From the Guidelines
Heart blocks are classified into three degrees of severity, with first-degree being the mildest and third-degree being the most severe, and the treatment approach varies based on the degree and symptoms of the block, with the most recent guidelines from 2019 providing the framework for management 1. The main difference between first, second, and third degree heart block lies in the severity of the conduction delay or block between the atria and ventricles.
- First-degree heart block is characterized by a prolonged PR interval (>200 ms) but with 1:1 atrioventricular conduction, meaning all electrical impulses still reach the ventricles, and it rarely causes symptoms or requires treatment.
- Second-degree heart block is divided into two types: Mobitz type I (Wenckebach), where the PR interval progressively lengthens until a beat is dropped, and Mobitz type II, where some atrial impulses fail to reach the ventricles without PR interval changes, with Mobitz type II being more serious and potentially requiring a pacemaker.
- Third-degree heart block (complete heart block) is the most severe form, with complete electrical dissociation between the atria and ventricles, leading to symptoms like fatigue, dizziness, syncope, and typically necessitating permanent pacemaker implantation. Key considerations in managing heart blocks include identifying the underlying cause, assessing the severity of symptoms, and determining the appropriate treatment, which may range from monitoring to pacemaker implantation, based on guidelines that prioritize patient outcomes in terms of morbidity, mortality, and quality of life 1.
From the Research
Heart Block Degrees
The main difference between first, second, and third degree heart block lies in the severity of the blockage in the electrical conduction system of the heart.
- First degree heart block: This is the least severe form, where the electrical signal from the atria to the ventricles is delayed, but all signals still reach the ventricles 2.
- Second degree heart block: In this type, some of the electrical signals from the atria are blocked and do not reach the ventricles. There are two sub-types: Mobitz Type I (Wenckebach) and Mobitz Type II 3.
- Third degree heart block: This is the most severe form, also known as complete heart block, where no electrical signals from the atria reach the ventricles, and the ventricles beat at their own rate, often very slowly 3.
Clinical Implications
Third degree heart block is a cardiovascular emergency that requires prompt recognition and treatment, as it can lead to severe symptoms such as hypotension, syncope, and bradycardia 3.
- The management of third degree heart block includes accurate interpretation of a 12-lead EKG, assessment of hemodynamic stability, and time-sensitive pharmacologic or procedural intervention, such as administering IV atropine, attempting transcutaneous pacing, or placing a transvenous pacemaker 3.
Comparison of Heart Block Degrees
While first and second degree heart blocks may not always require immediate medical attention, third degree heart block is a life-threatening condition that necessitates urgent treatment 2, 3.