Emergency Medicine Hub / Cardiovascular Emergencies • 3 min read • 2 sections

Narrow Complex Tachycardias: SVT & Atrial Flutter

Comprehensive emergency evaluation and protocolized management of narrow-complex regular and irregular supraventricular tachycardias (QRS < 120 ms): Atrioventricular Nodal Re-entrant Tachycardia (AVNRT), Atrioventricular Reciprocating Tachycardia (AVRT), Atrial Flutter with fixed vs. variable conduction, and Focal Atrial Tachycardia; evidence-based REVERT modified Valsalva maneuver, rapid IV Adenosine administration technique, non-dihydropyridine calcium channel blockers vs. beta-blockers, synchronized electrical cardioversion, and avoiding the 'regular 150 bpm' atrial flutter misdiagnosis trap.

Resuscitation Quick Actions • First 2 Minutes

High-Acuity

The 150 bpm Flutter Rule

A regular narrow-complex tachycardia at 140–160 bpm is ATRIAL FLUTTER 2:1 until proven otherwise; slow the AV node to reveal sawtooth waves

REVERT Modified Valsalva

15-sec strain to 40 mmHg (blow into 10 mL syringe to move plunger), then IMMEDIATELY lay flat with legs elevated to 45° for 15 sec (43% success)

Adenosine Rapid Technique

6 mg rapid IV push via antecubital IV + immediate 20 mL NSS flush via stopcock; repeat with 12 mg rapid IV push in 1–2 min if no conversion

Adenosine Transplant Warning

Cardiac transplant patients or patients taking Dipyridamole/Carbamazepine have hypersensitive receptors: reduce Adenosine dose to 3 mg (1–3 mg)

Diltiazem (Cardizem) Dose

0.25 mg/kg IV over 2 min (~20 mg); if ineffective after 15 min, repeat with 0.35 mg/kg IV (~25 mg); then start 5–15 mg/hr infusion

Unstable Synchronized Shock

Synchronized cardioversion at 50–100J biphasic for unstable SVT; 50J biphasic for unstable Atrial Flutter

Bottom-Line Clinical Pearl

Any regular narrow-complex tachycardia running at exactly ~150 bpm is ATRIAL FLUTTER WITH 2:1 AV CONDUCTION until proven otherwise! In true flutter, an atrial rate of 300 bpm conducts through the AV node with 2:1 block, producing a ventricular rate of 150 bpm; look for inverted 'sawtooth' F-waves in inferior leads II, III, and aVF. For paroxysmal SVT (AVNRT/AVRT), the first-line non-pharmacologic intervention is the REVERT Modified Valsalva Maneuver (strain to 40 mmHg for 15 sec, then immediately supine with passive leg raise to 45 degrees for 15 sec), which converts > 43% of patients to sinus rhythm (vs 17% standard). If vagal maneuvers fail, administer rapid IV push Adenosine (6 mg then 12 mg through a proximal large-bore IV with a 20 mL rapid saline flush).

1. Differential Diagnosis of Narrow-Complex Tachycardias

Narrow-complex tachycardias originate at or above the bundle of His, possessing a QRS duration $< 120\text{ ms}$ (unless co-existing baseline bundle branch block or rate-dependent aberrancy is present). Diagnostic branching depends on ventricular regularity:

Ventricular Rhythm	Primary Electrocardiographic Entities	Underlying Electrophysiological Mechanism
Regular Narrow-Complex Tachycardia	1. AV Nodal Re-entrant Tachycardia (AVNRT) (60% of paroxysmal SVT)<br>2. Orthodromic AVRT (30% of paroxysmal SVT)<br>3. Atrial Flutter with fixed 2:1 conduction<br>4. Sinus Tachycardia (gradual onset/offset, normal P waves)<br>5. Focal Atrial Tachycardia (ectopic atrial focus 150–200 bpm)	Micro-re-entry within dual AV nodal pathways (fast and slow pathways in AVNRT); macro-re-entry using an extranodal accessory pathway (AVRT); macro-re-entrant cavotricuspid isthmus circuit (Atrial Flutter).
Irregular Narrow-Complex Tachycardia	1. Atrial Fibrillation (chaotic fibrillatory waves, no distinct P waves)<br>2. Atrial Flutter with variable AV block (irregular F-waves)<br>3. Multifocal Atrial Tachycardia (MAT) ($\ge 3$ distinct P-wave morphologies; COPD)	Multiple chaotic wandering micro-re-entrant wavelets in left atrium (AF); variable AV nodal refractoriness filtering flutter waves; multiple competing atrial pacemaker foci (MAT).

2. The REVERT Modified Valsalva Maneuver

The landmark REVERT randomized controlled trial demonstrated that the modified Valsalva maneuver achieves cardioversion to normal sinus rhythm in 43.7% of patients compared to just 17.0% with the standard maneuver:

Step Sequence	Physical Maneuver Technique	Physiologic Mechanism
Step 1: Strain Phase	Patient seated upright: forcefully blow into the nozzle of a 10 mL plastic syringe to move the plunger (generating an intra-thoracic pressure of 40 mmHg) for 15 seconds	Elevates intrathoracic pressure, decreasing venous return and stroke volume, activating sympathetic tone.
Step 2: Passive Leg Raise	At exactly 15 seconds, immediately lay the patient completely flat (supine) while an assistant elevates their legs to a $45^\circ$ angle for 15 seconds	Massive autologous blood autotransfusion abruptly increases venous return to the right atrium under normal atmospheric pressure.
Step 3: Repositioning	Return the patient to the upright seated position for 30 seconds to assess rhythm	Profound, intense vagal parasympathetic discharge terminates AV nodal re-entry without pharmacotherapy.

Critical Pitfall / Contraindication

THE 'REGULAR 150 BPM' ATRIAL FLUTTER TRAP: When encountering an ECG with a regular narrow-complex tachycardia at a rate between 140 and 160 bpm (most commonly exactly 150 bpm), never assume it is sinus tachycardia or standard AVNRT without proof. In classic counterclockwise Atrial Flutter, the atrial rate is 300 bpm. Because the healthy AV node can rarely conduct at 300 bpm, it defaults to a 2:1 block, yielding a ventricular rate of precisely 150 bpm. One flutter wave is buried directly inside the QRS/T wave and is easily missed! Administering Adenosine will temporarily block the AV node, unmasking the continuous 'sawtooth' flutter waves and establishing the diagnosis.

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