A 55-year-old woman with metastatic lung cancer presents with dyspnea and pedal edema. On examination, the JVP is at 10 cm, with a negative Kussmaul sign. The heart sounds are diminished and the lungs have bibasilar crackles. The ECG shows QRS complexes of variable height.For the above patient with shortness of breath and peripheral edema, select the most likely diagnosis.