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Clinical Scenario: The tachycardia recorded in the 12-lead ECG shown in the Figure was obtained from a 73-year-old woman who presented with shortness of breath. What is the rhythm most likely to be?
Interpretation: The first step in determining the cardiac rhythm on any 12-lead ECG is to examine the rhythm strip. A lead II rhythm strip is seen at the bottom of the 12-lead tracing shown here, obtained simultaneously with the recording of the ECG. Lead II is generally preferred as the best single lead for rhythm determination because P waves are usually most easily seen in this lead, and because by definition P waves must be positive in lead II for there to be sinus rhythm (unless there is lead misplacement or dextrocardia).
The lead II rhythm strip in this Figure manifests a rapid and nearly regular rate. The most helpful clue for determining the rhythm lies with the brief pause that is seen toward the end of the tracing (this is the somewhat longer R-R interval between the 6th and 7th to last complexes on the rhythm strip). Atrial activity is clearly lacking except during this pause. Close attention to the baseline during the pause reveals the presence of 3 negative, small amplitude deflections that are regularly spaced from one another in a gentle sawtooth pattern at a rate that is close to 300/minute (the first negative deflection is just before the T wave in the pause; the other 2 follow the T wave). These signs of atrial activity occurring at a rate that is close to 300/minute strongly suggest that the underlying rhythm is atrial flutter. With the possible exception of lead V1 (in which intermittent notching in various parts of the ST segment may represent flutter activity), this would mean that atrial flutter activity (at an atrial rate just under 300/minute with 2:1 AV conduction) is hidden throughout the rest of the tracing. One might seek to confirm this theory by noting the response to a vagal maneuver and/or from empiric use of Adenosine.
There are 2 take-home messages from interpretation of this ECG: 1) atrial flutter is by far the most commonly overlooked cardiac arrhythmia. As a result, one should always maintain a high index of suspicion for this arrhythmia (always assume that the cause of a regular SVT at a rate that is close to 150/minute is atrial flutter until proven otherwise); and 2) when confronted with a difficult-to-interpret tachycardia—look to any pause that may be present in the rhythm for clues to the etiology of the arrhythmia.