A heart block is a type of heart rhythm disorder, or arrhythmia, that occurs when the electrical signal traveling from the heart’s upper chambers (atria) to the lower chambers (ventricles) is delayed or interrupted. This pathway, controlled by the atrioventricular (AV) node, coordinates the heart’s pumping action. When a blockage occurs, the ventricles may contract less frequently or out of sync with the atria, which can lead to a dangerously slow heart rate, or bradycardia. Understanding the location and nature of this electrical failure is necessary to determine the severity of the block and the appropriate medical response.
Understanding the Heart’s Electrical Pathway
The heart’s rhythm originates in the sinoatrial (SA) node, often called the natural pacemaker, which sends an electrical impulse across the atria, causing them to contract. This atrial activity is represented by the P wave on an electrocardiogram (ECG). The impulse then travels to the atrioventricular (AV) node, which briefly delays the signal to allow the atria to empty blood into the ventricles.
After the delay at the AV node, the signal rapidly travels down the bundle of His and through the Purkinje fibers to the ventricles. The electrical activation of the ventricles, which prompts their contraction, is recorded as the QRS complex. A QRS complex must occur after every P wave in a normal heart rhythm, signifying successful conduction. The time it takes for the impulse to travel from the atria to the ventricles is the PR interval.
Blocks That Do Not Drop the QRS
First-degree AV block represents the mildest form of conduction delay. Every electrical impulse successfully passes from the atria to the ventricles, but the transit time is prolonged. This condition is characterized by a consistently extended PR interval on the ECG, meaning the QRS complex is never dropped, only delayed. This type of block is generally considered benign and rarely causes symptoms.
Second-degree AV block Type I (Wenckebach) involves a predictable pattern of increasing delay before a QRS complex is dropped. The PR interval progressively lengthens with each beat until one atrial impulse fails to conduct past the AV node, resulting in a P wave not followed by a QRS complex. The cycle then immediately resets with a short PR interval, creating a recognizable and stable pattern. This block is typically located within the AV node itself and often does not require intervention.
Second-Degree Type II: The Dropped Beat Block
The specific heart block characterized by the intermittent dropping of a QRS complex is Second-Degree AV Block Type II, also known as Mobitz II. The electrical failure occurs suddenly, without any prior warning or progressive lengthening of the PR interval. Crucially, the PR interval for all conducted beats remains constant.
In Mobitz II, the block is almost always located below the AV node, in the bundle of His or the bundle branches, indicating a structural problem in the heart’s lower conduction system. An atrial impulse (P wave) occurs, but the signal is abruptly blocked from reaching the ventricles, resulting in an unexpected, absent QRS complex. This block often presents in a fixed ratio, such as 2:1 or 3:1, meaning every second or third P wave is not conducted. The sudden, unpredictable nature of the dropped beat makes Mobitz II a serious condition with a high risk of progression to complete heart block.
Third-Degree Block: Complete Dissociation
Third-Degree AV Block, or Complete Heart Block, represents a total inability of the atrial impulses to pass through the AV node to the ventricles. This is not an intermittent dropped beat like Mobitz II, but a complete failure of communication between the upper and lower chambers. The atria and ventricles beat completely independently of one another, a state known as atrioventricular (AV) dissociation.
In this condition, the atria are driven by the SA node at a normal rate. The ventricles must rely on a slow, secondary escape pacemaker located lower in the conduction system. This escape rhythm generates a very slow, unreliable ventricular rate, typically between 20 and 40 beats per minute, which is insufficient to maintain proper circulation. On an ECG, this is identified by P waves and QRS complexes that march along at their own regular rates but show no relationship to each other.
Clinical Significance and Management
Second-Degree Type II and Third-Degree blocks result in bradycardia, which severely limits the heart’s ability to pump blood effectively. Patients frequently experience symptoms such as fatigue, dizziness, lightheadedness, or fainting (syncope) due to insufficient blood flow to the brain. The unpredictable dropping of beats in Mobitz II and the extremely slow rate in Third-Degree block carry a substantial risk of progressing to cardiac arrest.
Immediate intervention is often required for both Mobitz II and Complete Heart Block to ensure a reliable heart rate. The standard long-term treatment involves the implantation of a permanent pacemaker. This device continuously monitors the heart’s electrical activity and delivers an impulse directly to the ventricles when a beat is dropped or the rate falls too low. This pacing therapy effectively replaces the failed natural conduction system to restore a safe heart rhythm.