Stable vs Unstable Tachycardia

Street Context

Tachycardia on the monitor is not automatically an emergency cardioversion. A scared 28-year-old with sinus tach at 120 from pain and anxiety is not the same patient as a pale, hypotensive person with a regular wide-complex tachycardia at 190. Your job is to decide: is this rate causing (or about to cause) serious end-organ hypoperfusion?

Prehospital pressure is high — family wants “the shock,” or the opposite, “don’t shock Mom.” Stability is a clinical judgment based on the whole patient, not a single number on the NIBP.

What “Unstable” Usually Means (Education)

Educational ACLS-style framing treats instability as serious signs of poor perfusion likely due to the tachycardia. Common teaching markers include:

• Hypotension — shock-range BP for that patient (know their baseline when possible).
• Acutely altered mental status — not baseline dementia; new confusion, unresponsiveness, agitation from hypoperfusion.
• Shock signs — cool clammy skin, delayed cap refill, weak pulses, mottling.
• Ischemic chest pain — rate-related cardiac ischemia picture.
• Acute heart failure — acute pulmonary edema / severe dyspnea attributable to the rate.

Important: sinus tachycardia from fever, hypovolemia, pain, hypoxia, or PE is often “fast for a reason.” Shocking sinus tach is wrong. Treat the cause. True primary tachyarrhythmias (SVT, VT, AF with RVR, flutter) are where rate/rhythm interventions live.

Stable vs Unstable — How to Think on Scene

1. ABCs and pulse first — no pulse → arrest algorithm, not “stable tachycardia.”
2. Identify the rhythm family — narrow vs wide, regular vs irregular (use your systematic approach).
3. Ask whether the rate is the problem — 140 in septic shock may be compensatory; 220 regular narrow with AMS is more likely primary arrhythmia.
4. If unstable from a presumed tachyarrhythmia — prepare for synchronized cardioversion per protocol (pads on, sedation considerations if conscious per protocol, energy settings per protocol).
5. If stable — oxygen/support as needed, IV access, 12-lead when appropriate, vagal/med pathways only as protocol allows, continuous monitoring, destination planning.

Narrow vs Wide (Quick Layer)

• Regular narrow, very fast — often SVT teaching pathway; vagal maneuvers and adenosine only if your protocol and clinical picture support them in stable patients.
• Irregular narrow — AF with RVR is common; rate control agents only per protocol; unstable → synchronized cardioversion pathway.
• Wide and regular, unstable — treat as VT until proven otherwise in standard education frameworks.
• Wide and irregular — more complex (polymorphic VT/torsades, AF with aberrancy, WPW-related patterns) — follow protocol carefully; do not improvise unfamiliar drugs.

Key Points for Paramedics

• Stability is clinical — not “BP over 90 equals stable” as a rigid rule. Trend, mentation, and work of breathing matter.
• Synchronized cardioversion for unstable reentrant/ tachyarrhythmia pathways requires pads, sync mode on, sedation per protocol when the patient is conscious, and clear team roles.
• Do not cardiovert sinus tachycardia driven by underlying medical causes.
• Adenosine and antiarrhythmics are protocol drugs with specific indications, cautions (for example, irregular wide complexes), and monitoring requirements — this lesson does not authorize a dose.
• 12-lead when the patient is stable enough and protocol supports it — helps destination and ED handoff; do not delay life-saving therapy for a perfect tracing in a crashing patient.
• Communicate: “Unstable regular wide tach at 190 — preparing synchronized cardioversion per protocol” is better than “he’s in VT, I guess.”

Field Pitfalls

• Calling unstable solely because the rate is high while perfusion is excellent.
• Calling stable because BP is “okay once” while the patient is obtunded and mottled.
• Treating compensatory sinus tach as SVT with adenosine.
• Forgetting pulse checks and running a tachycardia algorithm on PEA/arrest.
• Unsynchronized defibrillation when sync cardioversion was indicated (or the reverse in VF).

Practice

For the next three tachycardias you see (or practice in CardioStrip), say out loud: rhythm family → likely primary vs compensatory → stable or unstable markers → next intervention class per your protocol.

Sources & Further Study

• ACLS tachycardia algorithm education concepts (stable vs unstable).
• Your local synchronized cardioversion and antiarrhythmic protocols.