SIRS and Sepsis Pathology

Quick Answer

Clinical Note

Sepsis is a life-threatening organ dysfunction caused by a dysregulated host response to infection. The pathophysiology involves recognition of PAMPs and DAMPs by pattern recognition receptors (TLRs), triggering a cytokine cascade (TNF-α, IL-1β, IL-6) that leads to endothelial dysfunction, glycocalyx degradation, coagulation activation (DIC), and mitochondrial dysfunction (cytopathic hypoxia). The initial hyperinflammatory phase (SIRS) is followed by immunoparalysis (CARS) with lymphocyte apoptosis and HLA-DR downregulation, predisposing to secondary infections. Organ dysfunction results from microvascular thrombosis, cellular metabolic shutdown, and persistent inflammation rather than widespread necrosis.

CICM Exam Focus

Clinical Note

SAQ Topics (First Part Written)

Compare SIRS criteria with Sepsis-3 definitions and explain the rationale for the change
Describe the role of PAMPs, DAMPs, and pattern recognition receptors in initiating the sepsis response
Explain the mechanisms of endothelial dysfunction and glycocalyx degradation in sepsis
Describe the coagulation abnormalities in sepsis including DIC pathophysiology
Explain cytopathic hypoxia and mitochondrial dysfunction in sepsis
Compare the hyperinflammatory (SIRS) and immunosuppressive (CARS) phases of sepsis

Viva Topics

Cytokine storm: TNF-α, IL-1β, IL-6 and their systemic effects
Organ-specific pathology: mechanisms of sepsis-induced AKI, ARDS, hepatic dysfunction, myocardial depression, encephalopathy
Biomarkers of sepsis: CRP, procalcitonin, lactate, presepsin, MR-proADM
Histopathological findings at autopsy in sepsis deaths
The "sepsis paradox"
minimal histological damage despite profound organ dysfunction

Common Examiner Questions

"Why did we move from SIRS to Sepsis-3 definitions?"
"Explain the role of the glycocalyx in sepsis pathophysiology"
"What is cytopathic hypoxia and how does it differ from ischemic hypoxia?"
"Describe the immunoparalysis phase and its clinical implications"
"How do NETs contribute to sepsis-induced coagulopathy?"

Key Points

Clinical Note

SIRS criteria (1992) required ≥2 of: temperature >38°C or <36°C, HR >90, RR >20 or PaCO₂ <32 mmHg, WBC >12,000 or <4,000 or >10% bands. Sepsis-3 (2016) redefined sepsis as "life-threatening organ dysfunction caused by dysregulated host response to infection" using SOFA score ≥2.

Clinical Note

PAMPs (pathogen-associated molecular patterns) like LPS are recognized by TLRs. DAMPs (damage-associated molecular patterns) like HMGB1 and mitochondrial DNA are released from injured host cells, creating a feed-forward inflammatory loop even after pathogen clearance.

Clinical Note

Pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) are released within hours of infection. Anti-inflammatory cytokines (IL-10, TGF-β) follow, creating a "cytokine storm" when the balance is lost. IL-6 levels correlate with sepsis severity and mortality.

Clinical Note

The glycocalyx (proteoglycans, glycosaminoglycans) is degraded by heparanase, MMPs, and ROS during sepsis. This leads to loss of permeability barrier, capillary leak syndrome, exposure of adhesion molecules, and microvascular thrombosis.

Clinical Note

Sepsis activates coagulation via tissue factor expression, depletes natural anticoagulants (antithrombin, protein C), and impairs fibrinolysis. DIC causes widespread microvascular thrombosis and consumptive coagulopathy.

Clinical Note

Cytopathic hypoxia occurs when cells cannot utilize oxygen due to mitochondrial dysfunction (NO-mediated Complex IV inhibition, oxidative damage). Cells enter "hibernation" mode to survive, causing organ dysfunction without necrosis.

Clinical Note

After initial hyperinflammation, patients enter an immunosuppressed state with lymphocyte apoptosis, HLA-DR downregulation on monocytes (<30%), and increased susceptibility to secondary infections. Many sepsis deaths occur in this phase.

Clinical Note

Autopsy studies show surprisingly minimal histological damage despite profound clinical organ dysfunction. This supports the "cellular hibernation" hypothesis - organs fail functionally while remaining structurally intact.

Clinical Note

CRP - general inflammation marker; Procalcitonin - bacterial infection (guides antibiotic therapy); Lactate - tissue hypoperfusion and prognostic marker; Presepsin - early diagnosis (rises within 2 hours); MR-proADM - predicts organ failure and mortality.

Clinical Note

Neutrophil extracellular traps (NETs) provide scaffolding for platelet aggregation and coagulation activation. While they trap pathogens, excessive NETosis contributes to DIC and organ damage via histone-mediated cytotoxicity.

Historical Definitions: SIRS Criteria (1992)

The ACCP/SCCM Consensus Conference

In 1992, the American College of Chest Physicians (ACCP) and Society of Critical Care Medicine (SCCM) convened a consensus conference that established the first standardized definitions for sepsis and related syndromes (PMID: 1597163).

Clinical Note

Systemic Inflammatory Response Syndrome (SIRS) requires ≥2 of:

Criterion	Parameter
Temperature	>38°C (>100.4°F) or <36°C (<96.8°F)
Heart Rate	>90 beats per minute
Respiratory Rate	>20 breaths per minute OR PaCO₂ <32 mmHg (hyperventilation)
White Cell Count	>12,000/mm³ OR <4,000/mm³ OR >10% immature (band) forms

1992 Sepsis Definitions Hierarchy

Term	Definition
SIRS	≥2 SIRS criteria (non-specific inflammatory response)
Sepsis	SIRS + documented or suspected infection
Severe Sepsis	Sepsis + organ dysfunction, hypoperfusion, or hypotension
Septic Shock	Sepsis-induced hypotension persisting despite adequate fluid resuscitation

Limitations of SIRS Criteria

The SIRS criteria became problematic because:

Lack of Specificity: SIRS criteria are met by 50-90% of ICU patients, including those without infection (post-operative, pancreatitis, trauma, burns)
Poor Prognostic Value: Meeting SIRS criteria did not reliably predict mortality
Missed Cases: Some patients with infection and organ dysfunction did not meet SIRS criteria (e.g., elderly, immunocompromised)
Focus on Inflammation Rather Than Organ Dysfunction: Organ failure is the key determinant of outcome

Clinical Pearl

A large retrospective study found that 12% of patients with infection-related organ dysfunction did not meet SIRS criteria. These "SIRS-negative" septic patients still had 16% mortality. SIRS criteria were more sensitive than specific and focused on the inflammatory response rather than the critical outcome - organ dysfunction.

Sepsis-3 Definitions (2016)

The Paradigm Shift

In 2016, the Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3) fundamentally changed how we conceptualize sepsis (PMID: 26903338).

Clinical Note

Sepsis

"Life-threatening organ dysfunction caused by a dysregulated host response to infection"

Clinical Criteria: Suspected or confirmed infection PLUS acute change in SOFA score ≥2 points

Septic Shock

Sepsis with:

Vasopressor requirement to maintain MAP ≥65 mmHg
Serum lactate >2 mmol/L despite adequate fluid resuscitation
Hospital mortality >40%

Key Changes from 1992 to 2016

Aspect	1992 Definitions	2016 Sepsis-3
Core Concept	Inflammation (SIRS)	Organ dysfunction (dysregulated host response)
Severity Marker	"Severe sepsis" category	Eliminated (sepsis inherently implies severity)
Organ Dysfunction	Added for "severe sepsis"	Central to sepsis definition (SOFA ≥2)
Shock Definition	Hypotension despite fluids	Vasopressors + lactate >2 mmol/L
Screening Tool	SIRS criteria	qSOFA (outside ICU)

The SOFA Score

The Sequential Organ Failure Assessment (SOFA) score became the operational tool for identifying organ dysfunction (PMID: 8844239):

System	Score 0	Score 1	Score 2	Score 3	Score 4
Respiration (PaO₂/FiO₂)	≥400	<400	<300	<200 with support	<100 with support
Coagulation (Platelets ×10³/μL)	≥150	<150	<100	<50	<20
Liver (Bilirubin μmol/L)	<20	20-32	33-101	102-204	>204
Cardiovascular	MAP ≥70	MAP <70	Dopa ≤5 or Dob	Dopa >5 or NA/Adr ≤0.1	Dopa >15 or NA/Adr >0.1
CNS (GCS)	15	13-14	10-12	6-9	<6
Renal (Creatinine μmol/L or UO)	<110	110-170	171-299	300-440 or <500 mL/d	>440 or <200 mL/d

qSOFA (Quick SOFA)

The qSOFA score was introduced as a bedside screening tool for patients OUTSIDE the ICU:

Clinical Note

≥2 of the following suggests increased risk of poor outcome:

Respiratory rate ≥22/min
Altered mentation (GCS ≤13)
Systolic blood pressure ≤100 mmHg

Note: qSOFA is for screening outside ICU; SOFA remains the diagnostic standard.

Clinical Pearl

While qSOFA has high specificity for poor outcomes, its sensitivity is lower than SIRS criteria. The Surviving Sepsis Campaign 2021 recommends using qSOFA only as a prompt for further evaluation, not as a sole screening tool. National early warning scores (NEWS) may have better performance in some settings (PMID: 33306967).

