Left axis deviation (-45° to -90°) with qR in I and aVL, rS in II, III and aVF, and a QRS under 0.12 s.
The anterior fascicle of the left bundle is blocked, so the left ventricle activates from the posterior fascicle, from below upward. First you get a small r inferiorly and a small q in I and aVL, then large forces up and to the left. That adds up to left axis deviation of -45° to -90°, qR in I and aVL, rS in II, III and aVF, an R-wave peak time in aVL over 45 ms, and a QRS that's normal or slightly prolonged (80-110 ms).
Each is a standard 12-lead: 2.5 seconds of each lead in four columns, with 10 seconds of lead II along the bottom, drawn to scale (25 mm a second, 10 mm a millivolt). The measurements are under each one.
Right axis deviation with rS in I and aVL, qR in II, III and aVF, and a QRS under 0.12 s.
A QRS axis more negative than -30°: lead I positive, lead II and aVF mostly negative.
A QRS of 0.12 s or more with a deep, broad S (QS or rS) in V1 and a broad R with no Q wave in I, aVL, V5 and V6.
The ECG tool in the app gives you a new 12-lead every time and walks you through reading it: rate, QRS, axis, then where the ST is up and down. It's part of the Paramedic level, and you can try 5 ECGs a day free.
Try the 12-lead drill
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