ECG terminology, waves, and intervals

The medical assistant performs the ECG procedure. The ECG is read or interpreted by the provider. It is important for the medical assistant to understand the components of the ECG tracing. A medical assistant must be able to identify artifact in the recording. If artifact is seen, corrective action needs to be taken to get a clear ECG tracing. The medical assistant will also need to know how to identify life-threatening rhythms. When these are seen, it is important that the medical assistants get immediate help. Artifact and life-threatening rhythms are discussed later in the 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 hypercalcemia, hypokalemia, 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. The QT interval starts at the beginning of the Q wave and extends to the end of the T wave. 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.