What is the most likely diagnosis for a patient with chest pain, palpitations, generalized muscle aches, nonproductive cough, and fever, presenting with tachycardia, irregularly irregular rhythm, elevated cardiac troponin, and diffuse inspiratory crackles?

Acute Myopericarditis

The most likely diagnosis is acute myopericarditis, given the combination of pericarditic chest pain (constant, worse with lying down), pericardial friction rub (scratchy sound in systole and diastole), elevated troponin (184 ng/L), irregularly irregular rhythm suggesting atrial fibrillation, and systemic inflammatory features (fever, myalgias, leukocytosis). 1

Key Diagnostic Features Supporting Myopericarditis

Pericardial Inflammation Evidence

• Pericardial friction rub is pathognomonic for pericardial inflammation and is highly specific when present, though only audible in approximately one-third of pericarditis cases 2, 1
• The scratchy sound heard in both systole and diastole represents the classic bi- or triphasic friction rub characteristic of pericarditis 2
• Chest pain that worsens with lying down and is constant (rather than intermittent) is typical of pericarditis 2

Myocardial Involvement Evidence

• Elevated troponin (184 ng/L) indicates myocardial injury and occurs in up to 50% of acute pericarditis cases, defining myopericarditis 1
• The irregularly irregular rhythm (pulse 100-130 bpm) strongly suggests atrial fibrillation, which indicates myocardial involvement beyond isolated pericarditis 1
• Atrial fibrillation is a key distinguishing feature, as pure pericarditis would not typically cause significant arrhythmias 1

Systemic Inflammatory Features

• Fever (101°F for 48 hours), generalized muscle aches, and leukocytosis (14.2) support acute systemic inflammation consistent with viral myopericarditis 1
• The nonproductive cough and diffuse inspiratory crackles may represent associated pulmonary involvement or pleural inflammation 3

Why Not the Other Diagnoses

ST-Segment Elevation Myocardial Infarction - Excluded

• A 30-year-old with no cardiovascular risk factors and no past medical history has extremely low pre-test probability for acute coronary syndrome 3
• The presence of a pericardial friction rub is pathognomonic for pericardial inflammation, not coronary occlusion 1
• The ESC guidelines note that friction rub and irregularly irregular pulse are findings suggestive of alternative diagnoses to acute coronary syndrome 3
• While troponin is elevated, the clinical context (young age, friction rub, positional chest pain) points away from MI 3

Acute Pericarditis Alone - Insufficient

• Pure pericarditis would not typically cause elevated troponin or significant arrhythmias like atrial fibrillation 1
• The combination of troponin elevation AND atrial fibrillation indicates myocardial involvement, making this myopericarditis rather than isolated pericarditis 1

Adverse Vaccine Reaction - Too Nonspecific

• While the patient received influenza vaccine "a few weeks ago," this temporal relationship is too distant to be causally related 3
• The specific constellation of findings (friction rub, troponin elevation, atrial fibrillation) points to a defined cardiac inflammatory process rather than a nonspecific vaccine reaction 1

Immediate Management Priorities

Urgent Diagnostic Studies

• Urgent echocardiography is mandatory to assess for pericardial effusion, cardiac tamponade, and ventricular function 1
• ECG should be obtained immediately to document the atrial fibrillation and look for J-point elevation (typical in 60% of pericarditis cases) or PR depression 1
• Cardiac MRI with gadolinium should be considered for definitive diagnosis and to assess the extent of myocardial involvement 1, 4

Hospital Admission Required

• Hospital admission is indicated given the combination of elevated troponin, arrhythmia (atrial fibrillation), and fever 1
• The American Heart Association notes that troponin elevation in myopericarditis does not predict worse outcomes when left ventricular function is preserved, but monitoring is still essential 1

Critical Pitfalls to Avoid

• Do not assume this is an acute MI based solely on troponin elevation and chest pain - the friction rub, positional pain, and young age with no risk factors argue strongly against ACS 1, 4
• Do not dismiss the irregularly irregular rhythm - this atrial fibrillation indicates myocardial involvement and distinguishes myopericarditis from pure pericarditis 1
• Do not delay echocardiography - pericardial effusion with potential tamponade must be ruled out urgently 1
• The diffuse inspiratory crackles may represent pulmonary involvement but should not distract from the primary cardiac diagnosis 3