What is the most likely diagnosis and recommended acute management for a patient with a narrow‑complex tachycardia at 152 beats per minute, PR interval 132 ms, QRS duration 80 ms, and corrected QT interval approximately 400 ms?

Narrow Complex Tachycardia at 152 BPM: Diagnosis and Management

Most Likely Diagnosis

This ECG pattern is most consistent with atrioventricular nodal reentrant tachycardia (AVNRT), the most common mechanism when a regular narrow-complex tachycardia presents without clearly visible P waves. 1, 2

Diagnostic Reasoning

QRS Complex Analysis

• The QRS duration of 80 ms confirms this is a narrow-complex tachycardia (QRS <120 ms), indicating supraventricular origin. 1, 3
• Narrow QRS complexes almost always indicate supraventricular tachycardia rather than ventricular tachycardia. 2

P Wave Assessment

• The PR interval of 132 ms and identical atrial and ventricular rates (both 152 BPM) suggest 1:1 AV conduction. 1
• In typical AVNRT, P waves are usually hidden within or immediately after the QRS complex, appearing as pseudo-S waves in inferior leads or pseudo-R' waves in V1 rather than distinct P waves. 1, 2
• When no discrete P waves are visible on a regular narrow-complex tachycardia, AVNRT is the most common mechanism. 2

Rate and Regularity

• The regular rate of 152 BPM with equal atrial and ventricular rates rules out atrial fibrillation (which would be irregularly irregular). 2
• This rate is typical for paroxysmal supraventricular tachycardia. 1

Differential Diagnosis Considerations

The narrow-complex regular tachycardia differential includes:

• AVNRT (most likely): P waves hidden in or immediately after QRS, RP interval <90 ms. 1, 2
• AVRT (orthodromic): P wave typically visible in early ST segment, RP interval 90-200 ms. 1
• Atrial tachycardia: Distinct P waves with different morphology from sinus rhythm, often with RP > PR. 1
• Atrial flutter: Atrial rate would typically exceed ventricular rate with characteristic flutter waves. 1, 2

Acute Management Algorithm

Step 1: Assess Hemodynamic Stability

• If the patient shows hypotension, altered mental status, shock, chest pain, or acute heart failure, proceed immediately to synchronized cardioversion without delay. 1, 4
• Do not attempt pharmacologic conversion in unstable patients. 4

Step 2: For Hemodynamically Stable Patients

Initial Non-Pharmacologic Intervention

• Attempt vagal maneuvers first (Valsalva maneuver, carotid massage, or facial immersion in cold water) to terminate the arrhythmia. 1, 4
• These maneuvers should be performed while recording a 12-lead ECG to document response. 1

First-Line Pharmacologic Therapy

• Adenosine 6 mg rapid IV push is the preferred first-line agent due to its rapid onset and short half-life. 1, 4
• If the initial 6 mg dose is ineffective, give adenosine 12 mg rapid IV push. 4
• Follow each dose with a 20 mL saline flush and elevate the arm. 1

Advantages of adenosine:

• Rapid onset and short half-life minimize prolonged adverse effects. 1
• Can serve both diagnostic and therapeutic purposes by unmasking atrial activity if the rhythm does not terminate. 1, 4

Important adenosine precautions:

• Avoid in patients with severe bronchial asthma. 1
• Use with caution when diagnosis is unclear, as it may precipitate ventricular fibrillation in patients with coronary disease or cause rapid ventricular rates in pre-excited atrial fibrillation. 1, 4
• Patients on theophylline may require higher doses; effects are potentiated by dipyridamole. 1
• Higher rates of heart block may occur with concomitant carbamazepine. 1

Alternative Pharmacologic Options

If adenosine is contraindicated or ineffective:

• Intravenous diltiazem or verapamil (non-dihydropyridine calcium-channel blockers) are effective alternatives. 1
• Intravenous metoprolol (beta-blocker) may also be used. 1
• These longer-acting agents are particularly valuable for patients with frequent premature beats that may trigger early PSVT recurrence. 1

Step 3: If Pharmacologic Therapy Fails

• Proceed to synchronized cardioversion for refractory stable narrow-complex tachycardia. 1, 4
• Provide sedation to conscious patients before cardioversion. 4

Critical Pitfalls to Avoid

Medication Errors

• Never administer verapamil, diltiazem, or other AV-nodal blocking agents if there is any uncertainty about whether the rhythm is wide-complex tachycardia, as this can precipitate hemodynamic collapse in ventricular tachycardia. 4, 2
• Do not combine multiple antiarrhythmic agents due to additive toxicity risk. 4

Diagnostic Errors

• Automatic ECG analysis systems are unreliable and commonly suggest incorrect arrhythmia diagnoses—always interpret the ECG manually. 1
• Do not rely on hemodynamic stability alone to differentiate supraventricular from ventricular tachycardia. 2

Management Errors

• Do not delay cardioversion in unstable patients to attempt pharmacologic conversion. 4
• Adenosine may initiate transient atrial fibrillation in 1-15% of cases, which can be particularly problematic in patients with ventricular pre-excitation. 1

Post-Conversion Management

• Maintain continuous cardiac monitoring after successful conversion to detect recurrence. 4
• Obtain a 12-lead ECG in sinus rhythm to look for pre-excitation (delta waves) that would indicate Wolff-Parkinson-White syndrome. 1
• If pre-excitation is present on baseline ECG, refer urgently to a cardiac electrophysiologist, as these patients are at risk for sudden death if they develop atrial fibrillation. 1
• For patients with recurrent episodes despite acute management, referral to an arrhythmia specialist for consideration of catheter ablation is indicated. 1