What an ECG Actually Shows — and What It Doesn't

8 September 2026 · 3 min read

An ECG is the cheapest cardiac test in India — under ₹300 at most labs, often included in a routine check-up. It is also the test most likely to produce false reassurance or false alarm. Understanding what an ECG can and cannot tell you turns a scary-looking report into a useful data point rather than an appointment-worth of anxiety.

What an ECG shows

  • Heart rhythm — regular, irregular, too fast, too slow.
  • Signs of a past heart attack (Q waves in specific leads).
  • Ischaemia at the moment of the test (ST depression or elevation).
  • Conduction problems (heart block, bundle branch block).
  • Enlarged chambers (voltage criteria for LVH).
  • Electrolyte disturbances (hint at high or low potassium).

What an ECG does NOT show

  • A blockage in your arteries right now, unless it's causing symptoms at the moment of the test.
  • Your future heart attack risk.
  • Heart pumping strength (that's an echo).
  • Valve problems (also echo).
  • Intermittent arrhythmias that aren't happening right now.

The specific limitation that matters most

An ECG done at rest, when you feel fine, is normal in most people with significant coronary artery disease. A person can have a 90% blockage in a major artery and a perfectly normal resting ECG. This is why cardiologists order stress tests — the ECG under stress shows what the rest ECG cannot. A 'normal' ECG in a person with chest pain does NOT rule out serious cardiac disease.

Common ECG findings that scare people unnecessarily

  • Finding: Sinus bradycardia (HR 50-60) · What it usually means: Normal in fit adults
  • Finding: Sinus tachycardia (HR 100-110) · What it usually means: Anxiety, dehydration, caffeine
  • Finding: Right axis deviation · What it usually means: Often a normal variant
  • Finding: Early repolarisation · What it usually means: Usually benign, especially in young adults
  • Finding: Isolated premature beats · What it usually means: Common and usually benign
  • Finding: Left ventricular hypertrophy by voltage · What it usually means: Needs echo to confirm; often over-called

Common findings that matter

  • New ST depression or elevation.
  • Atrial fibrillation.
  • Any pathological Q waves suggesting past MI.
  • Second- or third-degree heart block.
  • Left bundle branch block, especially new.
  • Prolonged QT interval.

Building a personal ECG record

A resting ECG at 30, another at 40, and then annually from 50 gives you comparison. New findings on the current ECG that were not present on an old one are more meaningful than the same finding in isolation. Keep every ECG report (and the strip, if you can) in your record — it is small, and comparison across years is exactly what a cardiologist wants at any new visit.

An ECG in an emergency for chest pain is a triage tool, not a diagnostic tool. A normal ECG plus persistent chest pain is not reassurance — it is a reason for the next test, not to go home.

References

Free for 90 days, no card needed. After that, keeping the record costs ₹349 for the year.

General information, not medical advice. Always talk to a qualified doctor about your own care. Where this and your doctor disagree, your doctor is right.

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