 beats per minute (bpm) with consistent size and shape of the waveforms and lines described before.
Sinus bradycardia (SB) is almost exactly the same as NSR but the only thing that differs is the heart rate, which will be less than 60 bpm. While SB isn't technically "normal," it can be expected based on the circumstances. For example, if you are sleeping or resting, your heart rate may be slower than normal. Athletes will also typically have a slower heart rate because each beat is more efficient at pumping blood to the rest of the body. If you have symptoms such as dizziness, fatigue, or shortness of breath along with the SB, then it may be worth mentioning to your doctor.
On the opposite end, we have sinus tachycardia (ST) which differs with a heart rate of greater than 100 bpm. Unlike SB, ST is almost always a physiological response, a symptom of something else. Exercise, anxiety or stress, fevers, dehydration, blood loss, or stimulants like caffeine can all cause ST. So, if you are on a run and see your heart rate on the higher side, your heart is likely just doing what it should. But, if you have been at home for hours resting after your run and your heart rate is still about 100 bpm, it could be concerning.
When the rhythm gets interesting - ectopic beats
Ectopic beats, or ectopy, are beats that originate outside of the SA node, either rogue electrical activity from the atria or contraction of the ventricles outside of the normal conduction cycle. Ectopy is very common, with a majority of adults having at least occasional ectopic beats over a 24-hour period. They can be completely benign, or you may notice them before your smart device tells you what's going on.
Premature atrial contractions
A premature atrial contraction (PAC) is an early beat that originates somewhere in the atrial tissue, but not the SA node. The signal otherwise travels through the normal conduction path, and will look distinct on an ECG: the QRS or "beat" arrives earlier than expected with a different size and shape of the P wave. There may also be a brief pause (compensatory pause) after the PAC leading to the next normal beat as well.
PACs are very common even among healthy people. They are strongly associated with caffeine, alcohol, stress, fatigue, and stimulant medications, but they can occur without any obvious trigger. In isolation, they are almost always benign. If they occur frequently throughout the day or in runs (three or more PACs in a row) or if you feel any symptoms along with them, your doctor should know about it.
Premature ventricular contractions
A premature ventricular contraction (PVC) is an early beat that originates in the ventricular tissue, bypassing the upper half of the heart. Since the impulse does not travel through the normal fast-conduction system (the Bundle of His and Purkinje fibers), it spreads through the ventricular muscle more slowly, giving the QRS complex a very unique appearance that is wider (greater than 0.12 seconds) than a normal QRS complex, is usually not preceded by a P wave, and also sometimes followed by a compensatory pause.
Much like PACs, PVCs are also very common. The triggers are similar to the ones that can cause PACs, but they can be more concerning than PACs in certain contexts. The more often PVCs occur, they are more likely to cause structural damage to the heart over time. Runs of consecutive PVCs can also be much more serious than a run of PACs.
If your device is capturing what appear to be occasional wide, bizarre beats followed by a pause, you are likely seeing PVCs. Note their frequency, whether you have symptoms, and mention them at your next routine visit.
Cardiac arrhythmias worth knowing
Atrial Fibrillation
Atrial fibrillation (AFib) is by far the most common cardiac arrhythmia, affecting an estimated 37 million people worldwide. In AFib, instead of the SA node initiating a heart beat, the atria instead are bombarded with hundreds of electrical signals. So, instead of contracting fully, the atria end up just quivering.
The AV node still tries to do its job, blocking most of these impulses, but some will still get through to the ventricles. This causes the ventricles to contract in an irregular, disorganized pattern with tachycardic heart rate; if untreated, it can reach up to 200 bpm with a completely irregular rhythm or abnormal spacing between each QRS complex.
On an ECG, you won't see any P waves as the SA node has been essentially eliminated in this equation. Instead the "fibrillation" is noted in the baseline between QRS complexes, a representation of the atria quivering. There will also be no regular intervals in between beats, with no repeating patterns. The HR will also fluctuate greatly, with the number of beats per minute changing a lot every minute.
