cardiac tamponade physiology

PATHOPHYSIOLOGY - The pericardium is relatively non-compliant, so the pressure-volume curve is steep: a small rapid accumulation (about 100-200 mL of blood, as in trauma or ventricular rupture) can cause tamponade, while a slow accumulation (over weeks, as in malignancy or hypothyroidism) can reach 1-2 L before it becomes hemodynamically significant. - Rising intrapericardial pressure first exceeds right-sided pressures (the RA and RV have thinner walls and lower pressures). This compresses the chambers, so diastolic filling falls and RA pressure rises. - Eventually intrapericardial pressure, RA, RV, LA, and LV diastolic pressures all equalize (end-diastolic pressure equalization). Preload and stroke volume fall, and cardiac output falls (CO = SV x HR). - Compensation: sympathetic activation causes tachycardia and vasoconstriction. Because stroke volume is fixed, cardiac output depends on heart rate. Beta-blockers, hypovolemia, and positive pressure ventilation can all precipitate collapse. - Result: obstructive shock with raised JVP and clear lungs (no pulmonary edema, because the left heart is underfilled).

CAUSES - Malignancy (lung and breast cancer, lymphoma) - Idiopathic or viral pericarditis - Uremia - Post-MI free wall rupture (days 3-7), or Dressler syndrome - Aortic dissection (type A) with retrograde rupture into the pericardium - Trauma (penetrating chest injury) - Iatrogenic: cardiac surgery, central line, pacemaker or catheter perforation, anticoagulation - TB, hypothyroidism, SLE or other autoimmune disease, radiation

KEY MECHANISM OF PULSUS PARADOXUS - Pulsus paradoxus is a drop in systolic BP of more than 10 mmHg on inspiration. - Normal inspiration: negative intrathoracic pressure increases venous return to the RV. The RV expands, and the interventricular septum bows slightly toward the LV. - In tamponade, the pericardial space is fixed and the chambers compete for room (ventricular interdependence). The enlarged RV bulges the septum into the LV, so LV filling falls. Pooling of blood in the pulmonary vasculature during inspiration also reduces LV preload. LV stroke volume therefore falls, and systolic BP drops. - On expiration the process reverses. - Other causes: severe asthma or COPD, tension pneumothorax, constrictive pericarditis (less common), PE, and hypovolemic shock.

CLINICAL FINDINGS - Beck triad: hypotension, JVD, and muffled (distant) heart sounds. This occurs in only a minority of patients, so don't wait for all three. - Tachycardia, tachypnea, dyspnea, and clear lung fields. - Pulsus paradoxus. - JVP shows a preserved x descent but an absent y descent. Systolic filling persists, but diastolic filling is impeded because the heart cannot expand. In contrast, constrictive pericarditis has a prominent y descent and Kussmaul sign (JVP rises on inspiration). - Kussmaul sign is typically absent in tamponade.

DIAGNOSIS - ECG: low-voltage QRS, sinus tachycardia, and electrical alternans (beat-to-beat variation in QRS axis or amplitude from the heart swinging in the fluid; highly specific). Diffuse ST elevation and PR depression suggest accompanying pericarditis. - CXR: enlarged globular "water-bottle" cardiac silhouette (only if more than about 200-250 mL accumulates; may be normal in acute traumatic tamponade). - Echocardiography (best initial and diagnostic test): pericardial effusion, diastolic RA/RV collapse (RV diastolic collapse is specific), plethoric IVC, and exaggerated respiratory variation in mitral and tricuspid inflow velocities. - Right heart catheterization: equalization of diastolic pressures (RA, RV diastolic, PCWP). - Tamponade is a clinical diagnosis. Never delay treatment for imaging in an unstable patient (use FAST/bedside echo in trauma).

MANAGEMENT - Definitive: urgent pericardiocentesis (echo-guided, subxiphoid approach) for unstable patients. Pericardial window or thoracotomy for traumatic or hemorrhagic cases, and for recurrence. - Temporizing: IV fluid boluses to increase preload. Avoid diuretics, vasodilators, and nitrates. Avoid positive pressure ventilation where possible, because it lowers venous return. Inotropes (dobutamine) may help temporarily. - Treat the underlying cause (dialysis for uremia, reversal of anticoagulation, surgical repair of aortic dissection or free wall rupture).

HIGH-YIELD COMPARISONS - Tamponade vs constrictive pericarditis: both equalize diastolic pressures. Tamponade has pulsus paradoxus, absent y descent, and no Kussmaul sign. Constrictive pericarditis has Kussmaul sign, a prominent y descent, a pericardial knock, and pericardial calcification (TB, post-radiation, post-cardiac surgery). Constriction is treated with pericardiectomy. - Tamponade vs tension pneumothorax: both cause obstructive shock with JVD. Tension pneumothorax has unilateral absent breath sounds, tracheal deviation, and hyperresonance. - Tamponade vs cardiogenic shock: cardiogenic shock has pulmonary edema and a raised PCWP, while tamponade has clear lungs. - Hemodynamics: low CO, high SVR, high CVP/RA pressure, and low or normal PCWP (the PCWP is equalized at an elevated value in classic tamponade; with hypovolemia it may be lower).

BOARD PEARLS - Hypotension plus JVD plus clear lungs after chest trauma or MI should suggest tamponade. - Rapid accumulation of a small volume is more dangerous than slow accumulation of a large volume. - Hypovolemic patients (low-pressure tamponade) may lack JVD. - Do not give diuretics, and avoid intubation with positive pressure ventilation before drainage if possible.