9.2 Sinus Tachycardia vs SVT

Why This Discrimination Dominates PALS Scenarios

Few rhythm decisions generate more exam and megacode errors than confusing sinus tachycardia with supraventricular tachycardia (SVT). Both can produce very fast rates in infants and children. Only one is a primary electrical reentry problem that may need vagal maneuvers, adenosine, or cardioversion. The other is the heart’s appropriate response to an increased metabolic or circulatory demand. Cardioverting a febrile, dehydrated toddler in sinus tachycardia is the wrong therapy and can harm the child; delaying treatment of true SVT with poor perfusion is equally dangerous.

PALS expects you to use a bundle of clues—history, rate ranges, P waves, R-R pattern, and clinical context—not a single number in isolation.

Definitions (exam-level)

• Sinus tachycardia: Each QRS is preceded by a normal sinus P wave; the rate is fast for age because the sinus node is accelerating under physiologic stress (fever, hypovolemia, hypoxia, pain, anxiety, anemia, shock, medications such as bronchodilators).
• SVT (narrow-complex, most pediatric teaching cases): A reentrant supraventricular rhythm (commonly AV nodal reentrant tachycardia or AV reentrant tachycardia using an accessory pathway) that produces a usually regular, narrow QRS tachycardia, often without clearly visible normal sinus P waves. Onset is typically abrupt.

Wide-complex tachycardia and unstable management are covered in Chapter 10; this section focuses on narrow-complex discrimination and the "do not cardiovert sinus tach" rule.

Discrimination Table: History, Rate, ECG, Response

Heart-rate teaching thresholds — use carefully

PALS teaching commonly highlights:

• Infant SVT often ≥220/min
• Child SVT often ≥180/min

These are supportive thresholds, not absolute laws of nature. A severely shocked infant can approach very high sinus rates, and SVT can occasionally present at rates below the classic cutoff. Always integrate history + ECG morphology + variability. On exam items, when the vignette gives abrupt onset, rate ≥220 (infant) or ≥180 (child), no visible P waves, and rock-regular R-R, the answer is SVT. When the vignette gives fever and diarrhea, rate 170 in a toddler, visible sinus P waves, and a rate that varies, the answer is sinus tachycardia—treat volume and fever.

P waves and R-R variability in practice

Obtain a monitor strip and, when the child is stable enough, a 12-lead ECG. Look in multiple leads for P waves. In sinus tachycardia you should find upright P waves in the inferior leads (in normal situs) with a stable PR relationship. In SVT, P waves may be invisible in the T wave, inverted, or dissociated from a normal sinus pattern.

R-R variability is a high-yield bedside clue: print a long strip. Sinus tachycardia "breathes" and drifts. Classic reentrant SVT often looks metronomic.

Management Implications: Treat the Right Problem

Sinus tachycardia — treat the cause

Once you identify sinus tachycardia:

1. Do not perform synchronized cardioversion for rate control of sinus tachycardia.
2. Do not treat it as a primary arrhythmia with SVT drugs as the "cure."
3. Do search for and reverse the driver using the evaluate–identify–intervene loop:
   • Fever → antipyretic care and infection workup as indicated
   • Hypovolemia → isotonic fluid boluses with reassessment (Chapter 5)
   • Hypoxia / respiratory failure → oxygen and ventilation (Chapters 3–4)
   • Pain / anxiety → appropriate analgesia and calming measures
   • Shock / anemia / toxins → cause-specific therapy

As the cause improves, the sinus rate falls. That response is both therapy and confirmation of your diagnosis.

SVT — rhythm-directed therapy (preview)

If the rhythm is SVT:

• No cardiopulmonary compromise (stable enough): vagal maneuvers → adenosine pathway (Section 9.3).
• Cardiopulmonary compromise (unstable): synchronized cardioversion per PALS energies (Chapter 10), with sedation if the patient is conscious enough and it will not delay shock delivery excessively.

While you discriminate, still support ABCs: oxygen as needed, monitors, IV/IO access, and continuous reassessment. A child can transition from "stable SVT" to shock quickly, especially infants who present with heart-failure signs after hours of unrecognized SVT (poor feeding, irritability, hepatomegaly, rales).

Common exam traps

Clinical scenarios

Scenario A — Sinus tachycardia: A 2-year-old with 48 hours of vomiting and diarrhea has heart rate 172/min, dry mucous membranes, capillary refill 3 seconds, temperature 38.9°C. Monitor shows P waves before each QRS and mild R-R variability. Diagnosis: sinus tachycardia from hypovolemia/fever. Therapy: oxygen if needed, isotonic fluid boluses with reassessment, antipyretic care—not cardioversion.

Scenario B — SVT: A 3-week-old suddenly becomes pale and irritable. Heart rate is 280/min, R-R is fixed, no clear P waves. The infant has been feeding poorly for hours and now has delayed refill. Diagnosis: SVT (infant rate far above 220 teaching threshold). Therapy: support ABCs; if signs of poor perfusion/cardiopulmonary compromise, move toward urgent synchronized cardioversion; if relatively stable, ice-to-face vagal maneuver and prepare adenosine (9.3).

Master the discrimination table and you avoid the two lethal errors of this chapter: shocking sinus tach, and missing SVT.