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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| V̇O<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| V̇O<sub>2</sub>max]]. Reducing the exercise load to keep the Heart Rate constant will also mitigate, but not eliminate the reduction in [[VO2max| V̇O<sub>2</sub>max]]<ref name="Wingo-2006b"/>. Heart Rate drift and reduction in [[VO2max| V̇O<sub>2</sub>max]] is greater in hot than cooler conditions<ref name="Lafrenz-2008"/>. Heart Rate Drift also increases the [[Maximum Heart Rate]], with one study showing the average [[Maximum Heart Rate]] going from 189 BPM to 194 BPM after 45 minutes of exercise<ref name="Wingo-2005"/>.
=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| V̇O<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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=References=
<references>
<ref name="Plowman"> Sharon A. Plowman, Denise L. Smith, Exercise physiology for health, fitness, and performanc, 2011 !!date!!, Wolters Kluwer Health/Lippincott Williams Wilkins !!publisher!!, Philadelphia !!location!!, isbn 0-7817-7976-6, pages 134</ref>
<ref name="Romijn-1993"> JA. Romijn, EF. Coyle, LS. Sidossis, A. Gastaldelli, JF. Horowitz, E. Endert, RR. Wolfe, Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration., Am J Physiol, volume 265, issue 3 Pt 1, pages E380-91, Sep 1993, PMID [http://www.ncbi.nlm.nih.gov/pubmed/8214047 8214047]</ref>
<ref name="Heigenhauser-1983"> GJ. Heigenhauser, JR. Sutton, NL. Jones, Effect of glycogen depletion on the ventilatory response to exercise., J Appl Physiol, volume 54, issue 2, pages 470-4, Feb 1983, PMID [http://www.ncbi.nlm.nih.gov/pubmed/6833044 6833044]</ref>
<ref name="Wingo-2006a"> JE. Wingo, KJ. Cureton, Body cooling attenuates the decrease in maximal oxygen uptake associated with cardiovascular drift during heat stress., Eur J Appl Physiol, volume 98, issue 1, pages 97-104, Sep 2006, doi [http://dx.doi.org/10.1007/s00421-006-0249-y 10.1007/s00421-006-0249-y], PMID [http://www.ncbi.nlm.nih.gov/pubmed/16896737 16896737]</ref>
</references>
[[Category:Science]]
