I have a rapid narrow‑complex tachycardia (heart rate 143 bpm) with a short PR interval (PR) of 112 ms, normal QRS duration (QRS) of 80 ms and a markedly prolonged corrected QT interval (QTc) of 546 ms after recent troponin and D‑dimer testing; what does this electrocardiogram (ECG) indicate and what urgent management is required?

ECG Interpretation and Urgent Management

Your ECG shows a narrow-complex tachycardia (QRS 80 ms) at 143 bpm with a critically prolonged QTc of 546 ms and a short PR interval of 112 ms, which indicates supraventricular tachycardia (likely AVNRT or AVRT given the short PR) complicated by severe QT prolongation that places you at immediate risk for torsades de pointes and sudden cardiac death—you require urgent emergency department evaluation with continuous cardiac monitoring, immediate correction of any electrolyte abnormalities (particularly potassium and magnesium), discontinuation of all QT-prolonging medications, and cardiology consultation for both acute SVT management and assessment of underlying causes of QT prolongation. 1

Critical ECG Findings

Narrow-Complex Tachycardia Analysis

• Your QRS duration of 80 ms confirms this is a supraventricular tachycardia, not ventricular tachycardia, as narrow QRS (<120 ms) excludes ventricular origin 1
• The heart rate of 143 bpm with regular rhythm suggests either AVNRT (atrioventricular nodal reentrant tachycardia) or AVRT (atrioventricular reciprocating tachycardia) 1
• The short PR interval of 112 ms (normal is 120-200 ms) raises concern for an accessory pathway (pre-excitation), which would indicate AVRT or Wolff-Parkinson-White pattern 1
• A 12-lead ECG during tachycardia is essential to determine if P waves are buried in the QRS (suggesting AVNRT) or visible in the early ST segment (suggesting AVRT) 2, 1

Severely Prolonged QTc: The Immediate Danger

• Your QTc of 546 ms is grade 3 prolongation by CTCAE criteria (>501 ms) and represents a medical emergency 1
• QTc >500 ms is associated with significantly increased risk for torsades de pointes, a potentially fatal ventricular arrhythmia 1, 3
• The American Heart Association recommends using the Fridericia formula for QTc calculation, which your report appears to use, making this value reliable 4, 5
• Normal QTc is <450 ms in males and <460 ms in females; your value exceeds this by approximately 90-100 ms 1, 3

Urgent Management Algorithm

Immediate Actions (Within Minutes to Hours)

Step 1: Emergency Department Evaluation

• Proceed immediately to an emergency department for continuous cardiac monitoring 1
• Do not attempt vagal maneuvers or self-treatment given the severe QT prolongation 1
• The combination of SVT and severe QT prolongation creates compounded arrhythmia risk 1

Step 2: Electrolyte Correction (First Priority)

• Obtain immediate serum potassium and magnesium levels 1
• Correct hypokalemia to >4.0 mEq/L and hypomagnesemia to >2.0 mg/dL before any other intervention 1
• Administer 2g IV magnesium sulfate empirically even if serum magnesium is normal, as this is the first-line agent for preventing torsades de pointes 1

Step 3: Medication Review and Discontinuation

• Immediately discontinue all QT-prolonging medications including: ondansetron, domperidone, other antiemetics (palosetron, granisetron, prochlorperazine), psychiatric medications (escitalopram, venlafaxine, sertraline, mirtazapine, olanzapine), and any antiarrhythmic drugs 1
• Review recent troponin and D-dimer results mentioned in your history, as these suggest evaluation for acute coronary syndrome or pulmonary embolism, both of which can cause QT prolongation 1

Step 4: SVT Termination

• For hemodynamically stable narrow-complex SVT, adenosine 6 mg IV rapid push is first-line (can repeat 12 mg if needed) 1
• Critical caveat: If pre-excitation (WPW) is confirmed, adenosine could precipitate atrial fibrillation with rapid ventricular response and is relatively contraindicated 1, 2
• Alternative agents include IV diltiazem or metoprolol, but these must be used cautiously given QT prolongation 1
• If hemodynamically unstable (hypotension, chest pain, altered mental status), immediate synchronized cardioversion is indicated 1, 6

Mandatory Cardiology Referral Criteria Met

You meet multiple urgent referral criteria:

• Wide-complex tachycardia of unknown mechanism requires immediate electrophysiology consultation (though yours is narrow, the diagnostic uncertainty with short PR warrants urgent evaluation) 2, 1
• If baseline ECG shows pre-excitation (delta waves), this indicates WPW syndrome with risk of sudden death and requires immediate electrophysiology referral 2, 1
• QTc >500 ms requires cardiology evaluation to determine etiology (congenital long QT syndrome vs. acquired) 1, 3
• The combination of recurrent SVT and severe QT prolongation is high-risk and warrants inpatient evaluation 1, 2

Differential Diagnosis for QT Prolongation

Acquired Causes (Most Common)

• Medications: Review all current medications, supplements, and over-the-counter drugs for QT-prolonging agents 1
• Electrolyte abnormalities: Hypokalemia, hypomagnesemia, hypocalcemia 1, 3
• Cardiac conditions: Myocardial ischemia (correlate with troponin results), cardiomyopathy, heart failure 1
• Metabolic: Hypothyroidism, hypoglycemia 3

Congenital Long QT Syndrome

• If QT prolongation persists after correcting reversible causes, genetic testing for congenital LQTS should be pursued 3
• Family history of syncope, seizures, or sudden cardiac death in young relatives is highly suggestive 3
• Congenital LQTS requires beta-blocker therapy (nadolol or propranolol) and lifestyle modifications 3

Common Pitfalls to Avoid

Do not assume the tachycardia is benign despite narrow QRS—the severe QT prolongation creates substrate for life-threatening arrhythmias 1

Do not use class Ia or class Ic antiarrhythmic drugs (procainamide, flecainide, propafenone) for SVT in the setting of QT prolongation, as these will further prolong QT and increase torsades risk 1

Do not discharge from emergency care until QTc is <500 ms and SVT mechanism is identified, as 57% of patients with SVT experience episodes while driving, with 14% having syncope while driving 1, 2

Do not overlook drug-drug interactions—many commonly prescribed medications prolong QT, and the effect is additive 1

Follow-Up Testing Required

• Transthoracic echocardiogram to exclude structural heart disease and cardiomyopathy 2, 1
• Thyroid function tests, complete metabolic panel with calcium 3
• Repeat ECG after SVT termination and electrolyte correction to assess baseline QTc 1, 2
• If pre-excitation confirmed, electrophysiology study with possible catheter ablation (success rate 94-98%) 2, 1
• 24-hour Holter monitor or event recorder if arrhythmias are recurrent 2