You’re 30 minutes into your run and holding a strong, steady pace. Your stride feels smooth, your breathing is relaxed—overall, you’re feeling it. But then you glance at your watch and notice something odd: Despite running at the same effort for miles now, your heart rate has climbed nearly 10 beats per minute in the last 20 minutes. Um, what’s going on?

Chances are, you’re experiencing cardiovascular drift, a physiological event that occurs with running, cycling, and doing other endurance activities. Also known as cardiac drift, or cardio drift, this phenomenon may sound like a head-scratching occurrence, but it’s actually a “normal physiological response to prolonged exercise,” Tamanna Singh, M.D., sports cardiologist and co-director of the Sports Cardiology Center at the Cleveland Clinic in Cleveland, Ohio, tells Runner’s World.

Here, with the help of two experts, we explain what cardiac drift is, what causes it, ways to mitigate its effects, and signs heart rate fluctuations during exercise are worth a doctor visit.

What is cardiac drift?

Cardiac drift (also known as cardio drift) is the upward climb in heart rate over the course of a workout without any change in intensity, Fabio Comana, faculty lecturer at San Diego State University and master instructor for the National Academy of Sports Medicine, tells Runner’s World.

According to a 2012 review published in the journal Exercise and Sport Sciences Reviews, cardiac drift kicks in after about 10 minutes of exercise at a moderate intensity. That said, timing can vary based on factors including heat and humidity, as well as an athlete’s training and hydration status, says Singh.

As for how much of an uptick in heart rate you can expect with cardiac drift? Again, that hinges on the factors listed above, but it could be as high as a 15 percent increase over the course of your workout, according to Singh.

What causes cardiac drift?

There are a few different physiological effects that explain cardiac drift, says Comana. The primary one is that when you exercise, your body is trying to maintain cardiac output, which is basically how much blood the heart can pump out to the rest of the body, says Singh.

Cardiac output is driven by two things: heart rate and stroke volume (essentially the amount of blood your heart can pump out with each beat).

As you exercise, your stroke volume may decrease due to fluid loss caused by sweating, as well as muscle pump (basically, your muscles contracting), Comana explains. To make up for this decreased stroke volume, your heart rate increases so that cardiac output can stay the same.

Cardiac drift can also be chalked up, in part, to hormones. Exercise stimulates your sympathetic nervous system, which then activates the release of norepinephrine and epinephrine, Comana explains. These hormones build up in your bloodstream and cause your heart rate to gradually increase as you continue to exercise. This effect, says Comana, likely accounts for just a small rise in heart rate.

Finally, there’s the thermal regulatory effect, which basically means that as you exercise, you generate heat and your body has to devote resources to removing that heat. As you continue exercising—and thus keep generating more and more heat—your body has to work a little harder to lower your temperature. This increased work puts stress on the body that can ultimately manifest as a slight rise in heart rate, says Comana.

How does it influence run performance?

First, there’s the mental effect of cardiac drift: If you peep your watch midrun and notice your bpms have climbed, or perhaps you simply feel your heart pounding faster, you might start to think “I’m fatiguing” and lose your psychological drive to keep pushing in your workout, says Comana. In other words, you can psyche yourself into a slower pace.

Then, there’s the physiological effect: As you continue to exercise, there is a higher probability that your physical performance will start to taper off a little bit as your body fatigues from the physiological stress of an elevated heart rate, says Comana.

Can you reduce the effects of cardiac drift?

There are two effective strategies for minimizing cardiac drift, says Comana. The first: Control the environment you’re exercising in to minimize sweating, because fluids lost through sweat are a driver of cardiac drift. “Dry, cooler, breezy days with little humidity” are ideal, says Comana, pointing to 58 to 64 degrees as a “great temperature” for a workout. (Research implies temperatures between 46 and 59 might be most ideal for running.)

Of course, you can’t change the weather, but you can decide at what time of day you’re running and whether that run will occur outdoors or inside on a treadmill.

Second, stay on top of your hydration as that will mitigate how much your stroke volume declines and thus lessen the rise in heart rate due to cardiac drift, says Singh. It’s pretty much impossible to drink enough fluid during exercise to replace the amount you’re sweating out, says Comana, but you can hydrate aggressively before and during your run to combat fluid loss as much as possible.

Singh recommends downing an electrolyte drink the night before a run, especially if it’s a long run or run performed at a high effort. This can help you get a jumpstart on your hydration status as most of us are “probably inherently a little more dehydrated than we suspect,” she explains.

Hydration needs vary, but as a general rule of thumb, start with a baseline of 60 to 80 ounces per day and add fluids according to how long you’re training, as Runner’s World previously reported.

How can you adjust your workouts for cardiac drift?

In terms of how to adjust your workout for cardiac drift, consider first the goal of your workout, advises Singh. Is it to hit a particular pace? Main a specific effort level? Keep your heart rate in a certain zone?

If you’re striving for one of the first two goals, forget about your heart rate levels. Simply try to accept that cardio drift is a standard physiological response that happens with exercise and that you’re not “out of shape” or doing something wrong if you experience it. Instead, understand “heart rate is just a consequence of that work that you’re doing,” says Comana.

Now, if your goal is to hone in on a specific heart rate zone—maybe you’re fixed on that since you have a tendency to take your easy runs too fast, or perhaps you’ve been directed by a doctor to keep your bpms under a certain level—then be prepared for cardiac drift to happen. When it does, simply pull back your effort accordingly to allow your heart rate to decline, says Singh.

When should you see a doctor about heart rate changes during exercise?

Not all heart rate fluctuations during exercise are due to cardiac drift. Here are the scenarios where, barring inaccurate heart rate data, you should terminate your run and consider checking in with a doctor. (Note: Chest straps are the most accurate way to measure heart rate data, though the technology in wrist-based sensors is improving, says Singh.)

  • You notice on a steadily paced run that your heart rate is bouncing up and down.
  • Your heart rate climbs to an unusually high level during exercise.
  • Your heart rate drops significantly during exercise even though you are still working at a similar effort level.
  • You experience any of the following symptoms during exercise: racing heart, palpitations, chest pain, lightheadedness, or dizziness.
  • You feel like something’s off or have a question about your heart rate data. When in doubt, “it’s always better to just see a provider,” says Singh.

Contributing Writer

Jenny is a Boulder, Colorado-based health and fitness journalist. She’s been freelancing for Runner’s World since 2015 and especially loves to write human interest profiles, in-depth service pieces and stories that explore the intersection of exercise and mental health. Her work has also been published by SELF, Men’s Journal, and Condé Nast Traveler, among other outlets. When she’s not running or writing, Jenny enjoys coaching youth swimming, rereading Harry Potter, and buying too many houseplants. 

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