Why is this atypical atrial flutter from the left atrium?
We see a regular rhythm with narrow QRS complexes and a heart rate of approx. 75 bpm. Between 2 QRS complexes we see 4 atrial actions, so the atrial rate is about 300 bpm. At such high heart rates, we no longer speak of atrial tachycardia but of atrial flutter. How can the left atrial origin be derived? Lead I goes from the right to the left arm, with the positive electrode on the left arm. If the direction of excitation of the atria is the same as the vector of the lead, (toward the positive electrode), you see a positive flutter wave. So, when the atria are depolarizing from right to left, Lead I will have upright P waves. When the atrial flutter is from the left atrium, Lead I will have negative P waves. Since Lead V1 is on the patient's right side, it is the other way around: V1 will often (not always) have negative flutter waves with right atrial flutter, positive flutter waves with left atrial flutter.
If these findings are applied to this ECG, then it is clear that it must be left atrial flutter.