Postoperative Fever and Complications

• Immediate (0–24 hours) — Wind, and the emergencies. Most early fever is inflammatory from the surgical insult itself and needs no workup. But consider malignant hyperthermia (within hours of inhaled anaesthetic or succinylcholine, with rigidity, hypercarbia and rising temperature — treat with dantrolene), bacteraemia, and necrotising soft tissue infection of the wound, which can appear within hours and requires immediate surgical exploration.
• Postoperative day 1–2 — Wind. Atelectasis is traditionally cited, though the causal link to fever is contested; pneumonia is the more meaningful concern. Management is incentive spirometry, mobilisation and analgesia adequate to permit deep breathing.
• Day 3–5 — Water. Urinary tract infection, often catheter-associated. Remove the catheter as early as possible.
• Day 4–6 — Walking. Venous thromboembolism — deep vein thrombosis and pulmonary embolism. Prophylaxis and early mobilisation.
• Day 5–7 — Wound. Surgical site infection. Also the window for an anastomotic leak after bowel surgery, which presents with fever, tachycardia, pain and peritonism and is a surgical emergency.
• Beyond day 7 — Wonder drugs and What did we do. Drug fever, Clostridioides difficile colitis, deep collections and abscesses, and line infections.

Non-infectious inflammatory (the majority of early fever)

• Surgical tissue trauma: cut and cauterised tissue releases IL-1, IL-6 and TNF-α, which act as endogenous pyrogens independent of any pathogen.
• Haematoma/blood resorption, transfusion reactions, and drug fever: haem breakdown and hypersensitivity-driven cytokine release; drug fever typically appears after a week of exposure (beta-lactams, sulfonamides, phenytoin, heparin).
• Malignant hyperthermia: *RYR1*/*CACNA1S* mutation causing uncontrolled sarcoplasmic calcium release after volatile anaesthetic or succinylcholine.
• Endocrine/withdrawal mimics: thyroid storm, adrenal insufficiency, alcohol withdrawal.

Infectious (dominates after ~48–72 hours)

• Device-associated: catheter-associated UTI, central line-associated bloodstream infection, ventilator-associated pneumonia — each proportional to dwell time.
• Surgical site infection: skin flora (Staphylococcus aureus, streptococci) in clean cases; enteric Gram-negatives and anaerobes after colorectal or biliary surgery.
• Organ-space sepsis: anastomotic leak, intra-abdominal abscess, empyema.
• **Antibiotic-associated *Clostridioides difficile*** colitis from flora disruption.

Vascular/ischaemic

• Venous thromboembolism: clot-associated inflammation produces low-grade fever; immobility plus surgical hypercoagulability.

• Final common pathway: any pyrogen — bacterial lipopolysaccharide, damaged host tissue, or a drug-antigen complex — drives macrophage release of IL-1, IL-6 and TNF-α. These act on the organum vasculosum of the lamina terminalis to raise hypothalamic PGE2, elevating the thermoregulatory set point. Because the mechanism is shared, fever alone never distinguishes sterile inflammation from infection; only timing and site-specific findings do.
• Atelectasis: general anaesthesia abolishes sighing, reduces functional residual capacity below closing volume, and incisional pain splints the diaphragm. Dependent alveoli collapse, producing shunt physiology (hypoxaemia that does not correct fully with supplemental oxygen). Retained secretions in collapsed segments then seed pneumonia — the clinically meaningful sequel, since the atelectasis–fever link itself is contested.
• Catheter-associated UTI: the catheter provides a biofilm scaffold bypassing urethral defences; ascending uropathogens trigger urothelial cytokine release.
• Surgical site infection: bacteria inoculated at incision proliferate in a devascularised, haematoma-filled or ischaemic wound where neutrophil oxidative killing is impaired. Purulence under tension raises local pressure, producing pain out of proportion, erythema and eventual dehiscence.
• Anastomotic leak: tension, ischaemia at the bowel ends, or technical failure allows enteric contents into the peritoneum. Chemical then bacterial peritonitis produces massive third-spacing, systemic cytokinaemia, tachycardia preceding fever, and ileus. Localised leak walls off into an abscess; free leak causes peritonitis and septic shock.
• Venous thromboembolism: Virchow's triad is complete after surgery — endothelial injury, venous stasis from immobility and anaesthesia, and a hypercoagulable acute-phase state with elevated fibrinogen and factor VIII. Thrombus-associated inflammation causes low-grade fever; embolisation causes dead-space ventilation, hypoxaemia and right ventricular strain.
• Malignant hyperthermia: defective ryanodine receptor gating floods the myoplasm with calcium, driving continuous actin–myosin cycling and ATP hydrolysis. The result is heat generation, rigidity, rising end-tidal CO2, mixed acidosis, hyperkalaemia and rhabdomyolysis.
• Drug fever: type III/IV hypersensitivity with immune-complex or T-cell-mediated cytokine release; the patient looks disproportionately well for the temperature.

