Sinus Tachycardia vs SVT

Pathophysiology and Mechanisms

Sinus Tachycardia

• Driven by increased automaticity of sinoatrial (SA) node.
• Represents physiological response to extrinsic stressors (fever, hypovolemia, sepsis, anemia, pain, circulatory failure).
• Characterized by gradual acceleration (warm-up) and gradual deceleration (cool-down).
• Heart rate dynamically fluctuates with changes in autonomic tone.

Supraventricular Tachycardia (SVT)

• Encompasses paroxysmal or incessant tachycardias excluding ventricular tachycardia.
• Predominantly driven by re-entrant mechanisms (Atrioventricular Reciprocating Tachycardia [AVRT], Atrioventricular Nodal Reentry Tachycardia [AVNRT]).
• Occasionally driven by abnormal ectopic automaticity (Ectopic Atrial Tachycardia, Junctional Ectopic Tachycardia).
• Characterized by abrupt, sudden onset and termination.
• Heart rate remains rigidly fixed without beat-to-beat variability.

Clinical Presentation

Sinus Tachycardia

• Symptoms reflect underlying systemic illness or hemodynamic compromise.
• Resolves completely upon correction of underlying etiology.

SVT

• Infants manifest heart failure signs if prolonged: poor feeding, ashen color, tachypnea, hepatomegaly, irritability.
• Older children report palpitations, precordial discomfort, shortness of breath, dizziness.
• Exacerbated by caffeine, nonprescription decongestants, bronchodilators.
• In utero SVT causes hydrops fetalis.

Electrocardiographic (ECG) Differentiation

Feature	Sinus Tachycardia	Supraventricular Tachycardia (SVT)
Heart Rate	Variable; typically <230 bpm (rarely up to 240 bpm).	Fixed; 180-320 bpm (240-300 bpm in infants).
Rate Dynamics	Varies with respiration and autonomic tone.	Abrupt changes; rigidly regular rate.
P-Wave Presence	Always present.	Often hidden within QRS or T wave.
P-Wave Axis	Normal (Upright in leads I, aVF; inverted in aVR).	Abnormal or retrograde (Inverted in II, III, aVF).
P-QRS Relationship	Strict 1:1 conduction.	1:1 conduction usual, but variable in specific subtypes.
QRS Duration	Narrow.	Narrow (wide with aberrancy or antidromic AVRT).

Diagnostic Maneuvers (Adenosine Challenge)

Sinus Tachycardia Response

• Transient slowing of heart rate.
• Transient atrioventricular (AV) block occurs without tachycardia termination.
• Tachycardia resumes original rate immediately after adenosine metabolism.

SVT Response

• AVNRT/AVRT abruptly terminates.
• Retrograde P-wave often visible as final component before termination.
• Unmasks underlying flutter waves in atrial flutter.
• Unmasks abnormal atrial activity in ectopic atrial tachycardia via transient AV block.

Management Strategies

Sinus Tachycardia

• Treat underlying physiological trigger (fluids, antipyretics, antibiotics).
• Antiarrhythmic medications strictly contraindicated.

SVT (Acute Management)

• Vagal Maneuvers: First-line non-pharmacological therapy. Ice bag applied to face (infants); Valsalva maneuver, straining, or breath-holding (older children). Ocular pressure absolutely contraindicated.
• Pharmacotherapy: Intravenous Adenosine (0.1 mg/kg rapid IV push, maximum 6 mg initial dose). Administer via rapid push followed by rapid saline flush. Increases AV node refractoriness, blocking re-entrant circuits.
• Synchronized DC Cardioversion: Indicated for severe heart failure or hemodynamic instability. Dose: 0.5-2 J/kg.

SVT (Chronic Management)

• Infants: Often resolves spontaneously by 1 year of age. Medical therapy (Digoxin, beta-blockers) tapered by 12-18 months.
• Adolescents: Lower spontaneous resolution probability. Catheter ablation (radiofrequency or cryoablation) serves as definitive curative therapy, boasting 90-98% success rates.