To continue a recent theme, we offer this ECG, which appears on quick glance to be an atrial bigeminy. On closer inspection, we do not see ectopic P' waves in the second conducted beats of each pair. In fact, the P waves look the same in both the first and second beats. However, if you look very closely (we enlarged the screen), you may notice that the second T wave of each pair is slightly larger with a slightly different morphology than the first T wave. If this distortion of the second T waves represents a PAC, the PAC is non-conducted and concealed in the T wave.