The Fasting Runner's Collapse
A 20-year-old university athlete preparing for a 10 km race decides to train at altitude after fasting for 14 hours. At 3,200 m the partial pressure of inspired oxygen is roughly 40% lower than at sea level. Ninety minutes into the run he becomes confused and stops sweating despite a dry, hot day. His coach notes rapid deep breathing, a heart rate of 150 beats per minute, and warm flushed skin. A teammate had earlier given him a large bottle of a "sports drink" that turned out to be nearly pure water with no electrolytes. On arrival at the clinic his blood glucose is 2.9 mmol/L, sodium is 128 mmol/L, and hematocrit is elevated. The physician must decide whether the collapse is primarily hypoglycemia, exertional heat stroke, or dilutional hyponatremia, and whether immediate glucose, cooling, or fluid restriction is the correct first action.
1. Which single mechanism best explains the combination of confusion, low glucose, low sodium, and elevated hematocrit? Justify your choice by ruling out the alternatives.
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Best explanation: a mixed picture dominated by hypoglycemia plus dilutional hyponatremia, with altitude and heat as contributing stressors.
- Hypoglycemia (2.9 mmol/L): confusion and collapse fit, and a 14-hour fast depleted hepatic glycogen before exercise. This alone explains the confusion but not the low sodium.
- Dilutional hyponatremia (128 mmol/L): the large volume of near-electrolyte-free drink plus sweating (which loses salt as well as water) lowers serum sodium. This explains confusion and the absence of a clear heat-stroke picture.
- Exertional heat stroke is less likely as the primary cause: he stopped sweating and has warm skin, which is concerning, but core temperature is not reported as markedly elevated and the biochemical findings point elsewhere. Heat stress probably contributed rather than caused the event.
- Elevated hematocrit is consistent with dehydration and altitude-driven hemoconcentration, but it does not by itself explain the low sodium; it reflects the volume shifts, not the cause of collapse.
Conclusion: the findings are best unified by fasting-induced hypoglycemia compounded by water overload causing hyponatremia.
2. Given the values, what is the safest immediate first action, and what should be avoided?
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First action: check and correct glucose - give a measured dose of glucose (oral if conscious and able to swallow, otherwise intravenous) while starting cautious monitoring of sodium.
Reasoning:
- Hypoglycemia is immediately life-threatening to the brain, so it takes priority over slower processes.
- Sodium of 128 mmol/L is mild-to-moderate hyponatremia; rapid correction risks osmotic demyelination, so sodium should be corrected slowly and only under supervision.
- Avoid giving more free water or hypotonic fluids, and avoid rapid hypertonic saline unless there are severe neurological signs.
- Cooling should be applied if core temperature is genuinely elevated, but it is not the first step here.
Conclusion: treat the glucose first, restrict free water, and recheck sodium and glucose after correction.