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Atrial Bigeminy



Notice the pattern: a pair of beats, each of the pair fairly normal looking, occur close together, then a pause, another pair of beats, and so on.  The cause of the pattern: every sinus beat is followed by a PAC. In effect, there are two atrial pacemakers acting in alternation here.  Every beat has a P-wave initiating it, but notice that there are two different and alternating P-wave shapes, and two alternating P-R intervals as well.  The first type of P wave is followed by P-R intervals of 0.14 second.  The intervals between these R-waves and the next P-waves (the R-P intervals) are 0.65.   The P-R intervals following the second type of P-wave are 0.17, and the R-P intervals following are 1.10.  Notice: a shorter R-P interval produces a longer P-R interval.  Why?  The A-V node can be relatively refractory: it can transmit an impulse after a delay, and this delay can vary with how refractory it is.   The second beat in each pair demonstrate this: the shorter R-P means that the atrial impulse reaches the A-V node while it is still partially refractory, so the P-R interval is longer.  Finally, take note of the widened QRS (0.12 second) and the upward deflection in the V-lead:  these indicate a right bundle branch block.