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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| V̇O2maxV̇O<sub>2</sub>max]]) exercise. Heart Rate Drift is also called cardiovascular drift.
=Heart Rate Drift and V̇O<sub>2</sub>max=
Decreases in stroke volume of 10-16% and increases in heart rate of 12-16% are associated with a 9% to 19% reduction in [[VO2max| V̇O2maxV̇O<sub>2</sub>max]]<ref name="Ganio-2006"/><ref name="Wingo-2006b"/><ref name="Wingo-2006a"/><ref name="Wingo-2005"/>. Fluid intake<ref name="Ganio-2006"/> or body cooling<ref name="Wingo-2006a"/> will mitigate the reduction in stroke volume and [[VO2max| V̇O2maxV̇O<sub>2</sub>max]]. Reducing the exercise load to keep the Heart Rate constant will also mitigate, but not eliminate the reduction in [[VO2max| V̇O2maxV̇O<sub>2</sub>max]]<ref name="Wingo-2006b"/>. Heart Rate drift and reduction in [[VO2max| V̇O2maxV̇O<sub>2</sub>max]] is greater in hot than cooler conditions<ref name="Lafrenz-2008"/>.
=Mechanisms of Heart Rate Drift=
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| V̇O2maxV̇O<sub>2</sub>max]]. 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 (hypothermia), with each contributing about equally in most cases<ref name="Coyle-2001"/>.
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