Shock — Obstructive

Fundamental Mechanism of Obstruction

• Physical blockade of pulmonary or systemic venous return reduces ventricular preload despite elevated intrathoracic pressures
• Increased afterload on the right ventricle (RV) in PE and tension pneumothorax causes RV dilatation and septal shift, further compromising left ventricular (LV) filling
• The pressure-volume relationship becomes unfavorable: high filling pressures generate insufficient stroke volume output

Key Mechanism 1: Venous Return Obstruction

• In tension pneumothorax: positive pleural pressure collapses intrathoracic veins, physically preventing venous return
• In cardiac tamponade: fluid accumulation in the pericardial sac restricts ventricular diastolic filling; the pericardium acts as a fixed container limiting volume expansion
• Equalization of diastolic pressures (right atrial, RV end-diastolic, and pericardial pressures) occurs in tamponade, eliminating the normal pressure gradient for ventricular filling
• Kussmaul sign (paradoxical rise in CVP with inspiration) reflects the RV's inability to accommodate increased venous return

Key Mechanism 2: RV Afterload Crisis (PE, Tension Pneumothorax)

• Acute elevation of pulmonary vascular resistance (PVR) forces the RV to generate supranormal wall stress
• Increased RV wall tension impairs subendocardial perfusion, causing acute RV ischemia and dysfunction
• Interventricular septal shift results from RV dilatation, mechanically compromising LV cavity geometry and reducing LV preload
• Elevated RV pressures compress coronary arteries at the RV base, further worsening ischemia
• This creates a vicious cycle: hypoxia (from low cardiac output) and hypotension (from reduced systemic flow) worsen pulmonary vasoconstriction

Key Mechanism 3: Impaired Coronary and Systemic Perfusion

• Shock-induced hypotension (systolic BP <90 mmHg) reduces diastolic pressure, the primary driver of coronary perfusion
• In aortic dissection, the intimal flap may occlude major branch vessels (coronary ostia, subclavian, carotid, renal arteries)
• End-organ hypoperfusion triggers anaerobic metabolism, lactate accumulation, and metabolic acidosis
• Catecholamine surge increases heart rate and contractility but worsens PVR in PE, counterproductively increasing RV afterload
• Progressive multi-organ dysfunction develops if perfusion is not rapidly restored

Massive Pulmonary Embolism

• Risk factors: immobility (ICU, surgery, long flights), malignancy, hypercoagulable states (thrombophilia, OCPs), recent surgery/trauma, central venous catheters
• Accounts for ~50% of obstructive shock cases in hospital settings
• Acute obstruction of >30-40% of pulmonary vascular bed required for hemodynamic collapse

Tension Pneumothorax

• Causes: penetrating/blunt chest trauma (rib fractures, flail chest), barotrauma (mechanical ventilation, diving), bullae rupture in COPD/cystic fibrosis
• Can be primary (spontaneous in tall, thin individuals) or secondary (underlying lung disease)
• Occurs when a one-way valve effect allows continued air entry into pleural space during inspiration without escape during expiration

Acute Cardiac Tamponade

• Etiology: pericardial effusion (malignancy, uremia, post-cardiac surgery, viral pericarditis, tuberculous pericarditis, trauma, anticoagulation-related bleeding)
• Rate of fluid accumulation is more critical than total volume; rapid accumulation of even 200 mL can cause tamponade, whereas chronic effusions may tolerate >2 L
• Post-cardiac surgery tamponade often presents with persistent drainage or sudden cessation of drainage (clot formation)

Aortic Dissection with Rupture/Aortic Rupture

• Risk factors: uncontrolled hypertension (most common), atherosclerosis, Marfan syndrome, Ehlers-Danlos syndrome, bicuspid aortic valve, pregnancy
• Rupture into pleural or pericardial space causes catastrophic hemorrhage
• Type A dissections (ascending aorta involvement) have highest mortality if untreated

Other Causes

• Acute severe mitral stenosis with atrial fibrillation/rapid ventricular response
• Prosthetic valve thrombosis (mechanical valve malfunction)
• Acute right ventricular infarction with severe RV dysfunction
• Pulmonary hypertensive crisis (ARDS, severe pneumonia)
• Constrictive pericarditis (chronic obstruction; rare acute presentation)

