Atrial flutter, variable conduction
The lower level of the AV node blocks some of the waves the upper level passes, but not all. The ratio changes between 2:1 and 4:1, so the rhythm is irregular.
On the trace
- Measure several R-R intervals. Short ones of about 465 ms and long ones of about 725 ms, in no fixed order. The rhythm is irregular.
- Count the flutter waves between two QRS complexes. Two before a short interval, four before a long one: the ratio changes between 2:1 and 4:1. Now and then it is 3:1.
- Compare a long R-R with a short one. The long one is about one and a half to two times the short one. Atrial fibrillation has no such relation.
- Measure the flutter cycle. 200 ms: 300 per minute, the same on every beat.
- Measure the QRS. 96 ms, narrow.
Telling it apart
| 2:1 | Variable | |
|---|---|---|
| Ventricular rate | 150 | About 100 |
| Rhythm | Regular | Irregular |
| R-R intervals | All 400 ms | About 465 or about 725 ms |
| Sawtooth | Partly hidden | Easy to see in the long intervals |
What it means
The lower level of the AV node is close to the edge between passing every wave the upper level sends and blocking every second one. Small changes in vagal tone, for example with breathing, move it across that edge from beat to beat. The long R-R is not exactly double the short one, because the node also conducts more slowly just before it blocks. The irregular rhythm can look like atrial fibrillation: look for the sawtooth in the long intervals.
Full articleAtrial FlutterHow to recognise it, why it happens, what it means and what to do.