Sinus Rhythm

                Normal sinus rhythm results from cardiac impulses that originate in the SA node and continue on to the AV node, Bundle of His, left and right bundle branches and Purkinje system.   Since the SA node has the fastest impulse rate, it typically overrides the other pacemaker cells in the heart.  Sinus rhythm (normal sinus impulses are formed) includes:

• Sinus rhythm - regular rhythm; a P wave for every QRS complex; consistent P-R interval.  (Click on the thumbnail images below for corresponding normal sinus rhythm images)

• Sinus Tachycardia - heart rate increased above the normal range for the species

• Sinus Bradycardia - heart rate decreased below the normal range for the species

• Artifact - a normal rhythm can appear abnormal due to patient movement, positioning, electrical interference, etc.

• Sinus arrhythmia - irregular rhythm; normal P wave for every QRS complex; relatively constant P-R interval; usually due to changes in vagal tone

• Respiratory - typically, increased heart rate on inspiration and decreased on expiration; common in the dog (especially young, fit individuals with high vagal tone).

• Wandering Pacemaker - shift of the pacemaker from the SA node; also common in dogs with high vagal tone.