Metabolic response to trauma

Phases of the response to injury

• Hormones raised: cortisol and ACTH, ADH (water retention, oliguria), aldosterone (sodium retained, potassium lost), catecholamines, glucagon, growth hormone. Insulin action falls, and T3 falls (sick euthyroid).
• IL-6 is the main driver of the acute phase response (CRP rises); IL-1 and TNF add fever.
• Skeletal muscle responds by proteolysis, releasing alanine and glutamine for hepatic gluconeogenesis and gut fuel, so patients waste muscle.
• Post-operative sodium and water retention means hypotonic fluids readily cause hyponatraemia.
• The same hormones (ADH, aldosterone, thirst) are maximal in severe dehydration. The kidney must still pass about 400–500 mL of urine a day to excrete the daily solute load, even at maximal concentration. Pre-renal dehydration raises urea out of proportion to creatinine (for example urea 70 mg/dL with creatinine 0.8 mg/dL).
• Enhanced recovery (regional anaesthesia, minimally invasive surgery, early feeding, warming, good analgesia) blunts the response.

Body fluids and maintenance

• Total body water is about 60% of weight (42 L in a 70 kg man): two-thirds intracellular (28 L), one-third extracellular (14 L), of which about three-quarters is interstitial and one-quarter plasma (3.5 L). Blood volume is about 70 mL/kg in adults.
• Daily maintenance: water 25–30 mL/kg; sodium and potassium about 1 mmol/kg each; 50–100 g glucose to prevent ketosis. Children: 4-2-1 mL/kg/h rule.
• Insensible loss is about 500–800 mL a day and rises about 10–15% for each degree of fever.

• Of 1 L of 0.9% saline about 250 mL stays in the plasma (it spreads through the extracellular fluid); of 1 L of 5% dextrose only about 70–100 mL stays, because water spreads through all compartments.

Burns

• Oedema in the first hours is caused mainly by increased capillary permeability: heat damage and histamine, prostaglandins and kinins open endothelial gaps, so protein-rich fluid leaks into tissue. Hypoproteinaemia adds to it later. Fluid loss is greatest in the first 8–12 hours.

• Pain means nerves survive: a painless burn is deeper, not milder.
• Rule of nines (adult): head 9%, each arm 9%, front of trunk 18%, back of trunk 18%, each leg 18%, perineum 1%. Front of trunk plus one arm = 27%. The patient's palm with fingers is about 1%. Children have relatively larger heads.
• Parkland formula: 4 mL × weight (kg) × %TBSA of Hartmann's in 24 hours, half in the first 8 hours from the time of the burn (not arrival) and half in the next 16. A 70 kg man with 30% burns needs 8,400 mL, so 4,200 mL in the first 8 hours. Titrate to urine output of about 0.5 mL/kg/h in adults.

Crush injury and rhabdomyolysis

• Crushed muscle releases myoglobin, potassium, phosphate and CK. Myoglobin precipitates in acidic, concentrated urine and is toxic to tubules, causing acute kidney injury; dark urine with CK of tens of thousands confirms it.
• The most important kidney-protecting step is generous IV fluid early (ideally started before release), aiming for high urine output. Releasing the crush can cause sudden hyperkalaemic cardiac arrest, so monitor ECG and potassium.

Sodium

• Hyponatraemia (under 135 mmol/L): assess volume. Hypovolaemic (vomiting, diuretics, Addison disease): give saline. Euvolaemic (SIADH, hypothyroidism, post-operative ADH with hypotonic fluids): restrict fluid. Hypervolaemic (heart failure, cirrhosis, nephrotic): restrict ± diuretic. Seizures: hypertonic 3% saline bolus. Correct chronic hyponatraemia by no more than 8–10 mmol/L in 24 hours to avoid osmotic demyelination.
• Hypernatraemia: water deficit (elderly, diabetes insipidus); correct slowly to avoid cerebral oedema.
• Hyperglycaemia draws water out of cells and lowers sodium; hyperlipidaemia and paraproteins cause pseudohyponatraemia.

Potassium

• Hypokalaemia: vomiting, diarrhoea, fistulas, diuretics, insulin, beta-2 agonists, alkalosis, low magnesium. Weakness, ileus, arrhythmia; ECG flat T waves and U waves. Give IV KCl no faster than 10 mmol/h peripherally.
• Hyperkalaemia: AKI, ACE inhibitors, potassium-sparing diuretics, rhabdomyolysis, burns, tumour lysis, massive transfusion, acidosis, suxamethonium; haemolysed sample. ECG: tall tented T, flat P, wide QRS, sine wave.
• Treatment order: IV calcium gluconate first to stabilise the myocardium (it does not lower potassium); insulin with dextrose and nebulised salbutamol shift potassium into cells; then remove it (binders, dialysis). Refractory hyperkalaemia or fluid overload in AKI needs dialysis.

Calcium

• Hypercalcaemia of malignancy (PTHrP, bone metastases), for example 3.6 mmol/L (14.4 mg/dL): patients are dehydrated because calcium causes nephrogenic diabetes insipidus. First rehydrate with IV 0.9% saline; the most effective next step is an IV bisphosphonate (zoledronic acid or pamidronate), which inhibits osteoclasts and acts over 2–4 days. Calcitonin acts within hours but briefly.

Gastrointestinal losses and acid–base

• In gastric outlet obstruction the kidney first retains sodium. Once potassium is depleted, it exchanges sodium for H+ instead, producing paradoxical aciduria despite alkalosis. Treat with 0.9% saline plus KCl.

• Anion gap = Na+ − (Cl− + HCO3−), normally about 8–12. Raised: lactate, ketones, uraemia, salicylate, methanol. Normal: diarrhoea, fistulas, renal tubular acidosis, ureterosigmoidostomy, excess saline.

Access and nutrition

• In a shocked patient with no venous access, use intraosseous access (proximal tibia or humerus).
• Feed enterally where possible. Parenteral nutrition risks line sepsis, hyperglycaemia, refeeding hypophosphataemia and cholestasis.

Remember Calcium protects the heart, insulin moves potassium, only dialysis or binders remove it.

Exam trap Parkland's first half is given in the first 8 hours from the burn, not from arrival.

Exam trap A painless, white, leathery burn is full thickness; blistered, painful, blanching skin is superficial partial thickness.

How FCPS asks about this

• Oedema after major burns: increased capillary permeability.
• Parkland for 70 kg with 30% burns: 8,400 mL in 24 hours, 4,200 mL in the first 8 hours from the burn.
• Rule of nines for front of trunk and one arm: 27%.
• Fluid with potassium, calcium and lactate: Hartmann's solution.
• Crush injury with CK 60,000 and dark urine: generous IV fluids to protect the kidneys.
• Hypercalcaemia of malignancy after rehydration: IV bisphosphonate.
• Vomiting from pyloric obstruction: hypochloraemic hypokalaemic metabolic alkalosis with paradoxical aciduria; replace with saline and potassium.