On a 12-lead ECG, the net QRS deflection is equally tall and positive in lead I and lead aVF, with intermediate amplitudes in all other limb leads. What is the mean electrical axis of ventricular depolarization?

• A +30 degrees
• B +90 degrees
• C +60 degrees
• D +45 degrees ✓

Explanation

Lead I looks along 0 degrees and lead aVF along +90 degrees. When the net deflection is equally positive in both leads, the mean vector lies exactly midway between them, at +45 degrees, which lies within the normal adult range of roughly -30 to +90 degrees. An axis of +60 degrees would show its tallest positive deflection in lead II, and +90 degrees would make lead aVF clearly dominant over lead I.

Reference: Ganong's Review of Medical Physiology, 26th ed.

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