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