ECG Basics · Lesson 9 · Leads, axis and conduction

Electrical axis in two leads

The axis is the average direction of ventricular depolarisation in the frontal plane. Two leads give you the quadrant in seconds.

The quadrant method

Look at the net QRS in lead I and aVF: is the area above the baseline larger (positive) or smaller (negative) than the area below? Lead I positive points the axis to the left half of the circle; aVF positive points it downward. Both positive is a normal axis.

Axis explorer

Drag the needle (or use the arrow keys or the slider). The six limb leads redraw for that axis: each is the engine's normal beat recomputed with only the frontal QRS axis changed.

  • Normal
  • Left deviation
  • Right deviation
  • Extreme

Outlined: I and aVF (the quadrant check). Teal: the most positive lead, which lies closest to the axis. Amber: the most equiphasic lead, which lies nearly at right angles to it. One beat per lead, magnified ×3 · 25 mm/s · 10 mm/mV · Junkers engine · SIMULATED

AxisRangeCommon causes
Normal−30° to +90°—
Left deviation−30° to −90°Left anterior fascicular block, inferior MI, LVH, paced or ventricular beats
Right deviation+90° to +180°Right ventricular strain or hypertrophy, left posterior fascicular block, lateral MI; can be normal in children and tall, slim adults
Extreme−90° to ±180°Ventricular rhythm, severe conduction disease, or swapped leads

Refining it

Find the limb lead where the QRS is most equiphasic (as much up as down). The axis lies at right angles to that lead, on the side of the leads that are positive. For example, if aVL is equiphasic and II is positive, the axis is near +60°. Try it in the explorer: set +60° and look at aVL, then set −30° and look at lead II.

The live simulator keeps three model axes per rhythm (Variant 1–3: about +60°, +15° and +85°). The explorer's other angles are teaching beats computed by the same engine for this lesson only.
See it move in the live simulatorNormal axis

References

  1. Kligfield P, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part I: the electrocardiogram and its technology. Circulation 2007;115:1306–1324.
  2. Surawicz B, et al. Recommendations for the standardization and interpretation of the electrocardiogram, Part III: intraventricular conduction disturbances. J Am Coll Cardiol 2009;53:976–981.