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.
· The QRS is wide. Normally, that finding favors the diagnosis of ventricular rhythm, but interventricular conduction delays like bundle branch block can widen a junctional QRS. There is a small R’ in V1 and a wide little S wave in I and V6, favoring the diagnosis of right bundle branch block.· The frontal plane axis is abnormal. This can be a sign of a ventricular rhythm, because the axis reflects the direction of the electrical flow in the ventricles. But, this left axis deviation is also seen in left anterior fascicular block, which is frequently paired with RBBB.· The slow rate is just about on the border between the intrinsic rates of the junctional and ventricular pacemakers. For those who care for patients in an emergency or primary care setting, it is important to emphasize that the origin of the block is not as important as supporting the patient’s need for rate as part of the important cardiac output equation. Another consideration for this patient is “what caused the AV block”? She is very old, and that certainly is a factor. But, on close inspection, we see ST changes. Specifically, there are flattened, slightly depressed ST segments in II, III, and aVF. AVR has the same flat shape, with some elevation. V1 has a very subtle ST changes, as well. This is a sign of diffuse ischemia (chronic or acute). The machine read the QTc as prolonged, but I measured using an online measuring tool, and got 464.8 with the Bazett equation.
ECGs should always be approached in a systematic manner. When possible, patient presentation and symptoms should inform one’s decisions.
EDIT: Please read the comment below from David Richley and Ken Grauer. They bring up a very good argument for "high-grade" AVB, which is a good conversation to have with more advanced students.
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