Rather, if you understand a few basic ECG principles and the sequence of atrial and ventricular depolarization, you can predict the normal ECG patterns in each lead. As the sample ECG in Figure showed, the patterns in various leads can appear to be different, and even opposite of each other. For example, in some, the P waves are positive upward ; in others they are negative downward.
Finally, the T waves are positive in some leads and negative in others. Two related and key questions, therefore, are: What determines this variety in the appearance of ECG complexes in the different leads, and how does the same cycle of cardiac electrical activity produce such different patterns in these leads? A positive upward deflection appears in any lead if the wave of depolarization spreads toward the positive pole of that lead.
Similarly, if the ventricular stimulation path is directed to the left, a positive deflection R wave is seen in lead I see Fig. A negative downward deflection appears in any lead if the wave of depolarization spreads toward the negative pole of that lead or away from the positive pole. Thus, if the atrial stimulation path spreads downward and to the left, a negative P wave is seen in lead aVR see Figs.
If the ventricular stimulation path is directed entirely away from the positive pole of any lead, a negative QRS complex QS deflection is seen see Fig. If the mean depolarization path is directed at right angles perpendicular to any lead, a small biphasic deflection consisting of positive and negative deflections of equal size is usually seen. If the atrial stimulation path spreads at right angles to any lead, a biphasic P wave is seen in that lead. If the ventricular stimulation path spreads at right angles to any lead, the QRS complex is biphasic see Fig.
Figure A, A positive complex is seen in any lead if the wave of depolarization spreads toward the positive pole of that lead. B, A negative complex is seen if the depolarization wave spreads toward the negative pole away from the positive pole of the lead.
C, A biphasic partly positive, partly negative complex is seen if the mean direction of the wave is at right angles perpendicular to the lead. These three basic laws apply to both the P wave atrial depolarization and the QRS complex ventricular depolarization.
Figure With normal sinus rhythm the atrial depolarization wave arrow spreads from the right atrium downward toward the atrioventricular AV junction and left leg. In summary, when the mean depolarization wave spreads toward the positive pole of any lead, it produces a positive upward deflection.
When it spreads toward the negative pole away from the positive pole of any lead, it produces a negative downward deflection.
Ventricular rate can be calculated by determining the time interval between QRS complexes. Click here to see how ventricular rate is calculated. The duration of the QRS complex is normally 0.
This relatively short duration indicates that ventricular depolarization normally occurs very rapidly. This can occur with bundle branch 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 heart, thereby increasing the time for depolarization and the duration of the QRS complex.
The shape of the QRS complex in the above figure is idealized. In fact, the shape changes depending on which recording electrodes are being used. The shape also changes when there is abnormal conduction of electrical impulses within the ventricles. 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 action potentials. The ST segment is very important in the diagnosis of ventricular ischemia or hypoxia because under those conditions, the ST segment can become either depressed or elevated.
The T wave represents ventricular repolarization. Generally, the T wave exhibits a positive deflection. The reason 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 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 3 repolarization before the subendocardial cells. Wave forms that are above and below the Isoelectric line are called biphasic.
P Wave. Electrical impulses originating in the SA node trigger atrial depolarization. The normal P wave is no more than 0. The direction of electrical activity is from SA to AV node. The P wave is a representation of the time it takes for atrial depolarization.
It is viewed normally as small and curved with a positive deflection. Seen at it's tallest on lead II. T Wave. Ventricular repolarization which follows ventricular depolarization, is represented by the T wave. Its shape is rounded and taller and wider than the P wave. It is also more sensitive to physiologic and hormonal changes in shape but usually presents as a positive deflection 5 mm in height. U Wave. It is of the same deflection as T Wave and similar to shape to P Wave.
The U Wave is thought to represent late repolarization of the Purkinje fibers in the Ventricles and is more often not shown on a rhythm strip.
Intervals and Segments. Interval refers to the length of a wave plus the isoelectric line that follows it. The length of an interval ends when another wave begins. They are named by using the letters of both waves on either side. Intervals contain waves. Segments refers to the baseline between the end of one wave and the beginning of the next wave.
Segments are the lines between waves. See diagram below. PR Interval. Is the length along the baseline from the beginning of the P wave to the beginning of the QRS complex. This Is normally 0. QT Interval. Is the beginning of the QRS complex to the end of the T wave. In the presence of a U wave the measure should be from the beginning of the QRS complex to the end of the U wave. ST Segment.
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