# Electrocardiogram (ECG) An *electrocardiogram (ECG or EKG)* is a recording of the electrical impulses that travel through the heart's conduction system during the cardiac cycle. The instrument used to record these impulses is called an *electrocardiograph*. The electrical changes, resulting from the depolarization and repolarization of cardiac muscle fibers, are detected by electrodes placed on the skin. ## ECG Waveforms and Intervals The standard ECG waveform consists of specific deflections (waves) and time intervals that correspond to electrical events in the heart. - **P Wave**: Represents the *depolarization of the atria*, which is the electrical signal that triggers atrial contraction (systole). During this wave, the ventricles are in diastole (relaxed). - **QRS Complex**: Represents the *depolarization of the ventricles*. It is composed of three parts: - **Q wave**: The initial downward deflection. - **R wave**: The large, sharp upward spike. - **S wave**: The downward deflection following the R wave. This complex precedes ventricular contraction (systole), during which blood is ejected from the heart. - **T Wave**: Represents *ventricular repolarization*, the electrical recovery of the ventricles. This event occurs just before the ventricles relax (diastole). ## Segments and Intervals These are the time periods between the waves, which are crucial for assessing the heart's function. - **P-R Interval**: The time from the beginning of the P wave to the beginning of the QRS complex. It indicates the time taken for the electrical impulse to travel from the SA node (atria) to the ventricles. - **S-T Segment**: The period between the end of the S wave and the beginning of the T wave. Represents the time when the ventricles are completely depolarized. - **J (junction) Point**: The specific point where the QRS complex ends and the ST segment begins. ## Normal ECG Interval Durations | Interval/Duration | Normal Time (seconds) | | :---------------- | :-------------------- | | **PR interval** | 0.12 - 0.20 sec | | **QRS duration** | 0.08 - 0.12 sec | | **QT interval** | 0.4 - 0.43 sec | | **RR interval** | 0.6 - 1.0 sec | ## Uses of an Electrocardiogram An ECG is a painless, non-invasive diagnostic test used to assess the heart's electrical activity and overall function. It can help detect: - *Arrhythmias* (irregular heartbeats). - *Conduction defects* (problems with the electrical pathways). - Abnormal size and position of heart chambers. - Damage to the heart muscle (e.g., from a heart attack). - *Impaired blood flow* to the heart muscle (ischemia). - The effects of cardiac medications. - The function of artificial pacemakers.