I was looking at a heart rate file from a local athlete during their 20 mile long run. I am sure the temperature increased some during this 3 hour effort but I am also sure that dehydration played a role as well.
What is cardiac drift? From Wikipedia:
Cardiovascular drift (CVD, CVdrift) is the phenomenon where some cardiovascular responses begin a time dependent change, or "drift" after around 10 minutes of exercise in a warm or neutral environment. It is characterized by decreases in mean arterial pressure and stroke volume and a parallel increase in heart rate. It is influenced by many factors, most notably the ambient temperature, hydration and the amount of muscle tissue activated during exercise. To promote cooling, blood flow to the skin is increased, resulting in a shift in fluids from blood plasma to the skin tissue. This results in a decrease in pulmonary arterial pressure and reduced stroke volume in the heart. To maintain cardiac output at reduced pressure, the heart rate must be increased.
When it is hot and humid and you are exercising and sweating more than 10 – 20 minutes your heart rate will start to drift up at the same pace. If you are training by heart rate you will need to make an adjustment to keep the same pace. In the graph, after the initial warm-up period, you can see the heart rate gradually creep up. From mile 7 to 20 the pace was within 5 seconds per mile and on a flat course. In those 13 miles the heart rate went from the mid / high 150’s to mid / high 160’s. That is substantial and must be accounted for on your tempo runs.
This is another reason to not look at the heart rate monitor during your races.