Collapse on the march: heat exhaustion or heat stroke?
During a summer field exercise (34 °C, 60% relative humidity, wet-bulb globe temperature in the high-risk band) a 21-year-old cadet collapses 40 minutes into a 6 km march carrying 15 kg. He is confused, answers questions slowly and cannot stand unaided. Core temperature 41.3 °C, pulse 132/min, blood pressure 96/58 mmHg, respiratory rate 30/min. His skin is hot and, at the moment of examination, dry. He has passed only a small volume of dark urine since dawn. The section has 8 L of drinking water, four ice packs, a shaded truck and one medic. The nearest ambulance is 25 minutes away and no intravenous fluids are available. A fellow cadet says: 'He isn't sweating, so it isn't serious - he just needs to sit in the shade and drink.'
1. Which findings separate heat stroke from heat exhaustion in this casualty, and why is the absence of sweating a weak argument either way?
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This is exertional heat stroke. The discriminating features are:
- Core temperature 41.3 °C - hyperthermia above roughly 40 °C is the first pillar of the definition.
- Central nervous system dysfunction - confusion and inability to stand. Heat exhaustion causes weakness and faintness but not altered consciousness; this single finding does most of the work.
- Cardiovascular decompensation - BP 96/58 with a pulse of 132, not the simple postural faintness that resolves on lying down.
- Dark, scanty urine since dawn - dehydration plus probable myoglobinuria from muscle breakdown.
The sweating argument is weak in both directions. Dry skin is a late sign of hypothalamic thermoregulatory failure. In exertional heat stroke - the young, exercising, sweating patient - sweating is often still present, which is why case definitions rest on CNS signs and core temperature rather than skin dryness. Relying on 'dry skin' would delay cooling in exactly the group where delay is most lethal. Conversely, the argument that sweating proves heat exhaustion is also wrong: the sweating cadet who is confused at 41 °C is the classic heat-stroke casualty.
2. Give your first 15 minutes in priority order, with the reasoning for each action and for what you deliberately would not do.
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- Call for evacuation now, in parallel with cooling. The ambulance is 25 minutes away, so the request is on the critical path. This is not a 'stabilise then call' situation.
- Cool immediately. Remove clothing, move to shade, and use cold water immersion if any container and water can be improvised; otherwise ice packs to neck, axillae and groin plus fanning and wetting the skin. Target core temperature around 38.5–39 °C and stop active cooling there. Reasoning: survival and organ damage track time spent above 40 °C, so cooling is the definitive treatment, not a comfort measure.
- Do not give oral fluids to a confused casualty. Swallowing is unreliable and aspiration is a real risk; the fluid deficit is for the receiving facility to correct. Small sips only if consciousness is fully regained.
- Do not give antipyretics. Paracetamol and aspirin act on the hypothalamic set-point, which is not the problem in exertional heat stroke; they will not lower the temperature and add hepatic risk.
- Check and monitor: airway, breathing, conscious level, pulse, BP every 5 minutes; check capillary glucose if a glucometer exists, because hypoglycaemia mimics the confusion.
- Position supine with legs elevated for the hypotension, while keeping access for cooling.
The governing principle is cool first, transport second - a cooler casualty transported at 25 minutes does better than a normothermic-looking casualty who was cooled only on arrival.
3. Two hours later he is cooler and more alert but has still passed no urine. Name the two most likely complications and the observation that would alert you first.
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Most likely complications:
- Acute kidney injury from rhabdomyolysis and hypovolaemia. Damaged muscle releases myoglobin, which is nephrotoxic, and the pre-renal insult from dehydration and vasodilatation compounds it. The dark urine already reported is the clue.
- Electrolyte and coagulation disturbance - hyperkalaemia (from muscle breakdown), sodium abnormalities, and disseminated intravascular coagulation or hepatic injury, which are typical of prolonged hyperthermia.
First alert: continuing oliguria with dark, tea- or cola-coloured urine despite rehydration, together with a pulse that stays high - that combination distinguishes established renal injury from simple dehydration, which should produce a diuresis once volume is restored. Secondary alerts would be oozing from cannula or venepuncture sites (coagulopathy), a seizure (hyponatraemia), or an irregular rhythm (hyperkalaemia).
The judgement being tested is that 'he looks better' is not the endpoint; urine output and conscious state carry the prognostic information.