By source, with the mechanism that generates the sign

• Atelectasis/pneumonia: decreased breath sounds and dullness at the bases from collapsed dependent alveoli; pneumonia adds purulent sputum, focal crackles or bronchial breathing, leukocytosis and hypoxaemia.
• Catheter-associated UTI: suprapubic tenderness, dysuria after catheter removal, cloudy urine; costovertebral angle tenderness and rigors imply pyelonephritis or bacteraemia. Elderly patients may present only with delirium.
• Surgical site infection: incisional erythema, induration, warmth, tenderness and purulent drainage; fluctuance signals a drainable collection. Pain out of proportion, dishwater-grey drainage, bullae, crepitus, skin anaesthesia or rapidly advancing erythema are the red flags for necrotising soft tissue infection.
• Anastomotic leak: classically day 5–7 after bowel surgery with fever, tachycardia that precedes fever, abdominal pain, peritonism, prolonged ileus, feculent or bilious drain output, and new atrial fibrillation or oliguria. In an elderly or immunosuppressed patient the only sign may be unexplained tachycardia and a rising leukocyte count.
• Deep vein thrombosis/pulmonary embolism: unilateral calf swelling, warmth and tenderness; PE presents with pleuritic chest pain, dyspnoea, tachycardia, hypoxaemia and a low-grade temperature. Hypotension with a distended jugular venous pulse indicates massive PE.
• Malignant hyperthermia: intraoperative or early postoperative masseter/generalised rigidity, rising end-tidal CO2 despite increased minute ventilation, tachycardia, dark cola-coloured urine, and temperature rising late rather than first.
• ***C. difficile* colitis**: watery diarrhoea, cramping, leukocytosis that can be striking, after antibiotic exposure.
• Drug fever: fever with relative bradycardia in a comfortable-appearing patient, sometimes with rash or eosinophilia, and a negative site-directed evaluation.
• Line infection: rigors timed to line flushing, exit-site erythema or purulence.

Step 2 — targeted initial testing

• Chest radiograph for respiratory signs: plate-like basilar opacities with volume loss favour atelectasis; a new consolidative infiltrate with purulent secretions favours pneumonia.
• Urinalysis with culture for suspected UTI: pyuria plus significant bacteriuria in a symptomatic patient. Asymptomatic bacteriuria should not be treated — an IDSA position reaffirmed in the 2019 asymptomatic bacteriuria guideline.
• Two sets of blood cultures from separate venipuncture sites, drawn before antibiotics when the patient is toxic, rigoring, or has a central line; when catheter-related bloodstream infection is suspected, draw one set peripherally and one through the catheter so that differential time to positivity can be assessed.
• Wound culture only from purulent material, not from surface swabs of intact skin.
• Stool NAAT plus toxin EIA in a two-step algorithm for diarrhoea, per IDSA/SHEA guidance — NAAT alone cannot distinguish colonisation from disease.
• Complete blood count, lactate, and basic metabolic panel to gauge severity.

Step 3 — confirmatory imaging

• CT of the abdomen and pelvis with intravenous and enteral contrast is the study of choice for suspected anastomotic leak or intra-abdominal abscess: extraluminal contrast, free air beyond the expected postoperative window, or a rim-enhancing collection.
• CT pulmonary angiography for suspected PE and compression duplex ultrasonography for DVT, with the Wells criteria used to set pretest probability. D-dimer is nearly useless postoperatively because it is almost always elevated.
• Surgical exploration is the gold standard for necrotising soft tissue infection; imaging must never delay it, and the LRINEC score is supportive at best and cannot exclude the diagnosis.
• Malignant hyperthermia is a clinical diagnosis (rising end-tidal CO2, rigidity, mixed acidosis, hyperkalaemia, elevated creatine kinase, myoglobinuria); the caffeine–halothane contracture test on fresh muscle biopsy confirms susceptibility later.

Immediate stabilisation

• Sepsis or septic shock: obtain cultures, give broad-spectrum antibiotics early, and resuscitate with crystalloid, escalating to norepinephrine as first-line vasopressor — the Surviving Sepsis Campaign 2021 framework. Source control (drainage, debridement, line removal) is as important as antibiotics.
• Malignant hyperthermia: stop the volatile agent and succinylcholine, hyperventilate with 100% oxygen, and give dantrolene (2.5 mg/kg IV, repeated to effect) with active cooling and treatment of hyperkalaemia — per MHAUS protocols. Do not use calcium channel blockers with dantrolene.
• Necrotising soft tissue infection: emergent operative debridement plus broad-spectrum antibiotics (a beta-lactam/beta-lactamase inhibitor such as piperacillin-tazobactam, plus vancomycin, plus clindamycin for ribosomal toxin suppression), per IDSA/SIS 2014.

