After a pericardiocentesis what are signs of complications that require immediate medical attention?

Multiple. Changes in the patients symptoms such as passing out, shortness of breath, or weakness. The development of a rapid pulse or of a low blood pressure. A drop in the hemoglobin and hematocrit. The development of significant problems with the heart rhythm.
Several... If the procedure is over completed, the late concern is for recurrence of the pericardial effusion. Signs or symptoms of effusion recurrence could include: shortness of breath, chest pain, dizziness, light headedness, or anxiety.