Rhythms Originating from the Atrium

SA Node Irregularity

When the SA node fails to generate an impulse, a site outside the SA node but within the atria will generate an action potential faster than the SA node. The heart switches to the next fastest inherent rate beyond the SA node. 

If the SA node isn’t functioning, the atrial focus initially takes over, becoming the predominant pacemaker. The atria generates impulses which travel towards the Purkinje fibers via the atrioventricular junction before heading to the ventricles the same way they would if the signal came from the SA node. 

If the rate is faster than 100 beats per minute (bpm), the term “atrial tachycardia” describes the rhythm. If the rate is less than 100 bpm, it’s an “ectopic atrial” rhythm.

Impulses originating in the atrium change the normal morphology of the P wave. The atrial P wave is slower than the sinus P wave, and its morphology may be flattened, notched, or peaked. It has a saw-tooth or biphasic appearance.

A biphasic wave may produce a positive and negative deflection within one P wave. Rhythms originating from the atria will also have a narrow QRS complex because the impulses are generated above the ventricles.

Wandering Pacemaker

A wandering pacemaker has different foci of impulse generation throughout the atrium, switching between the sinoatrial node, atrium, and atrioventricular junction. The resulting rhythm is a combination of sinus and atrial beats. 

An impulse originating from the sinoatrial node presents a smooth, rounded P wave with an upward deflection. An impulse originating from the atrium presents a flattened, notched, or biphasic morphology. An impulse originating from the atrioventricular junction will result in a P wave which may look inverted or may be absent. 

The PR interval also varies as the impulse is generated from different foci.

P wave morphology changes from one complex to the next on a wandering pacemaker’s ECG strip.

• RATE: normal rate is between 60–100 bpm
• RHYTHM: varying R-R intervals due to changes in pacemaker sites
• P WAVE: morphology changes from one complex to the next. The P wave is followed by the QRS complex
• PR INTERVAL: varies but less than 0.20 seconds
• QRS COMPLEX: less than 0.12 seconds

Ectopic Beat

An ectopic beat is a single beat from a focus outside the SA node but within the conduction system.¹ They’re not an arrhythmia but an irritability in the conduction system, generating an impulse strong enough to override the impulse generated from the SA node. An ectopic beat may originate as an impulse from an escape mechanism. An ectopic beat arising from the atria is known as an atrial ectopic.

Premature Atrial Contraction

When irritability in the conduction system causes a beat to occur from an area in the atria other than the SA node, it’s a premature atrial contraction (PAC).

ECG strip detailing a premature atrial contraction (PAC).

Related Video – One Quick Question: What are PACs (Premature Atrial Contractions)? Video

PACs usually occur as a single beat. Clinicians should report the core or underlying rhythm and the type of ectopic beat (i.e., normal sinus rhythm with PAC) when reporting an ECG interpretation with a premature atrial contraction. A PAC may make the rhythm appear irregular, yet the clinician still measures R-R intervals on the 6-second strip to determine if the core rhythm is regular or irregular. 

The above strip details several PACs and the rhythm is quite irregular. PACs may appear in patterns, such as every other beat (bigeminy) or every third beat (trigeminy).

It is also possible a PAC occurs near the end of the QRS complex and the P wave, manifesting near the T wave. It may be entirely lost within the T wave. The below strip illustrates how the T wave appears more peaked than others.

Atrial Tachycardia

An irritable focus within the atria causes atrial tachycardia.³ The impulse generated is greater than the impulse from the SA node, the normal pacemaker of the heart. Individual complexes in atrial tachycardia have the characteristics of a PAC, including P wave morphology, PR-interval, and QRS complex. 

However, atrial tachycardia involves every beat instead of a single beat. Atrial flutter and multifocal atrial tachycardia are atrial tachycardia subtypes.

An ECG strip illustrating multifocal atrial tachycardia (MAT) with impulses originating in multiple foci within the atria. The arrows point to P waves with unusual morphology.

Atrial tachycardia is a regular arrhythmia with heart rates of 150–350 bpm.