Cardinal Symptoms

• Acute dyspnea (shortness of breath at rest or with minimal exertion)
• Chest pain (pleuritic in PE, tearing/ripping in aortic dissection, positional in pericarditis)
• Syncope or presyncope (from profound hypotension/reduced cerebral perfusion)
• Hemoptysis (in massive PE with pulmonary infarction)
• Sudden severe distress with sense of impending doom

Vital Sign Abnormalities

• Hypotension (systolic BP <90 mmHg, often refractory to fluid)
• Tachycardia (>100 bpm, sometimes >140 bpm in massive PE)
• Tachypnea (respiratory rate >20; often >30 in PE)
• Hypoxemia (oxygen saturation <90% on room air)
• Pulsus paradoxus (>10 mmHg drop in systolic BP during inspiration) — classic for tamponade but can occur in PE and pneumothorax

Physical Examination Findings

• Unilateral absent breath sounds on affected side
• Hyperresonance to percussion (tympanitic)
• Tracheal deviation away from affected side (late finding, indicates severe tension)
• Hypotension, JVD, muffled heart sounds (as shock worsens)
• Subcutaneous emphysema may be present

• Beck's triad: hypotension, elevated JVD, muffled heart sounds
• Kussmaul sign: paradoxical rise in JVD during inspiration (inspiratory increase in venous return cannot distend RV against pericardial constraint)
• Pulsus paradoxus: exaggerated drop in systolic pressure during inspiration (>10 mmHg)
• Pericardial friction rub (if acute pericarditis preceded effusion)
• Quiet precordium, no murmurs or gallops typically

• Focal unilateral findings are ABSENT (important distinction from pneumonia)
• Right ventricular heave (palpable parasternal lift from RV dilatation)
• Loud S2 (accentuated pulmonic component) from elevated PA pressure
• Right-sided S3 or S4 gallop
• Elevated JVD (from RV dysfunction/tricuspid regurgitation)
• Peripheral cyanosis (from low cardiac output)
• Signs of DVT: unilateral leg swelling, erythema, warmth

• Severe, tearing chest or back pain (pathognomonic if present)
• Blood pressure differential between arms (>20 mmHg systolic)
• Focal neurological deficits (if carotid involvement)
• Acute aortic regurgitation murmur (high-pitched early diastolic; if Type A involving aortic root)
• Syncope (from aortic rupture into pericardium or acute severe AR)
• Lower extremity pulse deficits (if iliac artery involved)

Clinical Recognition (First Priority)

• Index of suspicion is paramount; obstructive shock should be considered in any critically ill patient with hypotension, elevated JVD, and disproportionate hypoxemia
• Hemodynamic pattern: elevated CVP (>8 mmHg) with low cardiac output (cardiac index <2.2 L/min/m²)

Tension Pneumothorax (Clinical Diagnosis — Do NOT delay for imaging)

• Diagnosis is clinical: acute respiratory distress, hypotension, JVD, unilateral absent breath sounds, tracheal deviation
• Immediate needle decompression at 2nd intercostal space midclavicular line before imaging
• Chest X-ray (after decompression if time permits) shows collapsed lung with mediastinal shift and absent lung markings in affected hemithorax

Cardiac Tamponade

• Bedside echocardiography is gold standard: demonstrates pericardial effusion with diastolic collapse of RA and/or RV, confirming hemodynamic significance
• RA collapse >1/3 of cardiac cycle and RV diastolic collapse indicate tamponade physiology
• CVP elevation to >12 mmHg with equalization of pressures (RA = RV diastolic = pericardial pressure)
• Right heart catheterization (if echocardiography unavailable): characteristic "square root sign" in RV diastolic pressure tracing and pressure equalization
• Chest X-ray: enlarged cardiac silhouette ("water bottle heart") if effusion is large
• ECG: low voltage QRS complex (<5 mm in all limb leads) and electrical alternans (QRS amplitude variates beat-to-beat)
• Labs: elevated troponin if concurrent myocarditis; elevated BNP from stretch

