• Conduction disorders involving the sinus node are mostly caused by a block leading to sinus node dysfunction including sinus bradycardia, sinus pause or arrest, sinus exit block, tachy-brady syndrome.
• Sinus arrhythmia is irregular sinus rhythm that is mostly benign:
Respiratory sinus arrhythmia: sinus rate increases with inspiration and decreases with expiration, commonly seen in young healthy individuals
Ventriculophasic sinus arrhythmia: concurrent high grade AV block, P-P intervals are shorter when they enclose QRS complexes and longer when they don't enclose any QRS
• Sinus bradycardia and sinus tachycardia have normal P-QRS morphologies but slow (< 60 bpm) and fast (> 100 bpm) heart rate. They are caused by changes in rate of sinus pacemaking activity which are often physiologic.
Sinus bradycardia is seen in young, healthy individuals, after stimulation of parasympathetic system in Valsalva maneuver or during sleep
Sinus tachycardia is seen in situations where there is an enhanced sympathetic drive such as exercise, stress, or hypovolemia
• Pathologic manifestations of sinus bradycardia and tachycardia are chronotropic incompetence and inappropriate sinus tachycardia, respectively.
• Sinus node reentry tachycardia (SNRT) is an example of reentry-mediated tachyarrhythmia arising from the sinus node.
• Examples of conduction disorders involving the sinus node are listed below.
