Case 1: Ethel MacConkey - Surgical Site Infections

Thermoregulation: Mechanism

1. Temperature Sensors

• Central: In CNS (blood temp)

• Peripheral: In skin

2. Hypothalamus: Control Centre

• Integrate central + peripheral temp info → Maintain at 36.5-37.5ºC

Thermoregulation: Hypothalamus

Anterior: Hot = Heat dissipation

• SNS activation = Increase ACh = Vasodilation

• Sweating

• Behavioural changes (remove clothes, find shade)

• Arteriovenous anastomoses (AVAs) open

  • Blood flow through = Increase SA = Increase heat loss

Posterior: Cold = Heat conservation

• SNS activation = Increase norepinephrine = Vasoconstriction

• Increase muscle activity

  • Involuntary: Shivering

  • Voluntary: Walking/moving

• Lipolysis in brown adipose tissue = Produce heat (not ATP)

• AVAs close

  • Blood blocked = Decrease SA = Decrease heat loss

Febrile Response

> 38ºC

1. Inflammation/infection = Release pyrogens (cytokines)

2. Increase hypothalamic thermoregulatory centre set point = Increase body temp

3. Increase immune system activity + decrease pathogen growth

Skin: Anatomy

2 layers

1. Cutaneous: Superficial

2. Subcutaneous: Deep

Skin: Cutaneous

Epidermis + dermis

Cutaneous Skin: Epidermis

Nonvascularized

5 layers

1. Stratum corneum: Outermost

• Dead keratin-filled cells

2. Stratum lucidum

• Only in thick skin

• Keratinocytes

3. Stratum granulosum

• Keratohyalin

  • Waterproof

4. Stratum spinosum

• Dividing keratinocytes

  • Produce keratin

• Langerhans cells

5. Stratum basale: Innermost

• Stem cells

  • Migrate up + differentiate

• Melanocytes

• Merkel cells

Cutaneous Skin: Dermis

Contain blood vessels

For structural integrity

2 layers:

1. Papillary Dermis

• Contain…

  • Loose collagen fibres

  • Meissner corpuscles

    • Mechanoreceptors for fine-touch

  • Free nerve endings

  • Immune cells

• Supply nutrients to epidermis

2. Reticular Dermis

• Contain…

  • Dense collagen fibres

  • Fuffini corpuscles

    • Mechanoreceptors for pressure

  • Sweat + sebaceous glands

  • Hair roots

Skin: Subcutaneous

Contain…

• Fat: Protect from cold + trauma

• Superficial veins

• Free nerve endings

• Pacinian corpuscles

  • Mechanoreceptors for vibration + pressure

Skin Cells: Keratinocytes

Barrier against pathogens + environment

Store melanin → Melanocytes

• Protect against UV

Produce proinflammatory mediators

• Wound healing

Keratinocyte Dysfunction

Desquamation + Intercellular Junction Loss:

• Acne

• Blistering (pemphigus vulgaris, staphylococcal scalded skin syndrome)

Apoptosis: Sunburn

Increased Proliferation: Psoriasis

Skin Cells: Melanocytes

Synthesize + store melanin

• Dendritic processes transfer between keratinocytes

Stimulated by MSH, ACTH, UV

Skin Cells: Langerhans Cells

Macrophage (APC)

Skin Cells: Merkel Cells

In highly innervated areas

• Fingertips

Mechanoreceptors for deep static touch (shapes, edges)

Skin: Physiology

Physical Barrier: Protect against environment

Chemical Barrier:

• Acid mantle prevent pathogen growth

• Sweat + sebaceous glands

Cellular Barrier: Melanin = Protect against UV

Thermoregulation

Sensation

Vit D synthesis

Wound Healing

4 phases

1. Exudative

2. Resorptive

3. Proliferative

4. Maturation

Wound Healing: Exudative Phase

Day 1

Platelet aggregation = Form clot → Scab

Ruptured cells + platelets = Prostaglandins + thromboxane release = Local vasoconstriction + increase vessel permeability

Wound Healing: Resorptive Phase

Day 1-3

Chemotaxis = Recruit inflammatory cells

• Macrophages release growth factors + cytokines = Recruit more cells + stimulate fibroblast proliferation + resorb debris

Vasodilation

Wound margins proliferate

Wound Healing: Proliferative Phase

Day 3-7

Granulation tissue formation

• Smooth muscle migration + fibroplasia = Collagen synthesis

• Fibroblasts + epithelial cells release growth factors = Angiogenesis

• Wound contraction = Pull wound edges together

Epidermal cells migrate across collagen = Form layer over wound

• Secrete collagenase = Dissolve clot

Wound Healing: Maturation Phase

Weeks to years

Fibroblast proliferation + connective tissue remodelling = Form scar

• Macrophages release metalloproteinases + collagenases = Remodel collagen

• Remove excess collagen

• Reorganize collagen = Increase injury strength

No sweat + sebaceous gland regeneration

Bacteria: Shape

From MF1 Nabil Assad - Strep Pharyngitis

Cocci: Sphere/ellipse

• Mostly gram-positive

Bacilli: Rod/cylinder

• Most gram-negative

Vibro: curved

Spirilla: Spiral

Actinomycetes: Branching

Mycoplasmas: No stable morphology

Bacteria: Gram-Stain

From MF1 Nabil Assad - Strep Pharyngitis

Identify cell wall

Gram-Positive: Purple

• Thick cell wall

Gram-Negative: Pink

• Thin cell wall

Host Flora (Commensals)

