Read the ECG as a sequence of electrical events.

An ECG should be read as a sequence of electrical events. Before assigning a rhythm label or discussing possible abnormalities, identify the structures visible on the trace and define their boundaries precisely.

The terms wave, complex, segment, and interval have specific meanings. Using them accurately improves communication, supports reliable measurement, and prevents common interpretation errors.

This chapter focuses on the anatomy of the ECG trace: what each structure represents, where it begins and ends, and how it should be described.

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

A wave is a visible deflection away from the isoelectric baseline. A wave may be positive, negative, biphasic, or nearly isoelectric depending on the direction of electrical activity relative to the viewpoint of a specific ECG lead.

The main ECG waves are:

• P wave: atrial depolarisation
• Q wave: first negative deflection of the QRS complex before an R wave
• R wave: first positive deflection of the QRS complex
• S wave: negative deflection following an R wave
• T wave: ventricular repolarisation
• U wave: an occasional deflection after the T wave

A wave is not an interval. A wave describes a discrete deflection on the ECG trace. Its key features include onset, end, duration, amplitude, direction, shape, and consistency.

The P wave represents atrial depolarisation. In a typical sinus rhythm, it appears before each QRS complex because atrial activation normally precedes ventricular activation.

When examining the P wave, assess:

• Whether it is present
• Whether it has a consistent shape
• Whether it occurs before each QRS complex
• Its duration
• Its amplitude
• Its direction in relevant leads
• Its relationship to the QRS complex

Lead II is commonly useful for observing P waves because atrial activation is often clearly visible there. Lead V1 is also important because it may show a biphasic P wave and can help clarify atrial morphology.

To measure P-wave duration, begin at the first visible departure from the baseline and end at the final return to baseline. At standard speed, count the number of small squares and multiply by 40 ms.

A complex is a group of waves occurring together. The most important ECG complex is the QRS complex.

The QRS complex represents ventricular depolarisation. Although the ventricles are activated rapidly, the direction and sequence of activation cause the trace to form one or more deflections.

The QRS complex may be entirely positive, entirely negative, or biphasic. Its appearance depends on the lead being examined and the direction of the heart’s electrical forces.

QRS nomenclature is based on the sequence and direction of deflections, not on their absolute size.

A Q wave is the first negative deflection of the QRS complex before any positive deflection.

An R wave is the first positive deflection of the QRS complex.

An S wave is a negative deflection that follows an R wave.

If another positive deflection follows an S wave, it is called R prime, written as R′.

If another negative deflection follows R′, it may be called S prime, written as S′.

A QS complex is an entirely negative QRS complex with no positive R-wave deflection.

Examples of QRS naming include:

• qR: a small initial negative deflection followed by a positive deflection
• Rs: a positive deflection followed by a small negative deflection
• rS: a small positive deflection followed by a deep negative deflection
• RSR′: a positive deflection, then a negative deflection, then another positive deflection
• QS: a fully negative complex with no R wave

Lower-case letters are often used for relatively small deflections, while upper-case letters are used for larger deflections. This convention describes amplitude, but the basic name of the deflection still depends on its sequence and direction.

QRS duration is measured from the earliest onset of ventricular activation to the latest end of ventricular activation. In practical terms, measure from the first QRS deflection away from baseline to the final return to baseline.

At 25 mm/s:

• One small square equals 40 ms
• Three small squares equal 120 ms

A QRS duration below 120 ms is generally described as narrow. A QRS duration of 120 ms or more is generally described as wide.

When the QRS onset or offset differs between leads, use the earliest visible onset and latest visible offset across the relevant leads. Avoid measuring only the tallest part of the complex if initial or terminal low-amplitude components are visible elsewhere.

A segment is the portion of the ECG trace between two waves. Unlike a wave, a segment does not include the adjacent waves.

The main ECG segments are:

• PR segment: from the end of the P wave to the onset of the QRS complex
• ST segment: from the J point to the onset of the T wave
• TP segment: from the end of the T wave to the onset of the next P wave

The PR segment begins at the end of the P wave and ends at the beginning of the QRS complex. It represents the period after atrial depolarisation and before ventricular depolarisation becomes visible on the surface ECG.

The PR segment is different from the PR interval. The PR interval includes both the P wave and the PR segment.

