What is an ECG and what can it show about angina or a past heart attack? 

The Electrocardiogram (ECG) is one of the oldest and most common tools in cardiology. It provides a snapshot of the electrical activity that powers your heartbeat. While it is invaluable for diagnosing heart rhythm problems and acute heart attacks, its role in diagnosing stable angina or past heart events is more nuanced. Understanding what this squiggly line can, and cannot, tell your doctor is key to managing your expectations during a heart assessment. 

What We’ll Discuss in This Article 

• The definition and function of an Electrocardiogram (ECG). 

• Why a resting ECG might be normal even if you have angina. 

• Specific electrical patterns that indicate ischaemia (ST depression). 

• How ‘Q waves’ reveal permanent damage from old heart attacks. 

• The difference between resting, ambulatory (Holter), and stress ECGs. 

• Why a normal ECG does not completely rule out heart disease. 

• When further testing is required. 

What is an ECG? 

An electrocardiogram (ECG) is a simple, painless test that records the electrical signals in your heart. Sensors (electrodes) attached to your chest, arms, and legs detect the tiny electrical impulses that trigger each heartbeat, translating them into a graph on paper or a screen. It tells doctors how fast your heart is beating and whether the rhythm is steady or irregular. 

How It Works 

The heart is essentially an electrically driven pump. 

• The P Wave: Shows the electrical trigger from the top chambers (atria). 

• The QRS Complex: Shows the electrical firing of the main pumping chambers (ventricles). 

• The T Wave: Shows the heart ‘resetting’ for the next beat. 

The NHS describes the ECG as a key test used to check your heart’s rhythm and electrical activity, often taking just a few minutes to perform. 

Can an ECG detect angina? 

A resting ECG is often completely normal in people with stable angina because the heart muscle is not under stress while you are lying down. However, during an episode of chest pain or physical exertion (stress test), the ECG may show ‘ST-segment depression,’ which indicates that a part of the heart muscle is starving of oxygen (ischaemia). 

The ‘Silent’ Gap 

• Resting State: If your arteries are narrowed but not blocked, enough blood gets through at rest to keep the electrical signal normal. 

• Stress State: When you exercise, the narrowed artery can’t keep up. The electrical signal changes shape (ST depression) because the muscle is struggling. This is why doctors may order an exercise tolerance test or a 24-hour monitor. 

What can an ECG show about a past heart attack? 

An ECG can detect a past heart attack by identifying ‘pathological Q waves.’ When heart muscle dies and turns into scar tissue, it no longer conducts electricity. This ‘electrical hole’ changes the pattern of the QRS complex permanently. Additionally, persistent T-wave inversion (where the wave points down instead of up) can indicate areas of previous damage or strain. 

Signs of Old Damage 

• Pathological Q Waves: Deep, wide dips in the graph that act as a permanent footprint of a previous infarction. 

• Poor R-Wave Progression: The electrical signal doesn’t get stronger across the chest leads as it should, suggesting muscle loss in the front wall of the heart. 

• Blockages: Scarring can also block electrical pathways, leading to ‘Bundle Branch Blocks’ visible on the trace. 

Different Types of ECGs 

Because a standard 10-second ECG might miss intermittent problems, doctors use different formats. A ‘Holter monitor’ records your heart for 24–48 hours to catch angina or arrhythmias that happen during daily life. An ‘Exercise ECG’ (treadmill test) deliberately stresses the heart to provoke and record ischaemic changes that wouldn’t show up at rest. 

• Resting ECG: Standard snapshot. Good for acute attacks and permanent damage. 

• Ambulatory (Holter): The British Heart Foundation explains that this wearable device tracks your heart rhythm over a day or more to catch intermittent symptoms. 

• Stress ECG: Measures heart function under load. Less common now, often replaced by imaging stress tests. 

Triggers for Abnormal Readings 

An abnormal ECG reading is typically triggered by structural changes or acute stress. High blood pressure can thicken the heart muscle (Left Ventricular Hypertrophy), creating larger electrical voltages. Electrolyte imbalances (like low potassium) or certain medications can also alter the shape of the wave, mimicking heart disease. 

• Hypertrophy: A thickened heart muscle produces ‘taller’ spikes on the graph. 

• Electrolytes: Salt imbalances can stretch or shrink the T-wave. 

• Medication: Drugs like beta-blockers slow the rhythm, which is an expected change. 

Differentiation: ECG vs. Echocardiogram 

It is important to differentiate between an ECG (electrical) and an Echocardiogram (structural). An ECG looks at the wiring; an Echocardiogram (ultrasound) looks at the plumbing and moving parts. You can have a normal electrical system (ECG) but a weak pump or leaky valve (Echo), which is why both tests are often needed. 

Feature	ECG (Electrocardiogram)	Echocardiogram (Echo)
Measures	Electrical timing & rhythm	Structure, movement, valves
Detects	Arrhythmias, Ischaemia	Heart failure, Valve disease
Invasiveness	Stickers on skin	Gel & probe on chest
Time	5 minutes	30–45 minutes

Conclusion 

The ECG is a fundamental tool that provides a window into the electrical health of your heart. While it can reveal permanent scars from past heart attacks (Q waves) and acute distress during angina (ST depression), a normal resting ECG does not guarantee clear arteries. It is often just the first piece of the puzzle, used alongside blood tests and scans to build a complete picture of your cardiovascular health. 

If you have chest pain, do not rely on a previous ‘normal’ ECG to reassure you. Electrical changes can happen in minutes. Call 999 immediately. 

Authority Snapshot 

This article was written by Dr. Rebecca Fernandez, a UK-trained physician (MBBS) with extensive experience in cardiology and emergency medicine. Having interpreted thousands of ECGs for patients ranging from stable outpatients to critically ill trauma cases, Dr. Fernandez explains the capabilities and limitations of this fundamental heart test. This content has been reviewed to ensure alignment with NHS and NICE safety guidelines, helping you understand what your results actually mean. 

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