Atrial fibrillation (AFib) is a common heart rhythm disorder where the heart’s upper chambers, the atria, beat chaotically and irregularly. Instead of a coordinated contraction, the atria begin to quiver, which affects the heart’s ability to pump blood efficiently. An electrocardiogram (ECG) records the electrical signals that control the heart’s rhythm. The visual patterns on this tracing allow medical professionals to identify the heart’s electrical behavior, including the distinct signatures of AFib.
Reading the Normal Sinus Rhythm
Before recognizing the chaotic pattern of AFib, it is helpful to understand the appearance of a normal heart rhythm, known as a sinus rhythm, on an ECG. A single, complete heartbeat cycle is represented by a sequence of three distinct electrical deflections, or waves, on the graph. The first deflection is the P wave, which represents the electrical activation of the atria, signaling them to contract. Following a slight pause, the large, sharp spike of the QRS complex appears, showing the electrical activation of the ventricles, the heart’s main pumping chambers.
The final component is the T wave, which represents the electrical recovery or resting phase of the ventricles before the next beat begins. In a normal sinus rhythm, these three components follow each other in a consistent, predictable sequence, typically at a rate between 60 and 100 beats per minute. A key feature of this healthy pattern is that the distance between consecutive QRS complexes, known as the R-R interval, remains nearly identical.
The Three Hallmarks of Atrial Fibrillation
The ECG signature of AFib is characterized by three visual alterations that distinguish it from a normal rhythm. The most noticeable change is the rhythm’s complete unpredictability, known as an irregularly irregular pattern. This means the R-R interval—the time separating each ventricular contraction—is constantly changing and shows no repeating pattern or sequence. This random timing reflects the disorganized electrical signals bombarding the ventricles.
The second hallmark is the absence of discernible P waves, which are normally generated by the organized electrical firing of the atria. Because the atria are quivering instead of contracting effectively, the electrical signal that creates the P wave is completely lost. Instead of the smooth, rounded P wave, the baseline of the ECG tracing appears chaotic.
This chaotic appearance is caused by the third feature: the presence of fibrillatory waves, often called f-waves. These are small, rapid, erratic oscillations that replace the flat baseline between the QRS complexes. F-waves are the visual representation of the atrial muscle fibers firing independently at an extremely high rate, sometimes between 300 and 600 times per minute.
How AFib Types Appear Differently
AFib is categorized based on how long the chaotic rhythm lasts, and this classification impacts the ECG recording method. Paroxysmal AFib refers to episodes of the abnormal rhythm that start and stop spontaneously, typically resolving on their own within seven days. In a standard, short ECG recording, a person with paroxysmal AFib may show a perfectly normal sinus rhythm if the episode is not occurring at that exact moment. To diagnose this type, a longer-term monitoring device, such as a Holter monitor, is often necessary to capture the temporary switch from a normal rhythm to the classic AFib pattern.
In contrast, persistent AFib is continuously sustained beyond seven days and requires intervention to terminate. For this type, the entire ECG tracing will consistently display the three hallmarks, showing the irregularly irregular rhythm and f-waves throughout the duration of the recording.