Normal sinus rhythm (NSR)
The heart's normal rhythm, with each beat initiated by the sinoatrial (SA) node and conducted through the AV node and His-Purkinje system in the usual sequence.
Key ECG features
- ✓A P wave before every QRS
- ✓PR interval constant beat to beat
- •QRS narrow and uniform
- ±Regular, or cyclically irregular in dogs
✓ required · • typical · ± may be present
ECG characteristics
A P wave precedes every QRS, the PR interval is constant from beat to beat, and the QRS complexes are narrow and uniform. Rate and regularity vary with species and autonomic tone: dogs commonly show a normal respiratory variation (sinus arrhythmia), while a steadier rhythm is typical of cats.
This illustration is schematic. The grid is not drawn to scale.
Why it matters
It is the reference against which every ectopic or abnormal complex is judged. Establishing that the underlying rhythm is sinus is the first step in reading any tracing.
Differentiate from
- Sinus arrhythmiaCyclic respiratory variation in rate is a normal variant in dogs, not a departure from sinus rhythm - every beat is still sinus in origin.
Interactive simulator
Open in the ECG simulator
Watch this rhythm build the trace, beat by beat
How it's measured
PR interval
What it is, and how to measure it
QRS complex
What it is, and how to measure it
Practice questions
Spot the Abnormal Beat
Beginner · 2 questions feature this finding
Fast or Slow?
Beginner · 2 questions feature this finding
Normal Variant or Disease?
Beginner · 2 questions feature this finding
Example tracings
Cat · Domestic Shorthair · 7 years · Female (Recorded with Beecardia)Open in full viewer ↗
A regular sinus rhythm at 145 bpm with a P wave before every narrow QRS and no blocks - a normal feline ECG.
Cat · 12 years · Male (Recorded with Beecardia)Open in full viewer ↗
Regular sinus rhythm at 174 bpm with a P wave for every narrow QRS, alongside a left axis deviation.
Dog · Pomeranian · 5 years · Female (Recorded with Beecardia)Open in full viewer ↗
Sinus rhythm with normal waveform measurements and no ectopy, with a mild left axis deviation also present.