ECG Review-Shifting Sites
By Ken Grauer, MD
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Clinical Scenario: The ECG in the Figure was obtained from an asymptomatic 76-year-old woman. How do you interpret this tracing?
Interpretation: The most interesting part of this 12-lead ECG is the rhythm strip that appears at the bottom of the tracing. The rhythm is clearly irregular. Atrial activity is present, but it does not remain constant throughout the tracing. The obvious question is how to interpret the rhythm?
The easiest approach to arrhythmia interpretation when confronted with changing atrial activity and an irregular ventricular rhythm is to look first for the presence of an underlying rhythm. Recognition of the fact that the underlying rhythm in this case is sinus greatly facilitates interpretation of surrounding nonsinus activity. The complexes marked X (and possibly also the complex just before the first X-labeled beat) all occur at a regular rate (of about 75 beats/minute), and are all preceded by a similar looking upright P wave with a constant PR interval. This presumably reflects an underlying sinus rhythm. Atrial beat morphology subtly changes with the P wave marked Y, and beginning with the P wave marked Z is seen to take on a taller, more peaked shape for the remainder of the rhythm strip. The site of atrial pacemaker activity has therefore shifted from the sinus node to some other atrial site which manifests a similar P wave morphology to that seen at the very beginning of this tracing (ie, prior to the X-marked sinus beats).
The rhythm strip findings we have just described suggest a wandering atrial pacemaker as the etiology of the rhythm, in which P wave morphology intermittently changes reflecting rotation of the site of atrial pacemaker activity. An alternative explanation might be that the gradual acceleration of peaked P wave complexes toward the end of the rhythm strip reflects a "warm-up" phenomenon of an ectopic atrial tachycardia (EAT) arising from increased automaticity of some ectopic atrial site. Additional rhythm strip monitoring is needed to clarify this possibility.
Apart from the rhythm, nonspecific ST-T wave abnormalities are present in multiple leads on this ECG, but there are no acute changes. This tracing therefore provides a nice illustration of how helpful simultaneously recorded leads may be for assisting in complex rhythm interpretation, since the etiology of this arrhythmia would not be evident from inspection of the 12-lead ECG shown here without the accompanying rhythm strip.
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