Lect.6 Electrical axis and cardiac vector Cardiac vector: net result Vector that occurs during depolarization of the ventricles Figure:

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1 Lect.6 Electrical axis and cardiac vector Objectives: 1. State the relationship between the direction of cardiac vector with the direction (-ve, +ve) and amplitude of an ECG waves. 2. Draw diagram indicting the axes of limb leads. Cardiac vector: The cardiac vector is the net result of the directions of the spread of depolarization waves from the SA node through the atria, AV node, interventricular septum, ventricles, and to the apex of the heart, the last part of the heart to be depolarized is the base of the heart. Vector that occurs during depolarization of the ventricles When the cardiac impulse enters the ventricles through the AV bundle, the first part of the ventricles to become depolarized is the left endocardial surface of the septum, then this depolarization spreads rapidly to involve both endocardial surfaces of the septum, the endocardial surface of the two ventricles and finally it spreads through the ventricular muscle to the outside of the heart and thereafter, the heart vector points toward the base of the left ventricle. ventricles. Figure: Vectors that occur during depolarization of the

2 Figure: Gensis of the QRS complex. The first phase, directed from left to right across the septum, produce Q wave in V6 and an R wave in V1. The second phase, due to depolarization of the left ventricle from endocardium to epicardium, results in a tall R wave in V6 and a deep S wave in V1. Phase 3, depolarization of the basal parts of ventricles producing a terminal S wave in V6 and a terminal R wave in V1. Electrical axis of the ventricular QRS: In relation to the bipolar limb leads (I, II, III), the cardiac vector or axis can be calculated. The relationship among the six limbs leads, i.e., the axis of each standard bipolar and unipolar limb leads can be presented in the following diagram:

3 The mean electrical axis of the ventricular QRS The average direction of the depolarization waves in all ventricular muscle fibres is called the mean electric axis of the ventricles or mean ORS axis or vector. Depolarization in cardiac muscle fibres can be represented by an arrow pointing to the + ve direction. This arrow is called a vector. In normal heart, the direction of the cardiac vectors of the ventricles is normally toward the apex of the heart. In other words, the direction of the electrical potential is from the base of the ventricles toward the apex.. The mean ORS axis has a magnitude & direction, and is related to the anatomical axis of the heart. The normal electric axis is directed downwards and to the left between - 30 and +110 (average + 60). Clinically, the electrical axis of the heart is determined from the standard bipolar limb leads; lead I and lead III as follows: First, record the maximum potential (that of QRS wave; R wave) and polarity (+ve or ve), to determine the maximum potential, you might need to subtract the area of the negative wave from the area of the positive wave. This means that the net QRS deflection in each lead is calculated by subtracting the amplitude of the largest - ve wave in the QRS from that of the R wave. Second, a distance equal to the net deflection in each lead is drawn as an arrow on the corresponding axis of the bipolar limb lead (clock), starting from its zero potential point and pointing to the resulting

4 polarities (+ve or -ve). Third, draws perpendicular lines form both ends of the arrows; th apices of the two net potentials of lead I and III, the point of intersection of these two lines represents the mean electrical axis or the mean QRS vector of the ventricles. In a normal heart, the average direction of the vector of the heart during spread of the depolarization waves through the ventricles; the mean QRS vector, is about +59 degrees, as shown in the following figures: Figure: Determination of projected vectors in leads I, II, and III where vector A represent the instantaneous potential in the ventricles. Figure: The mean electrical axis of the heart (59 degree) plotted From lead I and lead III.

5 The cardiac vector affects the configuration of the ECG waves in the various leads. The normal direction of the mean QRS vector is downwards and to the left and is generally between 30 and +110 degrees. Axis deviation: Axis deviation occurs if the electric axis of the heart is beyond the normal range and it may be to the right or to the left. QRS axis further right that of +110 constitutes Right axis deviation (RAD), QRS axis left that of 30 constitutes Left axis deviation (LAD). Right axis deviation This normally occurs in vertical hearts (e.g. in tall slender subjects), but pathologically, it is common in right ventricular hypertrophy and right bundle branch block. In this case, the projection of the mean QRS axis is toward the -ve pole in lead I and toward the +ve pole in lead III, so in ECG, there are deep -ve waves (S waves) in lead I and high +ve waves (R waves) in lead III, as shown in the following figure.

6 Figure: Right axis deviation and a slightly prolonged QRS complex. Left axis deviation This normally occurs in horizontal hearts (e.g. in short obese subjects and pregnant women) but pathologically, it is common in left ventricular hypertrophy and left bundle branch block. In this case, the projection of the mean QRS axis is toward the +ve pole in lead I and toward the -ve pole in lead III, so in ECG there are high +ve waves (R waves) in lead I and deep -ve waves (S waves) in lead III.

7 Figure: Example of left axis deviation. Figure: left axis deviation caused by left bundle branch block. Note also the greatly prolonged QRS complex.