10.1 Unstable Tachycardia & Synchronized Cardioversion

The First Question in Every Tachycardia Scenario

When a monitor shows a fast rate, PALS does not start with drug names. It starts with two linked questions:

1. Does the child have a pulse?
2. Is there cardiopulmonary compromise that is likely caused by the tachyarrhythmia?

If there is no definite pulse, this is cardiac arrest—follow the VF/pVT or asystole/PEA pathways (Chapter 8). Unsynchronized defibrillation is the electrical therapy for shockable arrest rhythms. If there is a pulse and the child is unstable from the rhythm, the electrical therapy is synchronized cardioversion, not the arrest defibrillation sequence and not a long stable-SVT adenosine delay.

Chapter 9 covered bradycardia and stable narrow-complex SVT. This chapter owns the unstable branch and wide-complex pathways. Exam items love to switch one variable—pulse present vs absent, stable vs unstable, narrow vs wide—and expect you to change therapy immediately.

What “unstable” means in PALS (cardiopulmonary compromise)

Unstable tachycardia means the rapid rhythm is producing serious signs of inadequate perfusion (cardiopulmonary compromise). Classic findings include:

You do not need every finding. One serious sign attributable to the tachyarrhythmia is enough to call the child unstable. Mild anxiety alone, or a rate that is high only because of fever and dehydration with clear sinus P waves, is not an automatic cardioversion indication—that is sinus tachycardia care (Chapter 9).

Attribute signs to the rhythm

Ask: Are these serious signs likely caused by the tachyarrhythmia itself? A child can be tachycardic and hypotensive for other reasons (septic shock with sinus tach). In pure sinus tachycardia you treat the cause, not the rate with cardioversion. When the ECG shows a primary arrhythmia (SVT, VT, flutter, etc.) and perfusion is collapsing because the rate/rhythm prevents adequate output, electricity is the rapid fix.

Synchronized Cardioversion: When and How

Indication (exam-level rule)

If the child has a pulse and has serious signs of cardiopulmonary compromise likely due to tachyarrhythmia → perform synchronized cardioversion.

Do not spend minutes stacking vagal maneuvers or adenosine in a hypotensive, poorly responsive infant in SVT. Do not start an antiarrhythmic infusion as the first move in monomorphic VT with shock. Support ABCs in parallel, but the rhythm-specific priority is a timed shock.

Energy doses (memorize cold)

Pediatric PALS teaching for synchronized cardioversion:

Worked examples:

• 10 kg infant: first 5–10 J, then 20 J.
• 20 kg child: first 10–20 J, then 40 J.
• 40 kg child: first 20–40 J, then 80 J (respect device steps and local protocol while staying in the weight-based framework).

Contrast this with arrest defibrillation energies from Chapter 8 (2 J/kg first, then 4 J/kg, max ≤10 J/kg or adult device max). Mixing these tables is a frequent exam error:

Sedation

Cardioversion is painful. If the child is conscious enough to experience the shock, provide sedation/analgesia when it will not delay urgently needed cardioversion. If the child is rapidly deteriorating—obtunded, profoundly hypotensive—do not withhold the shock waiting for ideal procedural anesthesia. Airway equipment must be ready because sedatives can worsen hypoventilation in a low-output state.

Sync mode mechanics (why it matters)

In SYNC mode the defibrillator delivers energy on or immediately after the R wave, avoiding the vulnerable T-wave period (relative refractory period). A shock on the T wave can precipitate VF (R-on-T). Before you press shock:

1. Confirm the device is in synchronized mode (markers on R waves).
2. Confirm energy is set to 0.5–1 J/kg (first attempt).
3. Clear the patient; deliver the shock; reassess rhythm and pulse.
4. Re-enable SYNC after each shock if another synchronized attempt is needed—many devices revert to unsync after discharge.

If synchronization fails or critically delays therapy (e.g., the device cannot lock onto R waves because of polymorphic morphology or artifact), or if the rhythm becomes VF, switch to unsynchronized defibrillation at arrest energies and treat as the shockable arrest algorithm.

Team choreography

• Pads applied early; defibrillator at bedside for every tachyarrhythmia
• Oxygen, airway support, IV/IO access in parallel—not as serial delays
• Continuous monitor; print/record strips when possible
• Closed-loop communication: “Charging to 10 joules, synchronized—clear—shocking”
• Immediate reassessment: conversion? Still pulse? Still compromised?

Pulse vs No Pulse: The Electrical Therapy Split

This single distinction prevents catastrophic mismanagement:

If you cardiovert without checking for a pulse and the child is actually in pVT arrest, you may underdose and mishandle the algorithm. If you defibrillate at 2 J/kg unsynchronized a conscious child in monomorphic VT with a pulse, you risk R-on-T and unnecessary trauma. Pulse check first; stability second; QRS width third for the stable branch.

Narrow vs wide when unstable

For unstable tachycardia with a pulse, synchronized cardioversion is indicated for both many narrow-complex (e.g., SVT) and wide-complex (e.g., monomorphic VT) rhythms. You do not need to finish a complex ECG differential before the first shock when the child is crashing. QRS morphology still matters for post-conversion care, antiarrhythmic choice if the child stabilizes, and expert consultation—but electricity first when compromise is present.

Polymorphic VT / torsades is a special case: QRS morphology changes beat-to-beat, sync may be unreliable, and many protocols treat sustained polymorphic VT without a pulse—or with ineffective output—as defibrillation. Even with a tenuous pulse, if the rhythm is polymorphic and the child is unstable, teams often use high-energy unsynchronized shocks because sync cannot track a stable R wave (Section 10.2).

Common exam traps

Clinical scenario (synthesis)

A 6-year-old with known SVT arrives pale and barely responsive. Heart rate is 240/min, blood pressure is low for age, capillary refill is 4 seconds, and pulses are weak but present. The monitor shows a regular narrow tachycardia without clear P waves. This is unstable SVT with a pulse. Apply pads, set SYNC, select approximately 0.5–1 J/kg, sedate only if it will not delay, clear, and cardiovert. If the first shock fails and compromise persists, escalate to 2 J/kg synchronized. If the child loses the pulse and the monitor shows VF, immediately switch to unsynchronized defibrillation at arrest energies and start CPR.

Bottom line for 10.1: Pulse + serious signs from tachyarrhythmia → synchronized cardioversion at 0.5–1 J/kg then 2 J/kg; sedation if safe; sync on; no pulse → arrest defibrillation pathway. Master that split and you will not confuse Chapters 8, 9, and 10 on exam day.