Describe rhythm before you name it.

Describe the rhythm before naming it.

A reliable ECG rhythm assessment should follow a consistent sequence. It is tempting to identify a familiar pattern immediately, but premature diagnostic labeling can cause important details to be missed. A methodical description is more accurate, easier to communicate, and more useful for learning.

This chapter presents a five-step framework for rhythm assessment:

1. 01Regularity
2. 02Atrial activity
3. 03Atrioventricular relationship
4. 04QRS morphology
5. 05Rate and descriptive conclusion

The aim is to identify what is directly visible on the ECG before drawing broader conclusions.

This educational material is intended for learning purposes only. It is not a substitute for clinical assessment, diagnosis, or treatment decisions.

Begin by comparing consecutive R-R intervals across the entire available rhythm strip.

The R-R interval is the distance between two consecutive R waves. It represents the time between successive ventricular depolarisations.

A rhythm may be described as:

• Regular: consecutive R-R intervals are essentially equal
• Regularly irregular: R-R variation follows a repeating pattern
• Irregular: R-R intervals vary without an obvious repeating pattern

Do not judge regularity from only one or two beats. Compare several consecutive intervals over the longest available strip.

A useful practical method is to mark the distance between two R waves on a piece of paper or with digital calipers and move that distance across the strip. If the R waves repeatedly align with the marker, the rhythm is likely regular. If they do not, assess whether the variation is patterned or unpatterned.

Occasional premature beats can make an otherwise regular underlying rhythm appear irregular. When this occurs, describe both the underlying pattern and the premature event rather than relying on a single overall label.

Next, look for P waves or another repeating pattern of atrial activity.

Questions to ask include:

• Are P waves visible?
• Is there a P wave before each QRS complex?
• Do the P waves have a consistent shape?
• Are the P-P intervals regular?
• Are there more P waves than QRS complexes?
• Are there QRS complexes without visible preceding P waves?
• Is another atrial pattern present instead of distinct P waves?

The P wave represents atrial depolarisation. In a typical sinus rhythm, P waves are usually visible before QRS complexes and have a consistent morphology.

Lead II often provides a useful view of P waves, while V1 may help identify atrial activity when the P wave is subtle or biphasic. If P waves are difficult to see, inspect multiple leads rather than assuming that they are absent.

After identifying atrial and ventricular activity, determine how they relate to each other.

The key question is whether atrial depolarisation and ventricular depolarisation maintain a consistent relationship.

Assess the following:

• Does every P wave lead to a QRS complex?
• Is every QRS complex preceded by a P wave?
• Is there a one-to-one relationship between P waves and QRS complexes?
• Is the PR interval stable from beat to beat?
• Are there P waves that are not followed by QRS complexes?
• Are there QRS complexes that occur without a clearly preceding P wave?
• Do P waves and QRS complexes appear to occur independently?

A strong descriptive statement of a consistent atrioventricular relationship is:

“Each P wave is followed by a QRS complex, and the PR interval is stable.”

This describes the observation without immediately assigning a diagnostic label.

A stable PR interval means that the duration from P-wave onset to QRS onset is similar in consecutive cycles. Measure it in the lead where both landmarks are most clearly defined.

Now evaluate ventricular activation by examining the QRS complexes.

Describe:

• QRS duration: narrow or wide
• QRS morphology: consistent or variable
• Presence of premature complexes
• Relationship of unusual complexes to preceding atrial activity
• Whether the QRS appearance changes from beat to beat

At a paper speed of 25 mm/s, a QRS duration below 120 ms is generally described as narrow. A QRS duration of 120 ms or more is generally described as wide.

A narrow QRS complex indicates that ventricular activation appears rapid on the surface ECG. A wide QRS complex indicates that ventricular activation takes longer than usual, but it does not by itself establish the exact cause.

A premature beat occurs earlier than expected based on the underlying rhythm.

When a premature beat is present, describe:

• Whether the beat occurs early
• Whether its QRS morphology differs from the surrounding beats
• Whether a preceding P wave is visible
• Whether there is a pause after the premature beat
• Whether the underlying rhythm is otherwise regular

For example:

“An early beat with altered QRS morphology is present, followed by a pause. The surrounding rhythm is otherwise regular.”

This observation is more useful and more defensible than assigning a diagnosis without evaluating the full tracing.

A pause is an interval between beats that is longer than expected relative to the surrounding rhythm.

To describe a pause, compare it with the usual R-R interval. Consider whether the pause is approximately equal to a multiple of the baseline cycle length or whether it does not fit that pattern. Also assess whether atrial activity continues during the pause.

The term pause is descriptive. The mechanism cannot be established from duration alone and requires assessment of the full relationship between P waves and QRS complexes.

The method used to calculate rate depends on whether the rhythm is regular or irregular.

For a regular rhythm at 25 mm/s, use the small-square method:

For example, if there are 20 small squares between two consecutive R waves:

A shortcut method uses large squares:

For example, if there are 4 large squares between R waves:

The 1500 method is more precise because it uses small squares.

For an irregular rhythm, do not calculate the rate from one randomly selected R-R interval. A single interval may not represent the overall ventricular rate.

Instead, use a time-based strip:

1. 01Confirm that the rhythm strip spans 10 seconds.
2. 02Count all complete QRS complexes.
3. 03Multiply the number of QRS complexes by 6.
4. 04Report the result as an estimate.

For example, if 12 complete QRS complexes are present in 10 seconds:

If a 6-second rhythm strip is used, count the number of QRS complexes and multiply by 10. As with every calculation, record the method used and acknowledge variability in irregular rhythms.

Use this sequence each time:

1. 01Regularity

Compare R-R intervals throughout the strip. State whether the rhythm is regular, regularly irregular, or irregular.

1. 02Atrial activity

Identify P waves or another repeating atrial pattern. Describe their morphology and regularity.

1. 03Atrioventricular relationship

Determine whether P waves and QRS complexes have a stable relationship. Check whether each P is followed by a QRS and whether the PR interval remains consistent.

1. 04QRS morphology

Measure QRS duration and describe its morphology. Look for variation, premature beats, and unusual complexes.

1. 05Rate and conclusion

Calculate rate using a method appropriate to regularity. Then write a descriptive conclusion based on observed ECG features.

A high-quality rhythm conclusion should report visible features before giving a diagnostic name. It should be specific, reproducible, and appropriately cautious.

Example of a descriptive conclusion in a regular rhythm:

“Regular rhythm at approximately 75 beats per minute. A consistent P wave precedes every QRS complex. PR intervals are stable. QRS complexes are narrow.”

Example of a descriptive conclusion in an irregular rhythm:

“Irregular narrow-complex rhythm. R-R intervals vary across the strip. Ventricular rate was estimated from a 10-second strip. Atrial activity should be assessed in multiple leads because P waves are not clearly defined in the displayed lead.”

Example with an early beat:

“Predominantly regular rhythm with an isolated early complex of different morphology. The underlying rate is approximately 70 beats per minute.”

These statements distinguish direct ECG observations from diagnostic interpretation and clinical decision-making.

• Naming a rhythm before assessing R-R regularity
• Calculating an irregular rhythm from a single R-R interval
• Assuming every small deflection is a P wave
• Measuring PR intervals when P-wave onset is not clearly identified
• Ignoring QRS width and morphology
• Calling a rhythm regular after inspecting only two cycles
• Treating an isolated premature beat as the entire rhythm
• Confusing a descriptive ECG observation with a clinical diagnosis