The QRS complex labeled “1” is a PVC. There is no preceeding Pwave, and the 'pause' is fully compensatory. This means that the R-R interval for the 2 sinus beats on either side of the ventricular beat is exactly 2 times the normal RR interval. Put another way, the sinus node keeps firing right on schedule, and is unaffected by the PVC (the PVC is blocked from entering the AV junction going backward).
However, there is no normal beat from that middle sinus impulse because it is blocked at the AV junction by the PVC or the refractory state following. The next sinus impulse arises and conducts normally. The deflection labeled '2' is a PAC, distorting the T wave of the preceeding sinus beat. It is conducted normally, leading to a narrow complex beat. If it had occurred earlier in the cycle, it might have been conducted aberrantly, with a wide QRS complex.
NOTE: EKG's are for internal educational purposes of the University of Massachusetts Department of Family Medicine. Please do not forward without permission from Dr. Golding!
