The hallmark of Atrial Fibrillation is irregularity, both an irregular rhythm and irregular atrial electrical activity. AFib comes from an atrial origin, but not a sinus origin, so there will be no P Waves and the baseline will show fibrillation of the atria. Since the fibrillation of the atria is the origin of the electrical activity, randomly activating the AV Node, the complex below the atria (QRS-T) remains as described in the prototypical Normal Sinus Rhythm.
Fibrillation means disorganized electrical activity and is recorded on the EKG as constant, random, erratic deflections from the baseline.
Once a tracing is identified as having an irregular rhythm, fibrillation waves, and prototypical QRS-T complexes it can be called AFib. An Atrial Fibrillation EKG is:
An EKG tracing with an irregular rhythm of normal QRS-T complexes and fibrillation of the baseline.
Defining Characteristics* of AFib in Lead II
- Rate: N/A (See Rates of AFib below.)
- Rhythm: Irregularly Irregular | Random R-R Intervals.
- P Waves: Not present. (Non-sinus origin.)
- P-R Intervals: N/A
- QRS Complexes: Maximum 0.12sec (3mm) [3 small boxes].
- Q-T Intervals†: Approx. 0.42sec (10.5mm) [10.5 small boxes].
- T Waves: Amplitude less than 5mm (0.5mV) [5 small boxes] | Present after every QRS Complex, upright, & uniform.
* Changes in some features may not categorize a rhythm from AFib to something else and instead be mentioned after, such as: AFib with RvR, AFib with a LBBB, etc.
† Q-T Intervals are variable with HR, age, and sex.
Rates of AFib
There is no explicit rate definition for Atrial Fibrillation. Instead, tachycardic rates (HR over 100) are called AFib with a Rapid Ventricular Response while bradycardic rates (HR less than 60) are called AFib with a Slow Ventricular Response. Below are examples of Atrial Fibrillation with a slow (top) and rapid (bottom) ventricular response, respectively.
Most modern cardiac monitors calculate a heart rate by averaging three R-R intervals and dividing that by 60 seconds. Since a few R-R Intervals is generally only several seconds, irregular rhythms (including Atrial Fibrillation) tend to have a fluctuating heart rate displayed on the monitor. This means you will see the HR constantly moving up and down by a few beats in irregular rhythms.
Expected Rhythm of Atrial Fibrillation
Atrial Fibrillation is an irregularly irregular rhythm. This means that it does not have a steady beat like a metronome (irregular) and it has no repeating pattern (irregularly irregular.) On the EKG this is visible with random R-R Intervals. Above is an example AFib tracing with the random R-R Intervals marked in green.
Fibrillation waves in AFib
Fibrillation waves in Atrial Fibrillation are constant random deflections of both positive and negative amplitude. These waves are created by the chaotic and varied electrical signals that are fibrillation in the atria. As these random fibrillation waves activate the AV node, they are also responsible for the irregularly irregular ventricular rhythm described above. Below is a AFib monitor capture with the segments of fibrillation marked in grey.
The QRS-T Segment in Atrial Fibrillation
The QRS Complex and T Wave in AFib should be of the expected morphology described in Normal Sinus Rhythm. Since the pathophysiology of Atrial Fibrillation is limited to the atria (superior to the AV Node) features on the EKG representing ventricular activation remain prototypical. The below example AFib tracing has the QRS-T segment highlighted in green.
Below are several more examples of Atrial Fibrillation presented as monitor captures as well as on EGK graphs.
After becoming confident in identifying EKGs as Atrial Fibrillation head back to our EKG Rhythm Index to find information on another ECG. Otherwise practice interpreting novel EKGs with our EKG Generator:
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