The electrical axis classifications are summarized in Figure 2. Lastly, if the QRS complex is isoelectric or equiphasic in all leads with no dominant QRS deflection, it is considered an indeterminate axis. If the QRS axis happens to fall between -90 degrees and 180 degrees, this would be referred to as extreme axis deviation, whereby the ventricular vector is directed upward and to the right. The QRS vector would be directed downward and to the right. If the QRS axis falls between +90 degrees and 180 degrees, or beyond +100 degrees if the adult range is used, then RAD is present. In this case, the QRS vector is directed upward and to the left. If the QRS axis falls between -30 degrees and -90 degrees, it is considered LAD. The following axis classifications described are based on adults. This adult range is sometimes extended from -30 degrees to +100 degrees. The normal adult QRS axis is between -30 degrees and +90 degrees, which is directed downward and to the left. Between the ages of 8 to 16, the axis moves leftward with normal lying between 0 degrees to +120 degrees. At birth, the normal QRS axis lies between +30 degrees and +190 degrees. The QRS axis moves leftward throughout childhood and adolescence and into adulthood. There is some disagreement on the exact degrees that define each type, but some general cutoffs can be used for the QRS axis. If the QRS complex is isoelectric in a lead, then the ventricular axis is perpendicular (90 degrees) to that lead. Whereas a negative QRS complex in a lead has a ventricular axis that is approximately in the opposite direction to that lead. In general, a positive QRS complex in a lead has a ventricular axis that is approximately in the same direction going to that lead. This is done by interpreting the electrical signal (QRS complex) recorded at each electrode as positive, negative, or isoelectric and then considering their relationship with each other. The ventricular (QRS) axis is determined indirectly by evaluating the vectors produced under the electrodes. The ventricular (QRS) axis signifies the sum of all individual vectors generated by the depolarization waves of ventricular myocytes. Because the QRS complex is used to determine the ventricular axis, it is also referred to as the QRS axis. The ventricular axis can be determined by looking at the QRS complex, which represents ventricular depolarization. The normal ventricular axis is thus directed downward and slightly towards the left. When the electrical axis is discussed and taught, the ventricular axis is typically used in common clinical practice, although the atrial axis can be quite useful in clinical situations. Since the left ventricle makes up most of the heart muscle under normal circumstances thus, it generates the most electrical force visible on the EKG. Knowing the axis of each and how they interact can reflect certain pathology. Because each myocyte can produce an action potential, an axis for each wave and interval of the cardiac cycle can be determined. The sum of all the individual vectors generated by the depolarization waves makes up the electrical axis. In electrocardiology, a vector represents both the magnitude and direction of the action potential generated by an individual myocyte.
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