ECG Problems?

Question

Please help me with the questions if you can ..

1. How can a series of vectors be used to represent the sequence of depolarisation in the heart?

2. Why are unipolar limb leads used as well as bipolar limb leads?

Thank you!

Answer 1

1. With every heart beat, as the electrical wave spreads through the heart muscles, the direction of the dipolarization changes.  This "sweeping through" can be seen if you have multiple vectors facing different "directions".

Take Lead I as an example.  Because the initial depolarization of the septum is from L to R, this shows up in Lead I as a small negative deflection (Q).  However, the main depolarization of the left ventricular wall is down-and-to-the-right, so this shows up as a big positive deflection (R).

2. All the leads are meant to give doctors more information about the heart.  The bipolar leads (I, II, III) gives vectors of 0, +60 and +120 degrees in the coronal plane, and the unipolar leads (aVL, aVF, aVR) adds -30, +90 and -150 degrees in the same plane.  More vectors give you more information.

Nowadays, the "typical" ECG contains 12 leads of information.  However, as you know, sometimes doctors do 15-lead ECGs.  It is simply convention, rather than solid reason why some leads are chosen.  (In fact, most doctors I know don't even use the aVR.)

Hope this helps.