What an ECG Shows, and What It Does Not

An ECG records the electrical activity of your heart through electrodes on the skin, usually over about ten seconds. It shows three things well: the rhythm, how the electrical signal travels through the heart, and whether the muscle shows signs of strain, thickening or damage. It does not show how strongly the heart pumps, whether the valves work, or whether your coronary arteries are narrowed.

That gap between what people expect an ECG to prove and what it can actually show causes a great deal of false reassurance.

What it genuinely detects#

Rhythm problems are its main strength. Atrial fibrillation, an abnormally fast or slow rate, extra beats and heart block are all visible immediately. Conduction problems such as bundle branch block appear clearly.

It is also the first test in suspected heart attack, because a blocked artery changes the tracing in characteristic ways within minutes. Signs of a previous heart attack, of thickened heart muscle from long standing high blood pressure, and of certain electrolyte disturbances can all be present too.

What it cannot tell you#

An ECG at rest can be perfectly normal in someone with severely narrowed coronary arteries, because at rest the muscle is still getting enough blood. This is why exercise ECGs, CT coronary angiography and stress imaging exist.

It says nothing about pumping strength, which is what an echocardiogram measures. It says nothing about valves. And because it lasts ten seconds, it will miss an intermittent rhythm problem unless you happen to be having it at that moment. Palpitations that come and go are investigated with a Holter monitor or an event recorder for that reason.

Common phrases translated#

• Normal sinus rhythm: the expected finding
• Sinus arrhythmia: normal variation of rate with breathing, common in young people
• Sinus bradycardia: rate under 60, normal in athletes and during sleep
• Non-specific ST-T changes: extremely common, usually means little without symptoms
• Left ventricular hypertrophy by voltage: thickened muscle suggested, often over-called in thin people, confirmed by echocardiogram
• Poor R wave progression: frequently caused by electrode placement rather than disease
• Incomplete right bundle branch block: a common normal variant

Many machines print a computer interpretation at the top. These algorithms over-diagnose, particularly abnormalities in young and slim people, and are always meant to be confirmed by a clinician.

Why an old ECG is so valuable#

Comparison changes interpretation more than any single feature. A finding that has been present unchanged for ten years is usually your normal. The same finding appearing for the first time this month is a different matter entirely. If you have had ECGs before, ask for copies and keep them.

At work#

Pre-employment and periodic medicals often include an ECG, and abnormal computer readings on otherwise healthy young workers are common. The usual and appropriate response is clinical review and comparison, not immediate exclusion from work.

Putting it together#

An ECG is one piece of cardiovascular assessment, sitting alongside blood pressure, cholesterol, glucose and symptoms. The guide to understanding your blood test results explains how those laboratory numbers contribute to the same picture.