Electrocardiogram (ECG) Basics

Sheet Subject Physiology Done by Corrected by … Doctor Electrocardiogram (ECG) Basics - An electrocardiogram (ECG) is a simple test that can be used to check your heart's rhythm and electrical activity. Sensors attached to the skin are used to detect the electrical signals produced by your heart each time it beats. - Further explanation: when cardiac impulses pass through the heart, electrical current is spread into the adjacent tissues surrounding the heart. A small portion of the current spreads over the surface of the body. Electrodes of a galvanometer are placed on the skin on opposite sites of the heart records electrical potentials; this recording is known as ECG.  Since small portions reach the skin, a galvanometer with amplifiers is used. - There is no difference between an ECG and an EKG. ECG stands for electrocardiogram, and EKG is the German spelling for elektrokardiographie. 1|Page # 5 Depolarization versus Repolarization Waves: ‘understand’ - The figure below shows a single cardiac muscle fiber in the stages of depolarization and repolarization. Two electrodes are placed at the surface recording the potential difference: A- The first half of the fiber is under Depolarization, demonstrated by red positive charges inside and red negative charges outside, traveling from left to right. The other half is still polarized. The left electrode is in an area of negativity, and the right electrode is in an area of positivity, which causes the meter to record a positive potential difference. B- Complete depolarization of the fiber. The recording has returned to the zero baseline because both electrodes are now in areas of equal negativity (no potential difference). C- Repolarization starts, where positive charges return to the outside of the fiber in the first half. At this point, the left electrode is in an area of positivity, and the right electrode is in an area of negativity. Consequently, the recording, as shown, becomes negative. D- Complete repolarization of the fiber. Both electrodes are now in areas of positivity so that no potential difference is recorded between them. Thus, the recording returns once more to zero.  At the stages where complete polarization and depolarization occurs, there is no potential difference since the electrodes are in areas of equal charges. The zero baseline they reach in the recording is called the ‘isoelectric line’. 2|Page The Standardized Electrocardiogram: - The normal ECG is composed of a P-wave, a QRS complex, and a T-wave: 1- P-wave: is caused by atrial depolarization before their contraction begins. 2- QRS complex: caused by ventricle depolarization before their contraction via the bundle of His and Purkinje fibers. The normal duration of the QRS complex is less than 0.12s. If it becomes more than that, it may indicate a bundle branch block. It is important to know that not every QRS complex will contain all Q, R, and S waves. 3- T-wave: caused by ventricles repolarization. Although the T-wave is repolarization, its wave is upright because: Usually, the depolarization current of the ventricles starts from the base towards the apex and from the endothelium to the pericardium. Whereas its repolarization current happens in the opposite direction; from the apex to the base and from the pericardium to the endothelium. Theory: this phenomenon may be attributed to the intrinsic property of the ventricles’ fibers. In which ventricular depolarization is followed by contraction (ventricular systole) increasing the pressure in the ventricles. This presses the endothelium in the ventricles changing its ionic composition causing repolarization to start from the pericardium delaying it from the endocardium. Current moving in the opposite direction between electrodes causes the readings to flip, which explains why the repolarizing T-wave is upright. - As mentioned, both the P-wave and the QRS complex represent atrial and ventricular depolarizations respectively, and the T-wave represents the repolarization of the ventricles. There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization. The wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), thus it is masked by the much larger ventricular depolarization. - The cardiac cycle comprises a complete relaxation and contraction of both the atria and ventricles which lasts approximately 0.8 seconds. When cardiac cycles are irregular (i.e. not equal in duration) the heart’s rhythm is said to be irregular (arrhythmia). 3|Page - ECG paper is a grid where time is measured along the horizontal axis. It is divided into multiple squares: ➔ Each small square is 1 mm in length and represents 0.04 seconds. ➔ Each larger square (5 small squares) is 5 mm in length and represents 0.2 seconds. - The ECG paper moves at a standard speed of 25mm/sec. It has a heat-sensitive coating which is marked by a heated stylus for the recording. The stylus is attached to a moving coil galvanometer and movement of the stylus corresponds to the voltage detected by the galvanometer. - In ECG, there are intervals and segments: ‘follow on the figure above’ 1- Interval: it is a period measured in milliseconds starting from one wave to the other. Thus, an interval contains waves. a- PR interval: it reflects conduction through the AV node. The normal PR interval is between 0.12 - 0.2s in duration (3-5 small squares). If the PR interval is >0.2s, first-degree heart block is said to be present. b- RR interval: it reflects the time between heartbeats. The normal RR interval is 0.6 – 1 s → Therefore, per minute (60s), normal heartbeats ≈ 60 beats. c- QT interval: it represents the time taken for ventricular depolarization and repolarization. The normal QT interval shouldn’t exceed half the cardiac cycle; around 0.4 s. Interval Time (s) Small squares (0.04s) PR 0.12 - 0.2 3 - 5 (1 large square) RR 0.6 - 1 15 - 25 (4 large squares) QT 0.4 10 (2 large square) 4|Page 2- Segment: it is the isoelectric (flat) line between the end of a wave and the start of another. Thus, a segment doesn’t contain waves. The following segments correspond to phase 2 ‘plateau’ in the cardiac muscle action potential: a- ST segment: represent complete ventricular depolarization. The most important cause of ST segment abnormality (elevation or depression around the isoelectric line) is decreased blood flow which may cause myocardial ischemia or infarction. b- PR segment: represent complete atrial depolarization. ECG component Abnormality Indicates: PR >0.2s First degree heart block ST Elevated or depressed Decreased blood flow QRS >0.12s Bundle branch block Cardiac cycle Irregular intervals Arrhythmia Questions: Recall: we said that the RR interval can represent the time between beats. 1- If the RR interval = 15 small squares, what is the heart rate per minute? 1 small square = 0.04s 15 x 0.04s = 0.6s → 1 beat every 0.6s Per minute: 60 / 0.06 = 100 beat/min 2- If the RR interval = 25 small squares (1 second), what is the heart rate? Per minute: 60/1 = 60 beat/min 3- If the RR interval = 20 small squares, what is the heart rate? 20 x 0.04s = 0.8s Per minute: 60 / 0.8 = 75 beat/min 5|Page