Cardiovascular Drift and Critical Core Temperature: Factors Limiting Endurance Performance in the Heat?

Prolonged severe exercise stresses the cardiovascular system with the need to address the competing demands of blood flow to both the exercising muscles and the skin, while maintaining arterial blood pressure for adequate blood flow to the brain. With the imposition of an environmental heat load in addition to a high metabolic heat load, core temperature steadily increases along with a number of associated circulatory adjustments that inevitably signal an increasing strain on homeostatic control. Whether endurance performance under these conditions is limited by the rise in core temperature (critical core temperature hypothesis) or is limited by the inability to meet the competing demands for blood flow is debatable. The “critical core temperature” hypothesis is centered on an elevated body temperature providing peripheral afferent feedback from temperature-sensitive sites to the central nervous system (CNS), resulting in reduced CNS drive to the exercising muscles. Alternatively, exercise endurance in the heat may be limited by the rise in heart rate (HR) toward maximum HR that accompanies the increase in core temperature. The rise in HR is thought to compensate for a decrease in stroke volume (SV) as central venous pressure declines due to increased skin blood flow. However, recent research suggests that the upward drift in HR is the primary event that causes a decline in SV. Thus, the circulatory strain associated with HR approaching maximum and an inability to maintain cardiac output may signal fatigue in prolonged exercise in the heat.