Sudden‑Onset Supraventricular Tachycardia in Healthy Active Individuals

Etiology and Predominant Mechanisms

In physically active persons with abrupt SVT, the two most likely re‑entrant mechanisms are atrioventricular nodal re‑entrant tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT) that use an accessory pathway; both occur in structurally normal hearts and are often triggered by premature beats during or after exercise. 1
Re‑entry is the dominant electrophysiologic mechanism producing SVT in otherwise healthy, regularly exercising individuals. 1
The re‑entrant circuit requires a fixed pathway with unidirectional block in one limb and slowed conduction in another to sustain the tachycardia. 1
The majority of patients with these re‑entrant tachyarrhythmias have no structural cardiac disease; the abnormality is limited to the electrical wiring. 1

Premature atrial or ventricular ectopic beats that arise during or immediately after physical exertion can initiate the re‑entrant circuit responsible for SVT. 1

A systematic evaluation should rule out stimulant exposure (e.g., caffeine, energy drinks, pre‑workout supplements, decongestants, cocaine, amphetamines) as a precipitant of SVT. 2
Anemia can produce compensatory tachycardia that may trigger SVT episodes. 1
Acute infection or fever acts as a systemic stressor capable of provoking SVT. 2
The overall recommendation to assess for reversible contributors (including the above) is emphasized in contemporary practice guidelines. 3

Patients typically report a sudden, abrupt onset of palpitations that are not preceded by a gradual increase in heart rate. (Supported by multiple sources) 2
Episodes terminate abruptly, often after vagal maneuvers or spontaneously. 2
In active individuals, SVT may be precipitated by rapid postural changes, bending, or immediately after exercise. 1
After termination, some patients experience transient polyuria due to release of atrial natriuretic peptide. Both the termination pattern and polyuria have been documented. [2][1]

Obtaining a 12‑lead ECG during the tachycardic episode is essential to confirm SVT and to differentiate AVNRT from AVRT. This recommendation is endorsed by current cardiology societies. [2][3]
A resting ECG should be inspected for pre‑excitation (delta wave); its presence indicates an accessory pathway (Wolff‑Parkinson‑White syndrome) and mandates prompt electrophysiology referral because of the risk of sudden death if atrial fibrillation occurs. [2][3]
An echocardiogram is advised to definitively exclude structural heart disease, although most athletes with SVT have normal cardiac anatomy. [2][3]

Syncope is reported in roughly 15 % of SVT patients, typically occurring at the onset or termination of an episode; its presence does not necessarily imply underlying structural disease. [2][1]
When SVT episodes are frequent or prolonged over weeks to months, a tachycardia‑mediated cardiomyopathy can develop even in previously healthy hearts. [2][1]

Visible pre‑excitation on a resting ECG (delta wave) signifies Wolff‑Parkinson‑White syndrome and requires immediate electrophysiological evaluation because atrial fibrillation conducted over the accessory pathway can lead to sudden cardiac death. 2

The American College of Cardiology and Circulation journal suggest that supraventricular tachycardia is primarily caused by three main mechanisms: re-entry phenomena, abnormal automaticity, and triggered activity, with re-entry being the most common mechanism underlying most SVT subtypes 4, 5
Re-entry is the most common mechanism for SVT, involving repetitive electrical impulse conduction around a fixed obstacle in a defined circuit, requiring unidirectional conduction block in one limb of a circuit and slow conduction for both initiation and maintenance 4, 5
Abnormal automaticity occurs when tissues exhibit enhanced diastolic phase 4 depolarization, leading to increased firing rates compared to normal pacemaker cells, and can be located in the atria, AV junction, or vessels communicating directly with the atria 5
Triggered activity is associated with disturbances in repolarization, where afterdepolarizations can reach threshold and trigger early action potentials during repolarization 5

Atrioventricular Nodal Reentrant Tachycardia (AVNRT) is caused by a re-entry circuit within the AV node involving dual pathways (fast and slow) 6
Atrioventricular Reciprocating Tachycardia (AVRT) involves an accessory pathway between the atria and ventricles (bypass tract) 6
Atrial Tachycardia (AT) can be caused by enhanced automaticity, triggered activity, or micro-reentry within atrial tissue, and is characterized by a long RP interval with P wave morphology different from sinus rhythm 4, 5, 6
Atrial Flutter is a macro-reentrant circuit, typically around the tricuspid annulus, often associated with structural heart disease or precipitating events, and has an incidence that increases markedly with age 4, 5
Atrial Fibrillation with Rapid Ventricular Response often coexists with other forms of SVT, and is particularly dangerous in patients with accessory pathways 5, 7

Structural heart disease, including heart failure, hypertension, and valvular disease, is a predisposing factor for SVT 5, 7
Congenital heart disease, particularly Ebstein anomaly, Tetralogy of Fallot, transposition of great arteries, and atrial septal defects, is also a predisposing factor 7
Acute precipitating events, such as major surgery, pneumonia, and acute myocardial infarction, can trigger SVT 5
Chronic conditions, including heart failure, hypertension, and chronic lung disease, can contribute to the development of SVT 5
Metabolic disorders, such as hyperthyroidism and electrolyte abnormalities, can also contribute to SVT 8
Certain medications and substances, including stimulants, antiarrhythmics, and caffeine, can trigger SVT 8

Pre-excitation syndromes, such as Wolff-Parkinson-White, require prompt evaluation due to the risk of sudden cardiac death if atrial fibrillation develops 7
SVT in patients with congenital heart disease carries significantly higher risks of complications, including heart failure, stroke, and sudden cardiac death 7
Prolonged SVT can lead to tachycardia-mediated cardiomyopathy, especially when persistent for weeks to months 7