• Sinus node dysfunction (SND), also referred to as sick sinus syndrome (SSS), is defined as symptomatic bradycardia (heart rate < 60 bpm) resulting from the sinus node's problem with generating or transmitting electrical impulses.
• SND is often associated with abnormal resting ECG consistent with sinus bradycardia, sinus pause or arrest, or sinus exit block.
• Patients with SND can have normal resting ECG but due to chronotropic incompetence, unable to increase heart rate to at least 100-120 bpm, or to accelerate heart rate as quickly during exercise or treadmill exercise stress test.
• When there are alternating SND and SVT, it's called tachy-brady syndrome.
• Sinus bradycardia is seen in young, healthy individuals, after stimulation of parasympathetic system in Valsalva maneuver or during sleep
• Sinus pause arises when the SA node fails to pace for one cycle or more (pacemaking or P cells problem) leading to one missed cycle of P-QRS-T.
• A long pause may induce an escape beat or evolve into sinus arrest and life-threatening asystole.
• A sinus exit block arises when impulses sent from the sinus node fails to conduct within SA node or perinodal tissue (transitional or T cells problem) hence they “cannot exit”.
• First-degree sinus exit block is recorded as normal rhythm on ECG but EP study shows prolonged prolonged sinoatrial conduction time (SACT).
• In second-degree type I sinus exit block, there is progressive lengthening of transmission interval wihch pushes P-QRS complexes closer together.
• The resulting rhythm has progressively shortened P-P interval followed by dropped P-QRS-T beat.
• In second-degree type II sinus exit block, there is intermittently dropped P waves with consistent P-P intervals.
• In third-degree or complete sinus exit block, there is no impulse exiting SA node hence the resulting rhythm is similar to sinus pause or arrest.
• Often times, an escape beat from other automaticity foci will kick in to sustain ventricular depolarization; otherwise, fatal asystole might ensue.
• Tachy-brady syndrome has alternating runs of SND and SVT such as atrial fibrillation (most common), atrial flutter, atrial tachycardia, and less often, AVRT, and AVNRT.
• In addition, there is often long conversion sinus pause after termination of SVT (due to prior suppression of sinus node) or after electrical conversion of atrial fibrillation or atrial flutter.
