On this ECG we see Narrow Complex Tachycardia at a rate of almost 300/min.
The differential for this kind of fast tachycardia would be PSVT (AVRT ot AVNRT) and Atrial Flutter with 1:1 conduction.
You should always think of 1:1 Atrial Flutter when the rate is close to 300/min, and 2:1 Atrial Flutter when the rate is 150/min.
On this second ECG the diagnosis of Atrial Flutter is confirmed.
On this ECG the QRS rate is 150/min with P waves at a rate of 300/min best seen in Lead V1 as positive deflections, and in Lead II as negative deflections, so the rhythm is Atrial Flutter with 2:1 Conduction.
This second ECG is evidence that the rhythm in the first ECG is Atrial Flutter with 1:1 conduction.
READ MORE ABOUT: ECG Interpretation – Summary
