参考答案：C

解析：Cardiac tamponade is associated with decreased cardiac output, resulting in decreased blood pressure. Removing a small amount of blood may improve cardiac output and blood pressure. Pericardial blood doesn't clot rapidly because it's defibrinated by cardiac motion within the cardiac sac. If blood clots rapidly, the needle may have entered the heart. Clients with cardiac tamponade may have muffled heart sounds. If pericardiocentesis is effective, heart sounds become normal.