First-degree heart block is recognized by a prolonged PR interval without any dropped beats.
“Second-degree heart block is defined by one or more impulses not reaching the ventricles and is classified as type I and type II. Type I second-degree block, also known as Wenckebach or Mobitz I block, generally involves the AV node. It can be caused by medication effects, such as beta-blockers, digoxin, calcium channel blockers, adenosine, all of which slow the AV node. It may also be seen secondary to an inferior wall myocardial infarction or increased vagal tone. ECG findings depict progressive PR interval lengthening until a QRS complex is skipped, representing a dropped beat. Type I second-degree heart block is usually asymptomatic. Unless there is symptomatic bradycardia, this heart block does not necessitate treatment. If symptomatic bradycardia occurs, the inciting agent should be stopped or a pacemaker should be placed.
Type II second-degree heart block reveals random dropped QRS complexes without any changes in the PR interval.
Third-degree heart block demonstrates no relationship between the P waves and the QRS complexes.