Tachycardia Algorithm: Recognizing Stable vs. Unstable

Tachycardias are among the most common arrhythmias you encounter in clinical practice – whether in the emergency department, the intensive care unit, or the prehospital setting. The critical question is not primarily "What rhythm is this?" but rather "How is my patient doing?". The ACLS tachycardia algorithm from the American Heart Association provides you with a structured, reproducible approach that starts precisely with this initial clinical assessment. This article walks you systematically through the algorithm – from the initial stability assessment through the differentiation of narrow and wide complex tachycardias to specific treatment decisions including dosages.

Definition and Entry into the Algorithm

The ACLS tachycardia algorithm applies to any symptomatic tachycardia with a heart rate above 150/min. Heart rates between 100 and 150/min may also be clinically relevant but are less commonly the cause of hemodynamic instability – in these cases, the tachycardia is often secondary (e.g., due to fever, hypovolemia, pain). The algorithm therefore always begins with the same basic steps:

• Secure the airway, administer oxygen if indicated
• Establish monitoring: ECG (12-lead if possible), pulse oximetry, blood pressure
• Establish IV access
• Identify and treat reversible causes (Hs and Ts)

Only then comes the central decision point: Is the patient stable or unstable?

Stable vs. Unstable: The Key Clinical Decision

The distinction between hemodynamic stability and instability is the most important decision point in the entire algorithm. It determines whether you have time for a differentiated rhythm analysis or whether you must immediately proceed to electrical cardioversion.

Signs of Hemodynamic Instability

The AHA defines four cardinal signs of instability that you can remember with the acronym "SHAB" – with the critical caveat that these symptoms must be caused by the tachycardia:

• S – Severe hypotension (systolic blood pressure < 90 mmHg or signs of shock)
• H – Heart failure signs (acute pulmonary edema, jugular venous distension)
• A – Altered mental status (altered consciousness, confusion, syncope)
• B – Breast pain (chest pain – ischemic in character, indicating myocardial hypoperfusion)

A common clinical pitfall: Not every tachycardia with symptoms is unstable. Palpitations, malaise, or mild dyspnea without hemodynamic compromise indicate a stable situation. The decisive question is: Is the tachycardia causing significant end-organ hypoperfusion?

Unstable Tachycardia: Immediate Cardioversion

If there is clear hemodynamic instability, synchronized cardioversion is the treatment of choice – regardless of whether it is a narrow or wide complex tachycardia. You do not have time for a differentiated rhythm analysis or pharmacological therapy attempts in this situation.

Procedure for synchronized cardioversion:

1. Sedation/analgesia if the patient's condition allows (e.g., midazolam 1–2 mg IV or propofol 0.5–1 mg/kg IV titrated – caution: further blood pressure reduction in already unstable patients)
2. Activate sync mode on the defibrillator – verify that sync markers appear on the R-waves
3. Energy selection:
   • Narrow complex tachycardia (regular): Start with 50–100 J (biphasic)
   • Atrial fibrillation: Start with 120–200 J (biphasic)
   • Wide complex tachycardia: Start with 100 J (biphasic)
4. If unsuccessful: Escalate energy and cardiovert again
5. After each shock: Reactivate sync mode (many defibrillators automatically revert to asynchronous mode after delivery)

Important: If the tachycardia degenerates into ventricular fibrillation or pulseless ventricular tachycardia, you immediately switch to the cardiac arrest algorithm with asynchronous defibrillation.

A special case is unstable polymorphic ventricular tachycardia (Torsades de Pointes): Since there are no uniform R-waves for synchronization, asynchronous defibrillation is performed – just as with ventricular fibrillation.

Stable Tachycardia: Time for Rhythm Analysis

If the patient is hemodynamically stable, you have time for a systematic rhythm analysis. The key lies in assessing the QRS complex:

• Narrow QRS complex (< 120 ms): The impulse is conducted through the regular His-Purkinje system – the origin is supraventricular.
• Wide QRS complex (≥ 120 ms): The origin may be ventricular, or it may be a supraventricular tachycardia with aberrant conduction (bundle branch block, accessory pathway).