The ST segment begins at the J point and ends at the beginning of the T wave.

The J point marks the transition between the end of the QRS complex and the start of the ST segment. Accurate identification of this point is essential when describing ST-segment position.

The ST segment should be assessed relative to the isoelectric baseline, usually using the TP segment as a reference when it is clearly visible. Describe the segment before interpreting it. A structured description may include:

• Whether it is near the baseline, elevated, or depressed
• Its shape, such as horizontal, upsloping, or downsloping
• The leads in which the pattern appears
• Whether the finding is consistent across anatomically related leads
• Whether baseline artefact or unclear boundaries limit confidence

The TP segment extends from the end of the T wave to the beginning of the next P wave. In many ECG recordings, it provides a practical reference for the isoelectric baseline.

The TP segment may be difficult to identify at fast heart rates because the T wave and next P wave can occur close together. In such cases, baseline selection may be less certain and should be interpreted cautiously.

An interval includes one or more waves and may include a segment. Intervals are measured from a defined starting point to a defined ending point.

The main ECG intervals include:

• PR interval
• QT interval
• RR interval
• PP interval

The PR interval begins at the onset of the P wave and ends at the onset of the QRS complex. It includes the P wave and PR segment.

The PR interval reflects the time from the beginning of atrial activation to the beginning of ventricular activation. In adult ECG teaching, a typical PR interval is often described as approximately 120 to 200 ms.

When assessing the PR interval, consider both its duration and its consistency across consecutive beats. A stable PR interval supports a consistent atrioventricular relationship, but interpretation always requires assessment of the entire rhythm strip.

The QT interval begins at the onset of the QRS complex and ends at the end of the T wave. It includes ventricular depolarisation and ventricular repolarisation.

The QT interval does not begin at the P wave and does not end at QRS offset. It extends from the beginning of ventricular activation to the end of ventricular electrical recovery.

The end of the T wave should be identified where the final portion of the T wave returns to the baseline. If a U wave is separate and clearly visible, it is not routinely included in QT measurement. If T and U waves merge or the terminal T wave is indistinct, measurement uncertainty should be acknowledged.

The RR interval is the time between two consecutive R waves. It is commonly used to assess regularity and calculate ventricular rate.

In a regular rhythm, compare multiple RR intervals across the strip. In an irregular rhythm, one RR interval does not adequately represent the overall rate.

The PP interval is the time between two consecutive P waves. It can help assess atrial regularity when P waves are visible.

The PP interval and RR interval may be similar in a rhythm with a consistent one-to-one atrioventricular relationship. They may differ when atrial and ventricular activity are not consistently linked.

The transition zone refers to the area in the precordial leads where the QRS complex changes from predominantly negative to predominantly positive.

In the chest leads, early precordial leads such as V1 often show a relatively prominent S wave, while later leads such as V5 or V6 often show a more prominent R wave. The transition zone is typically described near the lead where the R and S waves have similar amplitude, often around V3 or V4.

The transition zone is a descriptive observation. Its position can vary between individuals and can be influenced by lead placement, body habitus, heart position, and technical factors. It should not be interpreted in isolation.

The electrical axis describes the overall direction of ventricular depolarisation in the frontal plane. It is usually estimated from the limb leads.

Axis assessment is based on the net direction of QRS complexes in selected limb leads. A simplified approach often begins by examining leads I and aVF, followed by additional leads when needed.

The axis is a vector concept, but the practical learning point is straightforward: the appearance of the QRS complex depends on whether the overall electrical activity moves toward, away from, or approximately perpendicular to the positive electrode of a given lead.

Axis should be assessed systematically and described in the context of the complete ECG. Technical factors, including incorrect limb-lead placement, can alter the apparent axis.

Use the same structured method for every ECG feature:

1. 01Locate the structure of interest.
2. 02Identify the earliest visible onset.
3. 03Identify the latest visible end.
4. 04Measure using small squares or electronic calipers.
5. 05Convert the result to milliseconds using the recording speed.
6. 06Describe duration, morphology, direction, and consistency.
7. 07State uncertainty when boundaries are unclear.

At a paper speed of 25 mm/s:

• One small square equals 40 ms
• Two small squares equal 80 ms
• Three small squares equal 120 ms
• Four small squares equal 160 ms
• Five small squares equal 200 ms