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.
Understanding ECG / EKG Wave Interpretation Reference
An ECG is a record of changing voltage at the skin, not a direct picture of the heart squeezing. Electrodes on the arms, legs, and chest compare electrical signals from different directions. This is why the same heartbeat can have a tall upward wave in one lead and a small or downward wave in another.
A standard twelve lead ECG gives several views of the same electrical event. The direction of a wave matters because an electrical signal moving toward a lead's positive electrode usually makes an upward deflection. Students should treat each lead as one camera angle rather than as a separate heartbeat.
The electrical sequence begins in a small group of cells called the sinoatrial node. Its signal spreads through the atria, then pauses briefly at the atrioventricular node. That pause gives the ventricles time to fill before they contract.
The signal then travels rapidly through the His bundle, bundle branches, and Purkinje fibers. Fast travel through this specialized pathway helps both ventricles activate in a coordinated way.
A wider ventricular complex can suggest that the signal took a slower or unusual route through the ventricles. Atrial repolarization is usually not visible as its own wave because it occurs during the much larger ventricular complex.
Timing measurements depend on the paper speed and the grid. Before measuring anything, check that the tracing uses the standard speed of 25 millimetres per second. At that speed, five small boxes make one large box.
Counting boxes gives a quick estimate, but it is only reliable when the rhythm is regular. For example, four large boxes between consecutive R waves gives a rate of 75 beats per minute because 300 divided by four equals 75. The QT interval changes naturally as heart rate changes.
A faster heart rate usually shortens it. Clinicians may calculate a corrected QT value to account for rate, but this requires care because different correction methods can give different results.
A sensible reading order reduces mistakes. First check the recording quality. Muscle movement, loose electrodes, breathing, and electrical interference can create marks that resemble abnormal waves.
Next find the R waves, compare their spacing, then look for a P wave before each ventricular complex. Check whether the same pattern repeats. Finally measure intervals in a clear lead instead of estimating from a blurry section.
Wearable devices often record only one lead, so they can be useful for noticing a pattern but cannot replace a full clinical ECG. ECG findings need symptoms, medical history, examination, and sometimes repeat recordings before anyone can decide what they mean.