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 primary distinction between them lies in the degree of conduction block from the atria to the ventricles, with significant implications for morbidity, mortality, and quality of life. The key differences between these degrees are as follows:
- First-degree heart block is characterized by a prolonged PR interval (>0.20 seconds) on an ECG, indicating slowed conduction from the atria to the ventricles, but all electrical impulses still reach the ventricles, and this type rarely causes symptoms and typically doesn't require treatment 1.
- Second-degree heart block comes in two forms: 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 conduct to the ventricles without PR interval changes, with Mobitz type II being more serious and potentially requiring a pacemaker 1.
- Third-degree heart block, also called complete heart block, is the most severe form where no electrical impulses from the atria reach the ventricles, causing the atria and ventricles to beat independently, resulting in a very slow ventricular rate (typically 20-40 beats per minute) and symptoms like fatigue, dizziness, and syncope, and almost always requires permanent pacemaker implantation to maintain adequate heart rate and prevent sudden cardiac death 1.
The management and treatment of these conditions are guided by the severity of the block and the presence of symptoms, with a focus on preventing morbidity and mortality, and improving quality of life. For example, asymptomatic third-degree AV block at any anatomic site with average awake ventricular rates of 40 bpm or faster may not require immediate pacing, but symptomatic cases or those with underlying cardiomegaly or left ventricular dysfunction may benefit from pacemaker implantation 1.
In terms of specific recommendations, the most recent and highest quality study suggests that third-degree heart block almost always requires permanent pacemaker implantation, while second-degree heart block may require a pacemaker in certain cases, and first-degree heart block typically does not require treatment 1. The decision to implant a pacemaker should be based on individual patient characteristics, including the presence of symptoms, the severity of the block, and underlying cardiac disease, with the goal of improving quality of life and reducing morbidity and mortality.
Some key points to consider in the management of heart blocks include:
- The distinction between Mobitz type I and type II second-degree heart block, with type II being more serious and potentially requiring a pacemaker 1.
- The importance of considering underlying cardiac disease and patient symptoms in determining the need for pacemaker implantation 1.
- The role of electrophysiological study in diagnosing and managing heart blocks, particularly in cases where the diagnosis is unclear or the patient has underlying cardiac disease 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 of electrical impulses from the atria to the ventricles.
- First degree heart block: characterized by a delay in the electrical impulses from the atria to the ventricles, but all impulses are still conducted 2.
- Second degree heart block: some electrical impulses from the atria are blocked, and not all impulses are conducted to the ventricles.
- Third degree heart block: complete blockage of electrical impulses from the atria to the ventricles, resulting in no impulses being conducted.
Clinical Implications
First degree heart block is associated with an increased risk of atrial fibrillation and worse outcomes in patients with heart failure 2.
- The prevalence of first-degree atrioventricular block in the general population is approximately 4% 2.
- Cardiac pacing for any indication in patients with first-degree heart block is associated with worse outcomes compared with patients with normal atrioventricular conduction 2.
Syncope and Sudden Cardiac Death
Syncope, or fainting, can be a warning sign for sudden cardiac death, regardless of its origin 3, 4.
- The actuarial incidence of sudden death by 1 year was significantly greater in patients with syncope than in those without syncope 3.
- Syncope may be due to more worrisome conditions, such as cardiac structural disease or channelopathies, and can be an indicator of increased morbidity and mortality risk, including sudden cardiac death 4.