Solution to ECG from 1/26/2014 Key features to recognize: -She is no longer bradycardic but remains in sinus rhythm. -The PR interval starts out prolonged and continues to increase throughout the ECG. -The 8th p wave does not conduct; the 9th p wave does conduct with the shortest PR interval of the ECG thus far. (Seen most clearly on the V1 rhythm strip). -After that, the PR interval again starts slowly prolonging. If you guessed Wenkebach on the patient’s first ECG from 1/22, give yourself a “hindsight is 20-20” pat on the back. The correct answer is B. Three days ago, she had second-degree AV block with 2 to 1 block. Now the block is happening so infrequently you only see it once on a 10-second ECG; it is at least 8 to 1 conduction if not more. The features on this ECG are classic for Wenkebach: the gradually lengthening PR interval, the dropped beat, followed by “resetting” of the PR interval. It’s worth noting the clinical picture also matters – if you increase the heart rate of a Wenkebaching patient (with exercise, dobutamine), the block will become less frequent as parasympathetic tone is overtaken by sympathetic drive. In a Mobitz Type II patient, the opposite occurs – the block WORSENS, as the sinus rate increases but not the conduction through the bundle of His. That patient will become very sympathetic with exertion and needs a pacemaker. Choice A is incorrect – It’s true that even the shortest PR interval is prolonged, but the dropped beat makes this at least 2nd-degree AV block. Choice C is incorrect – Because the problem is structural, there is no change in the PR interval to reflect parasympathetic tone; just dropped beats from failed infranodal conduction. Choice D is incorrect – At first glance, there might appear to be AV dissociation in the first half of the ECG since the PR interval elongates so slowly. But after the dropped beat, there is clear conduction with a lengthening PR interval that is classic for Wenkebach. Next ECG in the next 24 hours. Keep up the good work!
Posted on: Wed, 29 Jan 2014 01:21:58 +0000
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