[SHOW JUST THE TOP STRIP AND LADDER DIAGRAM] The ECG strip and accompanying ladder diagram shown here is an example of Mobitz type I 2nd degree AV block with Wenckebach periodicity. It is from a 25 year old male who feinted after donating a pint of blood. There is sinus bradycardias (rate of 50), progressive prolongation of the PR interval (0.24, 0.28 and 0.44 seconds) folowed by a P wave that is blocked within th AV junction. This results in 4:3 AV block. Note that the RR interval encompassing the blocked P wave of 2.10 seconds. is less than twice the preceding RR interval (2 x 1.3 =2.6 seconds) This is characteristic of type I 2nd degree AV block which, in this patient, was due to an increase in vagal tone associated with the phlebotomy.