An electrocardiogram (ECG or EKG) is a non-invasive medical test that records the heart’s electrical activity, translating these signals into a visual tracing. Understanding the heart’s rhythm on an ECG provides insights into its health and helps healthcare professionals assess cardiac function and identify irregularities.
The Building Blocks of an ECG
An ECG tracing consists of characteristic waves, segments, and intervals, each reflecting specific electrical events. The P wave is the first deflection, representing atrial depolarization, the electrical activation and contraction of the heart’s upper chambers. This impulse originates in the sinoatrial node and spreads across the atria.
The QRS complex follows the P wave, indicating ventricular depolarization, the electrical activation and contraction of the heart’s lower chambers. This complex is typically larger than the P wave due to the greater muscle mass of the ventricles. The T wave represents ventricular repolarization, the electrical recovery or relaxation of the ventricles. Atrial repolarization also occurs but is usually hidden within the QRS complex due to its larger electrical magnitude.
Key ECG measurements include the PR interval, QRS duration, and QT interval. The PR interval measures the time from the beginning of atrial depolarization to the start of ventricular depolarization, typically ranging from 0.12 to 0.20 seconds. This interval reflects conduction time through the atrioventricular node. The QRS duration measures the time for ventricular depolarization, normally between 0.08 and 0.12 seconds.
The ST segment is the flat line between the end of the S wave of the QRS complex and the beginning of the T wave. It represents the period when ventricles are fully depolarized, corresponding to the plateau phase of the action potential. The QT interval measures the total time for ventricular depolarization and repolarization, from the beginning of the QRS complex to the end of the T wave. This interval can vary with heart rate and other factors.
A Step-by-Step Guide to Rhythm Interpretation
Interpreting an ECG rhythm involves a systematic approach. First, determine the heart rate. For regular rhythms, the “300-rule” involves counting large squares between two consecutive R waves and dividing 300 by that number to estimate beats per minute.
For irregular rhythms, the “6-second method” counts QRS complexes within a 6-second strip (typically 30 large squares) and multiplies that count by 10 for the heart rate. This method provides a reliable average for rhythms where the beat-to-beat interval varies.
Next, assess rhythm regularity by examining R-R intervals (the distance between consecutive R waves). Consistent intervals indicate a regular rhythm; varying intervals indicate an irregular one. An irregularly irregular rhythm shows no discernible pattern, while a regularly irregular rhythm exhibits a repeating pattern.
Evaluate P waves for their presence, shape, and relationship to the QRS complex. In a normal heart rhythm, a P wave should precede every QRS complex, indicating that the electrical impulse originated in the atria. P waves should also appear consistent in shape and direction.
Measure the PR interval, which should be 0.12 to 0.20 seconds. A consistent PR interval suggests predictable conduction from the atria to the ventricles. Next, measure the QRS duration, which should be 0.08 to 0.12 seconds. A narrow QRS complex indicates efficient electrical signal travel through the ventricles.
Finally, assess T waves for their morphology and relationship to the preceding QRS complex. Normal T waves are generally upright in most leads and follow the QRS complex.
Understanding Common Heart Rhythms
Applying systematic interpretation identifies common heart rhythms. Normal Sinus Rhythm (NSR) is the heart’s default rhythm, originating from the sinoatrial node. NSR presents as a regular rhythm with a heart rate between 60 and 100 beats per minute. Each QRS complex is preceded by a normal, upright P wave, with PR interval and QRS duration within normal ranges.
Sinus Bradycardia is a variation of sinus rhythm with a heart rate less than 60 beats per minute. It maintains normal sinus rhythm characteristics: a regular rhythm, a P wave preceding every QRS complex, and normal PR interval and QRS duration.
Sinus Tachycardia is a faster variation of sinus rhythm, with a heart rate greater than 100 beats per minute. The rhythm remains regular, with all other ECG features consistent with normal sinus rhythm. This rhythm is often a physiological response to stress or exercise.
Atrial Fibrillation (AFib) is characterized by a highly irregular rhythm and the absence of distinct P waves. The baseline often shows chaotic, erratic electrical activity known as fibrillatory waves. The ventricular rate can vary significantly, and QRS complexes are typically narrow unless there are other conduction abnormalities.
Ventricular Tachycardia (VT) is a potentially serious rhythm originating from the ventricles. It has a fast heart rate (100-250 beats per minute) and wide QRS complexes (greater than 0.12 seconds). P waves may be absent or appear dissociated from the QRS complexes, meaning they beat independently. The rhythm can be regular or slightly irregular, and its wide QRS appearance is a key distinguishing feature.
When to Seek Medical Attention
This information offers a foundational understanding of ECG interpretation for educational purposes only and should not replace professional medical diagnosis or treatment. Only a qualified healthcare professional can accurately interpret an ECG in the context of a complete medical history and physical examination.
Certain symptoms warrant immediate medical attention, as they could indicate a serious heart condition. These include sudden chest pain (pressure, tightness, or squeezing) that may spread to the arms, neck, jaw, or back. Shortness of breath, even without chest discomfort, also requires prompt evaluation.
Other symptoms include dizziness, lightheadedness, fainting, or sudden, unexplained sweating. If you experience heart palpitations accompanied by any of these symptoms, seek emergency medical help. Prompt assessment by a healthcare provider ensures appropriate diagnosis and timely intervention.