Massive Pulmonary Embolism

• Clinical Wells Score >6 or PERC criteria failure + D-dimer elevation (>500 ng/mL) creates high pretest probability
• CT pulmonary angiography (CTPA) is diagnostic gold standard: demonstrates pulmonary arterial filling defect
• Echocardiography shows: RV dilatation (RV/LV ratio >0.9), RV dysfunction (reduced tricuspid annular plane systolic excursion <16 mm), McConnell's sign (RV free wall hypokinesis with apical sparing), elevated TR velocity (elevated RV systolic pressure)
• Right heart catheterization: elevated mPA pressure (>30 mmHg systolic), elevated PVR (>3 Wood units)
• Troponin elevation and BNP elevation indicate RV myocardial injury; lactate >2 mmol/L signals tissue hypoperfusion
• ABG: hypoxemia (PaO2 <80 mmHg on room air), respiratory alkalosis (low PaCO2) from hyperventilation, or metabolic acidosis if shock is prolonged
• ECG: sinus tachycardia, S1Q3T3 pattern (large S wave in I, Q in III, inverted T in III) is classic but insensitive (~20% cases); more common findings are T wave inversions in anterior leads and right axis deviation

Aortic Dissection

• CT angiography of chest (gold standard): intimal flap dividing true and false lumens, with differential contrast opacification
• Transesophageal echocardiography (TEE): superior sensitivity (~95%) for ascending aortic involvement; demonstrates intimal flap and branch vessel involvement
• MRI: excellent for chronic dissection but too time-consuming in acute setting
• Chest X-ray: widened mediastinum (>8 cm), loss of aortic knob, left pleural effusion (from bleeding)
• Labs: elevated D-dimer (very high negative predictive value; >500 ng/mL strongly supports diagnosis)
• ECG: typically normal unless Type A dissection extends to coronary ostia (causing STEMI pattern)

Hemodynamic Monitoring (Swan-Ganz Catheterization if diagnosis unclear)

• CVP elevation >8 mmHg with low cardiac output (CI <2.2 L/min/m²)
• Pulmonary artery occlusion pressure (PAOP) normal or low (distinguishes obstructive from cardiogenic shock, where PAOP is elevated)
• "Square root sign" in RV or RA tracing: rapid rise in early diastole followed by plateau

Tension Pneumothorax (Immediate Life-Saving Measure)

• Emergent needle decompression at 2nd intercostal space, midclavicular line (large-bore needle, 14-16 gauge)
• Mechanism: relieves positive pleural pressure, restores venous return, immediately improves hemodynamics
• Expected result: gush of air, sudden BP improvement, decreased JVD
• Immediate tube thoracostomy (chest tube, 28-36 French) after needle decompression
• Supplemental oxygen and mechanical ventilation if needed (avoid high PEEP in early stages)
• Avoid positive pressure ventilation until decompression if possible, as it worsens tension

Massive Pulmonary Embolism

• Unfractionated heparin (UFH) preferred (t½ 60-90 min, reversible, can be used with thrombolysis)
• Bolus: 80 U/kg IV, then infusion 18 U/kg/hr titrated to aPTT 1.5-2.5× baseline
• Mechanism: inhibits Factors IIa and Xa, prevents thrombus propagation and allows fibrinolysis
• Thrombolysis (alteplase, 100 mg IV over 2 hours, or weight-based) for hemodynamically unstable/shock PE
• Reduces thrombus burden, lowers PVR, and improves RV function
• Reduces mortality from ~30% to ~10% in massive PE
• Contraindications: active bleeding, recent stroke, intracranial pathology
• Inferior vena cava (IVC) filter if anticoagulation absolutely contraindicated or thrombolysis failure
• Surgical embolectomy reserved for failed thrombolysis or thrombolysis contraindication with hemodynamic instability

• Cautious fluid administration: RV is preload-dependent, but excessive fluid worsens RV dilatation and septal shift; use small boluses (250 mL) and reassess
• Vasopressors if hypotension refractory to fluids:
• Dobutamine or milrinone preferred (inotropic + pulmonary vasodilatory effects)
• Norepinephrine if systemic hypotension severe; less ideal because increases PVR but may be necessary
• Inhaled pulmonary vasodilators (inhaled nitric oxide, epoprostenol): reduce RV afterload without systemic hypotension
• Supplemental oxygen to maintain SaO2 >90

Complications of the obstruction itself (all time-critical)

