The Player Who Drank Too Much Water in the Heat
During a two-hour tournament match at 38 °C a 70 kg player loses 2.4 kg. Over the match and the next six hours he drinks 3.5 L of plain water, eats nothing salty, and passes only 300 mL of urine. His urine osmolality is 850 mOsm/kg (typical range 300–600), plasma sodium has fallen from 140 to 134 mmol/L, and his body mass is now 0.8 kg above his pre-match value. He is alert and sweating but pale and cool, with goosebumps despite the heat and a heart rate 20 beats/min above his resting value. The coach proposes salt tablets and restricting fluid until thirst returns.
1. Explain how plasma sodium fell even though sweat is hypotonic to plasma. Use the urine and mass data in your reasoning.
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Sweating alone concentrates the plasma, because sweat contains less sodium (roughly 40–60 mmol/L) than plasma does, so water is lost proportionally faster than salt. Losing 2.4 kg of sweat would therefore tend to raise plasma sodium.
Two findings show that this was overridden. First, urine osmolality of 850 mOsm/kg indicates near-maximal ADH action: osmoreceptors and volume receptors have signalled a deficit, so the kidney retains water and excretes a small, concentrated volume (300 mL). Second, his mass is 0.8 kg above baseline, so intake (3.5 L) exceeded losses (2.4 kg sweat plus 0.3 L urine). That is approximately 1.1 L of net free water retained.
Total body sodium fell only modestly (about 100–140 mmol in sweat), whereas total body water rose, so dilution outweighs the sodium loss and plasma sodium drifts from 140 to 134 mmol/L. This is dilutional, exercise-associated hyponatraemia, and the retained free water can move into brain cells.
2. Evaluate the coach's plan of salt tablets plus fluid restriction until thirst returns.
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Fluid restriction is reasonable in direction but unsafe as stated, and salt tablets alone are the wrong tool.
Restricting fluid makes sense because the problem is water excess, not water deficit, and further plain water would worsen the dilution. But 'wait until thirst' is unreliable here: thirst is blunted during exertion and is also suppressed by the developing hyponatraemia itself, and in a hot environment a genuinely dehydrated athlete would be harmed by restriction. Salt tablets without water can cause gastric irritation and vomiting and do not correct the underlying problem quickly; they also encourage the belief that salt is always the answer.
A better plan: stop plain water, give small volumes of an isotonic oral rehydration solution with salty food, weigh him and check for symptoms (headache, nausea, confusion, unsteadiness), monitor urine output and colour, and escalate to a hospital for hypertonic saline if neurological signs appear. Future prevention is individualised: weigh before and after, and drink to replace roughly 100–150% of the deficit with fluid containing sodium.
3. He is pale, cool, goosebumped and tachycardic but alert and sweating. Decide whether this is heat stroke, and state what must be done first and why.
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He does not meet the case definition for exertional heat stroke as presented: that requires core temperature above about 40 °C together with central nervous system dysfunction (confusion, aggression, collapse, seizure). He is alert, which argues against it, and he is sweating and cool and pale, findings more consistent with heat exhaustion and volume depletion, possibly compounded by hyponatraemia.
The first action is therefore measurement, not assumption: take a core temperature (rectal is the field standard - oral and tympanic readings are unreliable after heat exposure) and lie him down in shade with legs elevated. If core temperature is above 40 °C or any confusion appears, treat as heat stroke immediately: cold-water immersion and emergency transfer, because survival depends on how quickly cooling starts. If he is alert and not vomiting, active cooling and cautious oral rehydration, then reassess. Note that goosebumps and pallor can also reflect an anticholinergic or antihistamine medication impairing sweating, so a medication history is part of the assessment.