This patient's clinical presentation is consistent with postpericardiotomy syndrome (PCIS) after recent CABG. PCIS is likely due to surgical damage of the mesothelial pericardial cells with resultant blood in the pericardial space, which causes an autoimmune response against the released cardiac antigens. The immune complexes deposit and cause inflammation in the pericardium, pleura, and lung.
PCIS presents similar to acute pericarditis with typical pleuritic chest pain, pericardial friction rub, ECG changes, and/or new or worsening pericardial effusion. Most patients after cardiac surgery have some baseline ECG changes (such as LBBB in this patient), making it difficult to interpret the ECG for evidence of pericarditis. Patients who develop PCIS usually respond to NSAIDs; steroids maybe used for refractory cases. Giving colchicine after cardiac surgery significantly decreases the incidence of PCIS.
Postoperative mediastinitis typically occurs within the first 2 weeks after surgery and presents with fever, tachycardia, chest pain, and signs of sternal wound infection, such as purulent discharge. This patient's normal appearing sternotomy site makes this less likely.