2.6 Abnormal Q Waves - ECGs of Healed Myocardial Infarctions

Question 1

Explain how the main 3 changes seen in an MI on an ECG evolve over time as the myocardium heals.

Answer 1

1. ST elevation - becomes flat again and T waves become inverted. ST elevation usually lasts for days to weeks.
2. Q waves appear, even for years after the original MI, due to scar tissue and impaired depolarisation. These may persist for years.
3. T waves - become inverted, may eventually become upright again. These often revert within a year.

Question 2

Explain the evolution of the changes in an ECG secondary to an MI over time.

Answer 2

Question 3

Explain the mechanism of Q wave formation.

Answer 3

Q in the QRS complex usually demonstrates depolarisation in the septum - in the direction towards the negative electrode. However, if enlarged is due to scar tissue - it is an ‘electrically quiet’ zone, where depolarisation is impaired.

Another way to expalin this:

Whilst the electrical stimulus passes through the bundle of His, and before it separates down the two bundle branches, it starts to depolarise the septum from left to right. This is only a small amount of conduction (hence the Q wave is less than 2 small squares), and it travels in the opposite direction to the main conduction (right to left) so the Q wave points in the opposite direction to the large QRS complex.

https://www.nottingham.ac.uk/nursing/practice/resources/cardiology/function/q_wave.php

Question 4

Describe the size of a normal Q wave.

Answer 4

Normal Q waves are <0.04s wide (<1 little box) and <2mm deep.

Question 5

What do the Q waves in this ECG represent?

Answer 5

The Q waves are pathological and located in II, III and aVF.

They indicate an old inferior MI.

Question 6

What do the Q waves in this ECG represent?

What other changes can you see?

Answer 6

Q waves in II, III and aVF demonstrate inferior MI.

T waves are inverted in V3-V6 and aVF - also indicate anterior and lateral involvement.

Question 7

What do the Q waves in this ECG represent?

What other changes do you see?

Answer 7

There are Q waves in V1-V3, indicating previous anterior/septal infarct.

Note that there is still mild ST elevation in these regions! This means perhaps the MI is not so old, i.e. may have occurred in the last few days or weeks.

T waves are also inverted in V4-V6.