Learn more about the components of the ECG complex and characteristics of a normal ECG beat.
The ECG complex includes P, Q, R, S and T waveforms. All this tracings represents the conduction of electrical impulses from the atria to the ventricles.
P wave represents the atrial depolarization in the ECG. In case of P wave interpretation examine its location, configuration and deflection. Normal P wave has the following features:
- it is located before the QRS complex
- it is 2 – 3 mm high
- it lasts 0,06 to 0,12 second
- it is most often rounded and upright
- it is upright in leads I, II, aVF and V2 to V6 and usually variable in leads III and aVL; inverted in lead aVR; variable in lead V1.
Abnormal P wave is peaked, notched or enlarged and may reflect atrial hypertrophy or enlargement associated with chronic obstructive pulmonary disease, emboli, valvular disease or heart failure. Inverted P wave may indicate reverse conduction from the AV junction toward the atria. Varying P waves indicate that the impulse may be coming from different sites.PR segment
The PR segment tracks the impulse from the atria through the AV node. Duration is the most important characteristic when evaluation PR segment. Modifications in the PR segment signal altered impulse formation or a conduction delay. Normal PR segm
ent has the following features:
- it includes P wave until the beginning of
- it lasts 0.12 to 0.2 seconds
The features above do not refer to children. Short PR segment signals abnormal impulse origination. Long or prolonged PR segment may represent a conduction delay through the atria or AV junction due to heart block.
The QRS complex follows the P wave and represents depolarization of the ventricles. After depolarization, the heart contracts, which means it ejects blood from the ventricles and pumps it through the arteries, creating pulse. The QRS represents intraventricular conduction time. When evaluating the QRS complex, pay special attention to:
- location: QRS is after the PR segment
- amplitude: QRS is 5 to 30 mm high but differs in each lead
- duration: 0.06 to 0.1 second or half of the PR interval
- configuration: P wave, R wave and S wave included
- deflection: positive in leads I, II, III, aVL, aVF and V4 to V6 and negative in leads aVR and V1 to V3.
When no P wave appears in the QRS complex, it may indicate ventricular arrhythmia. Deep and wide Q waves ma yrepresent myocardial infacrtion. In this case the Q-wave amplitude is 25% of the R-wave amplitude or the duration of the Q wave is 0.04 second or more. A widened QRS complex greater than 0.12 second may indicate ventricular conduction delay. Missing QRS may indicate AV block.
A normal ST segment has the following characteristics:
- location: extends from the S wave to the beginning of the T wave
- deflection: neither positive nor negative but may vary from -0.05 to +1 mm.
A change in the ST segment may indicate myocardial damage. An ST segment may become either elevated or depressed. The elevated ST segment my indicate myocardial injury, while a depressed ST may signal myocardial ischeamia.
T wave represents repolarization. Normal T wave features:
- it is after the S wave
- it is 0.5 mm in leads I, II and III and up to 10 mm in the precordial leads
- it is typically round and smooth
- and usually upright in leads I, II and V3 to V6; inverted in lead aVR; variable in all other leads
Tall, peaked or tented T waves indicate myocardial injury. Inverted T waves may signal myocardial ischeamia. Heavily notched or pointed T waves my indicate pericarditis in adults.
QT interval represents the time required during ventricular depolarization and repolarization in the ECG. The lenght of the QT interval varies accoding to the heart rate. The faster the heart rate is, the shorter the QT interval is. The normal QT:
- extends from the beginning of the QRS complex to the end of the T wave
- its duration varies according to age, sex and heart rate, usually lasts from 0.36 to 0.44 second.
Abnormality in duration represents myocardial problems. Prolonged QT my indicate the risk of a life-threatening arrhythmia known as torsades de pointes.
The U wave may not available on the ECG. If it is displayed, normally it follows T wave, it is upright and rounded.
Source: Wolters Kluwer | Lippincott Williams & Wilkins