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ECG Interpretation
Waves, Intervals, and Common Arrhythmias
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An electrocardiogram, or ECG, records the heart's electrical activity over time and helps clinicians detect rhythm problems, conduction delays, and signs of cardiac injury. For early medical students, ECG interpretation connects basic cardiac physiology to real patient care. Learning the normal waves and intervals makes it easier to spot dangerous abnormalities quickly. A systematic approach also reduces errors when reading complex tracings.
Each part of the ECG reflects a specific electrical event in the heart, from atrial depolarization to ventricular repolarization. The timing between waves shows how impulses move through the sinoatrial node, atrioventricular node, His Purkinje system, and ventricular muscle. Common arrhythmias change the rate, rhythm, wave appearance, or interval length in recognizable ways. By comparing a tracing to normal values, students can identify patterns such as atrial fibrillation, sinus bradycardia, tachycardia, and heart block.
Key Facts
- P wave = atrial depolarization; QRS complex = ventricular depolarization; T wave = ventricular repolarization.
- Normal heart rate from ECG: Heart rate = 300 / number of large boxes between consecutive R waves.
- Normal PR interval = 0.12 to 0.20 s, which equals 3 to 5 small boxes.
- Normal QRS duration < 0.12 s, which is less than 3 small boxes.
- Normal QT interval varies with heart rate; corrected QT can be estimated by QTc = QT / square root of RR.
- Standard ECG paper speed = 25 mm/s, so 1 small box = 0.04 s and 1 large box = 0.20 s.
Vocabulary
- Depolarization
- Depolarization is the electrical activation of cardiac cells that triggers contraction.
- Repolarization
- Repolarization is the electrical recovery of cardiac cells after activation.
- PR interval
- The PR interval is the time from the start of atrial depolarization to the start of ventricular depolarization.
- QRS complex
- The QRS complex is the ECG waveform produced by ventricular depolarization.
- Arrhythmia
- An arrhythmia is any abnormal heart rhythm caused by altered impulse formation or conduction.
Common Mistakes to Avoid
- Confusing electrical activity with mechanical contraction, which is wrong because the ECG shows voltage changes, not the actual pumping force of the heart.
- Calling every irregular rhythm atrial fibrillation, which is wrong because premature beats, sinus arrhythmia, and heart block can also produce irregular patterns and need different clues for diagnosis.
- Ignoring paper speed and box size, which is wrong because interval and rate calculations depend on knowing that small and large boxes represent fixed time values.
- Measuring the PR, QRS, or QT interval from inconsistent points, which is wrong because intervals must be measured from the true onset and end of the waveform to avoid false diagnoses.
Practice Questions
- 1 An ECG shows 4 large boxes between consecutive R waves. What is the heart rate in beats per minute?
- 2 A patient's PR interval measures 6 small boxes and the QRS duration measures 2 small boxes. Calculate each interval in seconds and state whether each is normal.
- 3 An ECG has no distinct P waves, an irregularly irregular rhythm, and narrow QRS complexes. Which common arrhythmia is most likely, and what feature supports your conclusion?