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.
· There is evidence of bundle branch (fascicular) block.
This is a second-degree AV block, Type II. Type II blocks are considered to be intermittent tri-fascicular blocks. That is, one or two of the three main fascicles are blocked, and the remaining one or two are intermittently blocked. When conduction proceeds down the working fascicle(s), there is a QRS complex. When the intermittent block occurs, all three fascicles are blocked, producing a temporary complete heart block (trifascicular block). In this example, it appears that the right bundle branch (one fascicle) is consistently blocked, and the left bundle branch (two fascicles) is intermittently blocked.
The most immediate concern for any patient with an AV block is rate. It is important to assess the patient’s perfusion status to determine if the rate is fast enough to maintain BP. Type I AV blocks often respond to atropine, but Type II AV blocks do not, and it may even make them worse. The safest approach is a temporary pacemaker, either transcutaneous or transvenous, to stabilize the patient. Type II AV blocks usually require a permanent implanted pacemaker, as they tend to worsen and lead to third-degree AV block with ventricular escape (rate < 40 bpm) or ventricular standstill.