It means that either the left atrium chamber is bigger that it should be, or the left atrial wall is thickened for some reason. The 'notched' P-wave that looks like am M is like that because of the enlargement. If it is slurred upward with a notch, it is usually W-P-W (Wolff-Parkingson-White) I don't understand why you say the EKG showed a Y. You would need to ask the pediatric cardiologist to explain the "WHY" of this abnormal EKG.
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