Microorganisms living on/in body

Cause no harm OR benefit body

Host Flora on Skin + Nose

Staph epidermis

• Gram+, coagulase- staphylococcus (CoNS)

• Increase with age

Streptococci: Pediatrics

S. aureus: Transient

Host Flora in Mouth + Oropharynx

Viridans group streptococci

• Gram+

• Cause bacterial endocarditis

S. mutans: Cause dental caries

Host Flora in Lungs

Neisseria catarrhalis

• Gram- cocci

Alpha-hemolytic streptococci

Staphylococci

Candida albicans

Host Flora in GI

E. coli

• Gram- bacillus

Enterococcus

Klebsiella

Clostridium

Bacteriodes

• Gram- bacilli

• Pediatrics

Bifidobacterium

• Pediatrics

Host Flora in Vagina

Lactobacillus acidophilus

• Gram+ bacillus

• Maintain pH

Saprophyticus

• Gram+ CoNS

Surgical Site Infections (SSI): Description

Infection of skin + soft tissue or organ/space at recent surgery site

• ≤ 30 days post-op

• ≤ 90 days post-op for prosthetics

SSI: Epidemiology

Most common nosocomial infection

Risk factors…

• Older age

• Longer surgery

• Diabetes

• Obesity

• Smoking

• Coexisting infection

• Immunocompromise

  • Corticosteroid therapy

  • Malnutrition

SSI: Etiology

≤ 48-72h: Pathogens causing necrotizing fasciitis

• GAS

  • Ex: S. pyogenes

• Clostridium spp.

  • Ex: C. perfringens

> 48-72h: Endogenous organisms at surgical site

• Skin: S. aureus

• Genital/GI: E. coli

> 30 days: Indolent organisms

• CoNS: Low virulence + non-aggressive

  • Produce biofilm

  • Ex: S. epidermis

Biofilm

Bacteria aggregation in self-produced ECM

Functions:

• Protect against host defence mechanisms + abs

• Facilitate colonization on prosthetics

SSI: Pathophysiology

1. Exposure

• Infectious agent gain entry to host

2. Adhesion + Invasion

• Pathogen bind host cell receptors = Anchor pathogen + penetration

• Pathogens multiply + colonize tissue/fluids

  • Virulence determinants promote survival

  • Ex: Biofilm

3. Immune system activation

• Clear infection

SSI: Clinical Presentation

Superficial tissue

• Purulent discharge

• Possible fever

• Local tenderness, erythema, warmth, edema

Deep tissue

• Purulent discharge

• Fever

• Local tenderness

• Wound dehiscence

  • Spontaneous wound edge separation

• Necrotizing fasciitis

Organ/space

• Purulent discharge

  • From drain

  • Absecess

• Fever

SSI: Investigations

Blood tests

Microbiological studies

Imaging

SSI Investigations: Blood Tests

CBC

Inflammatory markers

Creatinine

SSI Blood Tests: CBC

Cell Leukocytosis

• ≥ 11 × 10⁹/L

Thrombocytosis

SSI Blood Tests: Inflammatory Markers

Elevated ESR + CRP

SSI Blood Tests: Creatinine

Determine baseline renal function for antibio dosing

SSI Investigations: Microbiological Studies

Culture + gram stain of drainage

• Wound

• Abscess

SSI Investigations: Imaging

Assess deep tissue or organ space

US, CT, MRI

SSI: Treatment/Management

Surgical

Pharmacological

SSI Treatment: Surgical

Drain

Debridement

Closure after infection resolves

SSI Treatment: Pharmacological

Empiric antibios

Targeted antibios

• After culture results

SSI: Empiric Antibios

Indications:

• Erythema ≥ 5 cm from wound edge

• Fever ≥ 38.5ºC

• HR ≥ 110 BPM

• WBC ≥ 12 × 10⁹/L

Selection:

• No GI/GU involvement

  • Cefazolin

  • Oxacillin

  • MRSA: Vancomycin, daptomycin, linezolid

• GI/GU involvmenet

  • Ceftriaxone/levofloxacin + metronidazole

  • Carbapenem (meropenem)

• Necrotizing soft tissue

  • Broad-spectrum

    • Vancomycin

    • Fluoroquinolones

    • 3rd gen cephalosporin

      • Ceftriaxone

  • Penicillin + clindamycin

SSI: Complications

Wound dehiscence

Secondary hemorrhage

Sepsis + septic shock

SSI: Prevention

Control blood glucose

Smoking cessation

Antibio prophylaxis

Antibiotic Prophylaxis

Indications:

• High risk of infection

• Prosthetics

• Contamination risk

IV 30-60 mins before

Selection:

• Cefazolin: First-line

• + Metronidazole: GI surgeries

• Clindamycin or Vancomycin: Beta-lactam allergies