Sinus, Atrial & Junctional Rhythms

Objective 1.1.3 — Identify sinus, atrial, and junctional rhythms on a rhythm strip, including criteria, common causes, and the basic treatment approach.

This lesson is the rhythm catalog. Every rhythm here is identified by walking through the 5-step method from Lesson 2 and matching the answers to a known pattern. We'll focus on what makes each rhythm unique — the one or two findings that separate it from its neighbors.

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Recap — The 5-Step Method

See Lesson 2 for the full math behind each rate-calculation method.

Intrinsic pacemaker rates: SA node 60–100 · AV junction 40–60 · ventricular/Purkinje 20–40.

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Sinus Rhythms — General Characteristics

A normal heartbeat starts in the SA node and travels the normal conduction pathway.

ECG hallmarks of a sinus-origin rhythm:

• Positive P wave before each QRS complex
• P waves that look alike (uniform morphology)
• Consistent PR intervals
• Regular atrial and ventricular rhythm (usually)

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Normal Sinus Rhythm (NSR)

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Sinus Bradycardia

Common settings:

• Adults and children during sleep
• Well-conditioned athletes
• Present in up to 35% of people under 25 while at rest

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Sinus Tachycardia

If the SA node fires faster than normal for the patient's age. Begins and ends gradually. At very fast rates it may be hard to distinguish a P wave from a T wave; the QT interval normally shortens as the heart rate increases.

Common causes:

• Exercise
• Fever
• Pain
• Fear or anxiety
• Hypoxia

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Sinus Arrhythmia

The SA node fires irregularly — R-R intervals vary by ≥ 0.16 seconds (more than 4 small boxes).

When the rate is slower than normal it's called sinus brady-arrhythmia; when faster, sinus tachy-arrhythmia.

Two flavors

• Respiratory sinus arrhythmia — tied to phases of respiration and changes in intrathoracic pressure. Heart rate increases gradually during inspiration and decreases with expiration. Most often seen in children and adults under 30.
• Non-respiratory sinus arrhythmia — seen in older individuals and those with heart disease; may be due to medication effects.

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Sinus Arrest / Pause

The SA node fails to generate an impulse, and no escape pacemaker (AV junction or ventricles) takes over when it should. The result is absent PQRST complexes for one or more beats.

Common causes:

• Hypoxia
• Myocardial ischemia
• Myocardial infarction

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Atrial Rhythms — General Characteristics

An irritable site within the atria discharges before the next SA node impulse is due. The ectopic P wave can look very different from a sinus P wave.

ECG features the P wave may show:

• Biphasic (partly positive, partly negative)
• Flattened
• Notched
• Pointed
• Lost in the preceding T wave

Premature-Beat Patterns

These patterns describe how often premature beats fall — and apply equally to atrial, junctional, and ventricular ectopy.

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Premature Atrial Complex (PAC)

A single ectopic atrial beat that interrupts the underlying sinus rhythm.

Common causes:

• Atrial enlargement
• Emotional stress
• Electrolyte imbalance
• Stimulants (caffeine, tobacco, cocaine)

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Multiformed Atrial Rhythm (MAR)

Formerly known as Wandering Atrial Pacemaker. The pacemaker site shifts between the SA node, ectopic atrial sites, and the AV junction, so P-wave morphology changes from beat to beat.

Common causes:

• Digitalis toxicity
• Observed in healthy patients (athletes in particular)
• May be observed during sleep

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Multiformed Atrial Tachycardia (MAT)

MAR with a ventricular rate > 100 bpm. Also known as chaotic atrial tachycardia. May be a precursor to atrial fibrillation.

Common causes:

• Severe COPD (classic)
• Acute coronary syndromes
• Hypokalemia
• Hypomagnesemia

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Supraventricular Tachycardia (SVT)

An umbrella term for any tachycardia that begins above the bifurcation of the bundle of His — at the SA node, atrial tissue, or AV junction. SVT typically occurs at rest and is often paroxysmal ("paroxysmal" = starts or ends suddenly, e.g., PSVT, PAT).

Three SVT mechanisms

SVT criteria

Common causes:

• Excessive catecholamine release
• Digitalis toxicity
• Electrolyte imbalance
• Heart disease
• Stimulants (caffeine, albuterol, cocaine)

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Wolff-Parkinson-White (WPW) Syndrome

The most common type of pre-excitation syndrome. An accessory pathway lets the impulse bypass the AV node, pre-exciting the ventricles. Usually seen when the patient is not having tachycardia — most often manifests when it triggers AVRT. Noted to occur with atrial fibrillation. Most common cause of tachydysrhythmias in infants and children, with cases leading to sudden cardiac death.

