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Heart Rate Drift is the reduction in the amount of blood the heart pumps with each beat (Stroke Volume) and the resulting rise in [[Heart Rate]] as the heart attempts to maintain the flow of blood (Cardiac Output). This drift starts happening after about 10 minutes of moderate (50-75% [[VO2max| V̇O₂max]]) exercise. Heart Rate Drift is also called cardiovascular drift.Athletes should be aware of HR drift, which can often be seen due to a rise in HR for the same effort. The keys to limiting HR drift are keeping as cool as possible, hydrating appropriately, and taking on carbohydrates.

The traditional explanation for Heart Rate Drift is that blood flow to the skin is increased, which reduces available blood volume for the rest of the body and therefore the ability of the heart to fill with blood, reducing the stroke volume. However, in hydrated subjects, the stroke volume is maintained even though the blood flow to the skin increases<ref name="González-Alonso-2000"/>. An intriguing study gave subjects either a placebo or a beta blocker then looked at heart rate and stroke volume over the course of 60 min cycling at 57% [[VO2max| V̇O₂max]]. With the placebo, the subjects showed the classic heart rate drift, but with the beta blockers, the heart rate and cardiac output remained steady<ref name="Fritzsche-1999"/>. (In both conditions, the subjects received a carbohydrate drink to prevent dehydration and both conditions had the same perceived exertion.) There is reasonable evidence that the reduction in stroke volume is the result of dehydration and/or overheating, with each contributing about equally in most cases<ref name="Coyle-2001"/>. The putative mechanism behind dehydration and HR drift is that dehydration reduces the pressure of blood flowing into the heart, which reduces the amount the heart fills, which reduces the stroke volume.