Conceptual Overview

Atrial rhythms originate from the atrial myocardium or structures above the ventricles (supraventricular). They encompass a broad spectrum of dysrhythmias - from isolated premature beats that are nearly universal in adults to life-threatening tachyarrhythmias that compromise cardiac output.

The underlying mechanisms of atrial arrhythmias fall into three categories:

Enhanced automaticity: An ectopic atrial focus fires spontaneously at a rate exceeding the SA node (ectopic atrial tachycardia, MAT)
Triggered activity: Afterdepolarizations cause premature firing, often related to digoxin toxicity or electrolyte disturbances
Re-entry: A circuit of conduction sustains the arrhythmia (atrial flutter, AVNRT, AVRT)

For the emergency clinician, the critical questions when encountering an atrial rhythm are:

Is the patient hemodynamically stable or unstable?
Is the QRS narrow or wide?
Is the rhythm regular or irregular?
What is the ventricular rate?

Key concept: Supraventricular tachycardias (SVTs) are defined by their origin above the bundle of His. They typically produce narrow QRS complexes unless aberrant conduction or a pre-existing bundle branch block is present. A wide-complex tachycardia should be treated as ventricular tachycardia until proven otherwise.

Premature Atrial Complexes (PACs)

PACs are the most common atrial arrhythmia and are found in the majority of healthy adults on Holter monitoring. They arise from an ectopic atrial focus that fires before the next expected sinus beat.

ECG Characteristics

P wave morphology: Different from sinus P waves; shape depends on ectopic focus location
Timing: Occurs earlier than the expected sinus P wave
PR interval: May be shorter, normal, or longer than sinus PR depending on ectopic focus proximity to AV node
QRS: Typically narrow and identical to sinus beats
Compensatory pause: Usually incomplete (post-PAC interval less than two normal R-R intervals) because the PAC resets the SA node

Aberrantly Conducted PACs

When a PAC arrives early enough, it may encounter a partially refractory bundle branch (typically the right bundle, which has a longer refractory period). This produces a wide QRS that can mimic a PVC or ventricular tachycardia:

Ashman phenomenon: Aberrant conduction most likely when a short R-R interval follows a long R-R interval (the longer the preceding cycle, the longer the refractory period)
RBBB pattern: Most common aberrancy morphology (rsR' in V1)
Clue to PAC origin: Look for a preceding ectopic P wave buried in the T wave

Blocked PACs

Very early PACs may arrive when the AV node is still absolutely refractory, resulting in a P wave without a following QRS. Blocked PACs are a common cause of apparent pauses on telemetry - always look for a premature P wave deforming the preceding T wave.

PACs mimicking pathology: Frequent PACs in a bigeminal pattern can halve the effective heart rate if every other PAC is blocked. Blocked PACs are one of the most commonly missed causes of bradycardia on telemetry - scrutinize the T waves.

Atrial Fibrillation

Atrial fibrillation (AFib) is the most common sustained cardiac arrhythmia, affecting 2-3% of the population. It is characterized by chaotic, disorganized atrial activity with multiple wavefronts of re-entry circulating through the atrial myocardium simultaneously.

ECG Characteristics

Atrial activity: No discrete P waves; replaced by rapid, irregular fibrillatory waves (f waves) at 350-600 bpm
Ventricular response: Irregularly irregular R-R intervals (hallmark finding)
QRS: Typically narrow unless pre-existing BBB, aberrant conduction, or accessory pathway
Rate: Uncontrolled rates typically 100-180 bpm; may be slower with AV nodal disease or rate-controlling agents

Classification

Swipe to see more

Type	Definition	Management Implications
Paroxysmal	Self-terminating within 7 days (usually <48 hours)	Rhythm control often attempted; anticoagulation based on CHA₂DS₂-VASc
Persistent	Sustained >7 days; requires intervention to terminate	Cardioversion considered after adequate anticoagulation or TEE
Long-standing persistent	Continuous >12 months	Rate control strategy more common; ablation may be considered
Permanent	Decision made to not pursue rhythm control	Rate control and anticoagulation are mainstays

