Fast & Furious (Part 1): Narrow Complex Regular Tachycardia

Case Presentation

A 95-year-old woman presented to the Emergency Department with complaints of palpitations.

Her heart rate was 144 bpm.

An ECG was obtained, revealing a narrow-complex regular tachycardia.

What Is Your Diagnosis?

And why?

Narrow Complex Regular Tachycardia: Differential Diagnosis

This ECG represents a classic theme seen in many emergency cases: narrow-complex regular tachycardia.

The most common differentials include:

1. Sinus Tachycardia
   • Clear, single P wave before every QRS complex
2. Supraventricular Tachycardia (SVT)
   • Typically no clear P waves visible
3. Atrial Flutter with Fixed Block
   • Multiple P waves (flutter waves) before each QRS complex

Visual ECG examples can help distinguish these — but in some real-world cases like this one, they aren’t always reliable.

Why This Case Is Tricky

Initially, there appeared to be only one visible wave between each QRS complex.

That could suggest:

• P waves → Possibly Sinus Tachycardia with 1st degree AV block
• T waves → Possibly SVT

So which is it? Diagnosis is critical — the treatment paths are entirely different.

ECG Analysis

Clue #1: Heart Rate

• 144 bpm is relatively slow for SVT
• But it's just right for Atrial Flutter (classically around 150 bpm)

Clue #2: The “Bix Rule”

When you see a “P wave” exactly midway between two QRS complexes, assume there are two more flutter waves — one buried before and one after the QRS.

In this case, the “single wave” seen between QRS complexes was actually part of a flutter wave series.

Final Diagnosis: Atrial Flutter

• Atrial flutter with a fixed 2:1 block was the working diagnosis.
• The patient was allergic to beta-blockers, so she was treated with Verapamil (CCB).

• Post-treatment ECG revealed classic flutter waves in V1 with variable block.

Learning Points

1. Always consider atrial flutter when the heart rate is close to 150 bpm
2. Our brains are often trained to ignore inverted P waves, misreading them as T waves
   • Try flipping the ECG upside down when in doubt
3. Use the Bix Rule
   • If a “P wave” is seen midway between QRS complexes, suspect hidden flutter waves
4. The differential for narrow complex regular tachycardia = Sinus tachycardia, SVT, or Atrial Flutter with fixed block

Further Learning

Watch the full case discussion on YouTube

Key Takeaways

• Not all regular tachycardias are SVT — look deeper for flutter
• Heart rate near 150 is a huge clue for atrial flutter
• Inverted P waves ≠ T waves — flip the ECG when in doubt
• Clinical decisions depend on accurate rhythm recognition

FAQ – Fast & Furious (Part 1)

Q1: What are the most common causes of narrow-complex regular tachycardia?

The key differentials include sinus tachycardia, supraventricular tachycardia (SVT), and atrial flutter with fixed block.

Q2: How can I identify atrial flutter on ECG if flutter waves are not obvious?

Use the Bix Rule — if a P-like wave is exactly between QRS complexes, suspect hidden flutter waves before and after.

Q3: Why is heart rate important in differentiating SVT from atrial flutter?

A heart rate close to 150 bpm is highly suggestive of atrial flutter with 2:1 block, especially if the rhythm is regular.