The ECG displayed here is from a 24 year old highly trained marathon runner with no evidence of heart disease. It illustrates an atypical form of type 1 2nd degree AV block in which there is continuous variability in the PP, PR and RR intervals.
The PP interval between beats 1 and 2 and between beats 2 and 3 measures 0.84 seconds. It then lengthens to 1.08 seconds between beats 6 and 7.
The PR interval shortens from 0.38 seconds in beat 1 to 0.22 seconds in beat 6. It then lengthens to 0.32 seconds in beat 7 and the 8th P wave is blocked. The next P wave (#9) conducts with a PR interval of 0.16 seconds.
These changes in sinus rate and AV conduction reflect the phasic variations in autonomic balance present in younger individuals and in highly trained athletes.