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