Rate vs. Rhythm Control

Rate control targets: Resting HR <110 bpm (lenient) or <80 bpm (strict); agents include beta-blockers, calcium channel blockers (diltiazem/verapamil), digoxin
Rhythm control: Electrical or pharmacologic cardioversion; antiarrhythmics (amiodarone, flecainide, propafenone); catheter ablation for refractory cases
Acute unstable AFib with RVR: Synchronized cardioversion per ACLS

AFib with pre-excitation (WPW): AV nodal blockers (diltiazem, verapamil, digoxin, adenosine) are contraindicated in AFib with WPW. These agents block the AV node and force conduction down the accessory pathway, potentially causing ventricular fibrillation. Use procainamide or ibutilide, or perform electrical cardioversion.

The 48-hour rule: AFib of unclear or >48 hours' duration carries a higher risk of left atrial thrombus. Cardioversion should be preceded by 3 weeks of therapeutic anticoagulation OR a transesophageal echocardiogram (TEE) to rule out left atrial appendage thrombus, followed by 4 weeks of post-cardioversion anticoagulation.

Atrial Flutter

Atrial flutter is a macro-reentrant circuit within the atrium, most commonly involving the cavotricuspid isthmus (typical flutter). Unlike the chaotic activity of AFib, flutter produces organized, repetitive atrial depolarization at a consistent rate.

ECG Characteristics

Flutter waves (F waves): Sawtooth pattern best seen in leads II, III, aVF, and V1
Atrial rate: Typically 250-350 bpm (most commonly ~300 bpm)
AV conduction: Usually 2:1 block, yielding a ventricular rate of ~150 bpm; can be 3:1, 4:1, or variable
QRS: Narrow unless pre-existing conduction abnormality
Regularity: Regular with fixed conduction ratio; irregular with variable block

Typical vs. Atypical Flutter

Typical (counterclockwise): Most common; negative sawtooth F waves in inferior leads; positive in V1. Circuit travels counterclockwise around the tricuspid annulus through the cavotricuspid isthmus
Typical (clockwise/reverse): Less common; positive F waves in inferior leads; negative in V1
Atypical: Does not use the cavotricuspid isthmus; may involve left atrium or atrial scars. F wave morphology is variable

"Think 150, think flutter": Any regular narrow-complex tachycardia at ~150 bpm should raise suspicion for atrial flutter with 2:1 conduction. The second set of flutter waves often hides in the QRS or T wave. Use vagal maneuvers or adenosine to transiently increase AV block and unmask flutter waves - but remember, these interventions will not terminate flutter.

Flutter vs. AFib management overlap: Atrial flutter carries the same thromboembolic risk as atrial fibrillation. Anticoagulation decisions follow the same CHA₂DS₂-VASc scoring. Flutter is more amenable to catheter ablation (cavotricuspid isthmus ablation has >95% success rate for typical flutter).

Supraventricular Tachycardia (SVT)

SVT is an umbrella term for any tachyarrhythmia originating above the ventricles, but in clinical practice it most commonly refers to the paroxysmal re-entrant tachycardias: AVNRT and AVRT.

AV Nodal Re-entrant Tachycardia (AVNRT)

The most common regular SVT (~60% of cases). A re-entrant circuit within or near the AV node involving dual pathways (fast and slow):

Rate: 140-280 bpm (typically 150-250 bpm)
Regularity: Very regular
P waves: Usually buried in the QRS (simultaneous atrial and ventricular activation) or seen as pseudo-R' in V1 / pseudo-S in inferior leads
QRS: Narrow (unless aberrancy)
Onset/offset: Abrupt start and stop (paroxysmal)

AV Re-entrant Tachycardia (AVRT)

Uses an accessory pathway (bypass tract) as one limb of the re-entrant circuit (~30% of SVT cases). Associated with Wolff-Parkinson-White (WPW) syndrome:

