Cardiovascular drift

The basic premise of cardiovascular drift is that after 10 minutes of exercise, HR will rise in spite of no change in the work rate or oxygen cost of the exercise being performed (Coyle and Gonzales-Alonso, 2001).There are a number of factors, which continue to be debated, in this effect (Ajisaka, et al, 2000; Cheatham, et al., 2000; Coyle and Gonzales-Alonso, 2001). The agreed fact is that, for a given oxygen uptake, cardiac output must remain constant. What is still debatable is the cause of cardiovascular drift including:

• a decrease in peripheral vascular resistance that leads to a drop in blood pressure as a result of an increase in skin blood flow to meet the needs of thermo-regulation, in light of the fact that blood pressure is a function of the stroke volume component of cardiac output and total peripheral resistance;

• a decrease in plasma volume, due to either dehydration or a shift of fluid from the vascular to the interstitial tissues of the exercising muscle;

• a drop in stroke volume, either as a function of one or both of the factors above, where venous return has decreased and impairs the Frank-Starling mechanism, or possibly, and more simply that a raised HR reduces the time for ventricular filling.

From the exercise leader's perspective, the one undeniable point is that over the duration of an exercise rehabilitation session the target HR should allow for an upward drift in HR by as much as 10 beats-min-1.

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