• Sinus tachycardia (ST) is sinus rhythm with fast heart rate > 100 bpm.
• As rate increases, amplitude of P wave will increases due to enhanced activation of the sinus node and associated atrial depolarization.
• ST is caused by enhanced sympathetic activity in both physiologic (e.g. exercise, stress) and pathologic (e.g. infection, hypovolemia, hyperthyroidism, beta-agonist, stimulant use) conditions.
• Inappropriate sinus tachycardia (IST) is ST without underlying physiologic demands or triggers, caused by neurohormonal dysregulation and/or sinus node hyperactivity.
• Patients with IST have resting heart rate > 100 bpm and exhibit symptoms such as palpitations, lightheadedness, or weakness.
• Unlike SNRT, ST and IST don't start and terminate abruptly.
• Sinus node reentry tachycardia (SNRT) arises from micro-reentry involving the sinus node.
• The resulting ECG might appear similar to sinus tachycardia with identical P morphology. However, SNRT starts and terminates abruptly.
• Without known baseline sinus P wave morphology, SNRT can be mistaken for atrial tachycardia (which often has irregularity at onset "warm up" and termination "warm down").
