During sustained endurance efforts there is a phenomenon that occurs when the effort remains constant and the heart rate starts to climb. It is known as Cardiac Drift and is an indication that performance efficiency is decreasing. In other words, the effort required to sustain the constant output begins to increase.
We recently published an article about interval training and how core body temperature values can be used to balance effort during a training session; (ie. starting at higher than FTP and ending at lower than FTP to compensate for the rise in core body temperature). The effect is that the effort is constant rather than increasing.
This idea can also be used for cardiac drift in what would be called 'heat pacing'. It is simply slowing the pace to compensate for the increase in core body temperature with the intent that the heart is not working harder towards the end of a training workout. Rather, the pacing adjustment based on the core body temperature allows for a consistent heart rate without any cardiac drift.
Polar also has an excellent article covering cardiac drift and effects on training. In this article Polar write, "When core body temperature increases, a similar increase is seen in heart rate. Furthermore, when core body temperature increases the body responds by increasing skin blood flow to help control the temperature rise. The increase in skin blood flow occurs at the same time that the working muscles demand a large proportion of blood flow, which creates competition and demand from different parts of the body."
Cardiovascular drift (CVD, CVdrift) is the phenomenon where some cardiovascular responses begin a time dependent change, or "drift" after around 5-10 minutes of exercise in a warm or neutral environment (90 Fahrenheit+) without an increase in workload. It is characterised by decreases in mean arterial pressure and stroke volume and a parallel increase in heart rate. It has been shown that a reduction in stroke volume due to dehydration is almost always due to the increase in internal (core body) temperature. It is influenced by many factors, most notably the ambient temperature, internal core body 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.
The effects of cardiovascular drift in performance sports is mainly focused around a higher RPE, the Rate of Perceived Effort. A person training or racing person will feel like they are expending more energy when they are not. This creates a mental block that can inhibit further performance.
To prevent or reduce cardiovascular drift, the commonly recommended approaches are to consistently replace fluids (hydrate), maintain electrolyte balance during exercise, and acclimatise to the environment where you are training and racing. In this respect, Heat Training with the goal of Heat Adaption (aka Heat Acclimation) would also be beneficial to reduce cardio drift.
In summary, the impact of heat on performance is well established, so monitoring core body temperature is an advantage for athletes who are combating cardio drift for training and racing.