Orthodromic AVRT: Antegrade conduction through AV node, retrograde through accessory pathway. Narrow QRS; retrograde P waves visible after QRS
Antidromic AVRT: Antegrade through accessory pathway, retrograde through AV node. Wide QRS that mimics VTach - much less common and more dangerous
WPW pattern (sinus rhythm): Short PR (<0.12 sec), delta wave, wide QRS

Acute Management of SVT

Vagal maneuvers: Modified Valsalva (strain with leg elevation), carotid sinus massage, ice to face
Adenosine: 6 mg rapid IV push → 12 mg → 12 mg. Must be given rapidly with saline flush via proximal IV. Briefly blocks AV node conduction to terminate re-entrant circuit
If refractory: IV diltiazem (0.25 mg/kg) or IV verapamil (2.5-5 mg); IV metoprolol; synchronized cardioversion if hemodynamically unstable

Adenosine caution: Adenosine is diagnostic AND therapeutic for SVT. If the wide-complex tachycardia is actually VTach, adenosine will generally not convert it (though it is not harmful in most cases). However, adenosine is contraindicated in irregular wide-complex tachycardia suspicious for pre-excited AFib (WPW + AFib).

Multifocal Atrial Tachycardia (MAT)

MAT is an irregular supraventricular rhythm caused by multiple automatic foci within the atria firing independently. It is strongly associated with severe pulmonary disease.

ECG Criteria

P wave morphology: At least 3 distinct P wave morphologies in the same lead
Rate: >100 bpm (if <100 bpm, termed "wandering atrial pacemaker")
Rhythm: Irregularly irregular (commonly confused with AFib)
PR intervals: Variable (different conduction times from different foci)
PP and RR intervals: Variable
Isoelectric baseline: Present between P waves (unlike AFib)

Common Associations

COPD (most common - up to 50% of cases)
Acute exacerbation of chronic lung disease
Decompensated heart failure
Theophylline use
Hypomagnesemia, hypokalemia
Sepsis, critical illness

Management

Treat the underlying cause: Optimize oxygenation, correct electrolytes (especially Mg²⁺ and K⁺), treat infection
Magnesium sulfate: 2g IV is first-line even with normal serum Mg²⁺ levels
Rate control: Non-dihydropyridine calcium channel blockers (diltiazem/verapamil) if tolerated hemodynamically
Avoid: Cardioversion (ineffective), digoxin (may worsen), beta-blockers with caution in COPD patients

MAT vs. AFib: Both are irregularly irregular, but MAT has organized, identifiable P waves (≥3 morphologies) with an isoelectric baseline between them. AFib has no discrete P waves - just chaotic fibrillatory activity. This distinction matters because the management is completely different. Cardioversion does NOT work for MAT.

Other Atrial Rhythms

Ectopic Atrial Tachycardia (EAT)

A focal atrial rhythm originating from a single ectopic focus with enhanced automaticity:

Rate: 100-250 bpm
P wave: Abnormal morphology (not sinus); consistent beat-to-beat (unlike MAT)
Warm-up/cool-down: Gradual acceleration at onset and deceleration at termination (unlike abrupt onset/offset of re-entrant SVT)
Response to adenosine: May transiently slow or unmask P waves but typically does NOT terminate the rhythm (unlike AVNRT/AVRT)
Treatment: Beta-blockers, calcium channel blockers; ablation for refractory cases

Wandering Atrial Pacemaker

Same criteria as MAT (≥3 P wave morphologies, variable PR and PP intervals) but heart rate <100 bpm
Generally benign and often seen in young, healthy individuals or with increased vagal tone
No specific treatment required

Junctional Rhythms

Originate from the AV junction (AV node or bundle of His). Technically supraventricular but classified separately:

