Postoperative Pyrexia

The Concept

Postoperative fever is common, and the key to diagnosing its cause is timing — when after the operation the fever appears strongly suggests its source. A classic and highly examinable aid is the mnemonic of the '5 Ws', each linked to a typical postoperative day, which gives a structured approach to a very common clinical problem.

The '5 WS'

Timeline in More Detail

• Immediate (0–24 h) — the inflammatory response to surgical tissue trauma, a transfusion reaction, a pre-existing infection, or (rarely) malignant hyperthermia.
• Early (day 1–2) — atelectasis is the classic cause (basal collapse from shallow breathing/retained secretions).
• Day 3–5 — urinary tract infection (often catheter-related), chest infection, and cannula-site thrombophlebitis.
• Day 5–7 (and later) — wound infection, DVT, an anastomotic leak, or a collection/abscess.

Assessment & Management

Assessment is a focused history and examination directed at the likely sources — the chest, wound, legs, intravenous lines and abdomen — supported by investigations: full blood count, and cultures of blood, urine, sputum and wound as appropriate, a chest X-ray, and imaging (ultrasound/CT) to look for a collection. Management is to identify and treat the specific cause — chest physiotherapy for atelectasis, removing or changing infected lines/catheters, draining collections, and antibiotics for confirmed infection.

WHY Atelectasis Causes Early Fever

It is worth understanding why atelectasis is the classic cause of day 1–2 fever. After surgery — particularly abdominal or thoracic — shallow breathing (from pain), reduced coughing and retained secretions cause the small airways and alveoli at the lung bases to collapse. This collapsed, poorly-ventilated lung both mounts an inflammatory (febrile) response and predisposes to infection. This is precisely why early mobilisation, good analgesia, deep-breathing exercises and chest physiotherapy are emphasised — they re-expand the lung and both prevent and treat this early fever.

A Structured Approach to the Febrile Patient

In practice, a structured approach to the postoperative fever combines the timing (the '5 Ws') with a focused clinical search: examine the chest (atelectasis/pneumonia), the wound (infection), the calves (DVT), the intravenous and urinary lines (phlebitis, catheter UTI), and the abdomen (collection/leak). Investigations are then directed by the findings — cultures, chest X-ray, and cross-sectional imaging for a suspected deep collection. This disciplined approach avoids blindly starting antibiotics and instead finds and treats the actual source.

A Note on Sepsis Recognition

Whatever the source, the crucial skill is recognising when a postoperative fever signifies developing sepsis rather than a benign cause. Warning features include a rising or swinging fever with tachycardia, hypotension, a rising respiratory rate, confusion, reduced urine output and a rising lactate — captured by early warning scores. Suspected sepsis triggers the 'sepsis six' (oxygen, blood cultures, IV antibiotics, IV fluids, lactate measurement and monitoring urine output) alongside urgent source control (e.g. Draining a collection). Treating the fever as a potential early sign of sepsis, not merely a nuisance, can be life-saving.

A Note on Non-infective Causes

It is important not to assume every postoperative fever is infective. Non-infective causes include the normal inflammatory (cytokine) response to surgical trauma (typical in the first 24–48 hours), a transfusion reaction, drug fever, a haematoma resorbing, and — importantly — venous thromboembolism (DVT/PE), which is itself a non-infective cause of fever. Recognising these prevents the reflex, and sometimes harmful, prescription of antibiotics for every temperature, and directs attention instead to the true cause identified by timing and a focused clinical assessment.

• '5 Ws': Wind (atelectasis, d1–2), Water (UTI, d3–5), Walking (DVT, d4–6), Wound (infection, d5–7), Wonder drugs/lines (any time).
• Immediate fever (0–24 h) often the inflammatory response to surgery or a transfusion reaction.
• Later/swinging fever (day 5–7+) → think collection/abscess or anastomotic leak; culture and image.

The 5 Ws sequence by postoperative day guides the search.

• Postoperative fever: timing suggests the cause — remember the '5 Ws'.
• Wind (atelectasis/chest, day 1–2), Water (UTI, day 3–5), Walking (DVT, day 4–6), Wound (infection, day 5–7), Wonder drugs/lines (any time).
• Immediate fever (0–24 h): inflammatory response to surgery, transfusion reaction, pre-existing infection.
• Assess chest, wound, legs, lines, abdomen; investigate with FBC, cultures (blood/urine/sputum/wound), CXR, imaging for collections.
• Early fever often atelectasis; later/swinging fever → collection, abscess or anastomotic leak; treat the cause.