Atrial Ectopic Beats
Atrial ectopic beats are caused by a premature depolarisation of the atria originating from a focus outside of the sinus node. They are common and benign without any additional indication of heart abnormality.
Ectopics may occur in patterns, such as bigeminy (every other beat), trigeminy (every third beat), or quadrigeminy (every fourth beat). They may also occur in sequences, such as couplets, triplets, salvos (four to seven), or episodes (eight or more).
Because these beats are premature, the RR interval between the ectopic beat and the preceding sinus beat is shorter than the RR interval between two sinus beats. Ectopic beats from the same focus have similar, if not identical, P wave morphologies.
If the conduction through the atria does not reach the sinus node before it depolarises normally, then the sinus node continues to provide the underlying rhythm of the heart. The ectopic beat will still occur early, whilst the natural depolarisation of the node will not conduct due to the depolarised state of the atria. The atrial ectopic beat will be followed by a compensatory delay and the rhythm will then continue as before the ectopic beat.
In the ECG trace below, note the difference in the P wave morphologies (shaded) for the sinus beats (one, three, four, and six) and atrial ectopics (two and five). The underlying rhythm is retained (indicated by arrows) by the compensatory delay.
If the ectopic focus causes the sinus node to be depolarised earlier than would be expected, the natural pacemaker is reset and the succeeding RR interval will be as observed from the underlying rhythm. This is termed a noncompensated atrial ectopic.
In the following example, the RR intervals (shaded) following the atrial ectopic beats exhibit no compensatory delay, indicating that the sinus node has been rest.