Case Presentation

A 53-year-old male presented to the Emergency Department with palpitations.

His ECG showed a heart rate of 132 bpm.

What is your diagnosis?

And why?

Narrow Complex Regular Tachycardia – The Differential

This ECG falls under a classic and commonly misinterpreted theme:

Narrow-complex regular tachycardia

Most common differentials:

• Sinus Tachycardia: Clear P wave before every QRS.
• Supraventricular Tachycardia (SVT): No visible P waves.
• Atrial Flutter with 2:1 Block: Multiple P waves per QRS, typically at a rate near 150 bpm.

Why This ECG Can Be Misleading

At first glance, it seems like there are no P waves, making SVT a tempting diagnosis.

But that would be wrong in this case.

ECG Analysis

Upon closer inspection:

• Inverted P waves are seen just before each QRS.
• These P waves can be easily missed or mistaken for inverted T waves.
• When the ECG is flipped upside down, the pattern becomes clear.

2 flutter waves appear before each QRS, and they map perfectly.

ECG Tip

When in doubt, flip the ECG paper upside down.

It can help distinguish hidden P waves from misread T waves.

Final Diagnosis: Atrial Flutter with 2:1 Block

The patient was given a beta-blocker, which:

• Slowed the heart rate.
• Unmasked flutter waves.

He then spontaneously cardioverted.

He was discharged home after observation and referred for cardiology follow-up.

Learning Points

• Always consider atrial flutter when dealing with narrow-complex tachycardia around 130–150 bpm.
• Hidden flutter waves can easily be missed.
• Flipping the ECG may reveal patterns your brain tends to ignore.
• Diagnosis matters — treating as SVT vs flutter differs greatly.

Further Reading

This case continues the atrial flutter series from Part 1.

• Fast & Furious (Part 1): Atrial Flutter in the Elderly

Key Takeaways

• A heart rate around 132–150 bpm in regular narrow-complex tachycardia should always raise suspicion for atrial flutter.
• Don’t trust your eyes alone — look for subtle inverted P waves.
• Treating flutter as SVT may miss the underlying arrhythmia.
• Beta-blockers often reveal flutter waves and support diagnosis.

Frequently Asked Questions

What are the ECG clues that suggest atrial flutter over SVT?

Subtle inverted P waves before QRS complexes and a heart rate close to 150 bpm are strong clues. Flipping the ECG may help expose flutter waves.

How is atrial flutter with 2:1 block treated acutely?

Rate control with beta-blockers or calcium channel blockers is common. Some patients may spontaneously cardiovert.

Why is it important to differentiate SVT from atrial flutter?

Misdiagnosis can lead to inappropriate management. Flutter often requires different medications and rhythm control strategies.

Fast & Furious ECG Series – Part 2: Atrial Flutter Misdiagnosed as SVT

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:

• Sinus Tachycardia: Clear, single P wave before every QRS complex.
• Supraventricular Tachycardia (SVT): Typically no clear P waves visible.
• 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

• Always consider atrial flutter when the heart rate is close to 150 bpm.
• Our brains are often trained to ignore inverted P waves, misreading them as T waves.
• Try flipping the ECG upside down when in doubt.
• Use the Bix Rule.
• If a “P wave” is seen midway between QRS complexes, suspect hidden flutter waves.
• 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.

Frequently Asked Questions

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.

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.

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.