Brain Metabolism and Blood Flow
Despite the brain occupying only 2-3% of total body mass, the brain consumes around 20% of total body oxygen consumption.11 The brain has a very high overall metabolic rate, a single neuron consuming many hundred-fold the power consumption of an average body cell.14 This high level of metabolism causes the brain to normally operate at temperatures 0.3-0.8°C above core temperature, and this figure can rise substantially during increased demand periods, and particularly during hyperthermia.14 This temperature increase is normally managed by cerebral blood flow (CBF) bringing in cooler blood, so that there is a differential between arterial inflow and warmed venous outflow. In addition to providing oxygen and other needs to the brain, it also transports heat away from it.15 Recent publications on young adults have documented an increase in the cerebral metabolic rate of oxygen consumption (CMRO2) of 10-20% during hyperthermia.15, 16
Simple immersion and immersed exercise increases brain blood flow by significant amounts, although human studies to date have not yet included children.17-19 The studies demonstrating this used thermoneutral water temperatures rather than warm water. The normal increase in CBF from thermoneutral immersion is overwhelmed in the face of hyperthermia. The normal physiologic response to hyperthermia uses two mechanisms of cooling: hyperventilation and increasing cerebral blood flow. Hyperventilation typically produces a reduction in arterial CO2 pressure (PaCO2), which causes cerebral vasoconstriction.11 This is one factor in reducing CBF. A 2020 study of passive thyperthermia corrected for this respiratory alkalosis by using an end-tidal forcing apparatus to correct PaCO2 to normothermic levels and found essential the same effects on cerebral metabolism elevation and cerebral blood flow reduction.16 As a consequence of a reduction in cerebral blood flow with a simultaneous increase in cerebral metabolism there is very real potential for cellular injury to brain cells. When brain temperatures reach 40°to 41°C, particular cell lines are damaged and when reaching 42°C, irreversible damage occurs.3, 20 The duration of hyperthermia is particularly relevant in promoting cellular survival, so aggressive rapid cooling measures are essential.21