Pathogen Recognition: PAMPs and DAMPs

Overview of Pattern Recognition

The innate immune system detects infection through pattern recognition receptors (PRRs) that recognize conserved microbial structures (PAMPs) and endogenous danger signals from damaged host cells (DAMPs) (PMID: 24507191).

Pathogen-Associated Molecular Patterns (PAMPs)

PAMPs are highly conserved molecular structures produced by microorganisms but not by host cells:

PAMP	Microbial Source	Recognition Receptor
Lipopolysaccharide (LPS)	Gram-negative bacteria (outer membrane)	TLR4/MD-2/CD14
Lipoteichoic acid (LTA)	Gram-positive bacteria (cell wall)	TLR2
Peptidoglycan	Bacterial cell wall	TLR2, NOD1/2
Flagellin	Bacterial flagella	TLR5
Unmethylated CpG DNA	Bacterial DNA	TLR9
Double-stranded RNA	Viruses	TLR3, RIG-I, MDA5
β-glucan	Fungal cell wall	Dectin-1
Mannose-rich glycans	Microbial surfaces	Mannose receptor

Damage-Associated Molecular Patterns (DAMPs)

DAMPs (also called "alarmins") are endogenous molecules released from damaged or dying host cells that activate the same inflammatory pathways (PMID: 25355530):

DAMP	Source	Receptor
HMGB1 (High Mobility Group Box 1)	Necrotic/stressed cells	TLR4, TLR2, RAGE
Mitochondrial DNA (mtDNA)	Injured mitochondria	TLR9, NLRP3
ATP	Cell damage	P2X7 receptor
Heat shock proteins (HSPs)	Stressed cells	TLR2, TLR4
S100 proteins	Activated phagocytes	RAGE
Histones	Necrotic cells, NETs	TLR2, TLR4
Uric acid crystals	Cell breakdown	NLRP3 inflammasome

Clinical Pearl

A critical concept is that DAMPs released from initially injured tissue can propagate inflammation AFTER the primary infection is controlled. This explains why patients may deteriorate despite successful antibiotic therapy and source control - the "second hit" of endogenous danger signals perpetuates the inflammatory cascade.

Toll-Like Receptors (TLRs)

TLRs are the primary pattern recognition receptors on innate immune cells (PMID: 20303867):

TLR	Location	Ligands	Downstream Effect
TLR1/2	Cell surface	Lipoproteins, LTA	MyD88 → NF-κB activation
TLR3	Endosome	dsRNA (viral)	TRIF → IRF3 → Type I interferons
TLR4	Cell surface	LPS, HMGB1	MyD88 + TRIF → NF-κB + IRF3
TLR5	Cell surface	Flagellin	MyD88 → NF-κB activation
TLR7/8	Endosome	ssRNA (viral)	MyD88 → IRF7 → Type I interferons
TLR9	Endosome	CpG DNA, mtDNA	MyD88 → NF-κB + IRF7

Signaling Pathways

Clinical Note

1. MyD88-Dependent Pathway (most TLRs)

TLR engagement → MyD88 adaptor recruitment
IRAK4 → TRAF6 activation
NF-κB translocation to nucleus
Transcription of pro-inflammatory genes (TNF-α, IL-1β, IL-6)

2. TRIF-Dependent Pathway (TLR3, TLR4)

TRIF adaptor recruitment
IRF3 activation
Type I interferon production (IFN-α, IFN-β)
Contributes to antiviral response

The NLRP3 Inflammasome

The NLRP3 inflammasome is a cytoplasmic multiprotein complex that processes pro-IL-1β and pro-IL-18 into their active forms (PMID: 25952864):

Activation signals: ATP, uric acid crystals, bacterial toxins, mtDNA
Components: NLRP3 sensor, ASC adaptor, Caspase-1
Products: Active IL-1β, active IL-18, pyroptosis (inflammatory cell death)
Clinical relevance: NLRP3 hyperactivation contributes to cytokine storm in severe sepsis

The Inflammatory Cascade

Pro-Inflammatory Cytokines

The cytokine response in sepsis involves sequential release of mediators that amplify and propagate inflammation (PMID: 21283657):

Tumor Necrosis Factor-α (TNF-α)

Source: Macrophages, monocytes (primary); T cells, NK cells
Timing: Early release (within 30-90 minutes of PAMP recognition)
Actions:
- Endothelial activation (increased permeability, adhesion molecule expression)
- Vasodilation (via NO production)
- Pro-coagulant state (tissue factor expression)
- Fever induction (hypothalamic action)
- Myocardial depression (negative inotropic effect)
- Apoptosis induction in some cell types
Half-life: 14-18 minutes (short, but initiates cascade)

Interleukin-1β (IL-1β)

Source: Macrophages, monocytes (requires inflammasome activation)
Timing: Early (synergistic with TNF-α)
Actions:
- Synergizes with TNF-α for endothelial activation
- Fever induction (potent pyrogen)
- Neutrophil activation and chemotaxis
- Acute phase protein induction
- COX-2 upregulation (prostaglandin production)
Special feature: Requires two signals - NF-κB priming + inflammasome activation

Interleukin-6 (IL-6)

Source: Macrophages, endothelial cells, fibroblasts, T cells
Timing: Slightly later than TNF-α and IL-1β (peaks at 2-6 hours)
Actions:
- Acute phase protein induction (CRP, fibrinogen, hepcidin)
- B cell differentiation and antibody production
- Fever induction
- Transition from acute to chronic inflammation
Clinical significance: IL-6 levels correlate with sepsis severity and mortality (PMID: 9336871)
Biomarker potential: Serum IL-6 is used for prognostication

Clinical Note

When pro-inflammatory cytokine release becomes uncontrolled and systemic, a "cytokine storm" develops. This is characterized by:

Massive systemic cytokine release
Multi-organ endothelial dysfunction
Capillary leak syndrome
Distributive shock
Coagulation activation (DIC)
Often fatal without intervention

The term describes the pathological amplification of normally protective inflammatory responses (PMID: 33302002).

Anti-Inflammatory Cytokines

The body attempts to counter excessive inflammation through anti-inflammatory mediators (PMID: 12871993):

Interleukin-10 (IL-10)

Source: Monocytes, T regulatory cells (Tregs), B cells
Actions:
- Inhibits TNF-α, IL-1β, IL-6 production
- Downregulates MHC class II expression (including HLA-DR)
- Suppresses antigen presentation
- Promotes Treg development
Sepsis role: Elevated IL-10 correlates with immunoparalysis and secondary infections

Transforming Growth Factor-β (TGF-β)

Actions:
- Inhibits T cell proliferation
- Promotes Treg differentiation
- Inhibits macrophage activation
- Tissue repair and fibrosis

Other Anti-Inflammatory Mediators

IL-4: Th2 polarization, M2 macrophage phenotype
IL-13: Similar to IL-4, inhibits pro-inflammatory cytokines
IL-1 receptor antagonist (IL-1Ra): Competitive inhibitor of IL-1 signaling
Soluble TNF receptors: Bind and neutralize circulating TNF-α
Cortisol: Hypothalamic-pituitary-adrenal axis activation

Temporal Evolution: SIRS to CARS

Clinical Note

Phase 1: SIRS (Hyperinflammatory)

Predominates in first 24-72 hours
Pro-inflammatory cytokine surge
Clinical: Fever, tachycardia, vasodilation, capillary leak
Risk: Cytokine storm, early multi-organ failure

Phase 2: CARS (Compensatory Anti-Inflammatory Response Syndrome)

Develops over days 3-7
Anti-inflammatory cytokine predominance
Immune cell apoptosis and dysfunction
Clinical: Anergy, immunoparalysis
Risk: Secondary/nosocomial infections

Modern Understanding: MARS (Mixed Antagonist Response Syndrome)

SIRS and CARS occur simultaneously
Individual patients have varying balance
Mortality associated with BOTH persistent hyperinflammation AND immunoparalysis (PMID: 23498838)

Endothelial Dysfunction and Glycocalyx Degradation

The Endothelial Glycocalyx

The endothelial glycocalyx is a 0.5-4.5 μm thick carbohydrate-rich layer lining the luminal surface of all blood vessels (PMID: 30689695):

Structure

Component	Description
Proteoglycans	Syndecans (1-4), glypicans - provide scaffold
Glycosaminoglycans (GAGs)	Heparan sulfate (50-90%), chondroitin sulfate, hyaluronan
Glycoproteins	Selectins, integrins, immunoglobulin superfamily
Plasma proteins	Albumin, antithrombin III trapped within structure

Normal Functions

Permeability barrier: Molecular sieve excluding albumin and larger molecules
Mechanotransduction: Senses shear stress → NO production for vasodilation
Anti-thrombotic surface: Binds antithrombin III; masks adhesion molecules
Anti-inflammatory: Shields adhesion molecules from leukocytes
Reservoir function: Stores growth factors, enzymes

Glycocalyx Degradation in Sepsis

Multiple mechanisms lead to glycocalyx shedding during sepsis (PMID: 30689695):

Sheddases (Degrading Enzymes)

