How to read and ECG: Mnemonic for ECG reading fast

Introduction

A structured approach makes ECG interpretation systematic and reliable. A commonly used mnemonic is:

RRAHI

• R = Rhythm
• R = Rate
• A = Axis
• H = Hypertrophy
• I = Ischemia

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How to Calculate Heart Rate in Regular Rhythm ECG

For a regular rhythm, use either of the following methods:

Method 1 (Big boxes)

• Take any two R waves
• Count the number of big boxes between them
• Divide 300 by that number
• The result is the heart rate

Method 2 (Small boxes)

• Take any two R waves
• Count the number of small boxes between them
• Divide 1500 by that number
• The result is the heart rate

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How to Calculate Heart Rate in Irregular Rhythm ECG

For irregular rhythms, use a longer rhythm strip:

• Take 10 big boxes in the rhythm strip
• Count the number of QRS complexes (R waves) within those 10 big boxes
• Multiply that number by 30

👉 This gives the heart rate in beats per minute (bpm)

Completed Formula

Heart rate = (Number of QRS complexes in 10 big boxes) × 30

Why this works:

• 1 big box = 0.2 seconds
• 10 big boxes = 2 seconds
• 60 seconds ÷ 2 seconds = 30

So you are converting a 2-second sample into a 1-minute rate.

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How to Check Rhythm of an ECG

To determine the rhythm:

• Check if R–R intervals are regular or irregular
• Look for P waves before every QRS complex
• Ensure P waves have a consistent shape
• Confirm that each P wave is followed by a QRS

Normal (Sinus Rhythm) Features:

• Regular rhythm
• One P wave before each QRS
• Normal PR interval
• Heart rate between 60–100 bpm

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How to Check Axis of an ECG

Axis refers to the overall direction of electrical activity in the heart.

Basic Method (using limb leads):

• Look at Lead I and aVF

Lead I	aVF	Axis
+	+	Normal axis
+	–	Left axis deviation
–	+	Right axis deviation
–	–	Extreme axis deviation

Normal Axis:

• Between -30° to +90°

The electrical impulse normally:

• Starts from the right atrium
• Travels toward the left ventricle
• Occurs 60–100 times per minute

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How to Identify Myocardial Ischemia on ECG

Look for these three key changes:

• ST segment changes
  • Elevation → possible acute injury
  • Depression → ischemia
• T wave changes
  • Inversion
  • Flattening
• Q waves
  • May indicate previous myocardial infarction

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How to Identify Hypertrophy on ECG

Hypertrophy leads to increased voltage (amplitude) on ECG.

Left Ventricular Hypertrophy (LVH)

A commonly used method:

• S wave in V1 + R wave in V5 or V6 ≥ 35 mm

Additional clues:

• Deep S waves in V1
• Tall R waves in V5/V6
• Possible left axis deviation

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This completes the full ECG reading framework using the RRAHI approach, covering all essential steps: rate, rhythm, axis, hypertrophy, and ischemia.