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Biology
Grade 10-12
ECG / EKG Wave Interpretation Reference Cheat Sheet
A printable reference covering P waves, QRS complexes, T waves, PR interval, QT interval, heart rate, and rhythm interpretation for grades 10-12.
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An ECG shows depolarization and repolarization as repeating P waves, QRS complexes, and T waves. The PR interval reflects electrical travel from the atria through the AV node to the ventricles. The QRS complex shows ventricular depolarization, and the T wave shows ventricular repolarization. Rate, rhythm, and interval measurements are used together to describe what the tracing suggests.
Key Facts
- The P wave represents atrial depolarization, which is the electrical activation that usually leads to atrial contraction.
- The QRS complex represents ventricular depolarization and is normally narrow, about 0.06 to 0.10 seconds in many standard references.
- The T wave represents ventricular repolarization, which is the recovery phase of the ventricular muscle cells.
- The PR interval is measured from the start of the P wave to the start of the QRS complex and is commonly about 0.12 to 0.20 seconds.
- The QT interval is measured from the start of the QRS complex to the end of the T wave and represents ventricular depolarization plus repolarization.
- Heart rate can be estimated on a regular rhythm by using heart rate = 300 divided by the number of large boxes between two R waves.
- A regular rhythm has fairly even R-R intervals, while an irregular rhythm has noticeably changing distances between R waves.
- On standard ECG paper, one small box usually equals 0.04 seconds and one large box usually equals 0.20 seconds.
Vocabulary
- Electrocardiogram
- An electrocardiogram is a recording of the heart's electrical activity as waves and intervals over time.
- P wave
- The P wave is the small upward wave that usually represents electrical activation of the atria.
- QRS complex
- The QRS complex is the sharp wave group that represents electrical activation of the ventricles.
- T wave
- The T wave is the wave after the QRS complex that represents electrical recovery of the ventricles.
- PR interval
- The PR interval is the time from the beginning of atrial depolarization to the beginning of ventricular depolarization.
- R-R interval
- The R-R interval is the time between two neighboring R waves and is used to estimate heart rate and rhythm regularity.
Common Mistakes to Avoid
- Confusing depolarization with contraction is wrong because electrical activation happens just before the mechanical muscle contraction.
- Measuring the PR interval from the end of the P wave is wrong because the PR interval starts at the beginning of the P wave and ends at the beginning of the QRS complex.
- Using the 300 rule on an irregular rhythm can be misleading because that shortcut assumes the R-R intervals are fairly even.
- Calling every tall upward wave a P wave is wrong because the tall sharp upward deflection is often the R wave within the QRS complex.
- Ignoring ECG paper scale leads to wrong time measurements because small boxes and large boxes represent specific time intervals.
Practice Questions
- 1 A regular ECG rhythm has 4 large boxes between two R waves. Estimate the heart rate using heart rate = 300 divided by large boxes.
- 2 A PR interval measures 5 small boxes. If each small box is 0.04 seconds, what is the PR interval in seconds?
- 3 A QRS complex measures 2 small boxes wide. If each small box is 0.04 seconds, what is the QRS duration?
- 4 Explain why both the shape of the waves and the spacing between R waves are important when describing an ECG rhythm.