Distributive Shock (Reduced SVR vasodilated)

💡 Distributive shock is characterised by a loss of systemic vascular resistance (SVR) despite normal or increased cardiac output. It results in relative hypovolaemia due to peripheral vasodilation and abnormal blood flow distribution — seen in sepsis, anaphylaxis, and neurogenic shock.

🧠 About

• Definition: Acute circulatory failure due to inappropriate vasodilation and maldistribution of blood flow, leading to end-organ hypoperfusion.
• Typically presents with warm peripheries, bounding pulse, and low diastolic BP — unlike the cool, clammy appearance of hypovolaemic or cardiogenic shock.

🔬 Pathophysiology

• Systemic vasodilation → ↓ systemic vascular resistance (SVR) → ↓ effective circulating volume.
• Compensatory ↑ cardiac output (hyperdynamic phase) → followed by myocardial depression if untreated.
• Microcirculatory dysfunction and capillary leak → tissue oedema and hypoxia.
• Ultimately causes lactic acidosis and cellular energy failure.

🧩 Major Causes

• 1. Septic Shock: Bacterial infection, pancreatitis, burns, or toxic shock syndrome.
• 2. Anaphylactic Shock: Hypersensitivity reaction causing mast-cell-mediated vasodilation and bronchospasm.
• 3. Neurogenic Shock: Loss of sympathetic tone after spinal cord injury or brainstem damage.

📈 Base deficit > 5 mmol/L and rising lactate indicate tissue hypoperfusion. Serial lactate trends are the best markers of response to resuscitation.

🩺 Clinical Features

• Hypotension, tachycardia (may be absent in neurogenic shock).
• Bounding pulse, wide pulse pressure, warm, flushed peripheries.
• Altered mental status: confusion → obtundation → coma.
• Oliguria or anuria.
• Features of underlying cause:
  • Sepsis: Fever, rigors, cellulitis, pneumonia, peritonitis, gangrene.
  • Anaphylaxis: Urticaria, angioedema, bronchospasm, hypotension.
  • Neurogenic: Bradycardia, warm dry skin below lesion, absent reflexes.

🔎 Investigations

• FBC (anaemia, leucocytosis), U&E (AKI), LFTs, coagulation screen.
• Arterial blood gas and lactate – assess acidosis and perfusion.
• Blood cultures prior to antibiotics.
• ECG and troponin – exclude cardiogenic element.
• Chest X-ray, abdominal imaging (CT/US) to locate infection or bleeding source.
• Consider cortisol/thyroid studies if adrenal or endocrine cause suspected.

💊 Management Principles

• 1️⃣ Resuscitation: ABCs with high-flow oxygen; fluid resuscitation (crystalloids 30 mL/kg bolus if septic).
• 2️⃣ Identify and treat the cause:
  • Sepsis: “Sepsis Six” – oxygen, cultures, IV antibiotics, IV fluids, lactate, urine output monitoring.
  • Anaphylaxis: IM adrenaline 500 µg (repeat every 5 min if needed), IV fluids, hydrocortisone 200 mg, chlorphenamine 10 mg IV.
  • Neurogenic: Elevate legs, maintain MAP > 85 mmHg, give IV fluids, and use vasopressors (noradrenaline/phenylephrine); atropine for bradycardia.
• 3️⃣ Vasopressors and Inotropes:
  • Noradrenaline = first-line for septic or neurogenic shock once volume replete.
  • Adrenaline (low-dose infusion) if anaphylaxis refractory to IM therapy.
  • Consider vasopressin as adjunct if noradrenaline requirement high.
• 4️⃣ Monitoring:
  • Continuous ECG, arterial line, and urinary catheter.
  • Serial lactate and base deficit to track perfusion improvement.
  • Target MAP ≥ 65 mmHg and urine output ≥ 0.5 mL/kg/h.
• 5️⃣ Escalation: Seek early critical care input for persistent hypotension or multiorgan failure.

🧠 Clinical Pearls

• Warm peripheries and low diastolic BP are classic of distributive (vs cold, clammy in hypovolaemic).
• In sepsis, vasoplegia is driven by nitric oxide and inflammatory cytokines — volume alone is never enough.
• In neurogenic shock, HR may be low despite hypotension — don’t miss the absence of tachycardia.
• Always reassess: response = best diagnostic test in any form of shock.

📚 References

• NICE NG51 (2024): Sepsis: recognition, diagnosis, and early management.
• Resuscitation Council UK. Anaphylaxis Algorithm 2023.
• ATLS 10th Ed. Neurogenic and Distributive Shock Management.
• Marik PE. Vasopressors in distributive shock. Crit Care Med 2019.