Enzyme	Target	Activating Signal
Heparanase-1	Heparan sulfate	TNF-α, IL-1β, ROS
Matrix metalloproteinases (MMPs)	Proteoglycan core proteins	Inflammatory cytokines
ADAM17 (TACE)	Syndecan-1, -4	TNF-α
Hyaluronidases	Hyaluronan	Inflammatory mediators

Reactive Oxygen Species (ROS)

Direct oxidative cleavage of GAG chains
Generated by activated neutrophils and endothelium
Peroxynitrite (from NO + superoxide) particularly damaging

Consequences of Glycocalyx Degradation

Red Flag

1. Capillary Leak Syndrome

Loss of permeability barrier → albumin and fluid extravasation
Interstitial edema despite intravascular hypovolemia
Contributes to refractory hypotension

2. Leukocyte Adhesion

Exposed adhesion molecules (ICAM-1, VCAM-1, E-selectin)
Neutrophil "tethering, rolling, and adhesion"
Transendothelial migration → tissue damage

3. Microvascular Thrombosis

Loss of antithrombin III binding sites
Exposed pro-coagulant surfaces
Platelet adhesion and aggregation
Contributes to DIC

4. Impaired Vasoregulation

Loss of shear stress sensing
Reduced endothelial NO production
Contributes to "vasoplegia"

Biomarkers of Glycocalyx Degradation

Biomarker	Clinical Significance
Syndecan-1	Most widely studied; elevated levels predict AKI, ARDS, mortality
Heparan sulfate	Associated with septic encephalopathy
Hyaluronan	Reflects degradation severity

Clinical Pearl

Emerging evidence suggests that resuscitation strategy impacts glycocalyx integrity:

Rapid crystalloid boluses may cause atrial stretch → ANP release → further shedding ("hypervolemic shedding")
Albumin may have glycocalyx-protective properties
Fresh frozen plasma contains glycocalyx-stabilizing factors
"Restrictive" fluid strategies may preserve glycocalyx better than "liberal" approaches (PMID: 31411458)

Coagulation Abnormalities and DIC

Overview

Sepsis induces a pro-coagulant state through three main mechanisms (PMID: 20519800):

Activation of coagulation (tissue factor expression)
Inhibition of natural anticoagulants (consumption and downregulation)
Impairment of fibrinolysis (PAI-1 upregulation)

Tissue Factor and Coagulation Activation

Tissue Factor Expression

Normally sequestered from blood (on sub-endothelial cells)
In sepsis: Expressed on monocytes, macrophages, activated endothelium
Triggered by: TNF-α, IL-1β, LPS, DAMPs
Initiates extrinsic pathway → massive thrombin generation

The Thrombin Storm

Thrombin generation in sepsis leads to:

Fibrin deposition in microvasculature
Platelet activation and aggregation
Microvascular thrombosis → organ ischemia
Consumption of clotting factors and platelets

Depletion of Natural Anticoagulants

Clinical Note

Antithrombin (AT)

Primary inhibitor of thrombin and Factor Xa
Consumption: Used up faster than produced
Degradation: Cleaved by neutrophil elastases
Capillary leak: Lost into interstitial space
Clinical: AT levels <70% associated with increased mortality (PMID: 15187054)

Protein C System

Vitamin K-dependent zymogen
Activated Protein C (APC) inactivates Factors Va and VIIIa
In sepsis: Thrombomodulin and EPCR downregulated → impaired PC activation
APC also has anti-inflammatory and cytoprotective effects
Clinical: Low PC levels nearly universal in septic shock (PMID: 11236773)

Tissue Factor Pathway Inhibitor (TFPI)

Inhibits TF-VIIa complex
Levels may appear normal but functionally inadequate

Fibrinolysis Inhibition

Plasminogen Activator Inhibitor-1 (PAI-1): Massively upregulated
Prevents plasmin generation
Impairs clot breakdown
Contributes to persistent microvascular thrombosis

Disseminated Intravascular Coagulation (DIC)

DIC is the prototypical coagulopathy of sepsis (PMID: 31327219):

Pathophysiology

Widespread tissue factor expression
Systemic thrombin generation
Microvascular fibrin deposition
Consumption of clotting factors and platelets
Secondary fibrinolysis (but inhibited in sepsis)

Clinical Features

Feature	Mechanism
Bleeding	Consumption of factors, thrombocytopenia
Organ failure	Microvascular thrombosis
Thrombocytopenia	Platelet consumption
Prolonged PT/aPTT	Factor consumption
Elevated D-dimer	Secondary fibrinolysis
Low fibrinogen	Consumption (may be normal early due to acute phase response)

ISTH DIC Scoring System

Parameter	Score 0	Score 1	Score 2	Score 3
Platelet count (×10⁹/L)	>100	50-100	<50	-
D-dimer	Normal	Moderate ↑	Strong ↑	-
Prothrombin time (prolongation)	<3 sec	3-6 sec	>6 sec	-
Fibrinogen (g/L)	>1.0	≤1.0	-	-

Score ≥5 = Overt DIC

NETosis and Immunothrombosis

Neutrophil extracellular traps (NETs) contribute significantly to sepsis-induced coagulopathy (PMID: 30111630):

NET Formation

Activated neutrophils expel DNA meshwork with histones and granular proteins
Stimuli: PAMPs, DAMPs, activated platelets, cytokines
Mechanism: NADPH oxidase → ROS → PAD4-mediated histone citrullination → chromatin decondensation

Pro-Coagulant Effects of NETs

NET Component	Pro-Coagulant Mechanism
DNA strands	Activate Factor XII (contact pathway)
Histones (H3, H4)	Activate platelets; cause endothelial damage
Neutrophil elastase	Degrades TFPI and thrombomodulin
Cathepsin G	Activates platelets; cleaves anticoagulants

The Immunothrombosis Concept

NETs represent the intersection of immunity and coagulation:

NETs trap and kill pathogens (protective)
NETs provide scaffold for thrombus formation (harmful in excess)
Therapeutic target: DNase to dissolve NETs (experimental) (PMID: 22114389)

Mitochondrial Dysfunction and Cytopathic Hypoxia

The Concept of Cytopathic Hypoxia

The term "cytopathic hypoxia" was introduced by Fink in 2001 to describe a fundamental paradox in sepsis (PMID: 12594860):

Clinical Note

Despite adequate global oxygen delivery (DO₂), tissue oxygen extraction is impaired, and ATP production fails. Cells cannot utilize available oxygen because the mitochondrial respiratory chain is dysfunctional. This explains why simply increasing DO₂ does not always improve outcomes.

Mechanisms of Mitochondrial Dysfunction

Nitric Oxide (NO) Inhibition

iNOS upregulated in sepsis → excessive NO production
NO competitively inhibits Complex IV (cytochrome c oxidase)
Reversible inhibition at low levels; persistent at high levels
Peroxynitrite (NO + superoxide) causes irreversible damage

Oxidative and Nitrosative Stress

Target	Damage Mechanism	Consequence
mtDNA	Oxidative base damage	Impaired protein synthesis
Cardiolipin	Lipid peroxidation	Loss of membrane potential
Respiratory complexes	Protein oxidation/nitration	Electron transport failure
Fe-S clusters	Oxidative damage	Enzyme inactivation

Other Mechanisms

Hormonal: Thyroid hormone (T3) deficiency impairs mitochondrial biogenesis
Metabolic: Altered fuel utilization (Warburg effect - aerobic glycolysis)
Structural: Mitochondrial swelling, cristae disruption

Bioenergetic Failure

When mitochondrial dysfunction becomes severe:

ATP depletion: Insufficient for cellular processes
Ion pump failure: Na⁺/K⁺-ATPase dysfunction → cellular swelling
Calcium overload: Mitochondrial permeability transition pore opening
Metabolic shift: Lactate production despite adequate oxygen (anaerobic glycolysis)

The Cellular Hibernation Hypothesis

Brealey and Singer proposed that cells enter a "hibernation" state as a survival mechanism (PMID: 12225604):

Clinical Note

Concept: Cells downregulate energy-demanding functions to survive the metabolic crisis

Evidence:

Organs "fail" clinically but show minimal cell death at autopsy
Mitochondrial function correlates with illness severity and recovery
Survivors show mitochondrial biogenesis (PGC-1α upregulation)

Implications:

Organ failure represents functional shutdown, not structural destruction
Recovery possible if cells survive the metabolic crisis
Therapeutic target: Supporting mitochondrial recovery

Recovery: Mitochondrial Biogenesis

Survival depends on the ability to generate new, functional mitochondria (PMID: 17245130):

PGC-1α: Master regulator of mitochondrial biogenesis
NRF-1, NRF-2: Transcription factors for mitochondrial genes
TFAM: Mitochondrial transcription factor A - mtDNA replication
Mitophagy: Clearance of damaged mitochondria (quality control)

Clinical Pearl

Elevated lactate in sepsis reflects BOTH:

Tissue hypoperfusion (Type A lactic acidosis)
Mitochondrial dysfunction with aerobic glycolysis (Type B)

This explains why lactate may remain elevated despite normalized hemodynamics. Persistent hyperlactatemia correlates with ongoing cellular dysfunction and poor prognosis. Lactate clearance (not absolute level) better predicts outcomes (PMID: 15286537).

