An electrocardiogram (ECG or EKG) is a window into your heart’s electrical activity. Used by doctors to diagnose heart conditions, it’s a test that looks complex but can be broken down into simple parts. Whether you’re a student, a curious patient, or just brushing up, this guide explains the basics of ECG readings—without the jargon overload.
What Is an ECG? An ECG records the electrical signals that make your heart beat. Electrodes placed on your skin (usually arms, legs, and chest) capture this activity as waves on a graph. These waves tell a story about your heart’s rhythm, rate, and health.
4-Lead ECG: Uses electrodes on arms and legs for basic rhythm monitoring.
12-Lead ECG: Adds chest electrodes for a full diagnostic picture.
Key Parts of an ECG Reading Think of an ECG as a map with landmarks—waves, intervals, and axes. Here’s what they mean:
P Wave
What it shows: Atrial contraction (the upper chambers squeeze).
Normal: Small, rounded, 0.06–0.12 seconds.
Red Flags: Missing P waves (atrial fibrillation) or tall ones (atrial enlargement).
QRS Complex
What it shows: Ventricular contraction (the lower chambers pump).
Parts: Q (small dip), R (big peak), S (dip after).
Normal: 0.06–0.10 seconds.
Red Flags: Wide QRS (>0.12 seconds) could mean a conduction block.
T Wave
What it shows: Ventricles resetting electrically.
Normal: Upright, follows QRS direction.
Red Flags: Inverted (ischemia) or peaked (high potassium).
PR Interval
What it shows: Time from atrium to ventricle activation.
Normal: 0.12–0.20 seconds.
Red Flags: Too long (AV block) or too short (pre-excitation).
QT Interval
What it shows: Full ventricular cycle (contraction + reset).
Normal: 0.36–0.44 seconds (adjusted as QTc).
Red Flags: Prolonged QTc (>0.45 seconds) risks arrhythmias.
Axes
What it shows: Direction of electrical flow.
Normal QRS Axis: -30° to +90°.
Red Flags: Extreme shifts (e.g., <-30°) may hint at heart strain.
How to Interpret an ECG: 5 Easy Steps
Reading an ECG feels daunting, but a systematic approach simplifies it:
Check the Rate
Count large squares between R waves: 300 ÷ squares = beats per minute (bpm).
Example: 3 squares = 100 bpm. Normal is 60–100 bpm.
Assess the Rhythm
Regular (consistent R-R spacing) or irregular (e.g., atrial fibrillation)?
Look for P before every QRS for sinus rhythm.
Measure Intervals
PR, QRS, QT—use a ruler or machine printout. Compare to normal ranges.
Evaluate the Axis
QRS positive in leads I and aVF? Axis is normal (0 to +90°).
Shifts left (<-30°) or right (>90°) may signal issues.
Spot Abnormalities
ST elevation (heart attack), wide QRS (block), or odd T waves (ischemia).
Common ECG Findings
Heart Attack (MI): ST elevation in specific leads, new Q waves.
Atrial Fibrillation (AFib): Irregular rhythm, no clear P waves.
High Potassium: Tall, peaked T waves.
Bundle Branch Block (BBB): A widened QRS complex (>120 ms), suggesting delayed electrical conduction in the ventricles.
Ischemia or Heart Attack (Myocardial Infarction): ST-segment elevation, depression, or abnormal T-wave changes.
Why ECGs Matter
ECGs catch problems early—irregular rhythms, blocked arteries, or electrolyte imbalances. But they’re just one piece of the puzzle. Symptoms (chest pain, dizziness) and history (age, smoking) guide the full diagnosis.
When to Seek Medical Attention
If an ECG shows abnormal results, it does not always indicate a serious problem. However, you should consult a healthcare provider if you experience:
Chest pain
Shortness of breath
Dizziness or fainting
Palpitations or irregular heartbeats
Final Thoughts
Mastering ECGs takes time, but understanding the basics—waves, intervals, and patterns—gets you started. Whether it’s a 4-lead rhythm check or a 12-lead deep dive, you’re now equipped to decode the heart’s language.
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