▪ Intrinsic SA node abnormality (enhanced automaticity)

• There is an elevated intrinsic heart rate (i.e., in the absence of autonomic influence).

• IST frequently has an exaggerated heart rate response to epinephrine and exercise.

• It may have a genetic cause (mutation in gene encoding the ion channels involved in pacemaking).

▪ Abnormal autonomic regulation (dysautonomia: ↑ sympathetic nervous system [SNS]/↓ parasympathetic nervous system [PSNS])

• Onset of illness is often preceded by viral illness or trauma.

• Non-cardiac symptoms are typical of other dysautonomias.

• Symptoms of dysautonomia may persist after successful SA node ablation.

Sinus Node Reentrant Tachycardia (SNRT)

○SNRT is a rare paroxysmal (usually non-sustained) tachycardia similar or identical to sinus tachycardia.

○A heterogenous conduction through the SA node complex creates the substrate for a reentrant circuit between the SA node and the RA.

Postural Orthostatic Tachycardia Syndrome (POTS)

○Autonomic disorder is associated with orthostatic intolerance (symptoms on standing).

▪ ST only occurs with a postural change.

○Primary forms

▪ Partial dysautonomic form: The orthostatic heart rate rises >30 bpm above baseline or >120 bpm within 10 minutes of a tilt table test without a decrease in blood pressure (BP; <20/10 mm Hg).

▪ Hyperadrenergic form: The orthostatic heart rate rise is accompanied by hypertension.

○Secondary dysautonomia due to:

▪ Diabetes

▪ Amyloidosis

▪ Lupus

▪ Sarcoidosis

Epidemiology and Clinical Features

IST

○Patients are 90% female; the mean age of presentation is 38 years.

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