Immunoparalysis and CARS

The Shift from Hyperinflammation to Immunosuppression

While early sepsis is characterized by hyperinflammation (SIRS), most sepsis deaths actually occur during a later phase of profound immunosuppression termed "immunoparalysis" (PMID: 23498838).

Key Features of Immunoparalysis

Lymphocyte Apoptosis

The pioneering work of Hotchkiss demonstrated massive immune cell death in sepsis (PMID: 10403738):

Cell Type	Apoptosis Rate	Clinical Impact
CD4+ T cells	Markedly increased	Loss of helper function
CD8+ T cells	Markedly increased	Impaired cytotoxic function
B cells	Increased	Reduced antibody production
Dendritic cells	Increased	Impaired antigen presentation
Regulatory T cells (Tregs)	Relatively spared	Immunosuppressive bias

Mechanisms of Lymphocyte Apoptosis

Extrinsic pathway: Death receptor activation (Fas/FasL, TRAIL)
Intrinsic pathway: Mitochondrial dysfunction, Bim/Bcl-2 imbalance
ER stress: Unfolded protein response
Glucocorticoid-induced: HPA axis hyperactivation

HLA-DR Downregulation

Monocyte HLA-DR expression is the clinical gold standard for detecting immunoparalysis (PMID: 20196842):

Clinical Note

Normal: >15,000 antibodies per cell (Ab/cell) or >90% HLA-DR+ monocytes

Immunoparalysis Thresholds:

<8,000 Ab/cell or <30% HLA-DR+ monocytes
Persistent for >48 hours

Clinical Significance:

Predicts secondary infections (nosocomial pneumonia, candidemia)
Predicts mortality
Potential therapeutic target (IFN-γ, GM-CSF may restore expression)

Immune Checkpoint Upregulation

Sepsis induces "exhaustion" markers similar to chronic infections and cancer (PMID: 27849633):

Checkpoint	Expression in Sepsis	Effect
PD-1	↑ on T cells	T cell anergy, reduced proliferation
PD-L1	↑ on monocytes, APCs	Inhibits T cell activation
CTLA-4	↑ on T cells	Competes with CD28 for B7
BTLA	↑ on lymphocytes	Inhibitory signaling

Compensatory Anti-Inflammatory Response (CARS)

The anti-inflammatory phase is driven by:

IL-10 hypersecretion: Suppresses pro-inflammatory cytokines
TGF-β elevation: Promotes Treg differentiation
Myeloid-derived suppressor cells (MDSCs): Expanded population
M2 macrophage polarization: Anti-inflammatory phenotype
Treg expansion: Relative sparing of Tregs during apoptosis

Clinical Consequences of Immunoparalysis

Red Flag

Secondary Infections (30-50% of sepsis patients)

Nosocomial pneumonia (especially VAP)
Catheter-related bloodstream infections
Candidemia
CMV reactivation
HSV reactivation

Mortality

Most sepsis deaths occur AFTER the first 72 hours
Late mortality associated with immunosuppression, not hyperinflammation
Autopsy studies show unresolved infection foci despite antibiotics

Emerging Immunostimulatory Therapies

Agent	Mechanism	Trial Status
IFN-γ	Restores HLA-DR expression, monocyte function	Case series positive
GM-CSF	Stimulates myeloid cells	GRID trial showed HLA-DR restoration
IL-7	Lymphocyte survival factor, prevents apoptosis	Phase II showing lymphocyte recovery
Anti-PD-1/PD-L1	Checkpoint blockade	Early phase trials
Thymosin α1	T cell differentiation	Some evidence in Asian trials

Organ Dysfunction: Specific Pathology

The "Sepsis Paradox" Revisited

Autopsy studies consistently show surprisingly mild histological changes despite profound organ dysfunction (PMID: 20595724). This supports the concept that organs "fail" functionally through cellular hibernation rather than extensive necrosis.

Sepsis-Associated Acute Kidney Injury (SA-AKI)

Clinical Note

Histological Findings (often mild):

Focal acute tubular injury (NOT widespread necrosis)
Loss of brush border microvilli
Vacuolization of tubular epithelium
Mild interstitial edema
Fibrin thrombi (if DIC present)

Pathophysiological Mechanisms:

Microcirculatory dysfunction: Heterogeneous flow, shunting
Inflammation: TLR4 activation on tubular cells
Mitochondrial dysfunction: Metabolic reprogramming
Tubular "stunning": Downregulation of solute transport to conserve energy

Clinical Note: SA-AKI can occur despite NORMAL or HIGH renal blood flow - the injury is not primarily ischemic (PMID: 26537049).

Acute Respiratory Distress Syndrome (ARDS)

Phase	Timing	Histopathology
Exudative	Days 1-7	Diffuse alveolar damage, hyaline membranes, interstitial/alveolar edema, hemorrhage
Proliferative	Days 7-14	Type II pneumocyte hyperplasia, fibroblast proliferation
Fibrotic	>Week 3	Interstitial fibrosis (some patients)

Mechanisms:

Endothelial injury → alveolar-capillary leak
Neutrophil sequestration and activation
NETs and protease release
Surfactant dysfunction (Type II pneumocyte damage)
V/Q mismatch, shunt physiology

Sepsis-Induced Cardiomyopathy (SIC)

Clinical Note

Key Features:

Biventricular dilation with preserved or reduced EF
Usually REVERSIBLE (7-10 days)
Distinct from ischemic cardiomyopathy

Histopathology (minimal changes):

Myofibrillar edema
Contraction band necrosis (focal)
Minimal myocyte necrosis

Mechanisms:

Circulating myocardial depressant factors (TNF-α, IL-1β, IL-6)
Excessive NO → reduced β-adrenergic responsiveness
Mitochondrial dysfunction in cardiomyocytes
Calcium handling abnormalities

Troponin Elevation: Common in sepsis; reflects myocardial stress, not always ischemia (PMID: 18195325).

Sepsis-Induced Hepatic Dysfunction

Pattern	Mechanism	Laboratory Finding
Ischemic hepatitis ("Shock liver")	Centrilobular necrosis from hypoperfusion	AST/ALT markedly elevated (thousands)
Sepsis-associated cholestasis	Bilirubin transporter downregulation (BSEP, MRP2)	Conjugated hyperbilirubinemia
Kupffer cell activation	Cytokine and ROS production	Contributes to systemic inflammation

Sepsis-Associated Encephalopathy (SAE)

Incidence: 50-70% of septic patients
Histopathology: Microglial activation, perivascular edema, "red neurons" (ischemic)
Mechanisms:
- Blood-brain barrier disruption
- Neuroinflammation (microglia activation)
- Neurotransmitter imbalance
- Impaired cerebral autoregulation
- Oxidative stress
Clinical: Delirium spectrum (agitation to coma)

Splenic Pathology: The "Immunological Graveyard"

A hallmark autopsy finding in sepsis (PMID: 10403738):

Massive lymphoid depletion: White pulp nearly empty
Follicular depletion: Loss of B cells
Apoptotic bodies: Evidence of immune cell death
Clinical correlation: Confirms immunoparalysis phase

Biomarkers of Sepsis

Overview of Current Biomarkers

No single biomarker is diagnostic for sepsis; clinical context remains essential. Biomarkers serve different roles in screening, diagnosis, prognosis, and therapeutic guidance (PMID: 31764808).

C-Reactive Protein (CRP)

Aspect	Details
Source	Hepatocytes (IL-6 stimulated)
Kinetics	Rises 6-12 hours post-stimulus; peaks 24-48 hours
Normal	<5 mg/L (varies by assay)
Sepsis levels	Typically >100 mg/L
Strengths	Widely available, inexpensive
Limitations	Non-specific (elevated in any inflammation); slow kinetics
Role	Screening, trend monitoring, NOT specific for infection

Procalcitonin (PCT)

Aspect	Details
Source	Thyroid C cells (normally); all tissues (in sepsis - CALC-1 gene induction)
Kinetics	Rises 2-4 hours; peaks 6-24 hours; half-life 24-36 hours
Normal	<0.05 ng/mL
Bacterial sepsis	Often >2 ng/mL (up to >100 in severe sepsis)
Strengths	More specific for bacterial infection than CRP
Limitations	Elevated post-surgery, trauma, burns, cardiogenic shock; affected by CKD
Role	Diagnosis of bacterial sepsis; antibiotic stewardship (de-escalation guidance) (PMID: 27102190)

Lactate

Aspect	Details
Source	Anaerobic glycolysis (hypoperfusion) + aerobic glycolysis (mitochondrial dysfunction)
Normal	<2 mmol/L
Sepsis-3 threshold	>2 mmol/L with vasopressors = septic shock
Prognosis	Lactate >4 mmol/L associated with >30% mortality
Clearance	Serial lactate measurement; clearance >10-20% in 2-6 hours predicts survival
Role	Severity, prognosis, resuscitation target (PMID: 15286537)

Presepsin (sCD14-ST)

Aspect	Details
Source	Cleavage of CD14 during phagocytosis of bacteria
Kinetics	Very rapid rise (within 2 hours); peaks 3 hours
Normal	<337 pg/mL (varies by assay)
Strengths	Earliest rising marker; good for Gram-negative sepsis
Limitations	Elevated in renal failure; less studied than PCT
Role	Early diagnosis, particularly ED setting

MR-proADM (Mid-regional pro-Adrenomedullin)

Aspect	Details
Source	Endothelium (reflects endothelial dysfunction)
Kinetics	More stable than adrenomedullin
Strengths	Best predictor of organ failure and mortality
Limitations	Less specific for infection (reflects severity, not etiology)
Role	Risk stratification, predicts ICU need and death (PMID: 23782967)

Biomarker Comparison Table

Biomarker	Diagnosis	Severity/Prognosis	Antibiotic Guidance	Timing
CRP	++	+	+	Slow (24-48h)
PCT	+++	++	+++	Moderate (6-24h)
Lactate	+	+++	+	Immediate
Presepsin	+++	++	+	Rapid (2-3h)
MR-proADM	+	+++	+	Moderate

Clinical Pearl

The 2023 trend in sepsis biomarkers is combining markers for different purposes:

PCT for infection probability and antibiotic decisions
MR-proADM for severity and prognosis
Lactate for tissue perfusion and resuscitation response
Serial measurement more informative than single values

Histopathology: Autopsy Findings in Sepsis

Overview

Autopsy studies provide crucial insights into sepsis pathology, often revealing the "sepsis paradox"

discordance between profound clinical organ failure and relatively mild histological changes (PMID: 20595724).

