Sinus tachycardia commonly reflects increased sympathetic activity. Re-entry is a separate mechanism by which an electrical impulse can circulate repeatedly and sustain tachycardia.
Sinus tachycardia
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Sinus tachycardia usually results from increased sympathetic activity associated with exercise or emotion.
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Healthy young adults can reach a sinus rate of up to 200/min during intense exercise.
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Treatment is usually unnecessary, although sinus tachycardia may sometimes reflect an underlying disease.
Requirements for re-entry
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Re-entry can occur when two alternative pathways have different conducting properties. Examples include the atrioventricular node and an accessory pathway, or normal tissue and ischaemic tissue.
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In the illustrated mechanism, pathway A conducts slowly and recovers quickly, whereas pathway B conducts rapidly and recovers slowly.
Formation and persistence of the circuit
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During sinus rhythm, each impulse travels down both pathways before entering a common distal pathway.
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A premature impulse may encounter pathway A after recovery while pathway B remains refractory, allowing conduction selectively down pathway A.
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Pathway B may recover while the impulse travels down pathway A. The impulse can then travel retrogradely up pathway B and establish a closed re-entry circuit.
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The resulting tachycardia can continue until a change in conduction rates or electrical depolarisation interrupts the circuit.