Cardiac Output
Simple aquatic immersion produces a long-recognized increase in cardiac
output.6-10 This has not been studied in children
during immersion. In land-based exercise in warm environments children
do show increase in cardiac output but to a lesser degree than in
adults.4 In a warm or hot environment, the first
response of the body is to increase peripheral circulation, especially
to skin surfaces to facilitate sweating.3 The
magnitude of this response is substantial, increasing peripheral
circulation 15 cm3/min for each 0.01°C rise in body
temperature, with potentially a 25-fold increase in cutaneous blood
flow.3, 11 Children have higher peripheral blood flow
than adults in comparable high temperatures.4 While
core temperature rises far more slowly than skin temperature, it is
highly likely that the initial neurologic signaling from the skin is
what triggers this response because this happens far quicker to protect
thermal homeostasis. Work done in our laboratory demonstrated nearly a
1200% increase in peripheral circulation within the first 4 minutes of
warm water (39°C) immersion in young subjects, nearly double that seen
in an older cohort.12, 13 This increase in peripheral
blood flow is facilitated by the increase in cardiac output, but the
redistribution of blood reduces the blood flow to central structures and
the brain.3 The speed with which this occurred was
dramatic, with immediate increase in blood flow within seconds. In a
warm water environment, the heated blood returns uncooled and even
heated to the brain, signaling a demand for yet more peripheral blood
flow, creating a vicious cycle until maximal peripheral flow is
achieved.3