Regarding the diagnosis and treatment of cardiac tamponade, which of the following statements is/are true?

C. Approximately 15% of needle pericardiocenteses give a false-negative result

DISCUSSION: Cardiac tamponade is most frequently caused by penetrating thoracic injury, but may occasionally be observed following blunt thoracic trauma from cardiac chamber rupture, coronary artery laceration, or ascending dissection of an aortic tear. Accumulation of as little as 150 ml. of blood in the pericardium will sufficiently decrease diastolic filling to produce distended neck veins, cyanosis, and decreased cardiac output. Beck's classic triad of distended neck veins, muffled heart sounds, and hypotension is present in only one third of patients with tamponade. Pulsus paradoxicus is even less frequently discernible. Immediate temporary treatment consists of pericardiocentesis, which also provides a diagnosis. However, approximately 15% of pericardiocenteses give false-negative results because of a clotted hemopericardium. Therefore, echocardiography prior to needle aspiration is generally advisable if promptly available. In the patient in extremis, emergency thoracotomy with pericardiotomy and cardiac repair should be performed. Most patients with penetrating cardiac wounds do not require cardiopulmonary bypass to repair their injuries.