Narrow Complex Tachycardias: Regular vs. Irregular

Regular Narrow Complex Tachycardia

The most common causes of a regular narrow complex tachycardia are:

• AV nodal reentrant tachycardia (AVNRT) – the most common form of SVT
• AV reentrant tachycardia (AVRT) – via an accessory pathway (e.g., WPW syndrome)
• Atrial flutter with regular conduction (e.g., 2:1)
• Atrial tachycardia

Therapeutic approach:

1. Vagal maneuvers as the first step:

• Modified Valsalva maneuver (straining against a closed glottis for 15 seconds, then lying flat and passively raising the legs)
• Carotid sinus massage (caution: rule out carotid stenosis, perform unilaterally)
• Conversion rate of vagal maneuvers: approximately 20–25%, with the modified Valsalva up to 40%

2. Adenosine if vagal maneuvers are ineffective:

• First dose: 6 mg as a rapid IV bolus, followed by a 20 mL normal saline flush
• Administration via the most proximal vein possible (antecubital vein preferred)
• If no conversion after 1–2 minutes: 12 mg as a rapid IV bolus
• If needed, another 12 mg after an additional 1–2 minutes
• Caution: Contraindicated in known WPW with atrial fibrillation (risk of triggering ventricular fibrillation); in heart transplant recipients, halve the dose (3 mg initially)

Adenosine has a half-life of less than 10 seconds – side effects (flushing, chest tightness, brief asystole) are unpleasant but self-limiting. You should warn the patient about these beforehand.

3. Calcium channel blockers or beta-blockers if adenosine is ineffective:

• Verapamil: 2.5–5 mg IV over 2 minutes, may repeat with 5–10 mg after 15–30 minutes (maximum dose 20 mg)
• Diltiazem: 0.25 mg/kg IV over 2 minutes, may repeat with 0.35 mg/kg after 15 minutes
• Metoprolol: 5 mg IV slowly, repeatable every 5 minutes up to a maximum of 15 mg
• Important: Verapamil/diltiazem and beta-blockers should never be combined – risk of severe bradycardia and AV block

Irregular Narrow Complex Tachycardia

By far the most common cause is atrial fibrillation, less commonly atrial flutter with variable conduction or multifocal atrial tachycardia (MAT).

For stable atrial fibrillation, two strategies are available:

• Rate control (the initial approach in most cases): Beta-blockers or calcium channel blockers at the dosages listed above; alternatively, amiodarone if ventricular function is impaired
• Rhythm control (pharmacological cardioversion): e.g., amiodarone 150 mg IV over 10 minutes, then maintenance dose; alternatively flecainide or propafenone in structurally normal hearts (pill-in-the-pocket concept)

Caution: In atrial fibrillation lasting more than 48 hours (or of unknown duration), there is an increased risk of thromboembolism – cardioversion should only be performed after TEE exclusion of atrial thrombi or after adequate anticoagulation, unless the patient is hemodynamically unstable.

Wide Complex Tachycardias: The Critical Scenario

Fundamental Rule: When in Doubt, Assume Ventricular Tachycardia

Every wide complex tachycardia should be treated as ventricular tachycardia (VT) until proven otherwise. This rule is clinically life-saving, as misinterpreting a VT as SVT with aberrant conduction and subsequently administering verapamil or diltiazem can lead to fatal ventricular fibrillation.

