Obstructive Shock

Obstructive shock is circulatory failure from a mechanical block to venous return or to cardiac outflow, in a heart that is initially normal. The result is a drop in preload or a rise in afterload, so cardiac output falls despite normal or high filling pressures.

Main causes

• Tension pneumothorax: raised intrathoracic pressure kinks the great veins and shifts the mediastinum
• Cardiac tamponade: pericardial fluid under pressure restricts diastolic filling (trauma, malignancy, uremia, infection, aortic dissection, post-procedure)
• Massive pulmonary embolism: acute right ventricular failure from pulmonary vascular obstruction
• Others: severe aortic stenosis, constrictive pericarditis, caval compression in late pregnancy, dynamic hyperinflation with auto-PEEP, and abdominal compartment syndrome

Tamponade is more dangerous when fluid accumulates quickly (200 mL can be fatal), since a stretched pericardium tolerates slow accumulation. Mortality is high without immediate relief: massive pulmonary embolism 25 to 50%, and traumatic tamponade near 100% untreated. Complications: cardiac arrest, ischemic organ injury, and right ventricular failure.

• Hypotension with raised jugular venous pressure, tachycardia and dyspnea
• Tension pneumothorax: unilateral absent breath sounds, hyperresonance, tracheal deviation (late) and subcutaneous emphysema
• Tamponade: Beck triad (hypotension, muffled heart sounds, distended neck veins), pulsus paradoxus over 10 mmHg
• Pulmonary embolism: sudden dyspnea, pleuritic pain, syncope, loud second heart sound, right ventricular heave and a calf swelling
• Auto-PEEP: wheeze, air trapping, and hypotension after bagging a patient with obstructive lung disease
• Red flags: shock in a ventilated patient, recent trauma or chest procedure, cancer, and sudden collapse after surgery or long travel

1. Bedside ultrasound (the first investigation):
   • Pericardial effusion with right atrial and ventricular diastolic collapse, plethoric inferior vena cava.
   • Dilated right ventricle with septal flattening, McConnell sign, and visible thrombus in the right heart.
   • Absent lung sliding and a lung point in pneumothorax.
2. ECG: low voltage and electrical alternans (tamponade), S1Q3T3, right bundle branch block and T inversion in V1 to V4 (pulmonary embolism).
3. Chest radiograph only if the patient is stable (never delay decompression for a film).
4. CT pulmonary angiography when stable enough to travel. D-dimer is not useful in shock.
5. Troponin, natriuretic peptide, lactate, blood gases.
6. Lower limb compression ultrasound to find deep vein thrombosis when CT is impossible.

Differentials: cardiogenic shock, hypovolemic shock, septic shock, aortic dissection and anaphylaxis.

Relieve the obstruction immediately, then treat the cause.

1. Tension pneumothorax:
   • Needle decompression with a 14-gauge, 8 cm cannula in the fourth or fifth intercostal space at the anterior axillary line (or second space at the midclavicular line in small chest walls), then a chest tube (28 to 32 Fr) with underwater seal. Children: 18 to 22-gauge needle then a tube 16 to 24 Fr by size.
2. Tamponade:
   • Ultrasound-guided pericardiocentesis by the subxiphoid or apical route with an 18-gauge needle and a drain left for 24 to 72 hours.
   • Surgical window or thoracotomy for traumatic, purulent or clotted effusions.
   • IV 0.9% saline 500 mL as a temporizing measure. Avoid positive pressure ventilation and sedatives until drained.
3. Massive pulmonary embolism:
   • Systemic thrombolysis: alteplase 100 mg IV over 2 hours (50 mg bolus over 15 minutes in cardiac arrest), then unfractionated heparin 80 units/kg IV bolus and 18 units/kg/h infusion titrated to an activated partial thromboplastin time of 1.5 to 2.5 times control.
   • Contraindications: intracranial hemorrhage, recent major surgery or trauma within 3 weeks, active internal bleeding.
   • Catheter-directed therapy, surgical embolectomy or venoarterial extracorporeal membrane oxygenation when thrombolysis is contraindicated or fails.
   • Norepinephrine 0.05 to 0.5 micrograms/kg/min IV for hypotension, with cautious fluids (500 mL at most). Avoid intubation if possible, since induction can cause arrest.
   • Later anticoagulation: apixaban 10 mg twice daily for 7 days, then 5 mg twice daily for at least 3 months.
4. Auto-PEEP: disconnect the ventilator circuit for 30 to 60 seconds, apply chest compression to expel air, then reduce rate and tidal volume (6 mL/kg ideal body weight) with a longer expiratory time. Salbutamol 5 mg nebulized.
5. Abdominal compartment syndrome: decompressive laparotomy.
6. Supportive care: oxygen, careful monitoring and early surgical or intensive care input.

Follow-up for pulmonary embolism: echocardiography and functional review at 3 to 6 months for chronic thromboembolic disease. Review the cause of tamponade, such as malignancy or uremia, and counsel on recurrence.