Definition And Electrophysiology

Supra-ventricular tachycardia (SVT) encompasses paroxysmal or incessant tachycardias originating above or within the His bundle, excluding ventricular tachycardia.
Characterized by a narrow QRS complex, defined as <120 ms in adolescents and young adults, <100 ms in children, and typically <0.08 seconds in neonates.
Fetal SVT is characterized by sustained rates >220 beats/min with 1:1 atrioventricular (AV) conduction.

Arrhythmia Mechanisms And Subtypes

SVT is driven by two primary electrophysiological mechanisms: reentry and abnormal automaticity.

Mechanism	Electrophysiological Characteristics	Common Clinical Examples
Reentry	Requires two distinct pathways, a zone of conduction delay, and unidirectional block. Characterized by abrupt onset and termination with a regular cycle length.	Atrioventricular Reciprocating Tachycardia (AVRT), Atrioventricular Nodal Reentry Tachycardia (AVNRT), Atrial Flutter.
Abnormal Automaticity	Ectopic focus drives depolarization. Characterized by gradual acceleration (warm-up) and gradual deceleration (cool-down).	Ectopic Atrial Tachycardia (EAT), Junctional Ectopic Tachycardia (JET).

Atrioventricular Reciprocating Tachycardia (AVRT): Most common SVT mechanism in infants, utilizing an accessory bypass tract. Orthodromic AVRT (antegrade AV node, retrograde pathway) yields a narrow QRS, while antidromic AVRT (antegrade pathway, retrograde AV node) yields a wide QRS.
Atrioventricular Nodal Reentry Tachycardia (AVNRT): Utilizes dual functional pathways (fast and slow) within the AV node. Rare in infancy but common in adolescence.
Atrial Flutter: A macro-reentrant circuit within the atria. Common in neonates with normal hearts or older children with surgically stretched atria (e.g., Fontan procedure, Ebstein anomaly).
Junctional Ectopic Tachycardia (JET): Automatic focus near the His bundle, most commonly occurring postoperatively after congenital heart surgery due to surgical trauma or edema.

Unrecognized SVT for 6–24 hours may present with signs of heart failure.
Symptoms include poor feeding, ashen color, restlessness, irritability, tachypnea, and hepatomegaly.
Heart rates typically range from 240–300 beats/min.

Present with abrupt onset of palpitations, precordial discomfort, fatigue, dizziness, dyspnea, or syncope.
Heart rates are typically >180 beats/min.
Episodes may be exacerbated by extrinsic triggers such as caffeine or nonprescription decongestants.

SVT exhibits fixed, rapid rates lacking the normal beat-to-beat variability seen in sinus tachycardia.
P waves are visible in only 50–60% of neonates on a standard ECG, often hidden within the QRS or T wave.
An abnormal P-wave axis (e.g., inverted in leads II, III, and aVF) strongly suggests SVT and argues against sinus tachycardia.
A transesophageal lead can effectively detect P waves when the surface ECG is obscure.

Arrhythmia	P-Wave Characteristics	P-QRS Relationship	Response To Adenosine Challenge
Sinus Tachycardia	Normal morphology and axis	1:1	Transient slowing; transient AV block.
Ectopic Atrial Tachycardia	Abnormal, different from baseline	Usually 1:1	No effect on rate; transient AV block unmasks abnormal atrial activity.
Atrial Flutter	Saw-tooth appearance (>240/min)	2:1 or 1:1	Transient AV block unmasks underlying flutter waves; rarely terminates.
AVNRT / AVRT	Usually not visible or inverted	1:1	Abrupt termination.
Junctional Ectopic (JET)	Normal or inverted	AV dissociation diagnostic	No effect on rate; transient retrograde VA block.

Vagal Maneuvers: First-line non-pharmacological therapy for stable patients. Application of an ice bag to the entire face for 15–30 seconds in infants, or Valsalva maneuver in older children. Ocular pressure is strictly contraindicated.
Adenosine: Drug of choice for stable patients with AVRT or AVNRT. Administered as a rapid IV push of 0.1 mg/kg (up to 6 mg initial dose), increasing to 0.2 mg/kg (up to 12 mg) if ineffective. Must be followed by a rapid saline flush. Contraindicated in asthma/bronchospastic disease or without direct current (DC) cardioversion standby.
DC Cardioversion: Indicated immediately for hemodynamically unstable patients or those in urgent heart failure. Administered as a synchronized shock at 0.5–2 J/kg.

Medication	Class	Clinical Indications And Nuances
Beta-Blockers (Propranolol, Atenolol)	Class II	Mainstay chronic suppression for patients without antegrade accessory pathways (Non-WPW).
Digoxin	Class V	Effective first-line therapy in infants. Strictly contraindicated in WPW syndrome due to risk of rapid antegrade accessory pathway conduction precipitating ventricular fibrillation.
Verapamil	Class IV	Utilized in older children. Strictly contraindicated in infants <1 year due to profound bradycardia, hypotension, severe apnea, and cardiovascular collapse risk. Contraindicated in WPW.
Amiodarone / Sotalol / Flecainide	Class III / IC	Indicated for resistant or refractory SVT. Flecainide is strictly restricted to patients with structurally normal ventricular function.

Catheter Ablation: Definitive, potentially curative therapy utilizing radiofrequency or cryoablation. Cryoablation is preferred near the AV node to eliminate permanent heart block risk.
Success Rates: 90–98% depending on the pathway location.
Indications: Ventricular dysfunction, medically refractory SVT, avoidance of long-term medication, hemodynamic compromise during SVT, or WPW with syncope/resuscitated cardiac arrest.
Complications: AV block, perforation, thrombosis, with a higher incidence in children weighing <15 kg.

Maternal Transplacental Therapy: First-line treatment for fetal SVT without hydrops. Digoxin (targeting maternal trough 1.5–2.0 mcg/mL), flecainide, and sotalol are the most common agents utilized. Flecainide exhibits a highly successful conversion rate.
Hydropic Fetus: Reduced transplacental bioavailability necessitates combination therapy or alternative routes. Direct fetal intramuscular (IM) digoxin injection into the buttock or thigh rapidly decreases the time to conversion, effectively bypassing placental transfer issues. Amiodarone may be required for severe, refractory hydropic cases.