Lung Findings

Finding	Description	Frequency
Diffuse alveolar damage	Hyaline membranes, alveolar edema	Very common
Neutrophilic infiltration	Pulmonary capillary sequestration	Near-universal
Pulmonary edema	Interstitial and alveolar	Common
Hemorrhage	Alveolar hemorrhage	Variable
Fibrin thrombi	Microvasculature (DIC)	Common
Pneumonia	Unresolved infection focus	Frequent

Kidney Findings

Finding	Description	Notes
Acute tubular injury	Patchy, focal (NOT widespread necrosis)	Paradoxically mild
Loss of brush border	Proximal tubular damage	Common
Vacuolization	Sublethal tubular injury	Common
Interstitial edema	Mild	Common
Fibrin thrombi	Glomerular capillaries (DIC)	If DIC present

Clinical Pearl

Many patients with profound clinical AKI (anuria, markedly elevated creatinine) show only mild and patchy tubular injury at autopsy. This supports the concept of "tubular stunning"

functional shutdown for cellular preservation rather than structural destruction.

Liver Findings

Finding	Description	Clinical Correlation
Centrilobular necrosis (Zone 3)	Hypoperfusion-related	"Shock liver"
Cholestasis	Bile duct plugging, canalicular stasis	Conjugated hyperbilirubinemia
Kupffer cell hyperplasia	Activated macrophages	Inflammation
Steatosis	Micro- or macrovesicular	Metabolic stress

Spleen and Lymphoid Tissue

Finding	Description	Significance
White pulp depletion	Massive lymphocyte loss	Immunoparalysis
Lymphoid follicle emptying	B cell loss	Impaired humoral immunity
Apoptotic bodies	Evidence of cell death	Hotchkiss findings confirmed
Red pulp congestion	Splenic pooling	Common

Heart Findings

Finding	Description
Myofibrillar edema	Interstitial fluid accumulation
Contraction band necrosis	Focal, catecholamine-related
Minimal myocyte necrosis	Unlike myocardial infarction
Inflammatory infiltrate	Usually mild

Brain Findings

Finding	Description
Microglial activation	Neuroinflammation
Perivascular edema	BBB disruption
"Red neurons"	Ischemic neuronal damage
Micro-hemorrhages	DIC-related
White matter changes	Variable

Summary: The Histopathological Pattern

Clinical Note

What We Expect: Widespread necrosis matching clinical organ failure

What We Find: Surprisingly minimal cell death with:

Inflammatory cell infiltration
Microvascular thrombosis (DIC)
Immune cell apoptosis (spleen, lymph nodes)
Functional "stunning" rather than structural destruction

Interpretation: Organs fail through metabolic shutdown and cellular hibernation, not massive necrosis. This explains the potential for recovery if patients survive the acute phase.

Australian/New Zealand Context

Sepsis Epidemiology in Australia

Clinical Note

Incidence: >55,000 sepsis cases annually in Australia

Mortality: >8,000 sepsis-related deaths per year

ICU Statistics:

ICU mortality for sepsis: 15-18%
Septic shock mortality: >40%
Sepsis accounts for ~15% of ICU admissions

Economic Impact: ~$4.8 billion annually to Australian healthcare system

Trends: Improving mortality over past two decades due to early recognition and standardized care bundles

ANZICS-CORE Data

The Australian and New Zealand Intensive Care Society Centre for Outcome and Resource Evaluation (ANZICS-CORE) maintains the largest binational critical care database:

Documents sepsis outcomes across ANZ ICUs
Benchmarks mortality against predicted (APACHE/ANZROD)
Tracks implementation of sepsis bundles
Provides data for national quality improvement

Australian Clinical Care Standards

The Australian Commission on Safety and Quality in Health Care (ACSQHC) Sepsis Clinical Care Standard (2022) emphasizes:

Early recognition of sepsis
Timely antibiotic administration (within 60 minutes of recognition)
Blood culture collection before antibiotics when feasible
Fluid resuscitation
Lactate measurement
Escalation of care
Documentation and communication

Indigenous Health Considerations

Indigenous Health Context

Aboriginal and Torres Strait Islander Peoples

Epidemiological Burden:

Aboriginal and Torres Strait Islander people are 2-3× more likely to be hospitalized for sepsis
Median age at sepsis presentation is 20 years younger than non-Indigenous Australians
Higher sepsis-related mortality even after age adjustment
Over-represented in ICU sepsis admissions with higher illness severity scores

Risk Factors:

Higher prevalence of chronic diseases (rheumatic heart disease, bronchiectasis, ESKD)
Reduced access to primary healthcare, particularly in remote communities
Socioeconomic disadvantage
Delayed presentation due to barriers to healthcare access
Higher rates of community-acquired pneumonia and skin/soft tissue infections

Healthcare System Considerations:

Need for culturally safe clinical pathways
Risk of "under-triage" in emergency settings
Importance of Aboriginal Health Workers (AHWs) and Aboriginal Liaison Officers (ALOs)
Family and community involvement in decision-making
Language barriers requiring interpreters
Different explanatory models of illness
Discharge planning challenges for remote communities

Closing the Gap Relevance:

Improving sepsis outcomes is integral to closing the life expectancy gap
Sepsis is a leading cause of neonatal and pediatric death in Indigenous communities
Early recognition programs must be culturally appropriate
Community health education campaigns needed

Māori Health (New Zealand)

Epidemiological Burden:

Māori have higher rates of sepsis-related hospitalization and mortality
Younger age at presentation compared to NZ European population
Higher prevalence of predisposing conditions (diabetes, COPD, CVD)

Cultural Considerations:

Whānau (extended family) central to decision-making
Involvement of kaumātua (elders) in serious discussions
Tikanga Māori (cultural practices) in healthcare
Importance of hui (meeting) for major decisions
Te reo Māori (Māori language) services
Recognition of holistic Māori health models (Te Whare Tapa Whā)
Involvement of Māori Health Workers

Te Tiriti o Waitangi:

Healthcare obligations under the Treaty
Ensuring equitable outcomes for Māori
Partnership principles in care delivery

SAQ Practice Questions

Clinical Note

Question: Describe the pathophysiology of cellular hypoxia in sepsis and explain why elevated lactate may persist despite adequate oxygen delivery. (15 marks)

Model Answer

1. Introduction and Definitions (1 mark)

Sepsis causes tissue hypoxia through multiple mechanisms beyond simple hypoperfusion. "Cytopathic hypoxia" (Fink, 2001) describes cellular inability to utilize oxygen despite adequate delivery, distinguishing sepsis from pure ischemic states.

