An electrocardiogram, or ECG, records the heart’s electrical activity. This information is translated into a visual tracing, allowing for the assessment of heart rate and rhythm. Atrial fibrillation is a common irregular heartbeat, or arrhythmia, where the heart’s upper chambers beat chaotically. Understanding both normal and abnormal rhythms on an ECG is necessary to recognize this condition.
The Normal Heart Rhythm on an ECG
To understand what is abnormal, one must first recognize what is normal. A healthy heart’s electrical pattern on an ECG is called a normal sinus rhythm. This rhythm originates from the sinoatrial (SA) node, the heart’s natural pacemaker, located in the right atrium. A normal sinus rhythm on an ECG has a heart rate between 60 and 100 beats per minute and a consistent, predictable pattern.
Each heartbeat in a normal sinus rhythm is represented by a series of distinct waves. The first wave is the P wave, a small, smooth, upward curve that represents the electrical impulse spreading through the atria, causing them to contract. The P wave has a duration of less than 0.12 seconds and a low amplitude.
Following the P wave is the QRS complex, the most prominent part of the ECG tracing. This complex signifies the rapid depolarization of the right and left ventricles. The QRS complex is narrow, with a duration of 0.12 seconds or less, indicating the electrical impulse has traveled through the ventricles efficiently.
After the QRS complex is the T wave, which represents ventricular repolarization, the recovery phase of the ventricles. The T wave is a smooth, rounded, upward wave. The segment between the QRS complex and the T wave, the ST segment, is flat against the baseline, indicating the interval between ventricular depolarization and repolarization.
Key ECG Features of Atrial Fibrillation
The ECG of a person with atrial fibrillation looks markedly different from a normal sinus rhythm. The organized pattern is lost and replaced by a chaotic and irregular tracing. The primary change is the absence of discernible P waves. Because the atria are quivering chaotically instead of contracting in a coordinated way, the single, smooth P wave disappears.
In place of the P waves, the baseline of the ECG appears shaky or chaotic due to fibrillatory waves, sometimes called “f-waves.” These f-waves are a visual representation of the disorganized electrical activity within the atria, which can generate up to 600 electrical impulses per minute. These waves can be fine or coarse but are always irregular and lack a consistent pattern.
Another defining characteristic of atrial fibrillation is an “irregularly irregular” rhythm. This means the distance between consecutive QRS complexes, the R-R interval, is completely variable and unpredictable. The atrioventricular (AV) node acts as a filter, allowing only some of these impulses to pass to the ventricles. This erratic filtering causes the ventricles to beat at irregular intervals, reflected in the varying R-R distances.
The QRS complexes themselves are narrow (less than 0.12 seconds in duration), similar to a normal sinus rhythm. This is because the electrical impulses, once they pass the AV node, travel the normal conduction pathway. The combination of no P waves, a chaotic baseline with fibrillatory waves, and an irregularly irregular ventricular response is the presentation of atrial fibrillation on an ECG.
Variations in Atrial Fibrillation on an ECG
While the rhythm in atrial fibrillation is always irregular, the rate at which the ventricles beat can vary, which alters the ECG’s appearance. This variation is based on how many chaotic atrial impulses are conducted through the AV node to the ventricles. The speed of this ventricular response is a focus for classifying and managing the condition.
One common variation is atrial fibrillation with a rapid ventricular response (RVR), or “uncontrolled” atrial fibrillation. This occurs when the ventricular rate is greater than 100 beats per minute, sometimes exceeding 150 beats per minute. On an ECG, this appears as an irregularly irregular rhythm with QRS complexes that are very close together. Patients with RVR are more likely to experience symptoms like palpitations and shortness of breath.
Conversely, atrial fibrillation can present with a slower, or “controlled,” ventricular rate below 100 beats per minute. This is often the result of medications that slow conduction through the AV node. The ECG will still show no P waves and an irregularly irregular rhythm, but the QRS complexes will be more spaced out.
Distinguishing Atrial Fibrillation from Other Arrhythmias
The features of atrial fibrillation on an ECG help distinguish it from other arrhythmias like atrial flutter. In atrial flutter, the atria also beat rapidly, but in a more organized, regular pattern. This activity creates a “sawtooth” pattern of flutter waves on the ECG, which are more regular than the chaotic fibrillatory waves of atrial fibrillation.
Another arrhythmia to distinguish is sinus tachycardia. In sinus tachycardia, the heart rate is over 100 beats per minute, but the rhythm is regular and originates from the SA node. The ECG shows a normal P wave before every QRS complex, and the R-R intervals are consistent, unlike in atrial fibrillation.
The Role of ECG in Diagnosis and Monitoring
A standard 12-lead ECG is the main tool for diagnosing atrial fibrillation. This test provides a ten-second snapshot of the heart’s electrical activity from twelve angles and can identify the arrhythmia if it is present. For many individuals who experience persistent symptoms, a standard ECG is sufficient for diagnosis.
However, atrial fibrillation can be intermittent, a condition known as paroxysmal atrial fibrillation, where episodes start and stop spontaneously. In these cases, a person may have symptoms, but an in-office ECG might show a normal sinus rhythm. To capture these episodes, longer-term monitoring is necessary.
A Holter monitor is a portable device that records the heart’s rhythm continuously for 24 to 48 hours, allowing for the detection of infrequent arrhythmias. For more sporadic symptoms, an event monitor may be used. This device can be worn for several weeks or months and is activated by the patient when they feel symptoms, or it may automatically record an abnormal rhythm. These tools help confirm a diagnosis and guide treatment.