What is the differential diagnosis for a 2-month postpartum patient with persistent tachycardia who stopped metoprolol (beta blocker) 1 month ago and has a recent echocardiogram showing no cardiomyopathy with strong ejection fraction (EF)?

Differential Diagnosis for Postpartum Tachycardia with Normal Cardiac Function

In a 2-month postpartum patient with persistent tachycardia, normal ejection fraction, and recent beta-blocker discontinuation, the differential diagnosis must prioritize arrhythmias (particularly supraventricular tachycardia and focal atrial tachycardia), thyroid dysfunction, anemia, pulmonary embolism, and occult peripartum cardiomyopathy with arrhythmic presentation.

Primary Cardiac Arrhythmias

Supraventricular Tachycardia (SVT)

• Atrioventricular nodal re-entry tachycardia (AVNRT) and atrioventricular re-entry tachycardia (AVRT) become more frequent during pregnancy and may persist postpartum, particularly in young women 1.
• Both premature extra beats and sustained tachyarrhythmias can manifest for the first time during pregnancy and continue into the postpartum period 1.
• The abrupt discontinuation of metoprolol one month ago may have unmasked an underlying arrhythmia that was previously rate-controlled 2.

Focal Atrial Tachycardia

• Focal atrial tachycardia is particularly concerning as it can be drug-resistant, persistent, and associated with structural heart disease 1.
• This arrhythmia occurs in approximately 22-33 cases per 100,000 pregnancies and can lead to tachycardia-induced cardiomyopathy if left untreated 3.
• Persistent unexplained sinus tachycardia in the peripartum period may mask underlying focal atrial tachycardia 3.

Atrial Flutter/Fibrillation

• Atrial flutter and atrial fibrillation are rare during pregnancy unless structural heart disease or hyperthyroidism is present 1.
• A rapid ventricular response can lead to serious hemodynamic consequences 1.

Peripartum Cardiomyopathy (PPCM) with Arrhythmic Presentation

Occult PPCM

• PPCM can present with arrhythmias as the primary manifestation even when ejection fraction appears preserved on initial echocardiography 4.
• Premature ventricular contractions (PVCs) are the most common recorded arrhythmias in PPCM, followed by ventricular fibrillation, non-sustained ventricular tachycardia, atrial fibrillation, and cardiac sudden death 4.
• Patients with PPCM are as susceptible to arrhythmias as those with other cardiomyopathies, particularly if left ventricular systolic dysfunction becomes chronic 1.
• A single echocardiogram may not capture evolving cardiac dysfunction; serial imaging at 6 weeks, 6 months, and annually is recommended 1.

Beta-Blocker Withdrawal Effect

• Abrupt discontinuation of metoprolol in patients with underlying cardiomyopathy can cause severe exacerbation of symptoms, including ventricular arrhythmias 2.
• During withdrawal of metoprolol in dilated cardiomyopathy patients, heart rate increases and ejection fraction decreases significantly 5.

Endocrine and Metabolic Causes

Thyroid Dysfunction

• Postpartum thyroiditis occurs in 5-10% of women and can present with either hyperthyroid or hypothyroid phases 6.
• Hyperthyroidism causes tachycardia directly, while hypothyroidism causes QT prolongation and ventricular tachycardia risk 7.
• Thyrotoxicosis is a recognized cause of atrial flutter and atrial fibrillation in the peripartum period 1.
• Metoprolol may have masked tachycardia from hyperthyroidism, and abrupt withdrawal might precipitate a thyroid storm 2.

Pheochromocytoma

• Pheochromocytoma may present with ventricular tachycardia due to catecholamine excess and can cause reversible cardiomyopathy 7.
• Administration of beta-blockers alone in pheochromocytoma has been associated with paradoxical hypertension; if metoprolol was masking this condition, withdrawal could unmask symptoms 2.

Anemia and Blood Loss

• Postpartum anemia from obstetric hemorrhage is a common cause of compensatory tachycardia 8.

Thromboembolic Disease

Pulmonary Embolism

• Venous thromboembolism is a leading cause of maternal mortality and commonly presents with tachycardia in the postpartum period 1, 8.
• The postpartum period carries increased thrombotic risk for up to 12 weeks after delivery 1.

Catecholamine-Mediated Tachycardia

Inappropriate Sinus Tachycardia

• Catecholamine-mediated ventricular tachycardia occurs frequently in young women without overt structural disease and is typically beta-blocker sensitive 7.
• Discontinuation of metoprolol may have unmasked catecholamine-driven tachycardia 7.

Congenital Arrhythmia Syndromes

Long QT Syndrome

• Women have significantly higher risk of long QT-related ventricular tachycardia and torsades de pointes compared to men, with 70% of Long QT Registry subjects being female 7.
• Post-pubertal females experience predominant incidence of cardiac arrests and syncope from long QT syndrome 7.

Critical Diagnostic Approach

Immediate Evaluation

• Obtain a 12-lead ECG during sinus rhythm to identify QT prolongation, pre-excitation, P-wave morphology suggesting atrial tachycardia, or other structural abnormalities 7, 8.
• Measure thyroid-stimulating hormone (TSH), free T4, and free T3 to exclude thyroid dysfunction 1, 7.
• Check complete blood count to assess for anemia 8.
• Measure electrolytes (potassium, magnesium, calcium) as imbalances cause arrhythmogenesis 7.
• Consider D-dimer and imaging for pulmonary embolism if clinically indicated 1.

Cardiac-Specific Testing

• Measure B-type natriuretic peptide (BNP) or NT-proBNP, as all patients with PPCM have abnormal levels (mean 1727.2 fmol/mL vs. 339.5 fmol/mL in healthy postpartum women) 1.
• Repeat echocardiography is essential, as not all PPCM patients present with left ventricular dilatation initially, and cardiac function can deteriorate over time 1.
• Consider 24-hour Holter monitoring or event recorder to capture paroxysmal arrhythmias 8.
• Cardiac MRI may provide more accurate assessment of chamber volumes and detect myocarditis if echocardiography is inconclusive 1.

Family and Medication History

• Obtain family history of sudden cardiac death or long QT syndrome 7.
• Review the indication for metoprolol during pregnancy and the reason for discontinuation 2.

Common Pitfalls

• Do not assume normal ejection fraction excludes PPCM, as arrhythmic presentations can occur with preserved systolic function initially 4.
• Do not attribute persistent tachycardia solely to anxiety or deconditioning without excluding life-threatening causes 8.
• Do not overlook the possibility that metoprolol was controlling an underlying arrhythmia rather than treating physiologic pregnancy-related tachycardia 2, 5.
• Do not delay serial echocardiographic follow-up, as PPCM can evolve over months 1.