Source-specific first-line therapy

• Atelectasis: incentive spirometry, early mobilisation and analgesia sufficient to permit deep breathing — no antibiotics.
• Hospital-acquired pneumonia: empiric coverage per the ATS/IDSA 2016 HAP/VAP guideline, narrowed by culture.
• CAUTI: remove or exchange the catheter and treat with culture-directed antibiotics.
• Surgical site infection: open the incision and drain — this is the definitive step. Antibiotics are added for surrounding cellulitis or systemic signs (IDSA/SIS 2014). For MRSA, vancomycin is dosed to a 24-hour AUC/MIC of 400–600 per the 2020 IDSA/ASHP/PIDS/SIDP consensus; the old 15–20 mcg/mL trough target has been retired.
• C. difficile: stop the inciting antibiotic; oral fidaxomicin or oral vancomycin are preferred over metronidazole (IDSA/SHEA 2021 focused update). Avoid antimotility agents.
• VTE: therapeutic anticoagulation. Direct oral anticoagulants (for example apixaban or rivaroxaban) are first-line for most acute VTE under CHEST antithrombotic guidance, but low-molecular-weight heparin such as enoxaparin is favoured in the early postoperative period — parenteral, titratable, and reversible around further surgery — and remains a preferred option in malignancy-associated thrombosis. An IVC filter is reserved for absolute contraindication to anticoagulation.
• Anastomotic leak: NPO, resuscitation, broad-spectrum antibiotics, then percutaneous drainage for a contained collection or reoperation with washout and proximal diversion for free leak or peritonitis.

Emergencies — recognise and act immediately

• Septic shock from an undrained source: persistent hypotension despite fluids with rising lactate. Antibiotics without source control fail.
• Necrotising soft tissue infection: crepitus, bullae, dishwater drainage, pain out of proportion. Delay to debridement is the strongest driver of mortality.
• Free anastomotic leak with peritonitis: diffuse rigidity, free intraperitoneal contrast, haemodynamic instability — straight to the operating room.
• Fascial dehiscence: typically around postoperative day 5–8, heralded by sudden salmon-coloured serosanguinous drainage from the incision; evisceration requires cover with saline-soaked gauze and emergent repair.
• Massive pulmonary embolism: hypoxaemia with hypotension and right heart strain.
• Malignant hyperthermia sequelae: hyperkalaemic arrest, rhabdomyolysis with acute kidney injury, and disseminated intravascular coagulation.
• **Fulminant C. difficile colitis with toxic megacolon**: colonic dilatation, absent bowel sounds, peritonism — may require subtotal colectomy.

Complications of the underlying process

• Intra-abdominal abscess from a contained leak: swinging fever, persistent leukocytosis, rim-enhancing collection on CT.
• Enterocutaneous fistula after a leak drains through the wound: enteric content at the incision, with fluid, electrolyte and nutritional losses.
• ARDS from systemic inflammation: bilateral infiltrates with refractory hypoxaemia not explained by heart failure.
• Incisional hernia, the late structural consequence of wound infection or dehiscence.
• Post-thrombotic syndrome after DVT.

Complications of treatment

• ***C. difficile* infection** as a direct consequence of broad-spectrum antibiotics — the iatrogenic loop.
• Acute kidney injury, notably with vancomycin, aminoglycosides, and the vancomycin plus piperacillin-tazobactam combination; also contrast-associated injury after repeated CT.
• Bleeding on therapeutic anticoagulation, including wound haematoma and, after spinal anaesthesia, epidural haematoma with new lower-limb weakness — an emergency.
• Heparin-induced thrombocytopenia: platelet fall typically 5–10 days after heparin exposure with paradoxical thrombosis; stop all heparin and start a non-heparin anticoagulant such as argatroban.
• Antibiotic hypersensitivity, and red man syndrome with rapid vancomycin infusion (histamine release, not IgE-mediated).

• Timing is the test. Fever on postoperative day 1 in a comfortable patient after uncomplicated surgery needs mobilisation and spirometry, not blood cultures. Fever after day 4 almost always has a findable source.
• Tachycardia out of proportion, on day 5–7, after a bowel anastomosis is an anastomotic leak until proven otherwise. The single best next step is CT abdomen/pelvis with contrast in a stable patient, and immediate laparotomy in an unstable one with peritonitis. Do not blame atelectasis.
• Fever within hours of anaesthesia with rigidity and a rising end-tidal CO2 is malignant hyperthermia — the earliest sign is the ETCO2, not the temperature. Give dantrolene and stop the trigger; the family history of an anaesthetic death is the planted clue.
• ***Pain out of proportion to examination findings*, with crepitus or dishwater-grey drainage, means necrotising infection.** The next step is the operating room, not a CT scan and not the LRINEC score.
• Salmon-coloured serosanguinous wound drainage around day 5–8 is fascial dehiscence. Cover with moist sterile gauze and return to theatre.
• Vancomycin is dosed to a 24-hour AUC targeting AUC/MIC 400–600 (2020 IDSA/ASHP/PIDS/SIDP consensus). A trough-only goal of 15–20 mcg/mL is the obsolete distractor.
• Cefazolin remains the standard prophylactic agent, given within 60 minutes of incision and stopped at wound closure (CDC 2017; ASHP/IDSA/SIS/SHEA 2013). In reported penicillin allergy, true cross-reactivity is roughly 1–3%, driven by shared R1 side chains rather than the beta-lactam ring — the 10% figure is historical.
• Do not treat asymptomatic bacteriuria in a febrile postoperative patient (IDSA 2019); pyuria alone in a catheterised patient is not a diagnosis. Remove the catheter and keep looking.