Classic WPW triad on ECG

• Delta wave (slurred upstroke at the start of the QRS)
• Short PR interval
• Widened QRS complex

Common causes:

• Believed to be congenital
• More common in men than in women

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Atrial Flutter (A-Flutter)

An ectopic atrial rhythm in which an irritable site fires regularly at an extremely rapid rate. Usually paroxysmal but occasionally lasts hours. The ECG hallmark is a "saw-tooth" or flutter-wave baseline.

Two types

Common causes:

• Cardiac surgery
• Cardiomyopathy
• Pericarditis
• Myocarditis

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Atrial Fibrillation (A-Fib)

Caused by altered automaticity in one or several rapidly firing atrial sites, or re-entry involving one or more atrial circuits. Atrial sites fire at 400 – 600 bpm, causing the atrial muscles to quiver rather than contract effectively. Pooled blood in the atria can form clots → embolic stroke risk.

ECG hallmark: fibrillatory waves (a "squiggly" baseline) with QRS complexes scattered on top.

Common causes — usually occurs with known cardiovascular disease:

• CAD
• Hypertension
• Cardiomyopathy
• Alcohol abuse
• Obesity
• Medication side effects (antihistamines, beta-agonists, local anesthetics)

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Treatment — Atrial Rhythms

• Correct the underlying cause
• Consult cardiology
• Provide supplemental O₂ for symptomatic patients
• Synchronized cardioversion for persistent SVT / SVT-type dysrhythmias
• Adenosine for narrow-complex SVT
• Anticoagulation for A-fib present ≥ 48 hours (clot/stroke risk)

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Junctional Rhythms — General Characteristics

When the AV junction (AV node and non-branching portion of the bundle of His) paces the heart, the impulse must travel backward (retrograde) to depolarize the atria. If a P wave is seen, it will be inverted in leads II, III, and aVF because the impulse moves away from the positive electrode.

This is a slower mechanism than sinus or atrial pacing — intrinsic junctional rate is 40 – 60 bpm.

ECG hallmark — P waves are inverted, retrograde (after the QRS), or missing.

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Premature Junctional Complex (PJC)

A single early junctional beat that interrupts the underlying (typically sinus) rhythm. Because conduction through the ventricles proceeds normally, the QRS is usually narrow (≤ 0.11 s). Most patients are asymptomatic.

Common causes:

• Digitalis toxicity
• Electrolyte imbalance
• Stimulants
• Acute coronary syndromes

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Junctional Escape Beat (JEB) & Escape Rhythm

Unlike PJCs, junctional escape beats occur late — after the next expected sinus beat fails to come. Several sequential junctional escape beats form a junctional escape rhythm.

Patients may be asymptomatic or experience dizziness or syncope related to the slow rate.

Common causes:

• Acute coronary syndromes
• Medication effects (beta-blockers, digitalis, diltiazem)
• Hypoxia
• SA node disease

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Accelerated Junctional Rhythm

An ectopic rhythm caused by enhanced automaticity of the bundle of His, producing a regular ventricular response above the intrinsic junctional rate but not yet tachycardic. Same criteria as a junctional escape rhythm except for increased rate.

Common causes:

• Acute MI
• Digitalis toxicity
• Hypokalemia

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Junctional Tachycardia

An ectopic rhythm originating in the pacemaker cells of the bundle of His. Three or more sequential PJCs at a rate > 100 bpm.

Common causes:

• Acute coronary syndromes
• Digitalis toxicity
• Side effects of certain medications

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Treatment — Junctional Rhythms

• Correct the underlying cause
• Consult cardiology
• Provide supplemental O₂ for symptomatic patients (slow or fast rates)
• Temporary pacemaker (transcutaneous pacing) for slow junctional rhythms
• Adenosine for persistent junctional tachycardia (JT)

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High-Yield Recap

Sinus rhythms — what changes vs NSR?

Atrial rhythms — quick differentiator

Junctional rhythms — by rate

All junctional rhythms share the same P-wave story: inverted in II, III, aVF, or missing, or after the QRS. PR (when present) is ≤ 0.12 s. QRS is usually narrow.

Premature-beat patterns

Treatment shortcuts