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?
The net deflection of QRS wave plotted on lead I is -1.2 millivolts, and lead II is +1.2 millivolts. So the absolute value of the deflections is the same. Therefore, the mean electrical axis must be exactly halfway in between these two leads, which is halfway between the lead II axis of 60 degrees and the lead I negative axis of 180 degrees, which provides a value of 120 degrees
-30 degrees
+30 degrees
+60 degrees
+120 degrees
ECG