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Heart Rate Drift
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Imagine that you’re riding a 3 hour event in hot conditions. You’re taking in fluid with electrolytes appropriately, but your heart rate keeps rising slightly as the event progresses, even though you have maintained a constant effort (same wattage).
The rise in pulse for the same level of exertion is heart rate drift. As blood volume diminishes due to the loss of water by perspiration and exhalation, the blood becomes more viscous (thicker), and the heart must work harder for the same required blood flow— it beats faster to compensate. The body takes fluid primarily from the blood, so that blood volume is affected quickly when exercising in hot conditions. Except that one can also lose a great deal of fluid even under relatively cool conditions, such as a multi-hour endurance workout.
Losing as little as liter of blood volume can raise heart rate by several beats— and it’s easy to lose a litre of blood volume in under an hour on a hot day (and you can only absorb about a litre per hour). Loss of two liters can have much more effect, perhaps 10-15 beats. More than that, and you’re putting terrible stress on your heart, not to mention the possibility of heat stroke as your body loses the ability to cool itself.
Suppose you ride at a constant 250 watts. You might observe the following, in half hour intervals:
00:30: 132 +0
01:00: 133 +1
01:30: 135 +3
02:30: 140 +8
03:00: 147 +15 = dehydrated
The above assumes a constant effort, e.g. 250 watts. If the effort level (wattage) is rising also, then of course heart rate would rise commensurately.
The solution? An appropriate sports drink which enhances fluid absorption so as to maintain as much blood volume as possible.
Even so, the body cannot absorb much more than 1 liter per hour of fluid, so if you’re losing body fluid more rapidly than 1 liter per hour, then your hours of riding are strictly limited. In the Everest Challenge, I consumed about 8 litres of fluid in an 8.5 hour day, and I was still dehydrated by day’s end.
Further reading: Training, Lactate, Pulse Rate @AMAZON, by Peter Janssen.