Wide, deep Q waves in III and aVF (40 ms or more) with no acute ST elevation: the scar of an infarct that has already happened.
Dead muscle makes no electrical forces, so the first part of the QRS points away from the scar. Over an old inferior infarct that means a Q wave in III and aVF that is pathological: wider than 40 ms (one small box), deeper than 2 mm, or more than a quarter of the QRS. Small, narrow q waves in III or aVR are normal; these are not. The ST is back on the baseline and the T may stay inverted. Q waves tell you something happened, not when, so an old ECG is worth a lot. And a patient with symptoms can still be having a new infarct on top of an old one.
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.
ST elevation in II, III and aVF, with reciprocal ST depression in aVL.
A QRS axis more negative than -30°: lead I positive, lead II and aVF mostly negative.
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.
Sinus rhythm, a normal axis, normal intervals, R waves growing from V1 to V5, and no ST elevation or depression.
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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