What Do the Waves on an Ecg Correspond to
Electrocardiogram (EKG, ECG)
General Description
Every bit the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, simply too throughout the trunk. This electrical action generated by the eye can exist measured past an assortment of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG, or EKG). A "typical" ECG tracing is shown to the right. The different waves that comprise the ECG represent the sequence of depolarization and repolarization of the atria and ventricles. The ECG is recorded at a speed of 25 mm/sec (v large squares/sec), and the voltages are calibrated so that one mV = 10 mm (two large squares) in the vertical management. Therefore, each small one-mm foursquare represents 0.04 sec (xl msec) in fourth dimension and 0.ten mV in voltage. Because the recording speed is standardized, one tin calculate the heart rate from the intervals between different waves.
P wave (atrial depolarization)
The P wave represents the moving ridge of depolarization that spreads from the SA node throughout the atria, and is usually 0.08 to 0.10 seconds (80-100 ms) in elapsing. The cursory isoelectric (null voltage) period later the P wave represents the time in which the impulse is traveling within the AV node (where the conduction velocity is profoundly retarded) and the packet of His. Atrial rate can be calculated by determining the fourth dimension interval between P waves. Click hither to see how atrial rate is calculated.
The catamenia of time from the onset of the P moving ridge to the beginning of the QRS complex is termed the PR interval, which normally ranges from 0.12 to 0.xx seconds in elapsing. This interval represents the fourth dimension between the onset of atrial depolarization and the onset of ventricular depolarization. If the PR interval is >0.20 sec, at that place is an AV conduction block, which is chosen a first-degree heart block if each impulse from the atria is yet able to be conducted into the ventricles.
QRS circuitous (ventricular depolarization)
The QRS circuitous represents ventricular depolarization. Ventricular rate tin can be calculated by determining the fourth dimension interval betwixt QRS complexes. Click here to encounter how ventricular rate is calculated.
The duration of the QRS circuitous is normally 0.06 to 0.10 seconds. This relatively short duration indicates that ventricular depolarization normally occurs very speedily. If the QRS complex is prolonged (> 0.ten sec), conduction is impaired within the ventricles. This can occur with bundle co-operative blocks or whenever a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. Such an ectopic foci nearly always results in impulses being conducted over slower pathways within the eye, thereby increasing the time for depolarization and the duration of the QRS complex.
The shape of the QRS complex in the in a higher place effigy is idealized. In fact, the shape changes depending on which recording electrodes are being used. The shape also changes when at that place is abnormal conduction of electric impulses within the ventricles. The figure to the right summarizes the nomenclature used to define the dissimilar components of the QRS complex as may occur in different ECG recording leads and/or with abnormal conduction within the ventricles.
ST segment
The isoelectric period (ST segment) following the QRS and ending at the beginning of the T wave is the time at which both ventricles are completely depolarized. This segment roughly corresponds to the plateau phase of the ventricular activeness potentials. The ST segment is very important in the diagnosis of ventricular ischemia or hypoxia because under those weather condition, the ST segment can go either depressed or elevated.
T and U waves
The T moving ridge represents ventricular repolarization. By and large, the T wave exhibits a positive deflection. The reason for this is that the terminal cells to depolarize in the ventricles are the beginning to repolarize. This occurs because the last cells to depolarize are located in the subepicardial region of the ventricles and these cells have shorter action potentials than found in the subendocardial regions of the ventricular wall. So, although the depolarization of the subepicardial cells occurs after the subendocardial cells, the subepicardial cells undergo phase three repolarization earlier the subendocardial cells. Therefore, repolarization waves more often than not are oriented opposite of depolarization waves (green versus ruby-red arrows in figure), and repolarization waves moving away from a postive recording electrode produce a positive voltage.
The T moving ridge is longer in elapsing than the QRS complex that represents depolarization. The longer duration occurs considering conduction of the repolarization wave is slower than the wave of depolarization. The reason for this is that the repolarization wave does not utilize the high-velocity parcel branch and purkinje organisation, and therefore primarily relies on cell-to-cell conduction.
Sometimes a small positive U wave may be seen following the T wave (not shown in figure at top of page). This wave represents the last remnants of ventricular repolarization. Inverted T waves or prominent U waves indicates underlying pathology or atmospheric condition affecting repolarization.
QT interval
The QT interval represents the time for both ventricular depolarization and repolarization to occur, and therefore roughly estimates the duration of an average ventricular activity potential. This interval tin range from 0.20 to 0.xl seconds depending upon eye charge per unit. At high heart rates, ventricular action potentials shorten in duration, which decreases the QT interval. Because prolonged QT intervals can be diagnostic for susceptibility to sure types of tachyarrhythmias, information technology is important to determine if a given QT interval is excessively long. In practice, the QT interval is expressed as a "corrected QT (QTc)" by taking the QT interval and dividing it by the square root of the R-R interval (interval between ventricular depolarizations). This allows an assessment of the QT interval that is independent of heart rate. Normal corrected Q-c intervals are 0.44 seconds or less.
There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during ventricular depolarization. Because the wave of atrial repolarization is relatively small in amplitude (i.e., has low voltage), it is masked by the much larger ventricular-generated QRS complex.
ECG tracings recorded simultaneous from different electrodes placed on the torso produce dissimilar characteristic waveforms. To learn where ECG electrodes are placed, CLICK Hither.
Revised 12/08/xix
DISCLAIMER: These materials are for educational purposes merely, and are not a source of medical decision-making communication.
Source: https://cvphysiology.com/Arrhythmias/A009#:~:text=The%20different%20waves%20that%20comprise,squares)%20in%20the%20vertical%20direction.
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