The patient:  This ECG was obtained from a 91-year-old woman who was complaining of weakness.  Unfortunately, we have no other information. 

The ECG:  This ECG has something for your basic students, and even more for the more advanced learners.  The first thing  that anyone should notice is the slow rate.  The ventricular rate is around 35 bpm, and regular.  If the patient is showing signs of poor perfusion, we would stop here and prepare to increase the rate with a temporary pacemaker (transvenous or transcutaneous). Why is the rate so slow?  There is no P wave in front of each QRS, so this is not sinus bradycardia.  Rather, we see P waves at a rate of approximately 100 bpm, wit a very regular rhythm.  Beginners should “march out” the P waves with calipers or by marking a straight edge piece of paper.  There are 15 P waves on this ECG – some are buried within QRS complexes (QRS #3) or T waves (QRS #4).

 Because there are two distinct, regular rhythms, but they do not track with one another, we know this is possibly third-degree AV block (complete heart block).  Another clue is that there are no steady, repetitive PR intervals, which means there is no relationship between the atrial rhythm and the ventricular rhythm.

 For more advanced learners, it is helpful to try to identify the origin of the escape rhythm.  If it is junctional, the AV block is above the junction.  If the escape is ventricular, the AV block is below the junction.  A junctional rhythm is usually between 40 – 60 bpm, with a narrow QRS.  Ventricular escape rhythms are usually less than 40 bpm and with wide QRS complexes.   This ECG will be a little challenging on this front, because the rhythm has some characteristics of junctional rhythm and of ventricular rhythm.

This two-lead rhythm strip shows a normal sinus rhythm at about 63 bpm.  The P waves are regular. After the sixth P-QRS, there is a non-conducted P wave.  The normal rhythm then resumes.  The two most common reasons for a non-conducted P wave in the midst of a normal sinus rhythm are 1) non-conducted PAC, and 2) Wenckebach conduction. The first is easy to rule out.  The non-conducted P wave is not premature, so it is not a PAC.  The second one is a little harder when we only have a short strip to look at.  We are conditioned to look for progressively-prolonging PR intervals until a QRS is "dropped".  In this case, the progression is in very tiny increments that are hard to see unless you zoom in and measure.  But they ARE progressively prolonging.  An easy hack:  measure the last PRI before the dropped beat and the first one after the pause.  You will see that the cycle ends on a longer PRI (about .28 seconds) and the new cycle starts up with a PR interval of about .20 seconds.  Fortunately, this conduction ratio will have very little effect on the patient's heart rate.

The patient:  Unfortunately, we no longer have information on this patient, other than the fact that she went to the OR for a permanent pacemaker implantation.

The ECG:  The atrial rate (P waves) is 99 beats per minute. The P waves are regular and all alike (NSR). The ventricular rate (QRS complexes) is 33 bpm, and the QRS complexes are regular and all alike. The PR intervals, when A-V conduction occurs, are 162 ms (.16 seconds) and all alike. The QRS complexes are wide, at 122 ms (.12 seconds). There is right bundle branch block, but no left hemiblock, as the frontal plane axis is normal. The QTc is prolonged at 549 ms.  Many ST segments on this ECG have a “flat” appearance, rather than the normal concave up shape.

The failure of 2 out of every 3 P waves to conduct indicates a second-degree AV block. Type I is a block of the AV node, with progressive prolongation of the PR interval until ONE P wave fails to conduct. Type II AV block is a block of the intraventricular conduction system. Clues that a second-degree AV block is Type II include:

·        The PR intervals are all alike.

·        More than one consecutive P wave is not conducted.

·        A P wave that is NOT in the refractory period of the preceding beat is not conducted.