As the heart undergoes depolarization and repolarization, the electric curleas that are created spreview not just within the heart, yet additionally throughout the body.This electrical task generated by the heart deserve to be measured by an array of electrodes put on the body surconfront. The taped tracing is dubbed an electrocardiogram (ECG, or EKG). A "typical" ECG tracing is shown to the ideal. The various waves that consist of the ECG recurrent the sequence of depolarization and repolarization of the atria and also ventricles.The ECG is videotaped at a speed of 25 mm/sec (5 large squares/sec), and the voltages are calibrated so that 1 mV = 10 mm (2 big squares) in the vertical direction. Thus, each little 1-mm square represents 0.04 sec (40 msec) in time and 0.10 mV in voltage. Since the recording speed is standardized, one deserve to calculate the heart rate from the intervals in between different waves.

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P wave (atrial depolarization)

The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and is normally 0.08 to 0.10 seconds (80-100 ms) in duration. The brief isoelectrical (zero voltage) period after the P wave represents the moment in which the impulse is traveling within the AV node (wright here the conduction velocity is significantly retarded) and the bundle of His.Atrial rate deserve to be calculated by determining the moment interval between P waves.Click right here to see exactly how atrial price is calculated.

The period of time from the onset of the P wave to the beginning of the QRS facility is termed the PR interval, which normally arrays from 0.12 to 0.20 seconds in duration.This interval represents the time in between the oncollection of atrial depolarization and the onset of ventricular depolarization. If the PR interval is >0.20 sec, tbelow is an AV conduction block, which is dubbed a first-degree heart block if each impulse from the atria is still able to be performed into the ventricles.

QRS complex (ventricular depolarization)

The QRS facility represents ventricular depolarization.Ventricular price deserve to be calculated by determining the moment interval in between QRS complexes.Click below to check out just how ventricular rate is calculated.


The duration of the QRS facility is typically 0.06 to 0.10 secs. This fairly short duration suggests that ventricular depolarization generally occurs exceptionally swiftly. If the QRS facility is expanded (> 0.10 sec), conduction is impaired within the ventricles. This can occur through bundle branch blocks or whenever before a ventricular foci (abnormal pacemaker site) becomes the pacemaker driving the ventricle. Such an ectopic foci nearly constantly outcomes in impulses being conducted over sreduced pathmethods within the heart, thereby enhancing the moment for depolarization and also the duration of the QRS facility.

The form of the QRS facility in the above number is idealized. In reality, the shape alters depending on which recording electrodes are being used. The form additionally alters once there is abnormal conduction of electric impulses within the ventricles. The figure to the right summarizes the nomenclature used to define the different components of the QRS complicated as may occur in various ECG recording leads and/or via abnormal conduction within the ventricles.

ST segment

The isoelectrical period (ST segment) adhering to the QRS and finishing at the start of the T wave is the time at which both ventricles are totally depolarized. This segment approximately corresponds to the plateau phase of the ventricular activity potentials. The ST segment is very essential in the diagnosis of ventricular ischemia or hypoxia bereason under those problems, the ST segment deserve to end up being either depressed or elevated.

T and U waves


The T wave represents ventricular repolarization. Typically, the T wave exhibits a positive deflection. The factor for this is that the last cells to depolarize in the ventricles are the first to repolarize. This occurs because the last cells to depolarize are situated in the subepicardial region of the ventricles and also these cells have actually shorter activity 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 3 repolarization prior to the subendocardial cells. Because of this, repolarization waves mostly are oriented opposite of depolarization waves (green versus red arrows in figure), and repolarization waves moving ameans from a postive recording electrode produce a positive voltage.

The T wave is much longer in duration than the QRS facility that represents depolarization. The longer duration occurs because conduction of the repolarization wave is sreduced than the wave of depolarization. The reason for this is that the repolarization wave does not make use of the high-velocity bundle branch and also purkinje system, and therefore mostly depends on cell-to-cell conduction.

Sometimes a tiny positive U wave might be checked out following the T wave (not presented in figure at height of page). This wave represents the last remnants of ventricular repolarization.Inverted T waves or influential U waves indicates underlying pathology or conditions affecting repolarization.

QT interval

The QT interval represents the moment for both ventricular depolarization and also repolarization to take place, and also therefore about estimates the duration of an average ventricular activity potential. This interval have the right to array from 0.20 to 0.40 seconds relying on heart price. At high heart prices, ventricular activity potentials shorten in duration, which decreases the QT interval. Because prolonged QT intervals deserve to be diagnostic for susceptibility to particular forms of tachyarrhythmias, it is crucial to recognize if a offered QT interval is excessively long. In practice, the QT interval is expressed as a "corrected QT (QTc)" by taking the QT interval and also splitting it by the square root of the R-R interval (interval in 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 much less.

Tbelow is no distinctly visible wave representing atrial repolarization in the ECG because it occurs throughout ventricular depolarization. Because the wave of atrial repolarization is fairly little in amplitude (i.e., has actually low voltage), it is masked by the much larger ventricular-created QRS facility.

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ECG tracings taped simultaneous from different electrodes placed on the body produce different characteristic waveforms. To learn wbelow ECG electrodes are put, CLICK HERE.

DISCLAIMER: These materials are for educational functions just, and also are not a resource of clinical decision-making advice.