You’re sitting on the couch, maybe scrolling through your phone, when your smartwatch buzzes with a notification that feels a little ominous. It says your heart rate has dipped. You look down and see the number: 45.
That’s low. Like, really low.
Most medical charts tell you that a "normal" resting heart rate is between 60 and 100 beats per minute (bpm). When you see a resting heart rate of 45, the immediate gut reaction is usually a mix of "Am I a super-athlete?" and "Is my heart about to stop?" Honestly, the answer usually lies somewhere in the middle, depending entirely on who you are and how you feel in that moment.
The elite athlete vs. the average person
For a professional cyclist or a marathon runner, a resting heart rate of 45 is basically a badge of honor. It’s evidence of an incredibly efficient cardiovascular system. When the heart muscle is strong, it pumps a larger volume of blood with every single squeeze. This is known as stroke volume. Because the pump is so powerful, the heart doesn't need to beat as often to keep your oxygen levels steady.
Take someone like Miguel Induráin, the five-time Tour de France winner. His resting heart rate was famously reported to be around 28 bpm. Next to that, 45 looks like a sprint.
But most of us aren't Miguel Induráin.
If you’re a "weekend warrior" or someone who just hits the gym three times a week, hitting the mid-40s can be a bit of a surprise. It’s definitely possible for a fit person to have a naturally low pulse, but doctors generally call anything under 60 bpm bradycardia.
When 45 bpm becomes a medical concern
Context is everything. If you feel great, a lower-than-average pulse is usually just a quirk of your physiology or a result of your fitness. But if that resting heart rate of 45 is accompanied by a specific set of "red flag" symptoms, the tone of the conversation changes instantly.
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Are you feeling dizzy when you stand up?
Do you feel like you might faint?
Is there a weird "brain fog" that won't lift?
Does your chest feel tight?
According to the American Heart Association, bradycardia is only considered a clinical problem if it causes symptoms. If your heart isn't pumping enough oxygen-rich blood to your brain, you're going to feel it. Fast. Shortness of breath during a simple walk across the room is a huge indicator that a pulse of 45 isn't cutting it for your body’s needs.
Underlying causes that aren't related to fitness
Sometimes, the heart’s electrical system just starts to wear out. It's not a fun thought, but it happens. The Sinoatrial (SA) node—the body’s natural pacemaker—can sometimes misfire as we age. This is often called Sick Sinus Syndrome.
There are also metabolic issues to consider.
Hypothyroidism is a common culprit. When your thyroid is sluggish, everything slows down. Your metabolism drops, your temperature drops, and yes, your heart rate drops. It’s like your body is trying to run on a battery that’s at 5%.
Then there’s the medication factor.
Are you on Beta-blockers for high blood pressure? Maybe something for anxiety? Drugs like Metoprolol or Atenolol are designed to slow the heart down to take the pressure off the arterial walls. If you’re on these and seeing a resting heart rate of 45, it’s likely a direct result of the chemistry you’re putting into your system.
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The "Sleep Effect" and why time of day matters
You shouldn't judge your heart's health based on a single data point while you're half-asleep. During deep REM sleep, it is perfectly normal for the heart rate to tank. It’s the body’s way of recovering. Seeing a 45 on your Oura ring or Apple Watch data at 3:00 AM is wildly different from seeing it at 2:00 PM while you’re sitting at your desk after a cup of coffee.
If your daytime "awake" rate is 45, that’s when you start asking questions.
How doctors investigate a low pulse
If you go to a cardiologist because you're worried about your low heart rate, they aren't just going to take your pulse and send you home. They need to see the "electrical map" of your heart.
Usually, this starts with an EKG (Electrocardiogram). It’s that test where they stick the cold patches on your chest. It tells them if the rhythm is steady. A resting heart rate of 45 that is "Sinus Bradycardia" (meaning the rhythm is perfect, just slow) is much less concerning than a "Heart Block," where the electrical signal is getting lost between the top and bottom chambers of the heart.
Sometimes they’ll make you wear a Holter monitor. It’s basically a portable EKG you wear for 24 to 48 hours. It catches the dips and spikes while you go about your actual life—eating, stressing over emails, and sleeping. This gives a much more "human" picture of what’s going on than a 30-second snapshot in a sterile doctor’s office.
Electrolytes and the "Quiet" causes
We talk a lot about "serious" diseases, but sometimes a low heart rate is just about chemistry. Your heart runs on minerals.
Potassium.
Magnesium.
Calcium.
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If your electrolytes are out of whack—maybe you’ve been fasting, or you’re over-hydrating with plain water without replacing salts—your heart’s electrical conductivity can change. It’s surprisingly easy to mess this up.
Practical next steps for anyone seeing a 45
Don't panic. Seriously. Panic will just raise your heart rate anyway, which sort of defeats the purpose of checking your baseline.
First, do a "symptom check." If you feel energetic, clear-headed, and capable of exercise, you might just have a very efficient heart. However, you should still mention it at your next physical.
If you are feeling symptomatic, you need to track the data. Use a notebook or a notes app to record your heart rate along with what you were doing and how you felt. "Tuesday, 2 PM, 46 bpm, felt lightheaded" is infinitely more useful to a doctor than "I think my heart is slow sometimes."
Check your supplements. Are you taking high doses of something that could be affecting your rhythm?
Ultimately, a resting heart rate of 45 is a signal. For some, it’s a signal of peak physical conditioning. For others, it’s a whisper from the body that something—be it a thyroid issue, a medication side effect, or a natural aging of the heart's wiring—needs a closer look.
If you are concerned, schedule a basic EKG. It is a non-invasive, relatively cheap test that provides immediate clarity. In the meantime, keep an eye on your hydration and ensure you aren't over-training without adequate rest, as overtraining syndrome can occasionally manifest as an abnormally low or high resting heart rate depending on the person. Focus on how you feel more than the number on the screen. The body is usually much louder about its problems than a smartwatch is.