Central venous pressure values of 15 mmHg or higher suggest tamponade, particularly if they are rising over a short observation period. Routine chest radiographs may reveal left or right hemothorax, the most common clinical condition associated with tamponade from penetrating injuries. Hemomediastinum occasionally occurs. The 'water-bottle' appearance of the heart is rare in acute tamponade from penetrating cardiac injuries, but may occur in slowly developing pericardial effusions from medical conditions. Pulmonary congestion is usually absent in patients with tamponade compared with the pulmonary congestion and cardiac dilatation seen in patients with congestive heart failure. Non-specific ECG changes include low voltage, depressed or altered ST segments, and evidence suggestive of subepicardial ischemia. ECG patterns are rarely diagnostic of tamponade, but are performed routinely to exclude other cardiac conditions.

Echocardiography provides definitive diagnostic information. Normally, a single echo is reflected from the posterior wall of the ventricles; two distinct echoes from this area indicate pericardial fluid, and the space between these echoes is a reflection of the volume of fluid.

CT and magnetic resonance imaging of the chest accurately show small chambers of the heart and the pericardial effusion. Limited time precludes these studies in rapidly evolving tamponade after penetrating chest injuries. Angiocardiography with radio-opaque dye can outline the right atrial lumen and the thickness of the atrial wall; a wall thickness greater than 6 mm indicates increased pericardial fluid.

The diagnosis is confirmed if non-clotting blood is obtained by pericardiocentesis, but a negative tap does not rule out the diagnosis. False-negative taps were seen in 19 per cent of hemopericardium from penetrating chest injuries because of clotted pericardial blood

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