What is Cardiovascular Drift? If you've ever trained with a heart rate monitor you might have noticed that your heart rate increases as the workout progresses even though your speed and effort remain the same. What you’re experiencing is Cardiac drift. It's really a normal occurrence and can increase your heart rate up to 20 beats per minutes over a 30 minute period. You'll find it's more evident during hotter conditions and there are a few reasons that contribute to this. First, dehydration causes a lowering of blood plasma or volume forcing the heart to work harder to support working muscles and cool the body simultaneously. Correspondingly, the stroke volume of the heart decreases so that cardiac output and oxygen uptake remain the same, keeping your breathing and effort similar while heart rate elevates. Acclimating to the heat and humidity can help diminish Cardiac drift but it will still occur to some extent. It's important to remember low body fluids can still occur in cooler conditions so this is not a heat only occurrence. Hydration is a double edged sword against cardiovascular drift. It fights the increase in core body temperature and it maintains blood volume.
Another contributing issue is muscular fatigue. During extended training when slow twitch muscle fibers fatigue (endurance fibers) your body attempts to compensate by recruiting additional muscle types. These muscles are generally fast twitch fibers which possess more brute strength but generate more lactic acid which in turn can elevate heart rate. This process is kind of a built in survival mechanism in the human body.
Lastly relax, anxiety, stress and tension can be contributing factors.