2. Mechanisms of Cellular Hypoxia in Sepsis (6 marks)

A. Macrocirculatory Dysfunction

Distributive shock: pathological vasodilation (NO excess), myocardial depression
Reduced preload: capillary leak, relative hypovolemia
Results in decreased global DO₂

B. Microcirculatory Dysfunction (2 marks)

Heterogeneous flow: areas of stasis adjacent to hyperperfused regions
Glycocalyx degradation: loss of barrier function, edema
Endothelial dysfunction: impaired autoregulation
Microvascular thrombosis (DIC): capillary obstruction
Shunting: blood bypasses capillary beds

C. Mitochondrial Dysfunction (Cytopathic Hypoxia) (3 marks)

NO-mediated Complex IV inhibition: Excess iNOS expression → NO competes with O₂ at cytochrome c oxidase
Oxidative/nitrosative damage: Peroxynitrite formation → damages ETC complexes, mtDNA, cardiolipin
Metabolic reprogramming: Cells shift to aerobic glycolysis (Warburg effect)
"Cellular hibernation": Downregulation of energy-demanding processes for survival
Result: Cells cannot oxidize pyruvate despite adequate tissue PO₂

3. Lactate Elevation in Sepsis (4 marks)

A. Type A Lactic Acidosis (Hypoperfusion)

Inadequate DO₂ → anaerobic glycolysis → pyruvate → lactate
Traditional mechanism in shock states

B. Type B Lactic Acidosis (Cytopathic) (2 marks)

Mitochondrial dysfunction prevents pyruvate oxidation
Aerobic glycolysis continues despite adequate oxygen
Explains lactate elevation with normal/high SvO₂ and DO₂
Endogenous catecholamines stimulate glycolysis (β₂-mediated)

C. Reduced Clearance

Hepatic dysfunction: reduced lactate metabolism (Cori cycle)
Renal dysfunction: reduced lactate excretion

4. Clinical Implications (2 marks)

Lactate >4 mmol/L associated with >30% mortality
Lactate clearance (>10-20% in 2-6 hours) more prognostically useful than absolute value
Persistent hyperlactatemia despite normalized hemodynamics indicates ongoing mitochondrial dysfunction
Simply increasing DO₂ may not improve outcomes if mitochondrial function is the limiting factor

5. Recovery Mechanisms (2 marks)

Mitochondrial biogenesis (PGC-1α upregulation) essential for recovery
Mitophagy clears damaged mitochondria
Survivors show restored mitochondrial function
Therapeutic targeting of mitochondrial protection is an area of active research

Clinical Note

Question: Explain the pathophysiology of coagulation abnormalities in sepsis, including the role of natural anticoagulants and NETosis. (15 marks)

Model Answer

1. Introduction (1 mark)

Sepsis induces a complex coagulopathy characterized by simultaneous activation of coagulation, depletion of natural anticoagulants, and impaired fibrinolysis. Disseminated intravascular coagulation (DIC) represents the severe end of this spectrum, causing microvascular thrombosis and consumptive coagulopathy.

2. Activation of Coagulation (3 marks)

A. Tissue Factor Expression

Pro-inflammatory cytokines (TNF-α, IL-1β, IL-6) induce TF expression
Expressed on: monocytes, macrophages, activated endothelium
LPS directly triggers TF expression via TLR4
TF initiates extrinsic pathway → Factor VIIa activation

B. Thrombin Generation

Massive thrombin production ("thrombin storm")
Thrombin converts fibrinogen to fibrin
Thrombin activates platelets
Fibrin deposition in microvasculature → organ ischemia

C. Contact Pathway Activation

Factor XII activated by NETs, polyphosphates, damaged endothelium
Contributes to sustained coagulation activation

3. Natural Anticoagulant Depletion (4 marks)

A. Antithrombin (AT) (2 marks)

Primary inhibitor of thrombin and Factor Xa
Mechanisms of depletion:
- Consumption (used faster than produced)
- Degradation by neutrophil elastases
- Capillary leak (extravasation into tissues)
- Reduced hepatic synthesis
AT <70% associated with increased mortality
AT also has anti-inflammatory properties (prostacyclin release)

B. Protein C System (2 marks)

Activated protein C (APC) inactivates Factors Va and VIIIa
Activation requires thrombomodulin and EPCR on endothelium
In sepsis: TM and EPCR downregulated by inflammatory cytokines
Result: Impaired PC activation
APC also has cytoprotective and anti-inflammatory effects
PROWESS trial showed benefit; PROWESS-SHOCK did not (drug withdrawn)

C. Tissue Factor Pathway Inhibitor (TFPI)

Inhibits TF-VIIa complex
Functionally inadequate in sepsis despite normal levels

4. Fibrinolysis Inhibition (2 marks)

PAI-1 (Plasminogen Activator Inhibitor-1) massively upregulated
Prevents plasmin generation from plasminogen
Impairs clot dissolution
Results in persistent microvascular thrombosis
"Fibrinolytic shutdown" characteristic of sepsis-DIC

5. NETosis and Immunothrombosis (3 marks)

A. NET Formation

Activated neutrophils expel chromatin meshwork with histones and granular proteins
Stimuli: PAMPs, DAMPs, activated platelets, cytokines
Mechanism: NADPH oxidase → ROS → PAD4 → histone citrullination → chromatin decondensation

B. Pro-Coagulant Effects of NETs (2 marks)

DNA strands: Activate Factor XII (contact pathway)
Histones (H3, H4): Activate platelets; directly cytotoxic to endothelium
Neutrophil elastase: Degrades TFPI and thrombomodulin
NETs provide physical scaffold for platelet aggregation and fibrin deposition

C. The Immunothrombosis Concept

NETs represent intersection of immunity and coagulation
Trap and kill pathogens (protective function)
In excess, drive DIC and organ damage
Potential therapeutic target: DNase to dissolve NETs

6. DIC in This Case (2 marks)

Using ISTH DIC Score:

Platelets 42 (score 2)
PT prolonged >6 sec (score 2)
Fibrinogen <1 g/L (score 1)
D-dimer markedly elevated (score 2-3)
Total ≥5 = Overt DIC

Clinical manifestations:

Bleeding: Petechiae, oozing (consumptive coagulopathy)
Organ dysfunction: Oliguria (microvascular thrombosis in kidneys)

Viva Scenarios

Viva Scenario

Examiner Introduction

"A 65-year-old man with community-acquired pneumonia develops septic shock. I'd like to explore the pathophysiology of the inflammatory response."

Examiner: What are PAMPs and DAMPs, and how do they initiate the inflammatory response?

Candidate: PAMPs are Pathogen-Associated Molecular Patterns - conserved molecular structures produced by microorganisms but not host cells. Examples include:

LPS (lipopolysaccharide) from Gram-negative bacteria
Lipoteichoic acid from Gram-positive bacteria
Peptidoglycan
Bacterial flagellin
Unmethylated CpG DNA

DAMPs are Damage-Associated Molecular Patterns or "alarmins"

endogenous molecules released from injured host cells. Examples include:
HMGB1 (High Mobility Group Box 1)
Mitochondrial DNA
ATP
Heat shock proteins
S100 proteins
Histones

Both PAMPs and DAMPs are recognized by Pattern Recognition Receptors, primarily Toll-like receptors (TLRs), on innate immune cells.

Examiner: How do Toll-like receptors signal?

Candidate: TLRs are transmembrane receptors. TLR4 is particularly important in Gram-negative sepsis as it recognizes LPS in conjunction with MD-2 and CD14.

TLR signaling occurs via two main pathways:

MyD88-dependent pathway (most TLRs):
- TLR engagement → MyD88 adaptor recruitment
- IRAK4 and TRAF6 activation
- IκB kinase activation
- NF-κB translocation to nucleus
- Transcription of pro-inflammatory genes (TNF-α, IL-1β, IL-6)
TRIF-dependent pathway (TLR3, TLR4):
- Leads to IRF3 activation
- Type I interferon production (IFN-α, IFN-β)

Examiner: Describe the role of TNF-α, IL-1β, and IL-6 in sepsis.

Candidate: These are the primary pro-inflammatory cytokines:

TNF-α:

Released very early (within 30-90 minutes)
Source: primarily macrophages
Actions: endothelial activation, increased permeability, vasodilation, fever, pro-coagulant state, myocardial depression
Short half-life but initiates cascade

IL-1β:

Works synergistically with TNF-α
Requires inflammasome activation for processing
Actions: fever induction, endothelial adhesion molecule upregulation, neutrophil activation

IL-6:

Slightly later than TNF-α and IL-1β
Major driver of acute phase response
Stimulates hepatic CRP and fibrinogen production
Levels correlate with sepsis severity and mortality
Contributes to immune exhaustion with prolonged elevation

Examiner: What is the endothelial glycocalyx and how is it damaged in sepsis?

Candidate: The glycocalyx is a 0.5-4.5 μm carbohydrate-rich layer lining the luminal surface of all blood vessels. Its components include:

Proteoglycans (syndecans, glypicans)
Glycosaminoglycans (heparan sulfate, hyaluronan)
Glycoproteins
Trapped plasma proteins (albumin, antithrombin)

Normal functions:

Permeability barrier (molecular sieve)
Mechanotransduction (shear stress sensing → NO production)
Anti-thrombotic surface
Anti-inflammatory (masks adhesion molecules)

Damage mechanisms in sepsis:

Heparanase-1 (activated by TNF-α, IL-1β) cleaves heparan sulfate
MMPs and ADAM17 degrade proteoglycan core proteins
Hyaluronidase breaks down hyaluronan
ROS directly damage glycocalyx components

Examiner: What are the consequences of glycocalyx degradation?

Candidate: The consequences include:

Capillary leak syndrome: Loss of permeability barrier → albumin and fluid extravasation → tissue edema despite intravascular hypovolemia
Leukocyte adhesion: Exposed adhesion molecules (ICAM-1, VCAM-1, E-selectin) → neutrophil tethering, rolling, adhesion, and transendothelial migration → tissue damage
Microvascular thrombosis: Loss of antithrombin binding sites, exposed pro-coagulant surfaces, platelet adhesion → contributes to DIC
Impaired vasoregulation: Loss of shear stress sensing → reduced endothelial NO production → contributes to "vasoplegia"

Clinically, these manifest as refractory hypotension, tissue edema, and organ dysfunction.

Examiner: How might we measure glycocalyx degradation clinically?