AFib can be dangerous as the inefficient contractions of the heart chambers can cause blood to pool in the left atrium, increasing the risk for blood clots and stroke. Also, if the heart rate remains very high over time, this can cause a condition called cardiomyopathy which is a weakening and damage to the heart muscle. If your smart device alerts you to a possible AFib reading, please contact your healthcare provider as soon as possible. It's not a reason to panic, but it should be looked at sooner than later.
Supraventricular Tachycardia
Supraventricular tachycardia (SVT) is an umbrella term for several different arrhythmias that originate above ("supra") the ventricles and cause a sudden, rapid heart rate. AFib also falls under this category and shares those characteristics. A key difference is that on an ECG, SVT will look like more of a regular rhythm, with even spacing between QRS complexes. P waves may also be absent, but this is likely due to the high heart rate (up to the 200s) making them unnoticeable. SVT can start and stop abruptly, bringing along symptoms like dizziness, palpitations, and chest discomfort. Much like AFib, SVT might not be immediately concerning if it stops on its own, but should be brought up with your doctor ASAP.
Ventricular Tachycardia
Ventricular tachycardia (VT) is also a rapid rhythm but instead of above, it originates in the ventricles and causes them to contract at a rate typically between 100 and 250 bpm. Unlike SVT, which is usually benign in structurally normal hearts, VT is a rhythm that demands respect.
When the ventricles contract in a fast, disorganized, and inefficient way, the heart may not be able to maintain adequate blood pressure and circulation to the brain and body, causing dizziness, loss of consciousness, or collapse. Sustained VT can degenerate into ventricular fibrillation (VF), where the ventricles quiver chaotically with no effective pumping at all. Ventricular fibrillation is a cardiac arrest and is fatal without immediate intervention.
On an ECG, VT appears as a rapid, mostly regular tachycardia. The QRS complexes are bizarre-looking (known as wide-complex) and can be reminiscent of a run of PVCs, which can be an accurate description. If your device captures what appears to be a rapid, wide-complex tachycardia and you are symptomatic, this is a potential emergency. Call emergency services. A brief run of VT (known as non-sustained VT or NSVT) in an otherwise healthy person should still be brought up with a doctor immediately.
Conclusion
The purpose of this article is not to cause concern or unnecessarily make you see your doctor. Instead, it's to help make some sense of the very important information that you have one your wrist. The world of heart rhythms and cardiology is very large, almost a universe all in its own and truthfully, this article is really just a starter into that universe.
So, what to do with all of this information? It's important to not fixate on the numbers and readings coming from your screen, but instead use it to inform decisions. Smart devices were designed to give people the means to be proactive in their own health decisions. We've spent this time focusing on the heart, but it's also important to listen to your gut. If you see something that makes you feel strange, please bring it up with your doctor.
References:
The Cleveland Clinic
Chambers of the Heart, https://my.clevelandclinic.org/health/body/23074-heart-chambers
Heart Conduction System (Cardiac Conduction), https://my.clevelandclinic.org/health/body/21648-heart-conduction-system
Premature Ventricular Contractions, https://my.clevelandclinic.org/health/diseases/17381-premature-ventricular-contractions
Premature Atrial Contractions, https://my.clevelandclinic.org/health/diseases/21700-premature-atrial-contractions
Atrial Fibrillation (AFib), https://my.clevelandclinic.org/health/diseases/16765-atrial-fibrillation-afib
Supraventricular Tachycardia (SVT), https://my.clevelandclinic.org/health/diseases/22152-svt-supraventricular-tachycardia
Ventricular Tachycardia: Symptoms &Treatment, https://my.clevelandclinic.org/health/diseases/17616-ventricular-tachycardia
Sinus Rhythms Explained: A Comprehensive Guide to ECG Interpretation, retrieved from https://aclscertification.org/rhythms-originating-from-the-sinus-node/
Bridging clinical knowledge and AI: an interpretable transformer framework for ECG diagnosis, retrieved from https://www.nature.com/articles/s41746-025-02215-8
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