20.1 Sepsis-3 Definitions & the Sepsis Six
Key Takeaways
- Septic shock is defined as sepsis requiring vasopressors to maintain a mean arterial pressure of 65 mmHg or more with a lactate above 2 mmol/L despite adequate fluid resuscitation.
- The Sepsis Six comprises oxygen, blood cultures, intravenous antibiotics, intravenous fluids, lactate measurement and urine output monitoring, delivered within one hour.
- Blood cultures should be taken before antibiotics wherever this does not delay treatment, because they remain the key to organism-directed therapy.
Severe bacterial infections and systemic dysregulated inflammatory states represent core emergencies frequently tested in MRCP(UK) Part 1. Candidates must possess detailed command of consensus definitions (Sepsis-3), validated bedside risk tools, time-critical resuscitation protocols (the Sepsis Six), empirical antimicrobial choices stratified by host vulnerability, cerebrospinal fluid (CSF) cytochemical interpretation, and the toxin-mediated pathophysiology of toxic shock syndromes.
1. Sepsis-3 Definitions & Pathophysiological Principles
The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3) replaced the historical Systemic Inflammatory Response Syndrome (SIRS) criteria, which lacked both sensitivity and specificity for life-threatening pathology.
Core Consensus Definitions
- Sepsis: Defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. In clinical practice, organ dysfunction is operationalised as an acute change in total Sequential Organ Failure Assessment (SOFA) score $\ge 2\text{ points}$ attributable to the infectious insult. A baseline SOFA score is assumed to be zero in patients without pre-existing organ impairment. An acute increase of $\ge 2\text{ points}$ reflects an overall in-hospital mortality excess of approximately $10%$.
- Quick SOFA (qSOFA): A simplified bedside screening tool designed to identify patients outside the intensive care unit (ICU) with suspected infection who are at heightened risk of prolonged ICU stay or in-hospital death. It does not define sepsis itself, but flags high risk when $\ge 2$ of the following 3 criteria (HAT) are met:
- H — Hypotension: Systolic blood pressure $\le 100\text{ mmHg}$
- A — Altered mental status: Glasgow Coma Scale (GCS) score $< 15$
- T — Tachypnoea: Respiratory rate $\ge 22\text{ breaths/min}$
- Septic Shock: A subset of sepsis in which underlying circulatory and cellular/metabolic abnormalities are profound enough to substantially increase mortality ($> 40%$). Clinically identified by patients with sepsis who exhibit both:
- Persistent hypotension requiring vasopressors to maintain a Mean Arterial Pressure (MAP) $\ge 65\text{ mmHg}$, AND
- Serum lactate $> 2.0\text{ mmol/L}$ ($18\text{ mg/dL}$) despite adequate intravascular volume resuscitation.
Pathophysiological Cascade
Infection triggers recognition of pathogen-associated molecular patterns (PAMPs, such as Gram-negative lipopolysaccharide/endotoxin and Gram-positive peptidoglycan/lipoteichoic acid) by host pattern recognition receptors, particularly Toll-like receptor 4 (TLR-4) on monocytes and macrophages. This initiates massive transcription of pro-inflammatory cytokines: tumour necrosis factor-alpha (TNF-$\alpha$), interleukin-1 (IL-1), and interleukin-6 (IL-6).
This cytokine storm disrupts the endothelial glycocalyx, activates inducible nitric oxide synthase (iNOS) with profound systemic vasodilation, causes widespread capillary leak, and induces tissue factor expression on endothelial cells. Disseminated microvascular thrombosis ensues alongside impaired endogenous fibrinolysis (driven by elevated plasminogen activator inhibitor-1 [PAI-1]). Tissue dysoxia occurs not merely from hypoperfusion, but from direct mitochondrial dysfunction ('cellular hibernation'), explaining persistent hyperlactataemia even when macrovascular haemodynamics appear restored.
