Physiologic Basis of
the Abnormal ECG:
Disorders of Rhythm
Created by Diane R. Karius, Ph.D.
If you will recall from the lectures on the cardiac action potential, the cells in the different parts of the conduction pathway of the heart also show a pacemaker potential (prepotential). This means that they may take over from the SA node under the right conditions. In addition, ischemia can cause even the cardiac myocytes to become the heart's pacemaker. We will now discuss what the ECG looks like in those situations when the SA node is NOT the pacemaker for the heart.
You may think this is a rare occurrence but, in fact, we all experience this on a daily basis in the form of premature atrial contractions (PAC) and premature ventricular contractions (PVC). In both of these cases, a non-SA nodal cell in the the atria (PAC) or the ventricles (PVC) triggers the heart to contract on a single beat basis.
Premature Atrial Contractions: As noted above, these arise when an ectopic focus in the atria trigger the heart's beat. Because this starts in the atria, the bulk of the ECG looks normal - there will be a P wave before the QRS complex which comes before a T wave. Depending on the location of the ectopic focus in the atrium, the P wave may or may not look like the other P waves, but after that, the depolarization travels through the conduction pathway (AV node to Bundle of His to Purkinje fibers to ventricular myocytes) in a manner identical to what would happen if the SA node had started things. The following illustrations show you a PAC and how to distinguish it from the normal beats:
Premature Atrial Contraction: The green arrow illustrates a premature atrial contraction (beat). As you can see, the cardiac cycle looks basically normal (sometimes the P wave is not the same shape as other P waves) and these can be hard to pick out of a normal ECG. The identifying feature: The R-R interval between the PAC and the preceeding beat is shorter than any of the other R-R intervals, as shown below. In the figure below, the R-R interval of the normal beats (represented by the blue bars) is fairly consistent. The pink bar represents the R-R interval of the PAC and the preceeding beat is clearly shorter than normal.
To summarize, a PAC is characterized by:
- a normal progression from P to QRS to T wave
- the P wave may or may not look the same as other P waves in the recording (this is dependent on where the ectopic focus is in the atria)
- the PR interval is normal
- the RR interval between the PAC and the preceeding beat is shorter than normal.
Premature ventricular contractions: The good news is that premature ventricular contractions are EASY to spot - Here's a picture of one for us to work with:
I am certain that even without the 'asterisk sign', you could easily have pointed to that particular cardiac cycle on the ECG and told me that something 'happened' there! In the case of a PVC, the cardiac cycle is initiated by a ventricular source - usually a bit of myocardial tissue that is a little irritable (usually for no particular reason). Because we started with a ventricular myocyte, the depolarization spreads through the ventricles using cell-to-cell transmission rather than using the conduction pathway. My favorite analogy for this: the conduction pathway is the express bus - so it gets to where it is going quickly, while a ventricular ectopic focus is the local bus - we're going to have to stop at every corner along the way before we get to where we are going. This highlights one important distinction between the normal conduction pathway and a ventricular ectopic focus: the time it takes to depolarize the ventricle is greatly prolonged if you don't use the regular conduction pathway. Because of this, the QRS complex is very long (on an ECG, that produces a wide or a broad wave that doesn't come to a nice little point). Notice also that the T wave is very different than the other T waves. Since we did not depolarize the ventricle in the normal order (from septum to outer walls), we are not going to repolarize in the standard sequence (the plateaus won't be the right lengths. As you might expect, the RR interval is shorter than normal, just as we saw with the PAC. However, there are a few other differences... In a PVC, the RR interval between the PVC and the beat that came before it is shorter than normal (pink bar on top - the blue bar on the bottom is the length of the normal RR interval from preceeding cycles). Notice, however, that the RR interval between the PVC and the following beat is longer than normal (red bar on top). This is called a compensatory pause. The compensatory pause is the result of the fact that the SA node (which was driving the heart prior to this beat) depolarized as scheduled (somewhere in the middle of the PVC) but that depolarize didn't travel into the ventricles because the AV node was in its refractory period. The next normal beat (end of the red bar) doesn't occur until the SA node goes through another complete cycle, producing the pause.
Characteristics of a PVC
- RR interval is shorter than normal
- There is no P wave prior to the QRS complex (after all, this beat started in the venticle)
- The QRS complex is prolonged and bizarrely-shaped (because the depolarization did not use the conduction pathway).
- The T wave is abnormal (if depolarization doesn't follow the normal pathway, repolarization does not).
- There is a compensatory pause after the PVC.
As I noted above isolated PVCs are normal. Their frequency increases if the myocardium is ischemic (too little oxygen means not enough ATP to maintain a normal membrane potential).
The preceeding events were single events where an ectopic pacemaker temporarily 'stole' command of the heart from the SA node. We can also have periods (long or short) where the ectopic pacemaker takes over from the SA node completely. The electrocardiographic characteristics are generally the same, except that we have no normal beats to compare the RR interval to. These events can be characterized based on where the ectopic focus is:
- Supraventricular tachycardia: this is an atrial pacemaker that has taken over from the SA node. Since this is an atrial pacemaker, we see the usual sequence of waves with normal durations and appearances, except the P wave may be abnormally shaped. If the rate is high enough, the AV node may begin to fail to conduct, so we may see P waves that aren't followed by QRS complexes (I say 'may' because usually the heart rate at that point is so fast that the P waves and the T waves are happening 'on top' of one another. They can be very difficult to distinguish at that point. It is often difficult to decided between supraventricular and sinus tachycardia, especially at very high heart rates for the same reason - the shape of the P wave (which helps you decide) can be obscured by the T wave.
- A hint: If you get lucky, there may be a random slowing between two cardiac cycles. Look for those brief pauses between two beats - and check for a P wave. Hopefully, the rate will be just slow enough that you can separate the T waves from any potential P waves.
- Ventricular tachycardia: in this case, a ventricular ectopic focus has taken over from the SA node. Therefore, we see QRS complexes and T waves that are bizarrely shaped and prolonged. The P waves (if you can see them) have no relationship to those QRS complexes. The following picture is an example of ventricular tachycardia:
Ventricular tachycardia in an ECG - note that the QRS complexes are bizarrely-shaped with abnormal T waves. P waves (which aren't easily visible) have no association with the QRS complexes/T waves.