Clues suggesting VT (vs. SVT with aberration):

• AV dissociation (independent P-waves visible)
• Fusion beats, capture beats
• Concordance in the precordial leads (all QRS complexes pointing in the same direction)
• QRS width > 160 ms
• Extreme axis deviation (northwest axis)
• Positive Brugada or Vereckei criteria
• Known history of structural heart disease

Regular Wide Complex Tachycardia

For stable, monomorphic VT or wide complex tachycardia of uncertain origin:

• Amiodarone: 150 mg IV over 10 minutes, repeatable if needed. Then maintenance infusion: 1 mg/min over 6 hours, followed by 0.5 mg/min over 18 hours (maximum dose 2.2 g/24 h)
• Alternatively, procainamide: 20–50 mg/min IV until conversion, until hypotension occurs, until the QRS complex widens by > 50%, or until a maximum dose of 17 mg/kg is reached
• If SVT with aberration is confirmed (e.g., known bundle branch block, prior ECG available for comparison): Adenosine can be used as a diagnostic and therapeutic trial – but only under full monitoring and with defibrillation readiness

Special case – Torsades de Pointes:

• Magnesium sulfate 1–2 g IV over 15 minutes – effective even in the setting of normal magnesium levels
• Treat the underlying cause: discontinue QT-prolonging medications, correct electrolytes (target potassium > 4.0 mmol/L)
• If hemodynamically unstable: asynchronous defibrillation

Irregular Wide Complex Tachycardia

The most important differential diagnoses are:

• Atrial fibrillation with aberrant conduction (bundle branch block)
• Atrial fibrillation in WPW syndrome (antidromic conduction via the accessory pathway – wide, bizarre QRS complexes with varying morphology)
• Polymorphic VT

Atrial fibrillation in WPW syndrome is a particularly dangerous scenario: AV node-blocking agents (adenosine, verapamil, diltiazem, digoxin) are absolutely contraindicated, as they can accelerate conduction over the accessory pathway and trigger ventricular fibrillation. The treatment of choice is electrical cardioversion or – if hemodynamically stable – procainamide or ibutilide.

Common Pitfalls and Clinical Pearls

• Sinus tachycardia is not a diagnosis in itself, but a symptom. Treat the cause (hypovolemia, sepsis, pain, pulmonary embolism), not the rate.
• Adenosine as a diagnostic tool: Even if adenosine does not terminate an SVT, the transient AV block (a few seconds) can unmask the underlying rhythm – e.g., sawtooth pattern in atrial flutter or irregular atrial activity in atrial fibrillation.
• Forgetting sync mode: A classic error under stress. Unsynchronized shocks in a tachycardia with a pulse can hit the vulnerable window of the T-wave and trigger ventricular fibrillation.
• Failing to recognize instability: A patient with a rate of 180/min, a systolic blood pressure of 85 mmHg, and confusion needs cardioversion – not adenosine trials.
• Don't forget magnesium: In any polymorphic VT, especially with a prolonged QT interval, magnesium should be administered early.

Algorithm Summary at a Glance

1. Heart rate > 150/min + symptoms → Initiate the algorithm
2. Secure ABCs, monitoring, IV access, search for reversible causes
3. Unstable? (Hypotension, altered mental status, pulmonary edema, signs of ischemia)
   • Yes → Synchronized cardioversion (sedation if possible)
   • No → Proceed with rhythm analysis
4. Narrow QRS (< 120 ms)?
   • Regular → Vagal maneuvers → Adenosine → Calcium channel blockers/beta-blockers
   • Irregular → Suspect atrial fibrillation → Rate/rhythm control
5. Wide QRS (≥ 120 ms)?
   • Regular → Assume VT until proven otherwise → Amiodarone; if SVT is confirmed: adenosine possible
   • Irregular → Rule out WPW → No AV node blockers in WPW → Cardioversion or procainamide
6. With any deterioration: Re-evaluate, proceed to immediate cardioversion if needed

Practical Training

The tachycardia algorithm looks logical and straightforward on paper – the challenge lies in its execution under time pressure, when the monitor is alarming and the patient is becoming progressively more unstable. In the ACLS courses offered by Simulation Tirol, you train exactly these scenarios on high-fidelity simulators: rapid stability assessment, correct use of sync mode, differentiation of narrow and wide complex tachycardias, and confident application of pharmacological algorithms. The structured debriefing after each scenario helps you reflect on decision-making patterns and build confidence – so that in a real emergency, you act instead of hesitate.