atrial echo

a·tri·al ech·o

electrical reactivation of the atrium by a retrograde impulse returning from the A-V node while the antegrade impulse continues to the ventricle; characterized electrocardiographically, by a pair of P waves enclosing a QRS complex, the second P wave being opposite in polarity (usually inverted in lead II), indicating that it is the reverse (the retrograde pathway) of the pathway of the first P wave (the antegrade pathway).
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If the fast pathway ceases to be refractory while the impulse is still traversing the slow A-V nodal pathway, the impulse may travel retrogradely over the fast pathway to reach the atria, thus producing an atrial echo beat.
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