Block Party – ECG Case #5

A 68-year-old male with morbid obesity and no past medical history comes to the office complaining of fatigue and shortness of breath. He does not feel dizzy and has never passed out. He takes no medications and all of his laboratory evaluations are completely normal, including electrolytes, hemoglobin and thyroid studies. His ECG is below:

We see here that the heart rate is a bit slow. Remember how to determine heart rate on ECG. Looks like about 42 beats per minute. While that is indeed slow, quite often someone can actually have a heart rate in the 40s without any symptoms.

Recall that the symptoms of bradycardia do include fatigue and shortness of breath, but sometimes also congestive heart failure symptoms such as lower-extremity edema, paroxysmal nocturnal dyspnea (waking from sleep short of breath) and orthopnea (dyspnea while laying flat).

OK then, what is the diagnosis? There is a P wave before each QRS complex and the P wave is upright in lead II, thus it is a sinus rhythm. You can clearly see a second P wave after each QRS complex; however, this second P wave is NOT followed by a QRS, and thus is "blocked" or "non-conducted.” This ECG shows conduction of one P wave for each two P waves that are present, so it is called 2:1 AV block or sinus rhythm with 2:1 AV block. Here is a picture showing the conducted and non-conducted P waves:

What do we do about this? Is it the cause of his symptoms? Well, 2:1 AV block is actually a form of second-degree AV nodal block and occurs when every other P wave is not conducted through the AV node to get to the ventricles. Thus, every other P wave is NOT followed by a QRS complex, as we see here.

There are only two types of second-degree AV block, and thus 2:1 AV block can possibly be from either second-degree type I AV nodal block (Wenckebach) or second-degree type II AV nodal block. This distinction is crucial since the second-degree type I AV block is usually benign and does not usually cause symptoms, while the later requires implantation of a permanent pacemaker.

A general rule to remember is that if the PR interval of the conducted beat is prolonged AND the QRS complex is narrow, then it is most likely second-degree type I AV nodal block (Wenckebach). Alternatively, if the PR interval is normal and the QRS duration is prolonged, then it is most likely second-degree type II AV block and a pacemaker is probably warranted, which is actually what the ECG in this case looks like.

But a pacemaker is a big deal, and we don't want to put one in every patient unless we are sure they had irreversible symptomatic bradycardia. Therefore, how can we tell if our patient has second-degree type I or second-degree degree type II AV block?

Remember that second-degree type I AV nodal block is an issue in the AV node itself, which is subject to sympathetic and parasympathetic tone, while second-degree type II AV block is an "infranodal" conduction disease of the His-Purkinje system. Therefore, altering AV nodal conduction would have no effect.

To distinguish between the two potential rhythms when an ECG reveals 2:1 AV nodal block, a few different maneuvers can be employed:

  1. Carotid sinus massage or adenosine. This slows the sinus rate and allows the AV node more time to recover, thus reducing the block from 2:1 to 3:2 and unmasking any progressing, prolonging PR intervals that would indicate second-degree type I AV nodal block.
  2. Atropine administration. This enhances AV nodal conduction and could eliminate second-degree type I AV nodal block since it is due to slowed AV nodal conduction.
  3. Exercise ECG testing. This enhances AV nodal conduction and could eliminate second-degree type I AV nodal block since it is due to slowed AV nodal conduction.

This patient was put on a treadmill and this was the result:

Notice there is NO prolongation of the PR interval, then just a "dropped" or "non-conducted P wave.” He became very short of breath after just 2 minutes on a Bruce protocol and exercise had to be stopped. He received a pacemaker and all of his symptoms completely resolved.

- by Steven Lome, DO, RVT