Sinus Tachycardia vs SVT

Supraventricular tachycardia (SVT) is a broad term encompassing paroxysmal or incessant tachycardias originating above or within the His bundle, characteristically presenting with a narrow QRS complex.
Sinus tachycardia is a physiological acceleration of the heart rate originating from the normal sinus node, universally driven by an underlying systemic cause.
Differentiating between these two entities is critical; sinus tachycardia necessitates identifying and treating the underlying systemic illness (e.g., sepsis, hypovolemia), whereas SVT requires targeted antiarrhythmic interventions, vagal maneuvers, or direct current cardioversion.

Comparative Clinical and Electrocardiographic Features

Feature	Sinus Tachycardia	Supraventricular Tachycardia (SVT)
Onset and Termination	Gradual acceleration ("warm-up") and gradual deceleration ("cool-down").	Abrupt, sudden onset and abrupt termination (classic for re-entrant SVT mechanisms).
Heart Rate Characteristics	Variable rate that visibly fluctuates with changes in autonomic tone and respiration.	Rigidly fixed R-R interval showing almost no beat-to-beat variation.
Peak Heart Rate	Varies by age and physiological stress, though it can occasionally reach up to 240 beats/min.	Usually >180 beats/min in older children/adolescents, and frequently 240–300 beats/min in neonates and infants.
P-Wave Morphology (ECG)	Always present with a normal, upright axis (positive in leads I, II, and aVF).	P waves are often hidden within the QRS/T wave, or demonstrate an abnormal retrograde axis (inverted in inferior leads II, III, and aVF).
P-QRS Relationship (ECG)	Strict 1:1 AV conduction.	Typically 1:1, but the P wave location depends on the specific re-entrant circuit (e.g., short VA interval in AVRT).
Response to Adenosine or Vagal Maneuvers	Produces only a transient slowing of the heart rate, which gradually accelerates back to the previous tachycardic rate.	Results in abrupt and sudden termination of the tachycardia (specifically for AV node-dependent circuits like AVRT and AVNRT).
Underlying Etiology	Secondary to systemic stressors such as fever, circulatory failure, severe dehydration, or pain.	Primary electrical abnormality, such as a concealed accessory pathway or dual AV nodal physiology.

Echocardiography is heavily utilized in the assessment of SVT to rule out associated structural congenital heart diseases (such as Ebstein anomaly or congenitally corrected transposition of the great arteries) and to screen for tachycardia-induced cardiomyopathy (left ventricular dilation and systolic dysfunction) resulting from prolonged, unrecognized episodes.
During an invasive cardiac electrophysiology study (EPS), re-entrant SVT can be reliably induced and terminated using programmed electrical stimulation and overdrive pacing, whereas sinus tachycardia cannot be induced or terminated in this manner.