A 54-year-old male presents to clinic with complaints of palpitations. He says the palpitations started off as intermittent but has now become more constant. He also endorses occasional dizziness when he is working in the yard. The patient has a history of hypertension as well as coronary artery disease. Physical examination reveals an irregularly irregular rhythm. EKG performed in the office reveals absence of P-waves. Osteopathic structural examination would likely reveal which of the following diagnoses?