ILLUSTRATION of the PASSAGE of the ELECTRICAL IMPULSE to the LEFT LATERAL ACCESSORY PATHWAY from the SINUS NODE in the ORTHODROMIC TACHYCARDIA

1. Initially, the sinus node (in green color at the top of the right atrium, left side of picture) is excited, as indicated by the glowing yellow color in the sinus node. The excitation wave spreads down the right and left atria to reach the AV node, in which the fast channel is blocked, as indicated by the red color. So the excitation wave spreads to the green left accessory bundle in the left ventricular lateral wall, which in turn is excited as indicated by turning yellow in color. It sends an excitation impulse to the open slow channel (indicated in yellow) of the AV node.

2. As this excitation goes through the AV node and down the Right and Left Bundles to the bottom of each ventricle and completes excitation of the Ventricles by sending the excitation wave from the bottom of each ventricle up to the top of each, the red block in the fast channel of the AV node is still present.

3. Disappearance of red block in fast channel of the AV Node, which changes to green (normal state of rest) and hence is ready to allow the excitation wave from the top of the ventricles to pass retrograde through AV Node to excite the atria !

4. Passage of electrical stimulus (yellow)retrograde through the fast channel of the AV node to stimulate the atria retrogradely, since there is no Block (indicated by the absence of the red color and the change to a yellow color)!

5. Completing excitation of atria and Sinus Node in a retrograde fashion with the excitation waves reaching the top of the atria.