I just read Student RN Tiffany's woes regarding heart blocks and want to bless the blogosphere with my distilled knowledge on this subject, having pondered the differences at some length so I don't look totally stupid at work.
- First-degree blocks are easy: if the P-R interval is longer than 0.20 seconds, it's a first-degree AV block. Period. End of story. That's longer than one large box on the EKG paper.
- There are two types of second-degree blocks, which is stupid because why didn't they just make first-, second-, third-, and fourth-degree blocks? But I digress. Both involve dropped beats. You can tell them apart by whether the P-R interval is consistent. A type II second-degree block has consistent P-R intervals, but a type I (Wenkebach) block has longer and longer P-R intervals until the impulse just doesn't quite reach the ventricles and the QRS is dropped. So the P-R interval is normal (therefore not a first-degree block) in a type II block---it's just not always followed by a QRS complex---but progressively longer in Wenkebach.
- Third-degree (complete) heart blocks can be spotted in the wild pretty easily if you suspect them when you see a verrrrry slooooow ventricular rate. The ventricles have no access to the perky SA node and are left to their own devices to contract at 20 to 40 beats per minute. The atria, however, march along to their usual drummer at the normal sinus rate. To confirm your suspicions, march out the P waves (don't be fooled by P waves hiding inside the QRS complexes) to see if they're regular.
EKG classes and books tend to make this more difficult than it needs to be. Anything else, oh wise readers?