Collapsed After a Hot Race: Hyponatraemia or Heat Stroke?
A 42-year-old runner finished a 42 km race in 4 h 20 min on a day when the air temperature was 31 °C and relative humidity 85 %. He lost about 3.5 L of sweat, which he replaced by drinking about 5 L of plain water at the water stations. He finished, then became nauseous and confused, and 40 minutes later had a seizure. In hospital his plasma sodium was 126 mmol dm⁻³ (normal 135–145), his body mass was 1.8 kg higher than before the race, and his urine was copious and very dilute. His core temperature was 38.4 °C, his skin was cool and clammy, and his blood glucose was normal. His friends, who had read about heat stroke in runners, wanted to wrap him in ice while waiting for the ambulance.
1. Explain, using ADH and kidney function, why his plasma sodium fell to 126 mmol dm⁻³ even though sweat contains sodium chloride.
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What was lost and what was replaced. Sweat is dilute compared with plasma: it contains sodium chloride but proportionally more water. He lost about 3.5 L of sweat and drank 5 L of plain water, so he gained roughly 1.5–1.8 L of water net, which matches the 1.8 kg rise in body mass.
The normal ADH response. Water loss raises plasma osmolality. Osmoreceptors in the hypothalamus detect this, the posterior pituitary releases ADH, and the distal tubule and collecting duct become more permeable to water, so more water is reabsorbed and a small concentrated urine is produced. Thirst is stimulated at the same time.
What actually happened. Absorbing several litres of plain water lowered plasma osmolality. The osmoreceptors detected dilution, so ADH secretion was suppressed and the kidneys excreted large volumes of dilute urine - exactly what was measured. But the kidneys can excrete only about 0.8–1.0 L per hour, which was less than his intake, so water accumulated faster than it could be removed.
The result. His body water rose while his sodium content did not; the salt lost in sweat was never replaced. Plasma sodium was therefore diluted to 126 mmol dm⁻³. The problem is the ratio of water to sodium, not the total volume of fluid.
2. Explain why his brain cells swelled, and why that explains the confusion and seizure. Why would giving more plain water or wrapping him in ice not correct the problem?
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Why the cells swelled. With plasma sodium at 126 mmol dm⁻³, the plasma is more dilute than normal, so its water potential is higher than that of the cytoplasm. Water moves by osmosis across the cell membrane from the plasma into the cells. Human cells have only a partially permeable membrane and no cell wall, so they cannot resist by building turgor pressure; they swell.
Why the brain is especially vulnerable. The brain is enclosed in a rigid skull. Swelling of brain tissue raises intracranial pressure, producing headache, nausea, confusion, seizures and, if uncorrected, coma and death.
Why the friends' plan fails. Wrapping him in ice treats hyperthermia. His core temperature was only 38.4 °C, so cooling cannot fix a sodium concentration of 126 mmol dm⁻³. Giving more plain water would add to the dilution and make the swelling worse. Both actions treat the wrong problem.
The correct response is to stop oral water intake and get urgent hospital care, where the sodium is corrected slowly with close monitoring, because correcting too quickly can itself damage the brain.
3. Distinguish this collapse from heat stroke, using at least two measurements from the data, and state the correct first response at the roadside.
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Heat stroke. Core temperature above about 40 °C, hot dry skin, and altered mental state caused by the high temperature. This runner's core temperature was 38.4 °C and his skin was cool and clammy, so he does not fit the picture. In heat stroke, cooling is the priority; in this case cooling would waste time.
Hypoglycaemia. Can also cause confusion and seizures during endurance exercise, but his blood glucose was normal, so it is excluded.
Hyponatraemia. Supported by the plasma sodium of 126 mmol dm⁻³, the 1.8 kg weight gain, the copious dilute urine and the history of 5 L of plain water against a 3.5 L sweat loss. His confusion has a biochemical cause, not a thermal one.
First response. Stop him drinking, sit him down, do not leave him alone, and call emergency services, reporting that he drank several litres of water during a hot race. Do not force more water.
Prevention. At 85 % humidity sweat evaporates poorly, so sweat losses are high. Long, hot events call for fluids containing sodium (a sports drink or oral rehydration solution), taken to thirst rather than in large fixed volumes.