7.1 Severe Sepsis and Septic Shock Antimicrobial Selection

Antimicrobial Management in Sepsis and Septic Shock

Sepsis and septic shock are medical emergencies where time to effective antimicrobial therapy is a critical determinant of survival. The Surviving Sepsis Campaign (SSC) guidelines provide a robust framework for managing these conditions, with a strong emphasis on rapid administration of appropriate antimicrobials, source control, and hemodynamic resuscitation.

Timing of Antimicrobial Initiation

The timing of antimicrobial administration depends on the certainty of the sepsis diagnosis and the presence of shock:

• Definite or Probable Sepsis (with or without shock): Administer broad-spectrum IV antimicrobials immediately, ideally within 1 hour of recognition.
• Possible Sepsis WITH Shock: Administer antimicrobials immediately, within 1 hour.
• Possible Sepsis WITHOUT Shock: A rapid assessment (clinical evaluation, labs, imaging) should be performed to investigate infectious and non-infectious causes. If concern for infection persists, antimicrobials should be administered within 3 hours of initial recognition.

Delaying effective therapy in patients with septic shock increases mortality by approximately 4-9% for every hour of delay. Therefore, obtaining blood cultures before starting antibiotics is crucial, but it must not delay the administration of the drugs if cultures cannot be obtained promptly.

Empiric Antimicrobial Selection

Empiric therapy must be broad enough to cover all likely pathogens while avoiding unnecessary toxicity and the promotion of resistance. Selection is guided by the suspected site of infection, prior antimicrobial exposure, patient comorbidities, immune status, and the hospital's local antibiogram.

General Principles

• Monotherapy vs. Combination Therapy: For most patients with sepsis without shock, single-agent broad-spectrum therapy (e.g., piperacillin-tazobactam, cefepime, or meropenem) is sufficient if it covers the suspected pathogens. For patients with septic shock, combination therapy (using two different classes of antibiotics covering Gram-negative pathogens) is initially recommended until susceptibilities are known.
• MRSA Coverage: Empiric methicillin-resistant Staphylococcus aureus (MRSA) coverage (e.g., vancomycin, linezolid) is recommended if the patient has risk factors. These include prior MRSA infection/colonization, recent IV antibiotics, history of recurrent skin infections, severe necrotizing infections, or presence of indwelling devices.
• MDR Gram-Negative Coverage: Double coverage for Pseudomonas aeruginosa (e.g., beta-lactam + fluoroquinolone or aminoglycoside) is indicated in septic shock, especially for patients with a high risk of MDR organisms (recent hospitalization, prior broad-spectrum antibiotics, structural lung disease, or known colonization).
• Fungal Coverage: Empiric antifungal therapy (e.g., echinocandins) should be considered in high-risk patients, such as those with recent abdominal surgery (especially anastomotic leaks), long-term TPN, prolonged broad-spectrum antibiotics, or neutropenia.

Pharmacokinetics and Pharmacodynamics (PK/PD) in Sepsis

Critically ill patients undergo significant physiological changes that alter drug disposition, particularly for hydrophilic antimicrobials (beta-lactams, aminoglycosides, vancomycin).

Increased Volume of Distribution (Vd)

Endothelial dysfunction and capillary leak syndrome lead to third-spacing of fluids. Aggressive fluid resuscitation further exacerbates this. The result is a massively increased Vd for hydrophilic drugs. Clinical Implication: Patients require higher initial loading doses to achieve adequate peak concentrations or therapeutic levels rapidly, regardless of renal function.

Altered Clearance

• Augmented Renal Clearance (ARC): In the early, hyperdynamic phase of sepsis, increased cardiac output can lead to ARC (creatinine clearance > 130 mL/min). This causes faster elimination of renally cleared drugs, leading to subtherapeutic levels.
• Acute Kidney Injury (AKI): Conversely, many patients develop sepsis-associated AKI, requiring dose reductions for maintenance therapy. Clinical Implication: Frequent reassessment of renal function and therapeutic drug monitoring (TDM) are essential. While loading doses remain high, maintenance doses must be adjusted dynamically based on changing organ function.

Altered Protein Binding

Sepsis often causes hypoalbuminemia, which increases the unbound (active) fraction of highly protein-bound drugs (e.g., ceftriaxone, ertapenem). However, this unbound fraction is also cleared more rapidly by the kidneys, resulting in a shorter half-life.

PD Targets

• Beta-lactams (Time > MIC): Prolonged or continuous infusions are recommended after an initial loading dose to maximize the time the free drug concentration remains above the minimum inhibitory concentration (MIC).
• Aminoglycosides (Cmax / MIC): Extended-interval (once-daily) high-dose regimens are preferred to maximize concentration-dependent killing and reduce nephrotoxicity.
• Vancomycin (AUC / MIC): Target an AUC/MIC ratio of 400-600, guided by Bayesian forecasting or two-level PK monitoring.

Source Control and De-escalation

Source control (e.g., draining abscesses, removing infected lines, surgical debridement) is as critical as pharmacotherapy and should ideally be accomplished within 6-12 hours of diagnosis.

Once culture and susceptibility results return (typically at 48-72 hours), broad-spectrum empiric therapy must be de-escalated to targeted, narrow-spectrum agents. This reduces collateral damage to the microbiome and decreases the risk of Clostridioides difficile infection.

Duration of Therapy

A typical duration for most serious infections associated with sepsis is 7 to 10 days. Longer courses may be needed for specific conditions (e.g., endocarditis, osteomyelitis, undrainable abscesses, S. aureus bacteremia) or in severely immunosuppressed patients. Shorter courses (e.g., 5-7 days) may be appropriate for rapid clinical responders with effective source control (like intra-abdominal infections) or community-acquired pneumonia.

Role of Procalcitonin (PCT)

PCT levels rise in response to bacterial infections and decline with effective treatment. The guidelines suggest using serial PCT measurements to support shortening the duration of antimicrobial therapy in patients with sepsis, rather than using standard fixed durations. However, PCT should not be used to withhold initial empiric antibiotics in suspected sepsis.