Think of the heart’s electrical activity like a flock of birds in flight. While each bird (myocyte) flaps in its own rhythm, the flock moves in a general direction . This is your cardiac axis: the average direction of depolarisation through the ventricles.

On a healthy ECG, the axis should lie between –30° and +90°, pointing down and slightly left, the path of least resistance through the muscular left ventricle. Most of the depolarisation heads that way, so leads like I and aVF catch a strong signal.

But if the flock veers off course?

• Left Axis Deviation (LAD) means the electrical flow is swinging more leftward (–30° to –90°). Common causes include left anterior hemiblock or inferior myocardial infarction, both of which block or damage parts of the usual route.
• Right Axis Deviation (RAD) pushes the axis beyond +90°, indicating right-sided strain. This is seen in pulmonary embolism, chronic lung disease, or right ventricular hypertrophy.

Axis deviation isn’t a diagnosis, it’s a directional clue. Like a compass needle twitching in a storm, it tells you something’s not right in the terrain. And in ECG interpretation, it’s often your first nudge toward a bigger story.

Axis Tips: Are They Leaving or Returning?

One of the quickest ways to assess cardiac axis is to look at leads I and III and use a bit of wordplay to help lock it in.

• If lead I is positive (the QRS points upwards) and lead III is negative (the QRS points downward), the electrical activity is moving away from lead III and toward the left. Think:
  → “They’re Leaving” → L for Left Axis Deviation.
  This suggests the heart’s electrical axis is shifted left often due to left anterior hemiblock, left ventricular hypertrophy, or an old inferior MI.
• If lead I is negative and lead III is positive, the axis is drifting rightward. The QRS is moving away from lead I, and coming back toward lead III. Think:
  → “They’re Returning” → R for Right Axis Deviation.
  This could reflect right ventricular strain, a PE, or right ventricular hypertrophy.

This “Leaving vs Returning” mnemonic gives you a rapid visual clue without needing to calculate angles. A clinically handy trick when time (or brainpower!) is limited. Remember though: for precision, use leads I and aVF, but for speed, I and III never lie.