What are the causes of sudden onset supraventricular tachycardia in a healthy male who runs daily?

Sudden Onset SVT in a Healthy Active Male

In a healthy male runner experiencing sudden-onset SVT, the most likely causes are atrioventricular nodal reentrant tachycardia (AVNRT) or atrioventricular reciprocating tachycardia (AVRT) involving an accessory pathway, both of which are re-entrant mechanisms that occur in structurally normal hearts and are triggered by premature beats during or after exercise. 1, 2

Primary Mechanisms in Healthy Individuals

Re-entry is the dominant mechanism causing SVT in otherwise healthy, active individuals like daily runners. 1, 2 This occurs when:

• An electrical impulse conducts repetitively around a fixed circuit in the heart, requiring unidirectional conduction block in one pathway and slow conduction to maintain the arrhythmia 1, 2
• The circuit most commonly involves dual pathways within or around the AV node (AVNRT) or an accessory pathway between atria and ventricles (AVRT) 2, 3
• Most patients with these re-entrant tachycardias have structurally normal hearts—there is nothing wrong with the heart muscle itself, only the electrical wiring system 4, 5

Specific SVT Types in Athletes

AVNRT (Most Common)

• Accounts for the majority of paroxysmal SVT cases in healthy individuals 3
• Involves dual conducting pathways (fast and slow) confined to the AV node and surrounding atrial tissue 2, 3
• The mean age at first presentation for "lone" SVT (without structural heart disease) is 37 years 4

AVRT (Second Most Common)

• Depends on an anatomically distinct accessory pathway that bypasses the AV node 2, 3
• If pre-excitation (delta wave) is visible on resting ECG, this indicates Wolff-Parkinson-White syndrome and requires immediate electrophysiological evaluation due to risk of sudden death if atrial fibrillation develops 1, 2, 6

Exercise-Related Triggers

In a daily runner, specific precipitating factors include:

• Premature atrial or ventricular beats that occur during or immediately after exercise, which can initiate the re-entrant circuit 1
• Increased sympathetic tone from physical exertion 2
• Electrolyte shifts during intense exercise 2
• Dehydration affecting electrolyte balance 2

Critical Reversible Causes to Exclude

Even in apparently healthy individuals, evaluate for: 1, 2

• Stimulant use: caffeine, energy drinks, pre-workout supplements, decongestants (pseudoephedrine), or illicit drugs (cocaine, amphetamines)
• Hyperthyroidism: can present with isolated SVT before other symptoms
• Electrolyte abnormalities: hypokalemia, hypomagnesemia
• Anemia: causing compensatory tachycardia that may trigger SVT
• Infection or fever: systemic stressors

Clinical Presentation Pattern

The history will typically reveal: 1, 4

• Sudden, abrupt onset of palpitations (not gradual acceleration like sinus tachycardia)
• Sudden, abrupt termination (often with vagal maneuvers)
• Heart rate of 150-300 beats/min during episodes 4, 3
• Extremely regular rhythm after the first 10-20 beats, "like a metronome" 4
• Episodes may be triggered by sudden position changes, bending over, or immediately post-exercise 1
• Possible polyuria after episode termination (from atrial natriuretic peptide release) 1

Diagnostic Approach

Obtain a 12-lead ECG during tachycardia to confirm diagnosis and determine the specific SVT mechanism. 1 Key features:

• P waves are identifiable in only 60% of cases 4
• In AVNRT, P waves are typically hidden within the QRS complex 3, 7
• In AVRT, P waves always follow the QRS with R-P interval usually exceeding 70 msec 3, 7
• Check resting ECG for pre-excitation (delta wave), which indicates an accessory pathway and mandates specialist referral 1

An echocardiogram should be obtained to definitively exclude structural heart disease, though it is usually normal in this population. 1

Important Caveats

• Do not assume anxiety or panic attacks without ECG documentation—SVT often terminates before presentation and episodes may be erroneously attributed to psychological causes 8
• Syncope occurs in approximately 15% of SVT patients, usually at episode onset or termination, and does not necessarily indicate structural disease 1, 6
• If episodes are frequent or prolonged (weeks to months), tachycardia-mediated cardiomyopathy can develop even in previously healthy hearts 1, 9
• Women have twice the risk of developing SVT compared to men 4