An electrocardiogram (ECG) records the heart’s electrical activity. This non-invasive test involves placing electrodes on the skin to detect the small electrical changes that occur as the heart beats. An ECG assesses the heart’s electrical functioning, providing insights into its rate, rhythm, and the timing of electrical impulses.
The heart’s rhythm is controlled by electrical signals, which originate in the upper chambers and propagate to the lower chambers, coordinating contractions that pump blood. A “heart block” refers to a delay or interruption in this electrical signaling pathway. Such blocks can impact how efficiently the heart beats, and their presence can be identified through specific patterns on an ECG. This article will explore the distinct ECG characteristics that help identify various types of heart blocks.
Understanding Key ECG Intervals
Interpreting an ECG relies on understanding its fundamental components: specific waves and intervals. The P wave reflects atrial depolarization, the electrical activation of the heart’s upper chambers (atria). Following the P wave, the QRS complex represents ventricular depolarization, the electrical activation of the heart’s lower chambers (ventricles).
After the QRS complex, the T wave indicates ventricular repolarization, the electrical recovery of the ventricular muscle cells after contraction. The PR interval measures the time from the beginning of the P wave to the start of the QRS complex. This interval reflects the duration it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. A normal PR interval ranges from 0.12 to 0.20 seconds. The R-R interval, measured between consecutive R waves, indicates the regularity and rate of the ventricular beats.
Identifying First-Degree Heart Block
First-degree heart block is identified on an ECG by a consistently prolonged PR interval, exceeding 0.20 seconds. Visually, this appears as an extended gap between the P wave and the QRS complex. Despite this delay, every atrial impulse successfully conducts to the ventricles.
The rhythm remains regular, as the delay is consistent for every beat. While the PR interval is longer than normal, the P waves and QRS complexes appear normal in shape and duration. This pattern indicates slowed conduction through the AV node, not a complete block or intermittent failure of conduction.
Identifying Second-Degree Heart Blocks
Second-degree heart blocks present with more complex patterns on an ECG, as some atrial impulses fail to reach the ventricles, resulting in “dropped” QRS complexes. Two distinct types exist: Mobitz Type I and Mobitz Type II. Mobitz Type I, also known as Wenckebach phenomenon, is characterized by a progressive lengthening of the PR interval with each successive beat until a QRS complex is dropped. After the dropped beat, the cycle resets, and the PR interval returns to its initial duration before lengthening again. This creates a distinctive pattern of “grouped beats” where the R-R interval progressively shortens before the dropped beat.
Mobitz Type II heart block shows a consistent PR interval for all conducted beats, which may be normal or prolonged, but with intermittently dropped QRS complexes. The R-R interval will be irregular around the dropped beat, but conducted beats maintain a consistent R-R interval. Unlike Mobitz Type I, where the block occurs within the AV node, Mobitz Type II often indicates a block lower in the heart’s electrical conduction system, such as in the His-Purkinje system. Differentiating between these types is important because Mobitz Type II can be more serious and has a higher likelihood of progressing to a complete heart block.
Identifying Third-Degree Heart Block
Third-degree heart block, also known as complete heart block, represents a severe interruption in the electrical communication between the atria and ventricles. On an ECG, this is characterized by complete atrioventricular (AV) dissociation, meaning the atrial activity (P waves) and ventricular activity (QRS complexes) are independent. The P waves march at their own regular rate, typically faster than the ventricular rate, while the QRS complexes also appear at their own independent, slower regular rate. There is no consistent relationship between any P wave and the subsequent QRS complex.
P waves may appear before, during, or after QRS complexes, sometimes hidden within the QRS complex or distorting the T wave. The ventricular rate in a third-degree heart block is very slow, often maintained by an escape rhythm originating from a site lower in the conduction system. This independent rhythm of the atria and ventricles creates a distinct pattern on the ECG tracing.