Understanding Cardiac Drift- When the Heart Rate Doesn't Match the Effort

With the heat and humidity of summer quickly approaching, and the race season well under way, I’ve had some athletes and clients ask me why their heart rate tends to climb, even when their perceived effort remains the same. This is commonly known as ‘Cardiac Drift’ or the slightly more technical name, ‘Aerobic Decoupling’. Whether you are a seasoned athlete or an ‘average Joe or Jill’ looking to get outside and enjoy some exercise, it’s helpful to know what this sensation means and how we can avoid it. 

Let me say upfront I’m not a medical professional. But I’ve seen this happen a lot with athletes (and non-runners/triathletes) over the years. Essentially, cardiac drift is characterized by a higher heart rate than warranted for a specific work load. Typically we see this in runners and cyclists. For example, a runner might be doing a 90 minute, steady effort long run, yet 45 min into the effort, the heart rate begins to slowly climb from 140 bpm to nearly 170 bpm by the end of 90 minutes. While some drift may be normal (5-10 bpm) over the course of the effort, excessive cardiac drift likely has some specific causes. In my experience in working with athletes, there’s a few things that can be causing the heart rate to elevate WITHOUT a rise in effort: 

1. Deconditioning- this is a fancy way of saying, you’re going too hard, too fast, or too far for your current cardiovascular (aerobic) ability. When we haven’t built an appropriate aerobic base, the heart tends to work harder to keep up. This is why Zone 2 (easy, steady-state aerobic work) exercise is so important. Think of it as the foundation from which we build later fitness. 
2. Acute Medical Issues- an elevated HR can signal that there is something wrong with the heart, lungs, or other vital organ. While this is rare, it’s always important to stay on top of routine check ups and be aware of your family’s history of any medical issues that could compromise your ability to exercise. Being fit does not always mean being healthy, as we learned that 2019 Ironman World Champion Runner-Up Tim O’Donnell experienced a massive heart attack in early 2020 at the Challenge Miami triathlon. He has since recovered and returned to racing, but learned that a genetic condition led to his heart attack. 
3. Heat- this is probably the biggest factor affecting a healthy athlete. Heat can be external (air temperature, water temperature, humidity) and/or internal (pushing hard or going long). As we run longer and longer in warmer weather, we can expect that HR will climb. We don't want to blow up (meaning the HR keeps climbing regardless of our reduction in effort, walking, stopping, etc.) to where we can't continue, but we can expect it to climb. This is why we periodically do tests to determine an acceptable HR range of numbers that can help guide our effort. However, in the moment, it can be hard to tell if you should keep going or slow down for the HR to decrease. As a general rule, if the HR keeps climbing but your RPE (rate of perceived effort) is still fairly similar you may want to slow down and wait for that HR to go down or at least stabilize. If it gets too hot, the core temp isn't as able to regulate and the heart has to work harder to push blood around the body causing the elevated HR. In the case of heat, simply slow down and allow your body to recover. 
4. Hydration - as a general rule runners, triathletes, and most everyone else train while dehydrated. There’s no perfect metric to determine the perfect amount of fluids to ingest, but the best gauge of hydration is urine color during exercise. But that's hard to gauge unless you're stopping for bathroom breaks (it should be light yellow- think lemonade:). However, doing a sweat test is a good way to know from the outset what you need to consume during a run at a certain intensity and in certain conditions. (basically you exercise an hour in certain conditions and measure your output and input by weighing your before and after your run/ride/strength routine). If you are dehydrated, your blood volume can go down and the HR goes up. 
5. Fueling - as I’ve mentioned in the past, if an athlete is pushing hard much past 60-90 min, they are running on empty. The trick is not to let the 'tank get that low', but also not to over consume. As we run at higher intensities, we are burning more carbohydrate (sugars) and we become depleted faster, but regardless of the intensity if you run over 90 min without refueling, you're almost empty. Starting the run well-fueled is a good way to ensure you're starting with a full tank. Then, as a general rule 40-70 grams of carbs per hour is what we need to consume to keep us going. As we run longer (2,3,4 hrs) that needs to be on the higher side of the range (pushing toward the 70 g rather than the 40 g). Performance products (gels, chews, drinks) all have specific nutritional information to help an athlete gauge the quantity required during activity. However, some prefer to eat whole food during workouts which requires more diligence in terms of knowing how many carbohydrates are in each serving of food. For example many people love peanut butter and honey on tortilla- me included- but we need to be light on the peanut butter and heavy on the honey, not the reverse as peanut butter is high on fat and low on carb, and the honey is opposite- no fat and high mid-glycemic carb. The tortilla is higher in carbs but usually has a gram or two of fat, meaning the total calories can be high but the carb content medium. If we choose that option, we may also need to consume something else to get hit our hourly carb goal (60 ish). All of this takes lots of practice and trial and error, and nutrition needs to be practiced in training. 
6. Outside factors- this is a catch all term, but essentially I'm talking about going too hard for the conditions, your fitness on the day (meaning how recovered you are from your workouts), etc. If you've been sick OR are about to be sick, a high HR is an indicator. If you haven't slept well before or are experiencing atypical stress, the HR can be elevated. If you're running at altitude, in high humidity, or at night after a long day at work- the HR can be elevated. There are a whole host of reasons the HR can be elevated, it just takes paying attention to determine which one(s) may be contributing. 

Hopefully these tips shed some light on what happens when the HR gets too high too quickly, and FAILS to match the intensity of the workload. Again, some drift is normal, but If this happens to you on a regular basis and is excessive (more than 5-10 bpm), reach out to me directly and let’s see if we can determine the specific cause of your aerobic decoupling.