Obstructive Shock (Mechanical Obstruction)

🩸 Obstructive shock is caused by a mechanical obstruction that impedes either cardiac filling or forward blood flow. Despite a healthy myocardium, the heart cannot pump effectively → tissue hypoperfusion and cellular hypoxia.

🧠 About

• Obstructive shock arises from a physical impediment to cardiac filling or ejection, resulting in decreased cardiac output.
• The myocardium is intrinsically normal, but preload or outflow is restricted.
• It overlaps clinically with cardiogenic shock, but treatment targets the mechanical obstruction rather than contractility.

💡 Key point: A rising lactate or base deficit >5 mmol/L indicates tissue hypoxia. Serial lactate monitoring helps gauge resuscitation success and detect persistent obstruction.

🔬 Pathophysiology

• Obstruction reduces venous return or impedes ventricular emptying → ↓ stroke volume → ↓ cardiac output.
• Despite compensatory tachycardia, organ perfusion remains critically low.
• Common sites of obstruction: pericardial space (tamponade), pulmonary circuit (PE or tension PTX), or great vessels/valves.

🩺 Major Causes

• 💔 Cardiac Tamponade – fluid in pericardial sac compresses the heart → impaired diastolic filling → ↓ stroke volume.
• 🌬️ Tension Pneumothorax – increased intrathoracic pressure collapses veins and compresses heart/lungs.
• 🫁 Massive Pulmonary Embolism – obstruction of pulmonary arteries → acute right heart strain and reduced left filling.
• 🩸 IVC Obstruction – raised intra-abdominal pressure (e.g., pregnancy, tumour, thrombosis).
• 🫀 Critical Valve Stenosis – especially aortic stenosis causing fixed cardiac output.

⚕️ Clinical Features

• Severe hypotension, cold clammy skin, tachycardia.
• Raised JVP (common to all causes).
• Distended neck veins + muffled heart sounds + hypotension = Beck’s triad (tamponade).
• Absent breath sounds + tracheal deviation = tension pneumothorax.
• Acute dyspnoea, pleuritic pain, hypoxia, ECG S₁Q₃T₃ pattern = massive PE.
• Critical aortic stenosis – ejection systolic murmur, slow rising pulse, exertional syncope.

🔎 Investigations

• 📈 FBC/U&E/LFTs: baseline and to rule out metabolic causes.
• 🫀 ECG: RV strain in PE, low voltage in tamponade, LVH in aortic stenosis.
• 🩸 D-dimer: helpful if low-risk PE suspected; skip if high-risk → straight to CTPA.
• 🩻 CXR: may show mediastinal shift (tension PTX) or enlarged cardiac silhouette (tamponade).
• 🫧 Echo: immediate diagnostic tool for tamponade (diastolic collapse of right atrium/ventricle), RV dilatation in PE.
• 🧭 CT Pulmonary Angiogram: gold standard for PE diagnosis.

🩹 Management

• 🔹 Airway and oxygen: Maintain SpO₂ >94% (or 88–92% in COPD).
• 🔹 Volume resuscitation: Judicious IV crystalloid boluses (avoid overload in tamponade or RV failure).
• 🔹 Definitive cause-specific interventions:
  • Cardiac tamponade: Urgent pericardiocentesis - remove pericardial fluid to restore filling.
  • Tension pneumothorax: Immediate needle decompression (2nd intercostal space, midclavicular line) → insert chest drain.
  • Massive PE: Thrombolysis (alteplase) or embolectomy if contraindicated.
  • IVC obstruction: Relieve compression (e.g., left lateral position in pregnancy).
  • Critical aortic stenosis: Balloon valvuloplasty or TAVI/surgical replacement.
• 🔹 Monitoring: Arterial line, CVP, urine output, serial lactate.
• 🔹 Escalate early: Involve cardiology, ICU, and cardiothoracic teams as appropriate.

🧠 Clinical Pearls

• Obstructive shock is often rapidly fatal if missed - diagnosis must be clinical, not delayed by investigations.
• Fluids alone rarely restore perfusion until the mechanical obstruction is relieved.
• Echo is the single most valuable bedside test - use early and repeat after intervention.
• Always reassess for evolving tamponade, expanding pneumothorax, or clot propagation.

📚 References

• Resuscitation Council UK (ALS 2021) - Management of Shock and Pericardial Tamponade.
• ATLS 10th Ed. - Obstructive and Cardiogenic Shock Protocols.
• NICE NG158 - Venous thromboembolic diseases: diagnosis and management.
• Tintinalli JE et al., 2023. Emergency Medicine: A Comprehensive Study Guide.