Sinus bradycardia
A sinus rhythm slower than normal for the patient, with the SA node still setting the pace.
Key ECG features
- ✓Rate low for the patient
- ✓Normal upright P before every QRS
- ✓PR interval constant
- ±Regular, unless sinus arrhythmia is superimposed
✓ required · ± may be present
ECG characteristics
A normal, upright P wave precedes every QRS and the PR interval is constant - only the rate is low. The rhythm is regular unless sinus arrhythmia is superimposed on it.
This illustration is schematic. Dashed line is a normal rhythm. The grid is not drawn to scale.
Common causes
Often benign high vagal tone in fit, relaxed or sleeping animals. Pathological causes include hypothermia, hypothyroidism (a canine problem; the feline thyroid far more often drives the rate up instead), raised intracranial pressure, hyperkalemia, and the effects of sedative, anesthetic and some cardiac drugs.
Why it matters
Significance depends on whether it is symptomatic and whether it resolves with exercise or atropine. A fixed, symptomatic bradycardia that does not respond points toward sinus node dysfunction.
Differentiate from
- Junctional escape rhythmBoth are slow with a narrow QRS. Sinus bradycardia keeps a normal upright P wave before every QRS; a junctional escape rhythm has none, or a retrograde one.
- Sinus node dysfunctionA bradycardia that resolves with exercise or atropine is usually vagal. One that does not, and that correlates with signs, points to sinus node dysfunction as the underlying disorder.
- Third-degree (complete) AV blockComplete block is also slow, but there are more P waves than QRS complexes and they march independently.