Final answer:
Type 1 second-degree heart block involves progressive lengthening of the PR interval until a QRS complex is missed, while type 2 second-degree heart block involves consistent PR intervals but occasional blocked impulses without progressive lengthening.
Step-by-step explanation:
The difference between types 1 and 2 second-degree heart block lies in the extent to which impulses from the SA node reach the AV node. In type 1, also called Mobitz type 1 or Wenckebach, there is a progressive lengthening of the PR interval until an impulse is blocked and a QRS complex is missed. In type 2, also called Mobitz type 2, there is a consistent PR interval but occasional blocked impulses without progressive lengthening.
For example, in type 1 second-degree heart block, the PR interval may progressively lengthen from 0.18 seconds to 0.2 seconds, then to 0.24 seconds, until a QRS complex is missed. In type 2 second-degree heart block, the PR interval remains consistent, but some P waves are blocked and not followed by a QRS complex.