By Ken Grauer, MD
Figure. Rhythm strip from an elderly woman with cardiomyopathy and known bundle branch block.
Clinical Scenario: The rhythm strip shown in the figure was obtained from an elderly woman with a history of cardiomyopathy. She has a known history of bundle branch block (ie, RBBB). How would you interpret the rhythm?
Interpretation/Answer: The 5 key components of rhythm analysis are: i) Assessment of the presence and nature of arial activity; ii) determination if the QRS complex is wide or narrow; iii and iv) determination of rate and regularity; and v) looking to see if atrial activity is related to neighboring QRS complexes. Use of the saying, "Watch your P's and Q's, and the 3 R's" (rate, regularity, related) is an easy way to remember these 5 components.
In this example, P waves are present and precede each QRS complex. However, they are not related to neighboring QRS complexes, because the PR interval constantly changes (ie, P waves aren't "married" to the QRS). Not only is the PR interval too short to conduct, but it blends into the initial part of the QRS complex the latter part of the tracing. The atrial rate is variable. In contrast, the ventricular rate is regular, with an R-R interval of just over 7 large boxes in duration (corresponding to a ventricular rate of about 43/minute). The QRS complex is widened. Whether this reflects an AV nodal escape rhythm with the patient's chronic conduction defect (bundle branch block) pattern or a slightly accelerated ventricular escape rhythm is impossible to tell from this single rhythm strip.
Our interpretation of this rhythm is sinus bradycardia and arrhythmia, with resultant AV dissociation and QRS widening from either an AV nodal escape rhythm with bundle branch block or ventricular escape. The point to emphasize about this example of AV dissociation is that since P waves never have a chance to conduct at any point on this tracing. One cannot determine from this tracing alone if any degree of AV block is present. It could well be that if sinus node activity sped up, then normal AV conduction would resume. Or the patient could be in second or third degree AV block.
Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, is Associate Editor of Internal Medicine Alert.