Determination of Electrical Axis in ECG

Transcript of the video: When we talk about the electrical axis in the ECG, we usually mean the mean QRS axis. Even though it is possible to calculate the axis of each component of the QRS like the initial activation and later activation, we usually mean the mean QRS electrical axis. And, electrical axis can be calculated for the P wave, PR segment, QRS complex, ST segment and T wave. All can be calculated separately. Modern ECG machines, with in-built computerized diagnostic algorithm, usually display a clock face in the print out, which will indicate the axis of P wave, QRS and T wave. Along with that, the measured axis of each wave will also be displayed, in the printout of the measured values.

Simple method of calculating the QRS axis is by looking at lead I and aVF. The negative waves are subtracted from the amplitude of the positive waves, and the resultant taken, for lead I and aVF. And, lead I is given an axis of zero, and aVF an axis of +90. So, this amplitude, the sum of the amplitudes, negative is subtracted from the positive, is plotted along the graph. Will be plotted here, lead I and aVF will be plotted in this direction. And geometrically you can calculate the resultant. That is what we will get as the mean electrical QRS axis in an ECG. So, if both are positive, you can see that the QRS axis will be between lead I and aVF, 0 to 90 degrees. That is a normal axis, simply, this is the normal axis, if both are positive. If lead I is negative and aVF is positive, then QRS will be in this quadrant, beyond 90 degrees, but less than 180 degrees. That will be taken as right axis deviation in this quadrant. If lead I is positive, and aVF is negative, then, it will be in this quadrant, axis will be in this quadrant and it is taken as, simply as left axis deviation. In that, there is a division. If it is less than, say, minus 30 degrees, that is, in this region, it is taken as minor left axis deviation and if it is beyond that, it is taken as true left axis deviation. Sometimes, up to minus 30 will also be called as leftward axis. A true left axis, you need, in this quadrant, beyond minus 30 to minus 90. Now what happens if both are negative, both aVF and lead I are negative, then the electrical axis comes to this quadrant. It is known as North West axis or indeterminate electrical axis, indeterminate electrical axis. It can be an extreme left axis deviation, or an extreme right axis deviation.

From that highly simplified determination of QRS axis, we move on to a more accurate determination, using this hexaxial reference system, in which each lead has an axis given. This is the positive of lead I and this is lead II, aVF 90 degrees, lead III 120 degrees. In case of aVR, as all the complexes are inverted, you will have, if aVR is negative, it is +30, and if aVR is positive, then it becomes -150. So, in this hexaxial reference system, we look at which lead is having maximum QRS deflection. Suppose the maximum QRS deflection is seen in aVF, when we look at the lead perpendicular to that, in lead I, you will have minimum QRS deflection which may be sometimes equiphasic. In that case you know that the axis is along 90 degrees, minimum QRS deflection along lead I and maximum along aVF. Then it is +90 degrees. And when the QRS is almost equal, in two leads, for example, in aVF and lead II, then the axis will be somewhere in this direction. Maximum deflection is between lead II and aVF or rather, both are having equally tall QRS complexes. Then, if you look at the perpendicular region, this region, you will have minimum QRS deflection, that is in aVL and lead I, you will have minimum amplitude. That is the second way of finding out the axis. If you are not seeing a lead in which the QRS complexes are equiphasic, then you look at the transition. Suppose say, lead I is positive, and aVL is negative, then you know that the transition is between lead I and aVL. We will take it as say, -15 degrees. Then you look at the perpendicular location. The QRS axis will be here, between 60 and 90, it is +75. Suppose the transition is between aVL and lead III, aVL is positive, lead III is negative, then it will be -45, that is the transition zone, and QRS axis will be in this region, +45. So that is how you look at the QRS axis when there is no particular lead which is showing equiphasic QRS complexes.