As the love undergoes depolarization and also repolarization, the electrical currents the are produced spread not just within the heart, but likewise throughout the body.This electrical task generated through the heart have the right to be measured by range of electrodes placed on the human body surface. The recorded tracing is referred to as an electrocardiogram (ECG, or EKG). A "typical" ECG tracing is shown to the right. The various waves that consist of the ECG stand for the sequence of depolarization and also repolarization the the atria and also ventricles.The ECG is recorded at a rate of 25 mm/sec (5 big squares/sec), and the voltages are calibrated so the 1 mV = 10 mm (2 big squares) in the vertical direction. Therefore, each small 1-mm square represents 0.04 sec (40 msec) in time and 0.10 mV in voltage. Since the recording rate is standardized, one deserve to calculate the heart price from the intervals in between different waves.

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P tide (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 isoelectric (zero voltage) duration after the P tide represents the moment in i m sorry the advertise is traveling within the AV node (where the conduction velocity is substantially retarded) and the bundle that His.Atrial rate deserve to be calculation by determining the moment interval in between P waves.Click here to see exactly how atrial rate is calculated.

The period of time from the beginning of the P wave to the start of the QRS complicated is termed the PR interval, which typically ranges native 0.12 come 0.20 secs in duration.This interval to represent the time between the onset of atrial depolarization and also the beginning of ventricular depolarization. If the PR expression is >0.20 sec, over there is one AV conduction block, which is called a first-degree heart block if every impulse native the atria is tho able to be performed into the ventricles.

QRS complicated (ventricular depolarization)

The QRS facility represents ventricular depolarization.Ventricular rate have the right to be calculate by determining the moment interval between QRS complexes.Click here to see how ventricular rate is calculated.


The expression of the QRS facility is normally 0.06 come 0.10 seconds. This reasonably short duration shows that ventricular depolarization generally occurs really rapidly. If the QRS complex is lengthy (> 0.10 sec), conduction is impaired within the ventricles. This can take place with bundle branch blocks or at any time a ventricular foci (abnormal pacemaker site) becomes the pacemaker steering the ventricle. Together an ectopic foci nearly always results in impulses being performed over slow pathways in ~ the heart, thereby increasing the time for depolarization and also the expression of the QRS complex.

The shape of the QRS complex in the over figure is idealized. In fact, the shape alters depending ~ above which recording electrodes are being used. The shape also changes when there is abnormal conduction of electrical impulses in ~ the ventricles. The number to the ideal summarizes the nomenclature offered to define the different components of the QRS complicated as may take place in different ECG recording leads and/or with abnormal conduction within the ventricles.

ST segment

The isoelectric duration (ST segment) following the QRS and ending at the start of the T tide is the time at i beg your pardon both ventricles are totally depolarized. This segment approximately corresponds come the plateau step of the ventricular action potentials. The ST segment is really important in the diagnosis the ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.

T and also U waves


The T wave represents ventricular repolarization. Generally, the T wave exhibits a optimistic deflection. The reason for this is that the critical cells to depolarize in the ventricles space the first to repolarize. This occurs since the critical cells come depolarize are situated in the subepicardial region of the ventricles and these cells have much shorter action potentials than uncovered in the subendocardial regions of the ventricular wall. So, return the depolarization of the subepicardial cell occurs ~ the subendocardial cells, the subepicardial cell undergo step 3 repolarization before the subendocardial cells. Therefore, repolarization waves typically are oriented the opposite of depolarization tide (green matches red arrows in figure), and also repolarization waves moving away native a postive record electrode create a confident voltage.

The T wave is much longer in duration 보다 the QRS facility that to represent depolarization. The longer duration occurs due to the fact that conduction the the repolarization wave is slower 보다 the wave of depolarization. The factor for this is the the repolarization wave does not make use of the high-velocity bundle branch and also purkinje system, and therefore primarily relies ~ above cell-to-cell conduction.

Sometimes a tiny positive U wave may be seen following the T tide (not shown in figure at top of page). This tide represents the critical remnants that ventricular repolarization.Inverted T tide or significant U waves indicates underlying pathology or conditions affecting repolarization.

QT interval

The QT term represents the time for both ventricular depolarization and also repolarization come occur, and therefore around estimates the term of an average ventricular activity potential. This interval can variety from 0.20 come 0.40 seconds depending upon heart rate. At high heart rates, ventricular activity potentials shorten in duration, i beg your pardon decreases the QT interval. Because lengthy QT intervals can be diagnostic because that susceptibility to certain varieties of tachyarrhythmias, it is vital to determine if a offered QT term is excessively long. In practice, the QT term 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 enables an evaluate of the QT interval the is elevation of heart rate. Common corrected Q-c intervals room 0.44 seconds or less.

There is no patent visible tide representing atrial repolarization in the ECG because it occurs throughout ventricular depolarization. Since the wave of atrial repolarization is reasonably small in amplitude (i.e., has low voltage), the is masked by the much larger ventricular-generated QRS complex.

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ECG tracings tape-recorded simultaneous from different electrodes placed on the body develop different properties waveforms. To learn where ECG electrodes space placed, CLICK HERE.

DISCLAIMER: These products are for educational objectives only, and also are no a source of clinical decision-making advice.