Nice illustrative tracing by Dawn. I agree entirely that best policy is to first describe what is seen rather than venturing diagnoses. As per Dawn - there is a regular SVT at ~ 130/minute. There is atrial activity - but it is clearly not sinus activity since there is no consistent upright P wave in lead II. So best initial description of this rhythm is: "a regular SVT @130/minute without clear sinus activity".

• By way of perspective - it is helpful to remember the principal differential diagnosis for a regular SVT of uncertain etiology. Think of 3 entities as the most common causes: i) Sinus Tachycardia; ii) Atrial Flutter; and iii) PSVT (= AVNRT). These 3 make up >90% of what most providers encounter. Sinus Tach is not present here - because it looks like atrial activity before the QRS in lead II is negative. PSVT is also unlikely - because it looks like there is "extra" atrial activity - possibly 2 P waves for each QRS.
• This leaves us with AFlutter vs the possibility of Atrial Tachycardia with 2:1 conduction. I vote for AFlutter - because the rhythm "looks like flutter" in the long rhythm strip. Leads III and aVF simply show a sawtooth pattern characteristic of AFlutter.
• Calipers are helpful to confirm this. Set your calipers at precisely HALF the R-R interval. Then try to walk out this interval in each of the 12 leads on the tracing. In this ECG - I believe the negative point in lead II walks out perfectly. It is admittedly more difficult to walk out atrial activity in other leads - but the sawtooth pattern and being able to precisely walk about 2:1 atrial-to-ventricular activity in lead II define the rhythm. Further confirmation could be forthcoming with a vagal maneuver - but the pattern seen is characteristic enough that a vagal maneuver is probably not essential for diagnosis.
• Precise determination of rate becomes important in the differential diagnosis of reguar SVT rhythms. The easiest way to determine rate is by the "every-other-beat" method. Find a QRS that begins or ends on a heavy line. This occurs multiple times during the long rhythm strip. Then calculate the rate of "every-other-beat". The R-R interval for 2 beats is just under 5 large boxes. Therefore - HALF the ventricular rate is just over 60/minute (ie, ~ 65/minute). Since half the rate ~ 65/minute - this means that the actual rate must be twice this = 130/minute for the ventricular rate. Since there is 2:1 conduction - this means that the atrial rate = 260/minute. While untreated AFlutter most often presents with 2:1 AV conduction and a ventricular rate ~150/minute (300/2) - the range for atrial activity with untreated AFlutter ~ 250-350/minute - so an atrial rate of 260/minute IS within this range.
• The differential diagnosis between AFlutter vs ATach can be challenging. Newer theory suggests many cases overlap. Although slightly faster atrial rate and sawtooth vs flat baseline have traditionally been diagnostic criteria for differentiating between AFlutter vs ATach - newer findings suggest that neither rate nor baseline characteristics are definitive for distinguishing atypical forms of AFlutter from ATach. Clinically - this distinction is usually not of great importance - since both AFlutter and persistent ATach typically merit referral to EP for study and potential ablative cure.

Otherwise - while there may be some ST elevation in anterior precordial leads on this tracing - the ST segment looks to be of upward concavity and less like an acute injury pattern. Overlap with atrial activity probably contributes to the ST segment abnormality. Repeat ECG after termination of the tachycardia should be revealing. I suspect that precordial ST segments will not be of concern.

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