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.