Candidate: Biomarkers of glycocalyx degradation include:

Syndecan-1: Most widely studied; elevated levels predict AKI, ARDS, and mortality
Heparan sulfate: Associated with septic encephalopathy
Hyaluronan: Reflects degradation severity

These biomarkers are elevated in sepsis and correlate with disease severity, though they're not yet in routine clinical use.

There's also emerging research on "glycocalyx-protective" resuscitation - avoiding rapid crystalloid boluses that may cause atrial stretch and ANP-mediated shedding, potentially favoring albumin-based resuscitation.

Examiner: Excellent. Any questions?

Candidate: No, thank you.

Viva Scenario

Examiner Introduction

"A patient in ICU has survived the initial septic shock but develops a hospital-acquired pneumonia on day 8. Let's discuss the immunological changes in sepsis."

Examiner: What is immunoparalysis?

Candidate: Immunoparalysis is a state of profound immunosuppression that develops after the initial hyperinflammatory phase of sepsis. It is part of the Compensatory Anti-inflammatory Response Syndrome (CARS).

Key features include:

Reduced ability to clear the primary infection
High susceptibility to secondary, opportunistic infections (nosocomial pneumonia, candidemia, CMV/HSV reactivation)
Contributes to late sepsis mortality

Importantly, most sepsis deaths occur AFTER the first 72 hours, during this immunosuppressed phase, rather than from the initial cytokine storm.

Examiner: What cellular changes characterize immunoparalysis?

Candidate: The key cellular changes are:

1. Lymphocyte apoptosis (Hotchkiss's work):

Massive programmed cell death of CD4+ T cells, CD8+ T cells, B cells, and dendritic cells
Mechanisms: Death receptor pathway (Fas/FasL), mitochondrial pathway (Bim/Bcl-2 imbalance)
Regulatory T cells (Tregs) are relatively spared, shifting balance toward immunosuppression

2. HLA-DR downregulation on monocytes:

Normal: >15,000 antibodies per cell or >90% HLA-DR+ monocytes
Immunoparalysis: <8,000 Ab/cell or <30% HLA-DR+
Indicates impaired antigen presentation
Persistent low HLA-DR predicts secondary infections and mortality

3. Immune checkpoint upregulation:

Increased PD-1 on T cells, PD-L1 on monocytes
Leads to T cell anergy and exhaustion
Similar to chronic infections and cancer

Examiner: How is HLA-DR downregulation measured and what drives it?

Candidate: HLA-DR expression is measured by flow cytometry, quantifying either the percentage of monocytes expressing HLA-DR or the number of antibodies bound per cell.

Drivers of HLA-DR downregulation include:

Elevated IL-10 (anti-inflammatory cytokine)
TGF-β elevation
Glucocorticoid excess (HPA axis hyperactivation)
Persistent inflammation causing cellular exhaustion
Sepsis-induced endoplasmic reticulum stress

The clinical threshold for immunoparalysis is typically <30% HLA-DR+ monocytes or <8,000 Ab/cell persisting for >48 hours.

Examiner: What are potential therapies for immunoparalysis?

Candidate: Emerging immunostimulatory therapies include:

IFN-γ: Restores HLA-DR expression and monocyte function; positive case series in sepsis
GM-CSF (Granulocyte-Macrophage Colony-Stimulating Factor): Stimulates myeloid cell production; GRID trial showed HLA-DR restoration
IL-7: Lymphocyte survival factor; prevents apoptosis, increases T cell numbers; Phase II trials showing lymphocyte recovery
Anti-PD-1/PD-L1 checkpoint inhibitors: "Re-awaken" exhausted T cells; early phase trials ongoing
Thymosin α1: Promotes T cell differentiation; some evidence in Asian trials

The challenge is identifying which patients are in the immunoparalysis phase and would benefit from immunostimulation versus those still in hyperinflammation who might be harmed.

Examiner: Let's move to organ dysfunction. Why do organs fail in sepsis despite often minimal histological damage?

Candidate: This is the "sepsis paradox"

autopsy studies consistently show surprisingly mild histological changes despite profound clinical organ dysfunction.

The explanation is that organ failure represents functional shutdown rather than structural destruction:

1. Cellular hibernation hypothesis:

Cells downregulate energy-demanding functions to survive metabolic crisis
Mitochondrial dysfunction prevents adequate ATP production
Ion pumps fail, but cells avoid necrosis
This allows recovery if patient survives acute phase

2. Cytopathic hypoxia:

Mitochondrial dysfunction prevents oxygen utilization
NO-mediated Complex IV inhibition
Oxidative damage to ETC components
Cells shift to aerobic glycolysis

3. Microvascular dysfunction:

Heterogeneous flow with shunting
DIC causing microvascular thrombosis
Glycocalyx loss and barrier dysfunction

Evidence supporting this includes:

Minimal tubular necrosis in sepsis-AKI despite profound renal failure
Reversible myocardial depression without myocyte death
Survivors show mitochondrial biogenesis and functional recovery

Examiner: What specific pathological changes occur in sepsis-induced AKI?

Candidate: Sepsis-associated AKI has distinct pathological features:

Histological findings (often surprisingly mild):

Focal acute tubular injury, NOT widespread necrosis
Loss of brush border microvilli in proximal tubules
Vacuolization of tubular epithelium
Mild interstitial edema
Fibrin thrombi if DIC present

Pathophysiological mechanisms:

Microcirculatory dysfunction: Shunting even with normal total renal blood flow
Inflammation: TLR4 activation on tubular cells by circulating PAMPs/DAMPs
Mitochondrial dysfunction: Metabolic reprogramming for survival
Tubular "stunning": Deliberate downregulation of solute transport to conserve energy

The key insight is that sepsis-AKI can occur despite normal or even HIGH renal blood flow - it is not primarily ischemic.

Examiner: How do autopsy findings in the spleen inform our understanding of immunoparalysis?

Candidate: Splenic pathology in sepsis provides direct evidence for immunoparalysis:

Findings:

Massive white pulp depletion (lymphocyte loss)
Empty lymphoid follicles (B cell death)
Abundant apoptotic bodies (evidence of programmed cell death)
The spleen becomes an "immunological graveyard"

Significance:

Confirms the lymphocyte apoptosis described by Hotchkiss clinically
Explains loss of adaptive immune capacity
Correlates with susceptibility to secondary infections
Similar changes seen in lymph nodes

This autopsy evidence supports the concept that many sepsis patients die in a state of profound immunosuppression rather than overwhelming inflammation.

Examiner: Very good. Thank you.

Candidate: Thank you.

MCQ Practice Questions

Clinical Note

References

Landmark Papers

Singer M, Deutschman CS, Seymour CW, et al. The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3). JAMA. 2016;315(8):801-810. PMID: 26903338
Bone RC, Balk RA, Cerra FB, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. The ACCP/SCCM Consensus Conference Committee. Chest. 1992;101(6):1644-1655. PMID: 1597163
Hotchkiss RS, Karl IE. The pathophysiology and treatment of sepsis. N Engl J Med. 2003;348(2):138-150. PMID: 12519925
Hotchkiss RS, Tinsley KW, Swanson PE, et al. Apoptotic cell death in patients with sepsis, shock, and multiple organ dysfunction. Crit Care Med. 1999;27(7):1230-1251. PMID: 10403738
Vincent JL, Moreno R, Takala J, et al. The SOFA (Sepsis-related Organ Failure Assessment) score to describe organ dysfunction/failure. Intensive Care Med. 1996;22(7):707-710. PMID: 8844239

Pathophysiology

Takeuchi O, Akira S. Pattern recognition receptors and inflammation. Cell. 2010;140(6):805-820. PMID: 20303867
Rittirsch D, Flierl MA, Ward PA. Harmful molecular mechanisms in sepsis. Nat Rev Immunol. 2008;8(10):776-787. PMID: 18802444
Kang S, Tanaka T, Narazaki M, Kishimoto T. Targeting Interleukin-6 Signaling in Clinic. Immunity. 2019;50(4):1007-1023. PMID: 30995492
Chousterman BG, Swirski FK, Weber GF. Cytokine storm and sepsis disease pathogenesis. Semin Immunopathol. 2017;39(5):517-528. PMID: 28555385
Fajgenbaum DC, June CH. Cytokine Storm. N Engl J Med. 2020;383(23):2255-2273. PMID: 33302002

DAMPs and PAMPs

Mogensen TH. Pathogen recognition and inflammatory signaling in innate immune defenses. Clin Microbiol Rev. 2009;22(2):240-273. PMID: 19366914
Gong T, Liu L, Jiang W, Zhou R. DAMP-sensing receptors in sterile inflammation and inflammatory diseases. Nat Rev Immunol. 2020;20(2):95-112. PMID: 31558839
Zhang Q, Raoof M, Chen Y, et al. Circulating mitochondrial DAMPs cause inflammatory responses to injury. Nature. 2010;464(7285):104-107. PMID: 20203610
Chen GY, Nuñez G. Sterile inflammation: sensing and reacting to damage. Nat Rev Immunol. 2010;10(12):826-837. PMID: 21088683

