What is the definition of septic shock?

Definition of Septic Shock

Septic shock is defined as a subset of sepsis requiring vasopressor therapy to maintain mean arterial pressure ≥65 mmHg AND serum lactate level >2 mmol/L (>18 mg/dL) in the absence of hypovolemia. 1, 2, 3

Core Diagnostic Criteria

The current definition (Sepsis-3) requires both hemodynamic and metabolic criteria to be met simultaneously:

• Vasopressor requirement: Need for vasopressors to maintain MAP ≥65 mmHg despite adequate fluid resuscitation 1, 4
• Elevated lactate: Serum lactate >2 mmol/L (>18 mg/dL) persisting after volume resuscitation 1, 2
• Absence of hypovolemia: These criteria must be present after adequate volume resuscitation has been completed 1, 3

This combination is associated with hospital mortality rates exceeding 40%, significantly higher than sepsis alone 4.

Pathophysiological Characteristics

Septic shock represents the most severe manifestation of sepsis with three key abnormalities:

• Circulatory dysfunction: Profound vasodilation and increased vascular permeability leading to distributive shock 1, 3
• Cellular abnormalities: Altered cellular metabolism at the mitochondrial level 1, 3
• Metabolic derangements: Lactate accumulation from both impaired tissue perfusion and cellular metabolic dysfunction 1, 3
• Microcirculatory failure: Tissue hypoperfusion despite macrocirculatory support with vasopressors 1, 3

Clinical Identification Algorithm

To identify septic shock in practice, follow this sequence:

1. Confirm sepsis diagnosis: Life-threatening organ dysfunction (SOFA score increase ≥2 points) caused by dysregulated host response to infection 2, 4
2. Assess fluid resuscitation status: Ensure adequate volume resuscitation has been completed 1, 3
3. Check blood pressure: Persistent hypotension requiring vasopressors to maintain MAP ≥65 mmHg 1, 4
4. Measure lactate: Serum lactate must be >2 mmol/L despite fluid resuscitation 1, 2

Both criteria #3 and #4 must be present simultaneously after adequate fluid resuscitation to diagnose septic shock. 4

Critical Management Implications

Early recognition triggers specific interventions:

• Immediate vasopressor initiation: Norepinephrine is first-line when criteria are met 1, 3
• Target MAP 65-70 mmHg: This is the recommended hemodynamic goal 1, 3
• Avoid fluid overload: Excessive fluid administration after meeting shock criteria worsens outcomes 1, 3
• Serial lactate monitoring: Essential for both diagnosis and monitoring response to therapy 1

Common Diagnostic Pitfalls

Be vigilant to avoid these errors:

• Not measuring lactate levels: This is essential for the current definition and cannot be omitted 1
• Assuming normal blood pressure equals adequate perfusion: Patients on vasopressors may have normal MAP but persistent tissue hypoperfusion 1, 3
• Delaying vasopressors while giving excessive fluids: This increases mortality and should be avoided once shock criteria are met 1, 3
• Confusing with other distributive shock types: Septic shock specifically requires documented or suspected infection 1
• Using outdated SIRS criteria: The current definition does not rely on SIRS criteria, which have been abandoned 2, 5, 4

Evolution from Previous Definitions

The Sepsis-3 definition (2016) represents a significant departure from earlier frameworks:

• Eliminated "severe sepsis": This term is now considered redundant 2, 4
• Removed SIRS criteria: Previous definitions required SIRS, which lacked specificity 2, 5, 4
• Added lactate requirement: Earlier definitions focused only on refractory hypotension without metabolic criteria 4
• Emphasized organ dysfunction: Current definition prioritizes organ dysfunction over inflammation 2, 4