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Atrial Fibrillation (SVR)

caution
Lead II
25 mm/s10 mm/mV
HR72bpm
RR833ms
QT328ms
Type

Atrial fibrillation with a slow ventricular response: irregularly irregular, ventricular rate under 60 bpm.

Updated

On the trace

The atrial picture is that of any AFib: no P waves, a fibrillatory baseline, a narrow QRS. The slow rate changes what to watch for underneath it.

Try each step, then check it.
  1. The ventricular rate is under 60.
    Show 53 bpm here, and the gaps are long: R-R intervals from 670 ms to 3 s. The AV node is letting few impulses through.
  2. The gaps are still random.
    Show Consecutive intervals differ by about 490 ms. That randomness is what keeps this AFib rather than a bradycardia with organised atrial activity.
  3. If it goes slow and regular, stop calling it AFib.
    Show A regular slow rhythm over a fibrillatory baseline is regularised AFib: the node has stopped conducting altogether and an escape pacemaker has taken over. That is complete heart block, and it is managed differently.

How to recognise it

FeatureValueNotes
RhythmIrregularly irregularThe irregularity is the reassurance. Losing it is the red flag.
P wavesAbsentFibrillatory baseline, unchanged from any AFib.
Ventricular rateUnder 60 bpm53 here, with R-R intervals from 670 ms to 3 s.
QRSNarrow, 76 ms hereA wide escape at a regular slow rate points to complete AV block.

The strip

Thirty seconds of atrial fibrillation with a slow ventricular response in lead II, drawn by the simulator. Work through the steps above on it: count the rate, then check whether the long gaps are regular or random.
Go deeper

Background

A slow ventricular response points to intrinsic AV-nodal disease, high vagal tone, or too much rate-controlling drug.

Management

1

Review AV-nodal drugs

The commonest cause is treatment: a beta-blocker, calcium-channel blocker or digoxin doing too much. Reduce or stop the offending agent and reassess the rate before looking further.

2

Suspect conduction disease

If the rate is slow off drugs, or the rhythm regularises into complete heart block, the AV node itself is failing. Symptomatic bradycardia or complete block is a pacing question, not a drug one.

The slow rate is about the AV node and the drugs acting on it; the stroke risk is unchanged from any AFib and is decided separately.

Differential

3rd Degree (Complete) AV Block

The trap SVR sets. When the AV node stops conducting entirely, an escape pacemaker takes over at a slow, regular rate. The fibrillatory baseline is still there, but the rhythm has organised. Slow and irregular is SVR; slow and regular is complete block, and it is managed as such.

The full pictureAtrial Fibrillation (AFib)Mechanism, stroke risk and anticoagulation are the same as the base rhythm.

References

  1. 2023 ACC/AHA/ACCP/HRS Guideline for the Diagnosis and Management of Atrial Fibrillation — Circulation, 2023