What is the best treatment approach for a patient with unstable tachycardia, considering their underlying medical history and current clinical presentation?

Unstable Tachycardia: Immediate Synchronized Cardioversion

For any patient with unstable tachycardia showing signs of hemodynamic compromise (hypotension, altered mental status, chest pain, acute heart failure, or shock), proceed immediately to synchronized cardioversion without delay for pharmacologic therapy. 1, 2

Defining Instability

The key determination is whether the tachycardia is causing the patient's instability. Look for these specific signs of rate-related cardiovascular compromise: 1

• Acute altered mental status
• Ischemic chest discomfort
• Acute heart failure
• Hypotension or other signs of shock
• Severe dyspnea with respiratory distress

Critical Caveat on Heart Rate

If the ventricular rate is <150 beats/min in the absence of ventricular dysfunction, the tachycardia is more likely secondary to an underlying condition rather than the primary cause of instability—treat the underlying cause first. 1

Immediate Management Algorithm

Step 1: Initial Stabilization (Do Not Delay Cardioversion)

• Provide supplementary oxygen if hypoxemic or increased work of breathing 1
• Attach cardiac monitor and establish IV access 1
• Obtain 12-lead ECG only if it does not delay cardioversion 1
• Administer sedation if patient is conscious (but do not delay if extremely unstable) 1

Step 2: Immediate Synchronized Cardioversion

This is the definitive treatment for unstable tachycardia regardless of whether it is narrow-complex or wide-complex. 1, 2, 3

Energy Doses for Synchronized Cardioversion:

For narrow-complex SVT (atrial fibrillation, atrial flutter, other SVTs): 1

• Initial biphasic dose: 120-200 J
• Atrial flutter/other SVT: 50-100 J initially
• Increase stepwise if initial shock fails

For wide-complex tachycardia (presumed ventricular tachycardia): 1

• Monomorphic VT: 100 J initially (biphasic or monophasic)
• Increase stepwise if no response

For monophasic waveforms: 1

• Start at 200 J and increase stepwise

Step 3: Special Consideration for Narrow-Complex SVT

If the patient has a regular narrow-complex SVT and is NOT hypotensive, adenosine may be administered while preparing for synchronized cardioversion (Class IIb recommendation). 1 However, this should never delay cardioversion if the patient is truly unstable.

• First dose: 6 mg rapid IV push followed by normal saline flush 1
• Second dose: 12 mg if required 1
• Research supports adenosine can be effective even in unstable SVT patients, with conversion rates of 88-93% 4, 5, 6

Critical Pitfalls to Avoid

Do NOT Delay Cardioversion

The American College of Cardiology explicitly advises against delaying cardioversion in hemodynamically unstable patients to attempt pharmacological conversion or obtain additional diagnostic studies. 2, 3 Time is myocardium and brain.

Wide-Complex Tachycardia: Presume VT

An unstable patient with wide-complex tachycardia should be presumed to have ventricular tachycardia and treated with immediate cardioversion. 1, 2

Avoid These Medications in Unstable Patients

• Do NOT use calcium channel blockers (verapamil, diltiazem) for wide-complex tachycardia—they can cause hemodynamic collapse in VT 2, 3
• Do NOT use adenosine for irregular or polymorphic wide-complex tachycardia—it may cause degeneration to ventricular fibrillation 2, 3
• Avoid beta-blockers in hypotensive states 3

Precordial Thump Exception

For witnessed, monitored, unstable ventricular tachycardia, a precordial thump may be considered only if a defibrillator is not immediately ready (Class IIb). 1

Post-Cardioversion Management

After successful cardioversion: 3

• Admit to ICU/CCU for continuous monitoring
• Immediate cardiology/electrophysiology consultation
• Rapidly correct electrolyte abnormalities (potassium, magnesium, calcium)
• Consider electrophysiology study and possible ablation once stabilized

When Sinus Tachycardia is the Culprit

If the rhythm is determined to be sinus tachycardia, do not cardiovert. 1 Sinus tachycardia (rate >100 bpm, upper limit approximately 220 minus patient's age) is a physiologic response to underlying conditions such as:

• Fever
• Anemia
• Hypovolemia/shock
• Hypoxemia

Treat the underlying cause—"normalizing" a compensatory tachycardia can be detrimental when cardiac output depends on the rapid rate. 1