You admit Mr. Heyes who is a 45-year-old man undergoing trea…

You admit Mr. Heyes who is a 45-year-old man undergoing treatment for metastatic adenocarcinoma of unknown primary origin. He is admitted with acute dyspnea. On arrival he is in distress. He is hypotensive to 70/30 and tachycardic to the 140s. Jugular veins are distended and his lungs are clear. Electrocardiogram shows low QRS voltage and sinus tachycardia, and you have to strain to hear his very quiet heart sounds on precordial auscultation. Which of the following is likely to be DECREASED in this patient?