A 38-year-old male comes to the emergency room complaining of shortness of breath on exertion for a few weeks. His medical history is significant for malignant melanoma. Physical examination is significant for dyspnea, jugular venous distention, pericardial friction rub,muffled cardiac tones, and hypotension. ECG shows low-voltage electrical waves. You suspected pericardial tamponade in this patient and wanted to do a bed-side maneuver to establish the diagnosis. Of the following, the measurement of which would help you in reaching the correct diagnosis in this patient?
