Stable vs Unstable Tachycardia

Why this matters

Check for a pulse, then look at perfusion. A fast rate on the monitor is not automatically a cardioversion. A frightened 28-year-old with sinus tachycardia at 120 from pain and anxiety is not the same patient as a pale, hypotensive person with regular wide-complex tachycardia at 190. Ask one question: is the rate causing, or about to cause, poor blood flow to the organs?

The scene feels urgent. Family may push for a shock, or they may not want one. Stability is still a judgment of the whole patient, not one number on the automatic cuff. Do not use a single rate as the cutoff.

Clinical sequence

1. Is there a pulse? If not, this is an arrest pathway, not “stable tachycardia.”
2. Name the rhythm family: narrow or wide, and regular or irregular.
3. Ask whether this is a primary fast rhythm or compensatory sinus tach from fever, bleeding, pain, hypoxia, or PE.
4. Judge stable versus unstable from perfusion. Then choose the next kind of treatment per protocol.

What “Unstable” Usually Means

Unstable means serious signs that organs are not getting enough blood, and that the fast rate is the likely cause. Common teaching signs include:

• Hypotension. The BP is in a shock range for that patient. Know the baseline when you can.
• New altered mental status. This is not baseline dementia. Look for new confusion, unresponsiveness, or agitation from poor perfusion.
• Shock signs. Cool clammy skin, delayed capillary refill, weak pulses, or mottling.
• Ischemic chest pain that the rate is causing.
• Acute heart failure. New pulmonary edema or severe shortness of breath caused by the rate.

Important: sinus tachycardia from fever, low volume, pain, hypoxia, or pulmonary embolism (PE) is often fast for a reason. Do not shock sinus tachycardia. Treat the cause. Rate and rhythm treatment is for a primary fast rhythm: supraventricular tachycardia (SVT), ventricular tachycardia (VT), atrial fibrillation with a rapid ventricular response (AF with RVR), or atrial flutter.

Stable vs Unstable — How to Think on Scene

1. Airway, breathing, circulation, and a pulse come first. No pulse means the arrest pathway, not a stable-tachycardia plan.
2. Name the rhythm family: narrow or wide, regular or irregular. Use your systematic approach.
3. Ask whether the rate is the problem. A rate of 140 in septic shock may be compensatory. A regular narrow rate of 220 with altered mental status is more likely a primary arrhythmia.
4. If unstable from a presumed tachyarrhythmia, prepare synchronized cardioversion per protocol. Put the pads on. Sedate if the patient is awake and protocol allows it. Protocol owns the energy.
5. If the patient is stable, give oxygen if the oxygen saturation is low, get intravenous access, and get a 12-lead when it is appropriate. Use vagal maneuvers or drugs only if your protocol allows them. Keep monitoring and choose the hospital.

Narrow vs Wide (Quick Layer)

• Regular, narrow, and very fast is often the SVT pathway. Use vagal maneuvers and adenosine only if your protocol and the clinical picture support them in a stable patient.
• Irregular and narrow: AF with RVR is common. Use rate-control drugs only per protocol. If unstable, use synchronized cardioversion.
• Wide, regular, and unstable: treat it as VT until proven otherwise.
• Wide and irregular is more complex. It may be polymorphic VT or torsades, AF with aberrancy, or a WPW (Wolff-Parkinson-White) pattern. Follow protocol. Do not improvise a drug you do not know.

Key points

• Stability is clinical. “BP over 90 means stable” is not a rigid rule. The trend, mental status, and work of breathing matter.
• Synchronized cardioversion for an unstable tachyarrhythmia needs pads, sync mode on, sedation per protocol if the patient is awake, and clear team roles.
• Do not cardiovert sinus tachycardia that is driven by another medical cause.
• Adenosine and antiarrhythmics are protocol drugs. They have specific indications, cautions such as irregular wide complexes, and monitoring requirements. This lesson does not authorize a dose.
• Get a 12-lead when the patient is stable enough and protocol supports it. It helps you choose the hospital and hand off to the ED. Do not delay life-saving treatment for a perfect tracing in an unstable patient.
• Say it clearly: “Unstable regular wide tachycardia at 190. Preparing synchronized cardioversion per protocol.” That is better than “I guess this is VT.”

Common errors

• Calling the patient unstable only because the rate is high while perfusion is good.
• Calling the patient stable because one BP looked acceptable while they are hard to wake and mottled.
• Treating compensatory sinus tach as SVT and giving adenosine.
• Skipping the pulse check and running a tachycardia plan on PEA or arrest.
• Using an unsynchronized shock when the patient still has a pulse and needs synchronized cardioversion, or leaving sync mode on during ventricular fibrillation.

Practice

Related: Wide-Complex Tachycardia: VT vs Mimics, Systematic ECG Approach.

Sources & Further Study

Named guidelines for further study. They are not clinical orders and do not replace your protocol or medical direction.

• NASEMSO National Model EMS Clinical Guidelines (2022) — U.S. EMS model language; protocol still wins
• AHA 2025 Part 9: Adult Advanced Life Support — instability is perfusion, not a single rate cutoff; do not cardiovert compensatory sinus tach
• Instability is a clinical perfusion judgment, not a single rate cutoff. Do not cardiovert compensatory sinus tachycardia.
• Your synchronized cardioversion and antiarrhythmic protocols.