Before you interpret an ECG, learn to see its rhythm.

Before an electrocardiogram can be interpreted, its recording conditions and basic visual language must be understood. An ECG is a time-based recording of the heart’s electrical activity. It does not directly measure myocardial contraction, blood pressure, cardiac output, or coronary blood flow. Instead, it records electrical voltage differences detected by electrodes placed on the body surface.

Electrical activation of the heart normally begins in the atria, travels through the atrioventricular conduction system, and then activates the ventricles. The ECG trace represents these events as waves, complexes, segments, and intervals. Learning to identify their boundaries accurately is the foundation of all later ECG interpretation.

This educational material supports structured ECG learning. It is not intended for clinical decision-making, diagnosis, or treatment planning in individual patients.

A standard resting ECG is usually recorded using 12 leads. These leads do not represent 12 separate electrical events. Instead, they provide 12 viewpoints of the same cardiac electrical activity.

The standard 12 leads include:

• Three bipolar limb leads: I, II, and III
• Three augmented limb leads: aVR, aVL, and aVF
• Six precordial or chest leads: V1 to V6

Each lead views electrical activity from a different direction. A waveform may therefore appear positive, negative, biphasic, or nearly isoelectric depending on the lead being examined. This is expected and does not automatically indicate an abnormality.

Before measuring or describing an ECG, confirm the recording settings. Incorrect assumptions about paper speed or amplitude calibration can lead to inaccurate measurements and incorrect conclusions.

The standard paper speed is 25 mm/s.

At this speed:

• One small horizontal square equals 40 ms, or 0.04 seconds
• One large horizontal square equals 200 ms, or 0.20 seconds
• Five large horizontal squares equal 1 second
• Thirty large horizontal squares equal 6 seconds
• Fifty large horizontal squares equal 10 seconds

The paper speed determines how quickly the trace moves across the ECG display. If the speed changes, the time represented by each square also changes.

At 50 mm/s:

• One small square equals 20 ms
• One large square equals 100 ms

At 12.5 mm/s:

• One small square equals 80 ms
• One large square equals 400 ms

Always check the printed or displayed speed before calculating duration, rate, or interval values.

At this calibration:

• One small vertical square equals 1 mm, which corresponds to 0.1 mV
• One large vertical square equals 5 mm, which corresponds to 0.5 mV
• A calibration signal of 1 mV should produce a vertical deflection of 10 mm

Amplitude calibration is important when describing the size of waves and complexes. If the gain is reduced or increased, the visual height of a waveform changes even though the patient’s underlying electrical signal may be unchanged.

The isoelectric baseline is the reference level from which ECG deflections are assessed. In many recordings, the TP segment is used as the most practical baseline because it lies between the end of the T wave and the start of the next P wave.

The baseline is especially important when assessing the ST segment. Before describing ST elevation or depression, confirm that the baseline is stable and determine which reference segment is being used.

Signal noise, muscle activity, baseline wander, poor electrode contact, and movement can make the baseline difficult to identify. When the trace is unclear, acknowledge uncertainty rather than forcing a precise measurement.

The P wave represents atrial depolarisation. In simple terms, it reflects electrical activation of the atria before atrial contraction. The P wave is commonly assessed in lead II and V1 because these leads often provide useful views of atrial activity.

The QRS complex represents ventricular depolarisation. It reflects electrical activation of the ventricles and is typically the most prominent feature of the ECG. Ventricular contraction follows shortly after this electrical event.

The ST segment begins at the end of the QRS complex and ends at the beginning of the T wave. It represents an early phase of ventricular repolarisation. Its position should be assessed in relation to the isoelectric baseline.

The T wave represents ventricular repolarisation, the electrical recovery phase of the ventricles after depolarisation.

A U wave may sometimes be visible after the T wave. It is not present in every ECG and should not automatically be included in QT interval measurement.

A wave is a visible deflection from the baseline. Examples include the P wave and T wave.

A complex is a group of waves that occur together. The QRS complex may contain a Q wave, an R wave, an S wave, or more than one positive or negative deflection.

A segment is the portion of the trace between waves. Examples include the PR segment, ST segment, and TP segment.

An interval includes one or more waves and may also include a segment. For example, the PR interval includes the P wave and the PR segment. The QT interval includes the QRS complex, ST segment, and T wave.

Accurate measurement starts with clear boundaries. Select the lead in which the onset and end of the waveform are easiest to see. When appropriate, compare multiple leads, since the earliest onset or latest offset may be more visible in another lead.

Use the following general approach:

1. 01Identify the relevant waveform or interval.
2. 02Find its earliest visible onset.
3. 03Find its latest visible end.
4. 04Count small squares or use electronic calipers.
5. 05Convert the measurement to milliseconds according to the paper speed.
6. 06Describe any uncertainty caused by noise, low amplitude, merging waves, or unclear boundaries.

Measure the P wave from its first departure from the isoelectric baseline to its return to baseline. The P wave is often easiest to identify in lead II, but V1 may provide additional useful information about morphology.

When describing the P wave, consider:

• Presence or absence
• Duration
• Amplitude
• Direction
• Shape
• Consistency from beat to beat
• Relationship to the following QRS complex

The PR interval, also called the PQ interval in some conventions, is measured from the beginning of the P wave to the beginning of the QRS complex.

It includes atrial depolarisation and the time required for electrical conduction from the atria toward the ventricles through the atrioventricular conduction system.

Measure the PR interval in the lead where both the P-wave onset and QRS onset are most clearly defined. A typical adult PR interval is often described as approximately 120 to 200 ms, although interpretation must consider the full ECG and clinical context.

Measure QRS duration from the earliest onset of ventricular activation to the final end of ventricular activation. This means measuring from the first visible deflection of the QRS complex to the final return to baseline.

When measuring QRS duration, do not rely solely on one lead if the boundaries are unclear. The earliest QRS onset and latest QRS offset may be visible in different leads.

A QRS duration below 120 ms is generally described as narrow. A QRS duration of 120 ms or more is generally described as wide. This is a descriptive measurement, not a complete explanation of the underlying mechanism.

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

The J point is the junction between the end of the QRS complex and the beginning of the ST segment. It is a key landmark for assessing the ST segment.

Describe the ST segment relative to an appropriate baseline, commonly the TP segment. Assessment should include the amount of deviation, the shape of the segment, the leads involved, and the consistency of the finding across related leads.

The QT interval extends from the onset of the QRS complex to the end of the T wave. It represents the total time of ventricular depolarisation and repolarisation.

The QT interval varies with heart rate. It tends to shorten at faster rates and lengthen at slower rates. Therefore, a corrected QT interval, called QTc, is often used to account for heart-rate dependence.

A commonly used formula is Bazett’s formula:

In this formula, QT and RR should be expressed in seconds.

Bazett’s formula is easy to use and widely recognised, but it may overcorrect at fast heart rates and undercorrect at slow heart rates. QTc should therefore be interpreted cautiously and in the context of the recording conditions, rhythm, measurement quality, medication history, laboratory data, and clinical setting.

When determining the end of the T wave, identify the point where the terminal portion of the T wave returns to the isoelectric baseline. If the end is difficult to define, especially when T and U waves merge, document that uncertainty. A distinct U wave should not automatically be included in the QT interval.