A 55-year-old man underwent an ECG at an annual physical, and his net deflection (R wave minus Q or S wave) in standard limb lead I was -1.2 millivolts. Standard limb lead II has a net deflection of +1.2 millivolts. What is the mean electrical axis of his QRS?

**Core Concept**
The mean electrical axis (MEA) of the QRS complex represents the average direction of ventricular depolarization. It is calculated using the net deflections in standard limb leads I and II.

**Why the Correct Answer is Right**
To determine the MEA of the QRS, we use the formula: MEA = (°) + 30° × (lead I deflection / lead II deflection). In this case, lead I deflection is -1.2 mV and lead II deflection is +1.2 mV. Plugging in the values, MEA = (°) + 30° × (-1.2 / 1.2) = -30°. This indicates that the ventricles are being depolarized from a left-to-right direction.

**Why Each Wrong Option is Incorrect**
**Option A:** This option would result from a calculation error or incorrect lead deflection values. A MEA of +30° would indicate right-to-left ventricular depolarization, which is not consistent with the given lead deflections.

**Option B:** A MEA of -90° would indicate that the ventricles are being depolarized from a right-to-left direction, which is opposite to the calculated value.

**Option C:** A MEA of +90° would indicate that the ventricles are being depolarized from a left-to-right direction, which is consistent with the calculated value, but this option is not the correct answer.

**Clinical Pearl / High-Yield Fact**
In clinical practice, the MEA can be used to diagnose conditions such as left ventricular hypertrophy (LVH), where the MEA is typically greater than +90°, or right ventricular hypertrophy (RVH), where the MEA is typically less than -30°. A normal MEA is between -30° and +90°.

**Correct Answer:** C. -30°

✓ Correct Answer: D. +120 degrees