Junctional escape rhythm: 40-60 bpm; occurs when SA node fails or sinus rate drops below junctional rate
Accelerated junctional rhythm: 60-100 bpm; often benign, seen with digoxin use, post-cardiac surgery, inferior MI
Junctional tachycardia: >100 bpm; consider digoxin toxicity, myocarditis, post-surgical
P waves: Absent, retrograde (inverted in II, III, aVF), or buried in QRS
QRS: Narrow (unless pre-existing BBB)

Quick Reference

Swipe to see more

Rhythm	Rate	Regularity	Key ECG Feature
Sinus tachycardia	100-180	Regular	Normal sinus P before each QRS
AVNRT	150-250	Regular	No visible P waves; pseudo-R' in V1
AVRT (orthodromic)	150-250	Regular	Retrograde P after QRS; short RP interval
Atrial flutter	~150 (2:1)	Regular (fixed block)	Sawtooth F waves in II, III, aVF
Atrial fibrillation	Variable	Irregularly irregular	No P waves; chaotic baseline
MAT	>100	Irregularly irregular	≥3 P wave morphologies; variable PR
Ectopic atrial tachycardia	100-250	Regular	Abnormal P; warm-up/cool-down
Junctional	40-60	Regular	Absent/retrograde P; narrow QRS

Clinical Pearls

"Irregularly irregular = two diagnoses": When you see an irregularly irregular rhythm, your differential is short: atrial fibrillation or MAT. Look for organized P waves with an isoelectric baseline (MAT) vs. chaotic fibrillatory baseline (AFib). The distinction drives completely different management.

"Rate of 150 = flutter until proven otherwise": A regular narrow-complex tachycardia at exactly ~150 bpm is atrial flutter with 2:1 conduction until you can prove it isn't. Use adenosine or vagal maneuvers to unmask the flutter waves hiding in the T wave. This won't terminate flutter, but it will reveal the diagnosis.

"SVT that breaks with adenosine = re-entrant circuit": If adenosine terminates the arrhythmia, you've confirmed the AV node is part of the re-entrant circuit (AVNRT or AVRT). If adenosine only slows the ventricular rate without terminating, think atrial tachycardia, atrial flutter, or MAT - the arrhythmia doesn't depend on the AV node.

"Always check magnesium": Hypomagnesemia is a frequently overlooked cause of refractory atrial arrhythmias, particularly MAT. Empiric IV magnesium (2g) is safe and often effective, even when serum levels are normal (serum Mg²⁺ poorly reflects total body stores).

The WPW trap: Pre-excited atrial fibrillation (AFib conducting over an accessory pathway) produces an irregular wide-complex tachycardia with variable QRS morphology. Giving AV nodal blockers (diltiazem, verapamil, adenosine, digoxin) can be fatal. When in doubt, treat wide-complex tachycardia as ventricular tachycardia or use procainamide. If unstable, cardiovert.

Modified Valsalva technique: The REVERT trial showed that a modified Valsalva (blowing into a syringe for 15 seconds while sitting semi-recumbent, then immediately lying supine with passive leg raise for 15 seconds) has a 43% conversion rate for SVT vs. 17% for standard Valsalva. Try this before reaching for adenosine.

References

Page RL, Joglar JA, Caldwell MA, et al. 2015 ACC/AHA/HRS Guideline for the Management of Adult Patients With Supraventricular Tachycardia. Circulation. 2016;133(14):e506-e574.
January CT, Wann LS, Calkins H, et al. 2019 AHA/ACC/HRS Focused Update of the 2014 Guideline for Management of Patients With Atrial Fibrillation. Circulation. 2019;140(2):e125-e151.
Appelboam A, Reuben A, Mann C, et al. Postural modification to the standard Valsalva manoeuvre for emergency treatment of supraventricular tachycardias (REVERT): a randomised controlled trial. Lancet. 2015;386(10005):1747-1753.
Brugada J, Katritsis DG, Arbelo E, et al. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia. Eur Heart J. 2020;41(5):655-720.
ECG Library - LITFL - Life in the Fast Lane. https://litfl.com/ecg-library/