ECG terminology, waves, and intervals
The medical assistant performs the ECG procedure, and the provider reads or interprets it, so it’s important for the medical assistant to understand the components of the ECG tracing. This includes recognizing artifact in the recording so corrective action can be taken, and recognizing life-threatening rhythms so immediate help can be obtained. Artifact is covered in the Troubleshooting and evaluation chapter, and life-threatening rhythms are covered in the Rhythm analysis and arrhythmias chapter.
ECG terminology
It is important to understand the common terminology used to comprehend the ECG tracing:
- Isoelectric line: A straight line that is also called the baseline. It represents a period of time with no electrical activity.
- Deflection: Any movement away from the baseline in the tracing. The deflections reflect the heart’s electrical flow. Upward movement is called a positive deflection. Downward movement is a negative deflection.
- Wave: A deflection from the baseline.
- Complex: A form made up of many waves (e.g., QRS complex).
- Segment: A part of a line between two points (e.g., the ST segment starts at the end of the S wave and ends at the start of the T wave).
- Interval: A period of time between two points or events. During an interval, many waves can occur.
Each deflection in the ECG tracing corresponds to a part of the cardiac cycle. The ECG cycle consists of waveforms that are labeled P, Q, R, S, T, and U. The Q, R, and S waves are usually grouped together and are referred to as the QRS complex. The next section explores each part of the ECG in more detail.
ECG waves and segments
The P wave is the first deflection in the tracing. It is created from the electrical impulses moving through the right and left atria. The P wave appears as a small, rounded hill. The first part of the P wave reflects the impulse moving from the SA node to the AV node in the right atrium. The second part of the P wave reflects the impulse in the left atrial chamber. Electricity given off when the atrial cells are depolarized creates the P wave. Thus, the P wave represents atrial depolarization. During the P wave, the atrial chambers contract and the blood moves into the ventricles.
The PR segment follows the P wave and appears as an isoelectric line. The electrical impulse moves slowly through the AV node. This electricity is not picked up on the ECG tracing. The PR segment is the time between the end of the atrial depolarization and the start of the ventricular depolarization. During the PR segment, the atrial chambers finish contracting.
The QRS complex is the next wave in the tracing. The impulse moves from the AV node through the remaining conduction system structures. The electricity given off from the impulse moving down the septum and around the outer walls of the ventricles creates the QRS complex. The electricity from the activity in the ventricles masks the repolarization electricity given off from the atrial cells as they recover. The QRS complex activity can be summarized as ventricular depolarization and atrial repolarization. During the QRS complex, the ventricles start contracting, and the blood moves into the arteries.
Each wave in the QRS complex is different. The Q wave is a negative deflection and represents interventricular septal depolarization. The large, triangular-shaped R wave reflects the depolarization of most of the ventricular walls. The S wave is the final depolarization of the ventricular walls. Not all of these three waves may be seen. The positive and negative deflections of these waves can be different in the 12 leads, or pictures, of the ECG.
The ST segment follows the last wave in the QRS complex and ends at the start of the T wave. The J point is the point where the QRS complex ends, and the ST segment starts. The ST segment is an isoelectric line. There is no electrical activity in the heart during this segment. (Remember, the impulse moving through the conduction system finished in the QRS complex.) During the ST segment, the ventricles finish contracting.
Repolarization causes electrical activity that creates the remaining waves. The T wave follows the ST segment and appears as a smooth, rounded, asymmetric waveform. The electrical activity from ventricular repolarization creates the T wave.
The U wave may follow the T wave, but in many cases, it is not seen. The U wave is created from the repolarization of the Purkinje fibers. A U wave may also appear if the patient has hypokalemia, calcium disturbances, or digoxin toxicity.
ECG intervals
An interval is a period of time from a start point to an end point. It is not a wave. If you need to define what occurs during an interval, you will simply summarize the activities that happen for each wave and segment during the interval period of time.
The PR interval starts at the beginning of the P wave and ends at the start of the Q wave. It represents atrial depolarization and the delay of the impulse in the AV node. The QT interval starts at the beginning of the Q wave and extends to the end of the T wave. It represents ventricular depolarization and repolarization. During the QT interval, the atrial chambers move from repolarization to the polarized state. The ventricular chambers depolarize and then move into the repolarized state.
Normal durations
Knowing what each waveform represents is only half of reading a tracing. The provider also measures how long each one lasts, because a wave that takes too long means the impulse is being delayed somewhere in the conduction system. At the standard paper speed of 25mm/second, each small box is 0.04 second, so the number of small boxes multiplied by 0.04 gives the duration. The normal adult values are as follows:
| Component | Normal duration | Small boxes |
| P wave | 0.06-0.11 second | 1.5 to 2.75 |
| PR interval | 0.12-0.20 second | 3 to 5 |
| QRS complex | 0.06-0.10 second | 1.5 to 2.5 |
| QT interval | 0.36-0.44 second | 9 to 11 |
Example: Measuring a PR interval
A PR interval on a tracing spans 4 small boxes. At 0.04 second per small box, the duration is second.
Answer: second, which falls within the normal 0.12-0.20 second range.
A PR interval longer than 0.20 second means the impulse is being held up on its way to the ventricles. A QRS complex wider than 0.10 second means the impulse is being delayed as it spreads through the ventricles, and one of 0.12 second or more means the ventricles are not being depolarized through the normal pathway. The QT interval shortens as the heart rate rises and lengthens as it falls, so it is always judged against the patient’s heart rate rather than on its own.
The medical assistant does not interpret these measurements; the provider does. Knowing the normal values is still useful, because it is what allows the medical assistant to recognize a tracing that should be taken to the provider immediately rather than filed.