A 60 year old man with hypertension, untreated colon cancer, and a history of NSTEMI presents to the emergency department with immediate onset dyspnea two hours ago. He is short of breath at rest. He has a cough with streaky bloody sputum and associated chest pain while coughing or taking a deep breath. He has two pillow orthopnea. He says he has been taking all his prescribed medications but forgot to bring in a list. His social history is significant for being mostly bedbound due to obesity and living alone.
On exam, vital signs are heart rate 70, blood pressure 110/70, respiratory rate 26, and oxygen saturation 90%. You hear a regular rate and rhythm. He has rales and decreased breath sounds. He has some jugular venous distension. His ABG is 7.50/30/68. His CXR is unremarkable. His EKG is normal sinus rhythm with several old Q waves.
Challenge: When you present your most likely diagnosis to your attending, he says that the heart rate isn't consistent. How do you explain the patient's heart rate?