ECG Interpretation: Waves, Intervals, Norms and What They Mean

An electrocardiogram (ECG/EKG) is the most common heart test: fast, painless and widely available. But the report hands the patient a string of abbreviations and numbers — heart rate, PR, QRS, QT, axis — with little sense of what is normal and what is a warning. This guide explains the logic of reading an ECG: what each value measures, which ranges are normal and which findings need a doctor.

How an ECG Works and What It Shows

An ECG records the heart's electrical activity — the wave of excitation that travels across the atria and ventricles and makes them contract. A standard ECG uses 12 leads: each "looks" at the heart from a different angle, which helps localise where an abnormality is.

Know the limits of the method. An ECG shows rhythm, rate, conduction and indirect signs of ischaemia or strain. It does NOT directly show structure and pumping — wall thickness, valve function, ejection fraction: that is the job of a cardiac ultrasound (echocardiography). And a single ECG is a "snapshot" of a few seconds: it can miss a transient arrhythmia, which is why a 24-hour monitor (Holter) is used.

Waves, Segments and Intervals: P, QRS, T, PQ, QT

The ECG trace is made of repeating elements, each reflecting a phase of the heartbeat:

• P wave — atrial depolarisation. Normally positive, small, preceding every QRS complex.
• PR (PQ) interval — conduction time from atria to ventricles via the AV node. Normal 0.12–0.20 s. Prolongation means AV block; shortening points to pre-excitation.
• QRS complex — ventricular depolarisation. Normal width 0.06–0.10 s. A wide QRS (≥0.12 s) means bundle branch block or a ventricular origin of the rhythm.
• ST segment — key for ischaemia. Elevation or depression of ST may signal acute disruption of blood supply.
• T wave — ventricular repolarisation. An inverted or "coronary" T may indicate impaired repolarisation or ischaemia.
• QT interval — the total electrical "systole" of the ventricles; the rate-corrected QTc is used. Normal QTc is roughly up to 0.44 s in men and 0.46 s in women.

Heart Rate and Rhythm: Sinus Rhythm and Arrhythmias

The first things assessed are heart rate and the source of the rhythm.

"Sinus rhythm" is normal: the impulse arises in the sinus node, a P wave precedes every QRS, and the intervals are even. Resting heart rate in adults is normally 60–100 bpm. Below 60 is bradycardia (often normal in athletes and during sleep); above 100 is tachycardia (normal with exertion or stress).

Rhythm disturbances visible on an ECG include atrial fibrillation and flutter, supraventricular and ventricular extrasystoles, and AV blocks. A single extrasystole is often harmless, but newly detected atrial fibrillation or frequent ventricular events warrant review by a cardiologist.

The Heart's Electrical Axis

The electrical axis is the averaged direction of ventricular depolarisation. The normal axis lies roughly between −30° and +90°. "Left" or "right axis deviation" is not a diagnosis but a descriptor: it can be a normal variant (depending on body build and heart position) or accompany chamber hypertrophy, blocks, or right-heart strain (for example in lung disease or pulmonary embolism).

ECG Reference Values: A Quick Table

Norms in children differ (higher rate, different intervals), and older adults more often show "background" changes — so values are always read by age and in the context of symptoms.

Common ECG Conclusions and What They Mean

• Sinus arrhythmia — beat-to-beat variation with breathing; in young people this is a normal variant.
• Incomplete right bundle branch block (IRBBB) — a very common finding, usually harmless in healthy people.
• Repolarisation abnormality — non-specific; assessed together with symptoms and blood tests.
• Signs of left ventricular hypertrophy — a frequent companion of long-standing hypertension.
• Scar / signs of a previous infarction — a reason for further work-up.

The same word in a report means something completely different in an asymptomatic young adult versus a patient with chest pain — which is why an ECG is always read alongside the clinical picture.

When an ECG Needs Urgent Attention: Red Flags

Seek care immediately (or call emergency services) if the ECG shows or is accompanied by:

• ST-segment elevation with chest pain — a possible acute myocardial infarction;
• signs of acute coronary artery disease — ST depression, a "coronary" T with typical pain;
• wide complexes at a high rate (ventricular tachycardia);
• a markedly prolonged QT;
• new complete block or severe bradycardia with loss of consciousness.

In an acute coronary event, not only the ECG matters but also a blood troponin test — a marker of heart-muscle damage. And sharply altered T waves and arrhythmias are sometimes caused not by the heart but by electrolytes — for example an abnormal potassium level.

Why an ECG Is Not a Diagnosis, and How It Differs from Holter and Echo

An ECG is powerful but limited: it is a short electrical "snapshot". A normal ECG does not rule out heart disease, and a "scary" report often turns out to be a normal variant. Therefore:

• to catch transient arrhythmias, a 24-hour monitor (Holter) is used;
• to assess structure and pumping function — a cardiac ultrasound;
• when ischaemia is suspected — stress tests and blood work.

You can upload your own ECG and get a plain-language breakdown of the values with the ECG and imaging interpretation service — it does not replace a doctor, but it helps you understand what is normal in the report and what is worth taking to a cardiologist.

This article is for informational purposes and does not replace an in-person consultation. Interpreting an ECG and making a diagnosis is the job of a doctor.