In a 70-year-old patient four days after knee arthroplasty who is febrile, tachycardic, leukocytotic, and has a new right lower‑lobe infiltrate on chest X‑ray, what is the most appropriate initial step in management?

Management of Post-Operative Pneumonia in a 70-Year-Old After Knee Replacement

The most appropriate initial step is C: Broad-spectrum antibiotics, which must be administered within one hour of recognizing this post-operative pneumonia with systemic inflammatory response. 1, 2

Immediate Recognition and Action

This patient presents with a clear post-operative pneumonia 4 days after knee replacement, evidenced by:

• Fever (38.9°C) and tachycardia (HR 115) indicating systemic inflammatory response 3
• Leukocytosis (WBC 15,000) with radiographic infiltrate confirming pneumonia 3
• Timeline (96 hours post-op) places this firmly in the infectious fever category, as fever beyond 96 hours post-operatively is likely to represent infection 3

Delayed antibiotic administration increases mortality by 7.6% per hour in septic patients, making immediate empiric broad-spectrum antibiotics the critical first intervention. 1

Why Antibiotics Come First

The Evidence Hierarchy

Broad-spectrum antibiotics targeting both gram-positive and gram-negative organisms, including anti-MRSA coverage, should be initiated immediately—before any other intervention. 1, 4, 5

• Initial inadequate antibiotic therapy is associated with increased mortality, longer hospital stays, and elevated healthcare costs, and subsequent modification after culture results does not remedy the impact of the initial inadequate choice 4, 5
• For post-operative pneumonia in a 70-year-old, empiric coverage should include vancomycin (for MRSA and gram-positive organisms) plus either cefepime, piperacillin-tazobactam, or a carbapenem (for gram-negative organisms including Pseudomonas) 1, 5

Why Not the Other Options First?

IV fluids (Option B) are supportive but secondary: While fluid resuscitation with 30 mL/kg crystalloid is indicated if the patient develops hypotension or lactate ≥4 mmol/L, there is no evidence of shock in this case (HR 115 is elevated but not critically so, and no blood pressure or lactate values suggest shock). 1 Antibiotics address the underlying infection and take priority.

DVT prophylaxis (Option D) should already be in place: Post-operative DVT prophylaxis should have been initiated immediately after knee replacement surgery as standard care. 3 This is not an "initial step" for managing new fever and pneumonia—it's baseline post-operative care.

The Complete Initial Management Algorithm

Within the First Hour:

1. Obtain blood cultures (at least two sets) before antibiotics, but do not delay treatment beyond 45 minutes 1

2. Measure serum lactate as a marker of tissue hypoperfusion 1

3. Initiate empiric broad-spectrum antibiotics immediately:

   • Vancomycin (for MRSA coverage) PLUS
   • Cefepime, piperacillin-tazobactam, or meropenem (for gram-negative coverage including Pseudomonas) 1, 5

4. Assess for septic shock and provide fluid resuscitation if indicated (MAP <65 mmHg or lactate ≥4 mmol/L) 1

Within 24-48 Hours:

• De-escalate antimicrobial regimen daily once culture results are available and narrow spectrum accordingly 1, 6
• Repeat imaging if fever persists despite appropriate antibiotics 3

Critical Pitfalls to Avoid

Never delay antibiotics to obtain cultures if the patient is clinically unstable—blood cultures should not delay treatment beyond 45 minutes. 1 In this case with fever, tachycardia, leukocytosis, and infiltrate, the patient requires immediate empiric therapy.

Do not rely solely on chest radiographs for diagnosis—they are neither sensitive nor specific in early infection, but the presence of an infiltrate combined with clinical findings (fever, tachycardia, leukocytosis) is sufficient to diagnose pneumonia and initiate treatment. 1, 2

Avoid monotherapy in this high-risk post-operative patient—combination therapy with anti-MRSA and anti-Pseudomonal coverage is warranted given the severity and post-operative timing. 5, 7

Regarding the Knee Prosthesis

While this patient has a new knee replacement, the clinical picture clearly points to pneumonia (right lower lobe infiltrate) rather than prosthetic joint infection. 1, 2 However, maintain vigilance:

• If the patient develops knee pain, swelling, warmth, or erythema, obtain ESR and CRP immediately, as these have 93% sensitivity when combined for detecting prosthetic joint infection 2, 6
• Staphylococcus aureus bacteremia from pneumonia can seed the prosthetic joint, so if blood cultures grow S. aureus, orthopedic consultation and joint aspiration become necessary 2, 6