It is also difficult to state with certainty which of the two P waves is blocked and which is conducted to the ventricles. One possibility, depicted in the upper ladder diagram, is that the P wave superimposed on the T wave is conducted with a PR interval that is markedly prolonged, measuring 0.44 seconds. The other possibility, shown in then lower diagram is that the P wave immediately preceding the QRS complex is conducted with a PR interval of 0.12 seconds.