Sinus rhythm with a beat-to-beat variation in the P-P interval (the time between successive P waves), producing an irregular ventricular rate.
- Variation in the P-P interval of more than 120 ms (3 small boxes).
- The P-P interval gradually lengthens and shortens in a cyclical fashion, usually corresponding to the phases of the respiratory cycle.
- Normal sinus P waves with a constant morphology (i.e. no evidence of premature atrial contractions).
- Each QRS complex is preceded by a normal P wave.
- Normal P wave axis: P waves should be upright in leads I and II, inverted in aVR.
- The PR interval remains constant around 120 – 200 ms. (i.e. no evidence of Mobitz I AV block).
- QRS complexes are < 120 ms wide (unless a co-existent interventricular conduction delay is present).