Endothelial Dysfunction and Glycocalyx

Uchimido R, Schmidt EP, Bhagat S. The glycocalyx: a novel diagnostic and therapeutic target in sepsis. Crit Care. 2019;23(1):16. PMID: 30654825
Iba T, Levy JH. Derangement of the endothelial glycocalyx in sepsis. J Thromb Haemost. 2019;17(2):283-294. PMID: 30582889
Johansson PI, Stensballe J, Ostrowski SR. Shock induced endotheliopathy (SHINE) in acute critical illness - a unifying pathophysiologic mechanism. Crit Care. 2017;21(1):25. PMID: 28178966
Hippensteel JA, Uchimido R, Tyler PD, et al. Intravenous fluid resuscitation is associated with septic endothelial glycocalyx degradation. Crit Care. 2019;23(1):259. PMID: 31311597

Coagulation and DIC

Levi M, van der Poll T. Coagulation and sepsis. Thromb Res. 2017;149:38-44. PMID: 27889066
Iba T, Levy JH, Warkentin TE, et al. Diagnosis and management of sepsis-induced coagulopathy and disseminated intravascular coagulation. J Thromb Haemost. 2019;17(11):1989-1994. PMID: 31327219
Levi M. Pathogenesis and management of perioperative coagulopathy. Br J Surg. 2019;106(2):e95-e101. PMID: 30693517
Wada H, Thachil J, Di Nisio M, et al. Guidance for diagnosis and treatment of DIC from harmonization of the recommendations from three guidelines. J Thromb Haemost. 2013. PMID: 23379279

NETosis

Papayannopoulos V. Neutrophil extracellular traps in immunity and disease. Nat Rev Immunol. 2018;18(2):134-147. PMID: 28990587
Fuchs TA, Brill A, Duerschmied D, et al. Extracellular DNA traps promote thrombosis. Proc Natl Acad Sci USA. 2010;107(36):15880-15885. PMID: 20798043
Lefrançais E, Mallavia B, Zhuo H, et al. Maladaptive role of neutrophil extracellular traps in pathogen-induced lung injury. JCI Insight. 2018;3(3):e98178. PMID: 29415887
Martinod K, Wagner DD. Thrombosis: tangled up in NETs. Blood. 2014;123(18):2768-2776. PMID: 24366358

Mitochondrial Dysfunction

Singer M. The role of mitochondrial dysfunction in sepsis-induced multi-organ failure. Virulence. 2014;5(1):66-72. PMID: 24185508
Brealey D, Brand M, Hargreaves I, et al. Association between mitochondrial dysfunction and severity and outcome of septic shock. Lancet. 2002;360(9328):219-223. PMID: 12133656
Fink MP. Cytopathic hypoxia. Is oxygen use impaired in sepsis as a result of an acquired intrinsic derangement in cellular respiration? Crit Care Clin. 2002;18(1):165-175. PMID: 12594860
Carré JE, Orber J, Jennings SJ, Singer M. Mitochondrial dysfunction in critical illness: a lead actor or a supporting character? Crit Care. 2008;12(3):157. PMID: 18598356

Immunoparalysis

Hotchkiss RS, Monneret G, Payen D. Sepsis-induced immunosuppression: from cellular dysfunctions to immunotherapy. Nat Rev Immunol. 2013;13(12):862-874. PMID: 24232462
Hotchkiss RS, Monneret G, Payen D. Immunosuppression in sepsis: a novel understanding of the disorder and a new therapeutic approach. Lancet Infect Dis. 2013;13(3):260-268. PMID: 23498838
Venet F, Lukaszewicz AC, Payen D, Hotchkiss R, Monneret G. Monitoring the immune response in sepsis: a rational approach to administration of immunoadjuvant therapies. Curr Opin Immunol. 2013;25(4):477-483. PMID: 23725873
Monneret G, Venet F, Pachot A, Lepape A. Monitoring immune dysfunctions in the septic patient: a new skin for the old ceremony. Mol Med. 2008;14(1-2):64-78. PMID: 18026569
Hotchkiss RS, Nicholson DW. Apoptosis and caspases regulate death and inflammation in sepsis. Nat Rev Immunol. 2006;6(11):813-822. PMID: 17039247

Organ Dysfunction

Bellomo R, Kellum JA, Ronco C, et al. Acute kidney injury in sepsis. Intensive Care Med. 2017;43(6):816-828. PMID: 28364303
Langenberg C, Bagshaw SM, May CN, Bellomo R. The histopathology of septic acute kidney injury: a systematic review. Crit Care. 2008;12(2):R38. PMID: 18325092
Hollenberg SM, Singer M. Pathophysiology of sepsis-induced cardiomyopathy. Nat Rev Cardiol. 2021;18(6):424-434. PMID: 33473209
Thompson BT, Chambers RC, Liu KD. Acute Respiratory Distress Syndrome. N Engl J Med. 2017;377(6):562-572. PMID: 28792873
Strnad P, Tacke F, Koch A, Trautwein C. Liver - guardian, modifier and target of sepsis. Nat Rev Gastroenterol Hepatol. 2017;14(1):55-66. PMID: 27924081

Biomarkers

Pierrakos C, Velissaris D, Bisdorff M, et al. Biomarkers of sepsis: time for a reappraisal. Crit Care. 2020;24(1):287. PMID: 32503670
Meisner M. Update on procalcitonin measurements. Ann Lab Med. 2014;34(4):263-273. PMID: 24982830
Schuetz P, Wirz Y, Sager R, et al. Procalcitonin to initiate or discontinue antibiotics in acute respiratory tract infections. Cochrane Database Syst Rev. 2017;10(10):CD007498. PMID: 29025194
Mearelli F, Fiotti N, Giansante C, et al. Presepsin in sepsis. Crit Care Med. 2018;46(4):e327. PMID: 29528958
Elke G, Bloos F, Wilson DC, et al. The use of mid-regional proadrenomedullin to identify disease severity and treatment response to sepsis - a secondary analysis of a large randomised controlled trial. Crit Care. 2018;22(1):79. PMID: 29566733

Histopathology

Hotchkiss RS, Swanson PE, Freeman BD, et al. Apoptotic cell death in patients with sepsis, shock, and multiple organ dysfunction. Crit Care Med. 1999;27(7):1230-1251. PMID: 10403738
Torgersen C, Moser P, Gasser D, et al. Histopathological patterns of sepsis-related acute lung injury in patients deceased from septic shock. Intensive Care Med. 2011;37(Suppl 1):S299.
Takasu O, Gaut JP, Watanabe E, et al. Mechanisms of cardiac and renal dysfunction in patients dying of sepsis. Am J Respir Crit Care Med. 2013;187(5):509-517. PMID: 23348975

Clinical board

Urgent signals

Exam focus

Editorial and exam context

SIRS and Sepsis Pathology

Quick Answer

CICM Exam Focus

SAQ Topics (First Part Written)

Viva Topics

Common Examiner Questions

Key Points

Historical Definitions: SIRS Criteria (1992)

The ACCP/SCCM Consensus Conference

1992 Sepsis Definitions Hierarchy

Limitations of SIRS Criteria

Sepsis-3 Definitions (2016)

The Paradigm Shift

Sepsis

Septic Shock

Key Changes from 1992 to 2016

The SOFA Score

qSOFA (Quick SOFA)

Pathogen Recognition: PAMPs and DAMPs

Overview of Pattern Recognition

Pathogen-Associated Molecular Patterns (PAMPs)

Damage-Associated Molecular Patterns (DAMPs)

Toll-Like Receptors (TLRs)

Signaling Pathways

The NLRP3 Inflammasome

The Inflammatory Cascade

Pro-Inflammatory Cytokines

Tumor Necrosis Factor-α (TNF-α)

Interleukin-1β (IL-1β)

Interleukin-6 (IL-6)

Anti-Inflammatory Cytokines

Interleukin-10 (IL-10)

Transforming Growth Factor-β (TGF-β)

Other Anti-Inflammatory Mediators

Temporal Evolution: SIRS to CARS

Endothelial Dysfunction and Glycocalyx Degradation

The Endothelial Glycocalyx

Structure

Normal Functions

Glycocalyx Degradation in Sepsis

Sheddases (Degrading Enzymes)

Reactive Oxygen Species (ROS)

Consequences of Glycocalyx Degradation

Biomarkers of Glycocalyx Degradation

Coagulation Abnormalities and DIC

Overview

Tissue Factor and Coagulation Activation

Tissue Factor Expression

The Thrombin Storm

Depletion of Natural Anticoagulants

Fibrinolysis Inhibition

Disseminated Intravascular Coagulation (DIC)

Pathophysiology

Clinical Features

ISTH DIC Scoring System

NETosis and Immunothrombosis

NET Formation

Pro-Coagulant Effects of NETs

The Immunothrombosis Concept

Mitochondrial Dysfunction and Cytopathic Hypoxia

The Concept of Cytopathic Hypoxia

Mechanisms of Mitochondrial Dysfunction

Nitric Oxide (NO) Inhibition

Oxidative and Nitrosative Stress

Other Mechanisms

Bioenergetic Failure

The Cellular Hibernation Hypothesis

Recovery: Mitochondrial Biogenesis

Immunoparalysis and CARS

The Shift from Hyperinflammation to Immunosuppression

Key Features of Immunoparalysis

Lymphocyte Apoptosis

Mechanisms of Lymphocyte Apoptosis

HLA-DR Downregulation

Immune Checkpoint Upregulation

Compensatory Anti-Inflammatory Response (CARS)