Critical Care Trauma Centre


Question of the Week: October 15, 1999


Identify the following rhythm.



Sinus rhythm with 2nd degree heart block, Wenkebach phenomena (Type I Block)
(P - P regular, R - R irregular,  more P's than QRSs with a pattern of progressive prolongation of the PR interval until one QRS is dropped).

The red arrows indicate where the P's are located - note that the P - P is regular.  In a Wenkebach pattern, the P's are produced regularly, but the transmission of the wave is progressively prolonged at the AV node, making the PR intervals irregular.  This results in an irregular R - R interval.  The lengthening of the PR interval is noted by the horizontal lines.  Note that this pattern of progressive lengthening of the PR interval must be evident from each P wave (dropped beats are identified by vertical yellow line).  The cycle then begins again with the shortest PR interval.  This strip is not long enough to determine whether the block is regular (i.e. a repeating pattern of 5 P's to 4 QRS's - 5:4), or variable (i.e. a varying number of P's to QRS's, for example one 5:4 cycle, followed by a 6:5 cycle, followed by 3:2 cycle).

In sinus rhythm with second degree heart block, there are more P's than QRS's.  In other words, some P waves are conducted, while some are blocked.  The QRS's that are produced are related to the preceding P.  In Wenkebach, the PR relationship is characterized by the gradual lengthening of the interval until one beat is delayed long enough that it fails to get through the AV node.  This specific pattern indicates that the block is an AV node problem - or a Type I - 2nd degree heart block.

Clinical Significance
Generally, clinical symptoms are related to the frequency of dropped beats.  If the patient has a sinus rate of 60 and blocks every 3rd beat, they may be symptomatic (low BP and cardiac output) due to the reduced number of conducted beats (QRS rate would be 40).  If the sinus rate is high enough, and the frequency of dropped beats low, the individual may be asymptomatice.

Because the right coronary artery supplies the AV node in most individuals, Type I - 2nd degree heart blocks frequently occur in conjunction with right coronary artery ischemia, or following right ventricle, inferior wall or inferior - posterior wall infarctions (these walls are frequently supplied by the right coronary artery).  It is often a transient rhythm that resolves with correction of the ischemia .  Temporary pacing may be necessary if the ventricular rate becomes too slow.