2 ECG Interpretation Fundamentals

Learn a repeatable method for checking ECG calibration and tracing quality, estimating rate, assessing rhythm and intervals, and summarizing findings in clinical context.

Start with a systematic approach

A systematic ECG reading turns a tracing into a description of rate, rhythm, intervals, and key waveform findings. In an emergency, interpret the ECG alongside the patient’s symptoms and condition; interpretation must not delay immediate assessment or treatment.

Use measurements as part of the overall clinical assessment rather than as stand-alone conclusions. A single rhythm strip may not provide enough information to assess all 12-lead findings.

Check tracing quality and

A standard 12-lead ECG usually records at $\text{25 mm/s}$ and $\text{10 mm/mV}$ . At this speed, one small horizontal box, measuring $\text{1 mm}$ , represents $\text{0.04 s}$ ; one large box, measuring $\text{5 mm}$ , represents $\text{0.20 s}$ ; and five large boxes represent $\text{1 s}$ .

Confirm the paper speed and gain before measuring. Review the tracing for artifact, possible lead placement concerns, and whether it is long enough to assess rhythm. A typical 12-lead ECG includes a rhythm strip, often about $\text{10 s}$ long.

Estimate the heart rate

For a regular rhythm, measure the distance between consecutive R waves. The approximate rate can be calculated from either the number of large boxes or the number of small boxes between them:

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

\text{Approximate rate} = \frac{1500}{\text{small boxes between R waves}}\,\text{beats/min}

For example, when R waves are $\text{4}$ large boxes apart, the estimated rate is about $\text{75 beats/min}$ , using $\frac{300}{4}$ .

For an irregular rhythm, count the QRS complexes in a $\text{6 s}$ strip and multiply the count by $\text{10}$ to estimate the average . This estimate may be less precise when the rhythm varies considerably. In adults, a resting rate of roughly $\text{60–100 beats/min}$ is commonly described as normal, but interpretation depends on context, including symptoms, age, and clinical setting.

Assess rhythm and conduction

First determine whether the QRS complexes are regular, regularly irregular, or irregularly irregular. Then examine the P waves and how they relate to the QRS complexes:

Are P waves present, and do they have a consistent appearance?
Does each P wave precede a QRS, and does each QRS have a preceding P wave?
Is the consistent?
Is the rhythm regular or variable?

A typically has consistent P waves before each QRS, a stable , and a regular rhythm. Normal respiratory variation can cause mild rate changes. Missing or inconsistent P waves, varying PR intervals, or a changing relationship between P waves and QRS complexes call for closer assessment of rhythm and conduction.

Measure the intervals

Measure from the start of the first relevant waveform to the end of the second. At standard paper speed, each small box represents $\text{0.04 s}$ .

: From the start of the P wave to the start of the QRS; it represents atrial activation through conduction to the ventricles. The common adult reference range is $\text{0.12–0.20 s}$ . A prolonged indicates delayed atrioventricular conduction.
: From the start to the end of the QRS complex; it represents ventricular depolarization. It is usually $<\text{0.12 s}$ . A widened QRS indicates delayed or abnormal ventricular activation and should be interpreted in context.
: From the start of the QRS to the end of the T wave; it represents ventricular depolarization and repolarization. Its reference range depends on heart rate, so clinicians commonly consider the corrected QT, or , rather than raw QT.

Check automated values against the tracing and clinical context. Reference limits can vary by sex, age, measurement method, and clinical guidance. QT prolongation matters because it can be associated with risk of serious ventricular arrhythmia.

Use a repeatable reading sequence

Apply the same sequence to each tracing:

Confirm patient details, tracing quality, , and lead placement when possible.
Estimate the and note whether the rhythm is regular.
Examine regularity, P waves, PR consistency, and P-to-QRS relationships.
Measure PR, QRS, and QT or when interpretable.
Inspect P-wave and QRS morphology, ST segments, T waves, and any unusual deflections.
Compare with prior ECGs if available, and connect findings with symptoms, examination, and other clinical information.
Summarize the rate, rhythm, important interval or waveform findings, and any urgent concern.

A concise description might read:

Regular rhythm, approximately $\text{75 beats/min}$ ; a consistent P wave precedes each narrow QRS; about $\text{0.16 s}$ ; no obvious interval abnormality on this strip.

Describe what is visible without claiming more than the available tracing supports. Begin with , then assess rate, rhythm, and intervals before reviewing waveform and segment findings; interpret measurements in context and treat the ECG as one part of the patient’s overall assessment.