Burns assessment and resuscitation: depth, TBSA, referral and fluids

Burn depth: what you see and what it predicts

Depth is classified by how much dermis survives, and its practical job is to predict whether the wound will heal within about three weeks. Burns that take longer carry a high risk of hypertrophic scarring and are usually excised and grafted.

Depth is dynamic, and Jackson's zones explain why. A central zone of coagulation is already dead; a surrounding zone of stasis is under-perfused but salvageable; an outer zone of hyperaemia will recover. Hypovolaemia, infection, oedema and cooling tip stasis into necrosis, so a burn can deepen over the first 48–72 hours. Partial-thickness burns heal from surviving adnexa — hair follicles and sweat glands — which is why deep dermal burns heal slowly. Laser Doppler imaging after about 48 hours predicts healing potential better than clinical judgement alone.

Estimating TBSA

Only partial- and full-thickness burns count towards TBSA. Simple erythema is excluded, and including it is a classic cause of overestimating small burns and starting unnecessary resuscitation.

• Wallace rule of nines (adults): head and neck 9%, each arm 9%, front of trunk 18%, back of trunk 18%, each leg 18%, perineum 1%.
• Palmar method: the patient's palm with adducted fingers is about 1% TBSA — useful for small or scattered burns, and for very large burns by estimating the unburnt skin.
• Lund–Browder chart: the most accurate method and the standard in children, because it adjusts head and leg proportions for age.

In infants the head is about 18% and each leg about 14%. For each year after the first, take roughly 1% from the head and add 0.5% to each leg, reaching adult proportions at around ten years.

Who to refer to a burns service

The National Burn Care Referral Guidance used in England and Wales sets deliberately low thresholds, judged on size, depth, site, mechanism and patient. Refer:

• Burns of 3% TBSA or more in adults, or 2% or more in children
• All full-thickness and all circumferential burns
• Any burn not healed within 2 weeks
• Any burn where non-accidental injury is suspected

Discuss with a burns consultant: burns of the face, hands, feet, perineum or genitalia; chemical, electrical, friction and cold injuries; an unwell or febrile child with a burn; and comorbidities that may affect treatment or healing. Suspected inhalation injury is discussed too, once the airway is safe. Older UK and international lists use higher size thresholds — 10% TBSA in adults, 5% in children, full thickness over 5% — and still appear in some texts; when a question names the current national guidance, answer with it.

Fluid resuscitation

Formal intravenous resuscitation is usually started for burns over 15% TBSA in adults and over 10% in children. The Parkland formula estimates the first 24 hours of crystalloid — classically 4 mL × weight (kg) × %TBSA of Hartmann's solution — with half in the first 8 hours from the time of injury and the rest over the following 16.

The formula is a starting estimate, not a prescription. Patients often end up receiving more than it predicts, so many UK units start lower — 2–3 mL/kg/%TBSA, as in the modified Brooke formula — and titrate hourly against urine output. Children also need glucose-containing maintenance fluid on top of resuscitation, calculated by Holliday–Segar (100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, 20 mL/kg/day thereafter), with blood glucose checked.

Endpoints and fluid creep

Urine output is the main endpoint: aim for 0.5–1 mL/kg/h in adults and about 1 mL/kg/h in children, measured hourly through a urinary catheter. Low output with tachycardia, rising haematocrit and lactate means under-resuscitation — increase the crystalloid rate rather than reaching for a diuretic or vasopressor in a hypovolaemic patient.

Over-resuscitation is the opposite danger. Fluid creep causes pulmonary oedema, abdominal and limb compartment syndromes and deepening of the burn from oedema, so output persistently above target means the rate should come down. Electrical injury with myoglobinuria is the exception that needs a higher target, around 1–2 mL/kg/h in adults, until the urine clears.

Inhalation injury, carbon monoxide and cyanide

Suspect inhalation injury after any fire in an enclosed space and secure the airway early, because oedema worsens over the first 24–48 hours and can make later intubation impossible. Stridor is a late sign; do not wait for it.

• Facial burns, singed nasal hair or eyebrows, soot in the mouth or nose, carbonaceous sputum
• Hoarse voice, brassy cough, stridor, respiratory distress
• Confusion or reduced consciousness, which may reflect carbon monoxide or cyanide

Carbon monoxide

Binds haemoglobin far more avidly than oxygen and shifts the dissociation curve to the left. Pulse oximetry reads falsely normal, so measure carboxyhaemoglobin by co-oximetry. Give 100% oxygen at once: it cuts the half-life from about 4 hours on air to roughly 60–90 minutes.

Hydrogen cyanide

Released from burning plastics, foam, wool and silk. It blocks cytochrome c oxidase, causing histotoxic hypoxia with a high lactate despite adequate oxygen delivery. Treat with intravenous hydroxocobalamin, which binds cyanide as cyanocobalamin for renal excretion; it turns skin and urine red and interferes with some laboratory assays.

Escharotomy

Escharotomy is indicated when circumferential deep dermal or full-thickness eschar threatens perfusion of a limb or restricts ventilation. Inelastic eschar cannot expand as oedema builds beneath it, so it acts like a tourniquet.

• Limbs: mid-medial and mid-lateral incisions through the eschar into fat until it springs apart — not through deep fascia. Protect the ulnar nerve behind the medial epicondyle and the common peroneal nerve at the fibular neck.
• Chest: incisions along both anterior axillary lines joined by a transverse cut across the costal margins, releasing a ‘breastplate’ when airway pressures rise and tidal volumes fall.
• After release: if perfusion does not return, or the injury is electrical or crushing, suspect a true subfascial compartment syndrome and perform fasciotomy.