You might have noticed that sometimes during a long run, at a certain point in your run, your heart rate goes up, while the intensity of the run remains the same. This rise in heart rate is called cardiac drift. In this post I’ll dive into what is cardiac drift and how to deal with it
What is cardiac drift
Cardiac drift (or cardiovascular drift, or CVD, or CVDrift) refers to the upwards drift in heart rate during a longer workout, such as a long run (but it can also happen with cycling, or shorter runs) where the intensity of the workout has remained the same over the course of the run.
stroke volume vs heart rate
There’s actually two things to look at if you want to understand where this cardiact drift comes from: the amount of beats per minute, and the stroke volume (the amount of blood that you heart pumps out per beat).
During your long run, your heart pumps a certain amount of blood per beat (stroke volume) and it does so with a certain amount of beats per minute (for example 140 bpm). If your heart starts to pump less blood per beat (and therefore pumps less oxygen to your muscles per beat), than it did earlier, your heart needs to compensate for this. And it does so by pumping more often. It’s as easy as that. A lower volume per beat, means more beats per minute. At the end of the day you are doing your workout and your muscles need their fuel.
Generally, if your fitness increases over time, your heart muscle gets stronger and your stroke volume increases. This becomes visible in your runs: over time you can run the same pace with a lower heart rate.
Cardiact drift often happens during long runs in the heat (but is certainly not limited to heat). Running increases your core temperature (and hot weather speeds up that process) and you are losing body fluid by sweating it out.
And by sweating out all that fluid you are actually also decreasing the plasma in your blood (filter out the blood cells from your blood, and you have liquid left: that’s plasma). Less plasme means that there’s actually less blood in your body, and therefore less blood for your heart to pump: your stroke volume decreases, as there’s not so much to pump.
And, if your stroke volume decreases, your heart rate has to compensate for that by pumping more often.
That is what cardiac drift is. The upward drift because of the relationship with the decrease in stroke volume. Which often comes from a rise in core temperature.
Cooling the body with ice slurry?
I could not find any study online that said anything about stopping the proces completely. I guess dehydration with long endurance training and racing are just part of the sport.
Yes, I can hear you think: If I get to cool my core, problem solved. I’ve actually found a study on that. Researchers tested the cooling of the body with ice slurry. According to that study ice slurry did indeed attenuate cardiac drift but it did not have any more benefits to performance than drinking fluid does (in this study they compared the vo2max of both groups). If you have access to Pubmed you can read that publication here.
As said above, often cardiac drift is related to dehydration. The ‘best’ way to prevent cardiac drift is making sure your water uptake at least levels that of your fluid loss (by the way, there’s more reasons to drink water on your long runs. For example: it helps you recover much faster).