The Patient This ECG was obtained from a 76-year-old woman who had just lost responsiveness while under care of an EMS crew. This tracing was obtained before a trans-cutaneous pacemaker was applied.
The ECG The most noticeable feature of this ECG is the extremely slow rate. In fact, there are only two QRS complexes visible. The P waves are regular at a rate of about 145 bpm, so the underlying rhythm is sinus tachycardia. NONE of the P waves are conducted, as evidenced by the different “PR intervals” in the two QRS complexes seen. This is complete heart block at it’s most extreme – the QRS complexes represent a very insufficient escape rhythm. A pacemaker is the only immediate treatment for this rhythm. Unfortunately, we don’t know the outcome for this patient.
It is very difficult to make judgements about the QRS complexes with only two of them, visible in three leads each. The first set show the criteria for left ventricular hypertrophy. The QRS in aVL is 20 mm tall, and anything over 11 mm meets voltage criteria for LVH. In addition, the ST segments have the “strain” pattern, another sign of LVH. The second QRS complexes are not typical of a normal ECG or one with LVH. We cannot vouch for the precordial lead placement.
There is some disagreement in the literature regarding the naming of the blocks. Some consider “high-grade” to be only second-degree AVB with two or more consecutive non-conducted P waves. Others include third-degree AVB in the “high-grade” category. It can come down to just semantics – the RATE and how the patient responds to the rate is the primary consideration. Classification as to the origin of the block is helpful when determining the best long-term treatment decisions.
This is a good ECG for teaching the concept of AV dissociation, and how dangerous third-degree AV blocks and high-grade second-degree AV blocks can be.
