The recording on the left is in sinus rhythm and the one on the right in supraventricular tachycardia (AVNRT) (The X axis is in 25ms divisions.).
The coronary sinus (CS) recordings indicates how the left atrium is depolarized.
Note:
The excitation of the left atrium is from the A/V node (CS proximal (blue arrow) to CS distal (turquois arrow) in both sinus rhythm and AVNRT
In AVNRT the right atrium (RA {yellow arrow}) is excited last.