• Pulseless electrical activity arrest: the terminal event in untreated obstructive shock — the myocardium contracts but no blood is available to eject. Tamponade, tension pneumothorax, and thrombosis are three of the reversible Hs and Ts in the AHA 2020 ACLS algorithm; the arrest is non-shockable, unlike ventricular fibrillation/pulseless VT. Treat by relieving the obstruction, not by escalating epinephrine alone.
• Peri-intubation cardiovascular collapse: positive-pressure ventilation raises intrathoracic pressure and abolishes the last remnant of venous return. Signalled by profound hypotension immediately after induction. Decompress the chest or drain the pericardium before intubating when feasible.
• Shock liver, acute tubular necrosis, and mesenteric ischemia: prolonged low cardiac output; heralded by transaminases in the thousands, rising creatinine with muted urine output, and lactate that fails to clear after the obstruction is relieved.
• Chronic thromboembolic pulmonary hypertension after PE: unresolved organized thrombus; suspect with persistent exertional dyspnea months later — ventilation/perfusion scanning, not CTPA, is the preferred screening test.
• Extension of aortic dissection: retrograde extension produces tamponade, acute aortic regurgitation, or coronary ostial occlusion (classically the right coronary, giving an inferior STEMI pattern). Per the 2022 ACC/AHA Aortic Disease Guideline, type A dissection is a surgical emergency.

Complications of treatment

• Needle/tube thoracostomy injury: intercostal vessel laceration (insert over the superior rib margin), lung laceration, and persistent air leak. Re-expansion pulmonary edema follows rapid evacuation — unilateral infiltrate and hypoxemia after drainage.
• Pericardiocentesis injury: RV puncture, coronary laceration, or hemopericardium; echo or fluoroscopic guidance reduces risk. Pericardial decompression syndrome — paradoxical pulmonary edema or LV failure after drainage — is rare but an emergency.
• Thrombolysis for massive PE: intracranial hemorrhage is the feared complication; any new headache, seizure, or focal deficit mandates immediate head CT and cessation of lytic and heparin.
• Heparin-induced thrombocytopenia: platelet fall around days 5–10 with thrombosis, not bleeding. Stop all heparin and start a non-heparin anticoagulant such as argatroban (ASH 2018 VTE guidelines); do not start warfarin alone.
• IVC filter: migration, fracture, and caval thrombosis — retrieve when anticoagulation becomes safe.

• The hemodynamic fingerprint: high CVP with low cardiac output and a normal-to-low pulmonary capillary wedge pressure separates obstructive shock from cardiogenic shock (wedge high) and hypovolemic/distributive shock (CVP low). Tamponade is the exception in which all diastolic pressures equalize.
• Single best next step in tension pneumothorax: needle decompression, then tube thoracostomy. The stem that shows hypotension, unilateral absent breath sounds, and JVD is testing whether you will waste time on a chest x-ray — you should not. Note that trauma protocols (ATLS) also endorse the 4th–5th intercostal space at the anterior axillary line, since chest-wall thickness may exceed catheter length anteriorly.
• Tamponade buzzwords: Beck triad, pulsus paradoxus, electrical alternans, low-voltage QRS, and a water-bottle heart. Bedside echocardiography showing diastolic right-heart chamber collapse is the confirmatory step; pericardiocentesis is the therapeutic one. Fluids are a bridge, never the definitive treatment.
• The association examiners love: Kussmaul sign is most classically linked to constrictive pericarditis and RV infarction; pulsus paradoxus is the tamponade sign. Pulsus paradoxus may be absent in tamponade when an ASD, severe aortic regurgitation, or markedly elevated LVEDP is also present.
• McConnell sign (RV free-wall hypokinesis with a spared apex) plus a dilated RV in a hypotensive patient is massive PE until proven otherwise; per the AHA scientific statement on massive/submassive PE, systemic thrombolysis is indicated for hemodynamic instability without a bleeding contraindication.
• Common distractor — thrombolysis for submassive PE: a normotensive patient with RV strain and a positive troponin gets anticoagulation, with catheter-directed therapy considered; routine systemic lysis is not indicated.
• Aortic dissection sequencing: rate control with an IV beta blocker (esmolol) before a vasodilator, since vasodilator-first causes reflex tachycardia and increased aortic wall shear stress (2022 ACC/AHA Aortic Disease Guideline). Never anticoagulate or lyse a suspected dissection.
• RV physiology: in PE the RV is preload-dependent but easily overfilled — large-volume resuscitation worsens septal shift and drops cardiac output further.