1 ECG Fundamentals and a Systematic Reading Method

Learn how to verify, measure, and systematically interpret a 12-lead ECG while accounting for tracing quality and clinical context.

Purpose and clinical context

A 12- electrocardiogram records the heart’s electrical activity from 12 viewing angles. A consistent reading sequence helps reveal rhythm, conduction, and waveform abnormalities while reducing missed findings. Interpret the tracing alongside the patient’s symptoms, history, and prior ECGs; an ECG alone does not determine treatment.

Leads and placement

A is a view of electrical activity, whereas an is a sensor attached to the body. A standard 12- ECG uses 10 electrodes.

The six frontal-plane limb leads are I, II, III, aVR, aVL, and aVF. The six horizontal-plane chest leads are V1–V6. Place limb electrodes on the limbs when practical.

Chest positions

V1: fourth intercostal space at the right sternal border.
V2: fourth intercostal space at the left sternal border.
V3: midway between V2 and V4.
V4: fifth intercostal space at the midclavicular line.
V5: level with V4 at the anterior axillary line.
V6: level with V4 at the midaxillary line.

The limb leads view electrical forces in the frontal plane, while chest leads view them across the horizontal plane. Neighboring leads provide related views: II, III, and aVF are inferior; I and aVL are high lateral; V1–V2 are septal; V3–V4 are anterior; and V5–V6 are lateral.

Incorrect or inconsistent placement can alter the tracing and imitate disease; placing V1 or V2 too high is a particular concern. Check contact and signal quality, and document nonstandard placement.

Calibration and paper measurements

Check tracing speed and gain before measuring; standard settings are $25\ \mathrm{mm/s}$ and $10\ \mathrm{mm/mV}$ , unless the ECG states otherwise. At $25\ \mathrm{mm/s}$ , one small horizontal box, measuring $1\ \mathrm{mm}$ , represents $0.04\ \mathrm{s}$ , and one large box, measuring $5\ \mathrm{mm}$ , represents $0.20\ \mathrm{s}$ . At $10\ \mathrm{mm/mV}$ , $10\ \mathrm{mm}$ vertically represents $1\ \mathrm{mV}$ .

Waves, segments, and intervals

The P wave represents atrial depolarization. Assess its shape and whether it precedes each QRS. The , from the beginning of the P wave to the beginning of the QRS, reflects atrial-to-ventricular conduction; its typical adult range is $120\text{–}200\ \mathrm{ms}$ .

The represents ventricular depolarization. A duration under $120\ \mathrm{ms}$ is generally considered narrow. A wider complex suggests delayed ventricular activation and needs interpretation in context.

The ST segment runs from the end of the QRS, or J point, to the beginning of the T wave. Assess elevation or depression relative to the baseline, considering the leads involved and the clinical context. The T wave represents ventricular repolarization.

The runs from the beginning of the QRS to the end of the T wave and represents ventricular depolarization and repolarization. Because it varies with heart rate, assess the and the measurement method; automated values may require verification. Typical intervals are guides, not diagnoses: age, heart rate, medications, and clinical context affect interpretation.

Rate and rhythm

For a regular rhythm at standard paper speed, estimate ventricular rate by dividing $300$ by the number of large boxes between consecutive R waves:

\text{rate in beats/min} = \frac{300}{\text{large boxes between consecutive R waves}}

For example, four large boxes gives approximately $75\ \mathrm{beats/min}$ . The sequence $300\text{–}150\text{–}100\text{–}75\text{–}60\text{–}50$ is a quick visual aid.

For an irregular rhythm, count QRS complexes in a $10$ -second recording and multiply by $6$ . A longer strip can improve the estimate.

Then assess rhythm systematically: Is the R–R spacing regular? Are P waves present and similar? Does each P precede a QRS, and is the consistent? Is the QRS narrow or wide? Regularity, P-wave appearance, and the P-to-QRS relationship together help classify rhythm; do not identify a rhythm from rate alone.

Frontal

The describes the average direction of ventricular depolarization in the frontal plane. A common adult reference range is approximately $-30^\circ$ to $+90^\circ$ .

A quick quadrant check uses leads I and aVF:

QRS predominantly positive in both: usually a normal axis.
Negative in I and positive in aVF: rightward axis.
Positive in I and negative in aVF: possible leftward axis; inspect II to distinguish a clearly leftward axis from a borderline one.
Negative in both: extreme axis; verify placement and assess the ECG clinically.

This method estimates the axis. A more precise value can be calculated from the net QRS deflection in the limb leads.

A systematic reading sequence

Use the same sequence for each tracing:

Verify the tracing: confirm the patient and date; check speed, gain, artifact, and placement. Compare with prior ECGs when available.
Estimate rate: choose the method appropriate to a regular or irregular rhythm.
Describe rhythm: assess regularity, P waves, the PR relationship, and QRS width.
Determine axis: use the limb- quadrant method or calculate the axis.
Measure intervals: assess PR, QRS, and QT/.
Inspect morphology: assess P waves, QRS shape, R-wave progression across V1–V6, and T waves.
Review ST segments: look for abnormalities in anatomically related, or contiguous, leads and consider reciprocal changes.
Summarize: state the rate, rhythm, axis, key interval or waveform findings, and important limitations such as artifact or uncertain placement.

A concise description might read: “Regular rhythm, approximately $75\ \mathrm{beats/min}$ ; each P wave precedes a narrow QRS with a consistent ; axis appears normal. Intervals and ST–T segments should be assessed and reported from the tracing.” This describes observed features without claiming a diagnosis unsupported by the full clinical picture.

Reliable interpretation begins with correct placement and calibration. Identify what each wave and interval represents, select a rate method suited to rhythm regularity, and determine axis from the limb leads. Apply the sequence of quality, rate, rhythm, axis, intervals, morphology, and ST–T changes, then interpret findings in clinical context.