It is even possible that in the upper tracing (from the 23 year old disscussed in the preceding 2 pages), the sinus node rate is actually 88 per minute and that there is a fixed 2:1 relationship between the sinus node discharge (arrows) and atrial activation (i.e. 2nd degree SA block). Indeed, neither 1st degree nor fixed 2nd degree SA block can be made from the body surface ECG. In younger patients it is more likely that the bradycardia is due to slowing of the sinus node rate rather than to 2:1 SA block. In elderly individuals, the reverse is more likely.
The lower of the 2 tracings shown above is from an 83 year old male patient, The rate of 40 shown here is probably the result of 2:1 SA block but this diagnosis can not be established with certainty from this ECG.