How Do Myocardial Infarcts Create Q Waves?

Creating a Window into the Heart

The next several posts are extremely important because they may prevent you from missing posterior wall (PW) myocardial infarctions (MIs). 

The posts on May 30 (1) and April 4, 2024 (2) discussed myocardial de/re-polarization.  The key point from those posts was that the heart depolarizes (becomes negatively charged) from the inside out creating a positive vector and an upright R wave in the anterior  electrocardiographic leads.  The heart repolarizes (becomes positive again) from the outside in, but this also creates a positive vector and an upright T wave.  So both the R and T waves generally go in the same direction unless there is left ventricular wall thickening or other issues which change the T vector and may invert the T. 

So what creates Q waves.  If there is an anterior wall MI, the front of the heart is electrically silent.  This allows the electrodes to detect the depolarization in the back wall of the heart.  That vector is going away from the front of the heart and away from the anterior electrodes.  This produces a Q wave as shown in the figure.  The Q wave is much smaller than the R wave it replaces because the back wall of the heart is father away and electricity decreases rapidly with distance.

But as Rhett Butler, played by Clark Gable, said in the 1939 film classic, Gone with the Wind, "Frankly, my dear, I do not give a damn".  Why should you care?  You should care because knowing the window theory of Q waves will help diagnose posterior wall MIs, which are often missed even by good clinicians.  Posterior wall MIs will be discussed more in the next post.

1.https://pauldthompsonmd.substack.com/p/the-r-wave-and-t-wave-are-buddies?u

2. https://pauldthompsonmd.substack.com/p/i-hate-waiting-in-line?