This is a good ECG for demonstrating sinus brady and first-degree AV block. It shows the sinus node in the process of slowing down. For your more advanced students, there is left axis deviation due to left anterior fascicular block (left anterior hemiblock). The ST segments are flat, suggesting coronary artery disease. The fourth (bottom) channel is a good rhythm strip. Just crop the image. Please refer to Dr. Grauer's interesting post on teaching hemiblocks on our Ask The Expert page.
Left anterior fascicular block
This is a nice, clear right bundle branch block pattern: wide QRS, supraventricular rhythm (NSR), and rSR' pattern in V1. Wide little s waves in Leads I and V6 are also diagnostic. The left axis deviation indicates a left anterior fascicular block, since there is no other apparent reason for the left axis deviation, such as pathological Q waves or LVH. Left anterior fascicular block is a diagnosis of exclusion, also considering that RBBB and LAFB are often seen together (bifascicular block), since the two fascicles have the same blood supply.
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This series of ECGs was obtained from a 60-year-old man who was involved in a one-car accident. He sustained no injuries, but his blood alcohol level was far above the legal limit for intoxication at over 300 mmol/L. ECG No. 1 shows the ECG obtained by paramedics in the field, which they incorrectly interpreted to be atrial fibrillation. No medication was given. The ER physician obtained ECG No. 2, and considered sinus tachycardia as the diagnosis, but also, because of the fast rate and the fact that the rate had not changed for at least 15 minutes, he considered SVT or atrial flutter with 2:1 conduction. The ERP administered diltiazem (Cardizem) to the patient, which resulted in ECG No. 3. The transition to the slower rate was not captured on rhythm strips, but the nurse's notes showed a gradual change over 15 minutes from a rate of 160 to 105/min.
Usually, on the Instructors' Collection ECGs, we like to give the "answer". In this case, however, there will undoubtedly be some discussion regarding what went on. This discussion can be useful if you are teaching intermediate to advanced students. Questions to consider: 1) Is the fast rhythm an SVT and, if so, which one? 2) Is it sinus tachycardia and, if so, what are the effects of the car accident and the alcohol? 3) Is the left anterior fascicular block relevant? (Criteria are left axis deviation, slightly widened QRS complex at 110 ms, no other obvious reason for the axis deviation). 4) Is the ST elevation in the inferior wall during the tachycardia a sign of acute M.I.? The patient was lost to followup, so it is not known whether the ST changes were investigated. Note the flat ST segment and inverted T waves in V1 during the tachycardia that resolve when the rate decreases.
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A good example of aberrantly conducted premature beats (PACs or possibly PJCs) that are conducted with a left anterior fascicular block. The underlying rhythm is sinus at about 80/min. The timing of the premature beats is best seen in the Lead II rhythm strip at the bottom, as this ECG machine does not print the 12 leads in an uninterrupted manner. You will see interruptions each time the leads change.
The first beat on the ECG is one of the premature beats. You can observe the left axis deviation without pathological Q waves. Lead I shows the premature beats with an Rs pattern, and Leads II and III have rS. The early beats have caught the anterior fascicle of the left bundle branch refractory from the preceding beat. It recovers for the normally-timed sinus beats.
Lots of information in this ECG! The underlying rhythm is atrial fibrillation with a controlled rate. The QRS is .12 seconds in duration, with an rSR' pattern in V1 and a wide s wave in Leads I and V6, indicating right bundle branch block. In addition, the axis is leftward - Leads I and aVL are upright and Leads II, III, and aVF are negative. There is no other obvious reason for the left axis shift, and therefore, the diagnosis by exclusion is left anterior fascicular block. RBBB and LAFB often appear together, as the right bundle branch and the anterior fascicle of the left bundle share the same blood supply from the left coronary artery. ALSO, this patient has a right ventricular pacemaker, and is pacing appropriately when the atrial fib slows. Pacer spikes are not readily seen, but the width of the QRS, the axis of the wide QRS complexes (left), and the timing (after a pause) all support the paced rhythm diagnosis. V5 and V6 actually show a very tiny hint of a spike. The T wave inversions seen in the upright leads are common with RBBB, and are usually considered normal in this setting.