2. The Sepsis Six Care Bundle & Acute Resuscitation
The UK Sepsis Trust and Surviving Sepsis Campaign mandate that the Sepsis Six bundle be initiated and fully delivered within 1 hour of recognition of severe sepsis or septic shock. This rapid operational protocol halves mortality when delivered comprehensively.
| Operational Direction | Bundle Component | Clinical Standard & Physiological Target |
|---|---|---|
| IN (Deliver) | 1. Intravenous Fluid Resuscitation | Administer an initial bolus of $30\text{ mL/kg}$ of balanced crystalloid (e.g., Hartmann's solution or Plasma-Lyte) within the first 3 hours. Balanced crystalloids reduce hyperchloraemic metabolic acidosis and acute kidney injury compared to $0.9%$ normal saline. Re-assess volume responsiveness via dynamic parameters (capillary refill time, passive leg raise, stroke volume variation). |
| IN (Deliver) | 2. Intravenous Broad-Spectrum Antibiotics | Administer maximal therapeutic doses of broad-spectrum antimicrobials within 60 minutes of identification, targeted to the presumed focus and local antimicrobial resistance patterns. Each hour of delay in septic shock is associated with an incremental $7.6%$ reduction in survival. |
| IN (Deliver) | 3. Oxygen Therapy | Titrate supplemental oxygen to maintain target oxygen saturations of $94\text{–}98%$ in non-hypercapnic patients, or $88\text{–}92%$ in patients at risk of hypercapnic respiratory failure (e.g., severe COPD). |
| OUT (Obtain) | 4. Blood Cultures | Inoculate at least two sets of blood cultures (aerobic and anaerobic bottles) prior to antibiotic administration, provided this does not cause significant delay ($> 45\text{ minutes}$). Obtain additional cultures (sputum, urine, wound, CSF) as indicated. |
| OUT (Obtain) | 5. Blood Lactate Measurement | Measure baseline venous or arterial blood lactate. A level $> 2\text{ mmol/L}$ denotes cellular hypoperfusion; $> 4\text{ mmol/L}$ indicates critical metabolic failure. Serial lactate measurements every 2–4 hours guide resuscitation (aiming for $\ge 20%$ clearance every 2 hours). |
| OUT (Obtain) | 6. Strict Urine Output Monitoring | Insert an indwelling Foley urinary catheter with a urometer to monitor hourly urine output. The physiological threshold for renal hypoperfusion is $< 0.5\text{ mL/kg/h}$ for two consecutive hours. |
Advanced Vasoactive Support
When fluid resuscitation fails to restore a MAP $\ge 65\text{ mmHg}$, vasopressor therapy must be initiated without delay via a central venous catheter (or temporarily via an established wide-bore peripheral cannula):
- First-line Vasopressor: Noradrenaline (norepinephrine) is the agent of choice. It acts predominantly as an $\alpha_1$-adrenergic agonist with potent vasoconstrictor effects, supported by modest $\beta_1$-inotropic action, avoiding the profound tachycardia associated with adrenaline or dopamine.
- Second-line Vasopressor: Vasopressin (arginine vasopressin) titrated up to $0.03\text{ units/min}$ to reduce noradrenaline dosage requirements and correct endogenous vasopressin deficiency.
- Inotropic Support: Dobutamine is added when myocardial dysfunction persists (elevated cardiac filling pressures and low cardiac output) despite adequate MAP.
- Refractory Shock: Intravenous hydrocortisone ($200\text{ mg/day}$ administered as $50\text{ mg}$ 6-hourly or as a continuous infusion) is indicated only in septic shock refractory to fluid resuscitation and high-dose vasopressor therapy.
A 68-year-old woman with a history of recurrent urinary tract infections is admitted to the acute medical unit with acute confusion, rigors, and dysuria. On examination, she is lethargic with a GCS of 13. Her blood pressure is 82/46 mmHg, heart rate is 122 beats/min, respiratory rate is 26 breaths/min, and temperature is 39.1°C. Her peripheral capillary refill time is 4 seconds. An arterial blood gas confirms a pH of 7.31, PaO2 of 10.4 kPa on room air, PaCO2 of 4.1 kPa, standard bicarbonate of 16 mmol/L, and lactate of 3.6 mmol/L. She receives an immediate intravenous bolus of 30 mL/kg balanced crystalloid fluid over 90 minutes alongside empirical intravenous broad-spectrum antibiotics and blood cultures. A repeat assessment 30 minutes post-infusion reveals a persistent blood pressure of 84/48 mmHg and a repeat venous lactate of 3.4 mmol/L. How is her condition categorized under Sepsis-3 definitions, and what is the next most appropriate pharmacological intervention?