Cardiovascular Drift: Another Reason to Stay Hydrated

Running man with heart rate ECG

Cardiovascular drift occurs during prolonged exercise and is the result of an increasing heart rate and decreasing stroke volume. Cardiovascular drift results from a decrease in blood volume and sympathetic nervous system triggered increase in heart rate. Studies show ways to reduce cardiovascular drift are staying hydrated and biking at a lower pedal cadence.

Cardiovascular drift is the term that describes the physiological changes in heart function during prolonged exercise. During prolonged exercise, stroke volume steadily drops as the heart rate increases. Stroke volume is the amount of blood the heart pumps with every beat. Cardiac output is a function of stroke volume times heart rate. Therefore, cardiac output may remain constant during prolonged exercise despite the increase in heart rate due to compensation from  a drop in stroke volume. Often the decrease in stroke volume actually drops cardiac output despite the increasing heart rate.  When exercise physiologists encountered cardiovascular drift in the late 70's, a clear explanation of the cause of this phenomenon was lacking. However, three and a half decades of research on the subject has increased our understanding of cardiovascular drift.

In 1994 a study found that athletes who were well hydrated before an exercise bout displayed less cardiovascular drift (Influence of graded dehydration on hyperthermia and cardiovascular drift during exercise, 1992, Mountain SJ and Coyle EF). In this study, athletes were monitored on cycle ergometers at moderate intensity for two hours in a high temperature (91°F) and high humidity (50% humidity) environment. Participants were divided into the following groups based hydration before the exercise bout: no fluid, small fluid, moderate fluid and large fluid intake 24 hours prior to exercise bout. Although all four groups saw their body temperature rise over the course of the exercise bout, the groups with a lower level of hydration had a significantly higher core body temperature in the second hour of the cycling protocol. The figure below demonstrates the increase in core body temperature over the exercise bout.

Rectal temperature increases with exercise

Not only did the dehydrated groups have trouble controlling core body temperature, cardiovascular function was affected as well. All four groups saw a decrease in stoke volume and increase in heart rate over the course of the exercise bout. However, these changes had a graded response corresponding to hydration level. In the fully hydrated group, the increased heart rate was able to compensate for the decreased stroke volume and cardiac output was not affected. Cardiac output decreased over the course of the exercise bout in the other three less hydrated groups. The figure below shows cardiovascular data from this study.

Heart rate, cardiac output, stroke volume show cardiovascular drift

Two hypotheses emerged to explain why dehydration and hyperthermia (increase in core body temperature) bring on cardiovascular drift. One hypothesis said that cutaneous blood flow (blood flow in vessels in the skin) resulted in the drop in stroke volume. The idea is that as blood pools in veins in the skin less blood is available to return to the heart. An alternate hypothesis is that an increase in heart rate is responsible for the reduction in stroke volume. An increase in heart rate means there is less filling time for the ventricles; thus, stroke volume is reduced. The second hypothesis is supported by several studies (Cardiovascular Drift During Prolonged Exercise: New Perspectives, 2001, Coyle EF and Alonso JG). 

First, blood flow in the skin does not change after 30 minutes of steady exercise.  Thus, cutaneous blood flow can't explain the changes in blood flow. Second, a study that blocked sympathetic activation of the heart (via beta adrenergic receptor blockers) in humans cycling showed no cardiovascular drift. The sympathetic system accelerates heart rate, a chronotropic effect. Stroke volume and heart rate did not change after reaching a steady state in the participants with blocked sympathetic innervation of the heart. This supports the idea that an increasing heart rate is responsible for the decrease in stroke volume. In addition, hypovolemia (reduced blood volume) is thought to play a role.  Hypovolemia may occur when blood plasma is lost as sweat. A decrease in blood volume triggers a further increase in cardiac output because the body must maintain its blood pressure or mean arterial pressure (MAP). The body resists a drop in MAP by increasing cardiac output through increased sympathetic activity to the heart raising heart rate. An increase in core body temperature also triggers the sympathetic nervous system to increase cardiac output (via heart rate) for thermoregulation purposes. The figure below demonstrates the interplay between cardiovascular factors.

Hydration is a double edged sword against cardiovascular drift. It fights the increase in core body temperature and it maintains blood volume. For this reason, studies have found hydrating during exercise also reduces cardiovascular drift.  Hydrating replaces lost blood plasma to sweat, maintaining arterial pressure. The increase in body water helps resist changes to the core temperature.

Why is cardiovascular drift important?  Cardiac output decreases as stroke volume decreases.  In addition to pumping less blood with every beat, the heart is able to generate maximal force when fully stretched (this property is called the Frank-Starling law of the heart). Blood oxygenation decreases as cardiac output decreases.  This decreases cerebral blood oxygenation, likely a major input in fatigue.  Of interest to cyclers, a recent study found that an increase in pedal cadence, but not external workload, increased cardiovascular drift and an exercise-induced decrease in cerebral oxygen saturation (Cardiovascular drift and cerebral and muscle tissue oxygenation during prolonged cycling at different pedaling cadences, 2012, Kounalakis SN and Geladas ND).

This suggests that maintaining a lower pedal cadence is beneficial to cyclers in competition. The reason for this may be because muscle capillaries in active muscle are occluded during contraction (due to increased pressure of the surrounding contracting muscle fibers). At a higher pedal cadence the muscle capillaries would have less opportunity to fill between contractions. This study also demonstrates that maintaining cardiac output is essential for maintaining cerebral oxygen saturation, a key component of central fatigue.  

In conclusion, the best way to alleviate cardiovascular drift is by staying hydrated and, if you're on a bike, biking at a lower pedal cadence.

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