Of the six limb leads produced by the ECG machine (electrocardiograph), are you aware that only TWO of those leads are actually recorded from the patient? Were you also aware that the other four limb leads are generated by the machine itself? Leads I and II are the only two leads actually recorded from the patient. Leads III, aVR, aVL and aVF are calculated and generated by the electrocardiograph’s computer.
Without going into a lot of history and physiology, just know that a cardiac researcher and physiologist named Wilson attempted to create separate leads out of the LA, RA and LF electrodes. Each one would be a positive electrode with a shared, fixed negative pole created mathematically by the machine. He did not achieve his goal – but he came very close. Some years later, a scientist by the name of Goldberger realized what the problem was and corrected it. Now each of the limb electrodes (LA, RA, LF) is a lead in and of itself (aVL, aVR, aVF) – separate from Leads I, II and III.
Believe it or not, this makes understanding the way Lead III acts a lot simpler because we can now see that Lead III actually equals aVF – aVL. Just so you know there’s no magic here, this is how we arrive at that equation:
We begin with Einthoven’s Law (Equation) II = I + III or LF – RA = (LA – RA) + III
Next, we substitute the augmented leads for the electrode positions… aVF – aVR = (aVL – aVR ) + III
We moved (aVL – aVR ) to the other side and changed the signs… aVF – aVR – aVL + aVR = III
The positive aVR and the negative aVR cancel each other… aVF – aVR – aVL + aVR = III
Turning this equation around… III = aVF – aVL
Now comes the part that will help us understand the idiosyncratic actions of Lead III. Let’s just consider the QRS complexes for now…
When the QRS in aVF is positive and the QRS in aVL is negative, the QRS in Lead III will be positive.
When the QRS in aVL is positive and the QRS in aVF is negative, the QRS in Lead III will be negative.
If the QRS complexes in both aVF and aVL are positive, but aVL is more positive than aVF (i.e., has greater amplitude), the QRS in Lead III will be negative. The QRS in Lead III can be negative in spite of the fact that the QRS complexes in both aVF and aVL are positive!
If the QRS complexes in both aVF and aVL are negative, but the QRS in aVF is less negative than in aVL, the QRS in Lead III will be positive. The QRS in Lead III can be positive in spite of the fact that the QRS complexes in both aVF and aVL are negative!
A previous infarction can certainly affect this relationship. If there is a Q wave in Lead III and a Q wave in aVF and/or Lead II, then there is most likely a previous MI present. But if a Q wave is present only in Lead III, check Leads aVF and aVL to see if they can explain the morphology in Lead III.