This strip shows a second-degree AV block. During most of the strip, 2:1 conduction is present. At the beginning, however, two consecutive p waves are conducted, revealing progressive prolongation of the PR interval. This usually represents a Type I , or nodal, block: progressive refractoriness of the AV node. However, the wide QRS ( possibly left bundle branch block), and the fact that the non-conducted p waves are "out in the open" where they should have conducted, points to Type II - an intermittant tri-fascicular block. Wenckebach periods in patients with LBBB can be caused by progressive conduction delay in the right bundle branch.
Second-degree AV block with 2:1 conduction
This interesting ECG is a great one for your more advanced students who are ready to discuss the anatomical and physiological differences between the AV blocks, as opposed to just measuring PR intervals. It shows a sinus rhythm with an atrial rate of 72/minute. Second-degree AV block causes every other p wave to be blocked, resulting in a pulse rate of 36 beats per minute. In addition, the ECG shows right bundle branch block, as evidenced by the wide QRS (136 ms), rsR' pattern in V1, and the wide little S wave in Lead I.
When second-degree AVB conducts 2:1, it can sometimes be difficult to determine if the block is Type I (occuring above the Bundle of His), or Type II (occuring at or below the Bundle of His). This is because two p waves must be conducted in a row to see the tell-tale progressive prolongation of the PR interval seen in Type I (Wenkebach).
Two clues that this block is Type II are: 1) the presence of right bundle branch block. Type II blocks are sub-Hisian blocks, often in the fascicles, and the right bundle branch block is a fascicle block. Many Type II AV blocks show signs of right bundle branch block; 2) The non-conducted p waves occur well clear of the refractory periods of the preceding beats. In Type I blocks, the QRS is eventually dropped because the p wave occurs in the refractory beat of the preceding QRS. Only one beat is missed. In Type II blocks, p waves that SHOULD have conducted, don't. Sometimes, more than one p wave in a row will be non-conducted.
All our content is FREE & COPYRIGHT FREE for non-commercial use
Please be courteous and leave any watermark or author attribution on content you reproduce.