• RATE: 150–350 bpm
• RHYTHM: regular (may be irregular in MAT)
• P WAVE: atrial P wave is different in morphology from a sinus P wave. The P wave may be buried within a T wave.
• PR INTERVAL: 0.12–0.20 seconds
• QRS COMPLEX: less than 0.12 seconds

Atrial Flutter

When the atria are intensely irritable, a cardiac rate above 250–350 bpm may occur. The irritability causes a rhythm known as atrial flutter, represented by a saw-tooth appearance of the P waves due to the cyclic pattern of generated impulses creating a series of rapid atrial waves.

An anti-clockwise pattern of impulse generation reveals a negative deflection of the P wave on the LEAD II.

If the impulse generated flows in an anti-clockwise manner, LEAD II shows a negative deflection of P waves.

When the rapid rate occurs, the ventricles do not have enough time to adequately fill, so there is not enough blood volume ejected to sufficiently meet the body’s needs. Fortunately, the atrioventricular node (AV node) regulates the heart, blocking some impulses from reaching the ventricles. That allows the ventricles to fill for adequate ejection into the systemic circulation.

The AV node allows impulses to pass through every second, third, or fourth P wave.⁴ A 12-lead ECG will detail a rapid succession of P waves with a rate between 250 and 350 bpm. A QRS complex does not follow every P wave, only manifesting every second, third, or fourth saw-tooth wave. That causes a slower ventricular rate compared to the atrial rate. 

The QRS complex only manifests every two, three, or four saw-tooth waves, creating a 2:1, 3:1, or 4:1 block. If the R-R interval is irregular, variable blocks are occurring.

Atrial Fibrillation

During atrial fibrillation, the atria are so intensely irritable they no longer beat but merely quiver.⁵ The ECG tracing displays barely visible fibrillatory waves within the isoelectric line, known as F waves. The quivering atria generate an impulse up to 350 bpm.

There are no discernable P waves in atrial fibrillation. The ventricular rate is produced by the AV node blocking most of the generated impulses, producing an irregular R-R interval pattern. The F waves are generated in such a chaotic manner no discernable pattern is produced.

In atrial fibrillation, the R-R interval pattern is irregular.

Atrial fibrillation is easily recognized. The only two characteristic findings are the absence of P waves and irregular rhythms resulting from irregular R-R intervals. As the ventricular rate increases, R-R intervals may become closer and appear to have a regular rhythm. 

A rapid ventricular rate above 100 bpm (or uncontrolled ventricular response) may induce symptoms in the patient, while a ventricular rate lower than 100 bpm (or controlled ventricular response) is less likely to cause symptoms. 

Atrial fibrillation symptoms result from the quivering atria’s inability to fill with adequate blood, resulting in reduced cardiac output.

Supraventricular Tachycardia

Related Video – One Quick Question: What are the SVT (Supraventricular Tachycardia) Criteria?

Supraventricular tachycardia (sometimes referred to as paroxysmal supraventricular tachycardia, or PSVT) is a regular tachyarrhythmia with absent P waves, arising from either the atria or AV node.⁶ The rate is usually 150–250 bpm. 

Due to the rhythm’s rapidity, patients are often symptomatic due to decreased cardiac output. The rhythm is rapid, regular, absent of P waves, and easily recognized. The QRS complex is narrow as the rhythm arises above the ventricles.

Related Video – One Quick Question: What are the Differences Between ST (sinus tachycardia) and SVT?

Supraventricular tachycardia features absent P waves.

• RATE: 150–250 bpm
• RHYTHM: regular
• P WAVE: absent
• PR INTERVAL: cannot be measured
• QRS COMPLEX: less than 0.12 seconds.

Summary

The foci sometimes conduct impulses from areas other than the SA node, creating abnormal rhythms in the atrium. These rhythms include wandering pacemakers, ectopic beats, atrial tachycardia, atrial flutter, atrial fibrillation, and paroxysmal supraventricular tachycardia. They all have different rates, rhythms, P wave ECG placement, PR intervals, and QRS complexes. Clinicians should know the different values to effectively diagnose and treat patients.