Ekg Hegzaaxial System

1 1 M i M M * j M i ! i I 1 I [ I b net of zero lead HI

Fig. 7. The amplitudes of the lead I and II R waves are plotted along the corresponding leg of Einthoven's triangle starting at the midpoint and drawn with a length equal to the height of the R wave (units used to measure the amplitude can be arbitrary because the direction, not the magnitude, of the axis is important). The direction of the plot is toward the positive end of the lead if the R wave has a positive deflection and negative if it has a negative deflection. Perpendiculars from each point are then drawn into the triangle and meet at a point. A line drawn from the center of the triangle to this point gives the angle of the mean electrical axis. Because the normal activation sequence in the heart generally goes down and left, this is also the direction of the mean electrical axis in most people. The normal range is anywhere from 0° to +90°. Modified from L.R. Johnson (ed.), Essential Medical Physiology, 3rd Ed., 2003.

Lead Sagittal Plane The Heart

Fig. 8. (A) The augmented leads are shown on Einthoven's triangle along with the other three frontal plane leads (I, II, and III). (B) The hex axial reference system for all six limb leads is shown, with solid and dashed lines representing the bipolar and unipolar leads, respectively. aVL, voltage recorded between left arm limb lead and neutral reference lead; aVF, voltage recorded between left leg lead and neutral reference lead; aVR, voltage recorded between right arm limb lead and neutral reference lead; LA, left arm; LL, left leg; RA, right arm. Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

Fig. 8. (A) The augmented leads are shown on Einthoven's triangle along with the other three frontal plane leads (I, II, and III). (B) The hex axial reference system for all six limb leads is shown, with solid and dashed lines representing the bipolar and unipolar leads, respectively. aVL, voltage recorded between left arm limb lead and neutral reference lead; aVF, voltage recorded between left leg lead and neutral reference lead; aVR, voltage recorded between right arm limb lead and neutral reference lead; LA, left arm; LL, left leg; RA, right arm. Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003. Reproduced with permission of the McGraw-Hill Companies.

from under the QRS complexes from at least two leads are needed. However, it is easier and more commonly determined by an estimate using the deflection (positive or negative) and height of the R wave. Figure 7 shows a simple example of using leads I and II to find the electrical axis of the heart. It should be noted that, in the normal human heart, the electrical axis of the heart roughly corresponds to the anatomical orientation of the heart (from base to apex).

3.3. The 12-Lead ECG

Leads I, II, and III are the bipolar limb leads discussed thus far. There are three other leads that use the limb electrodes; these are the unipolar limb leads. Each of these leads uses an electrode pair that consists of one limb electrode and a "neutral reference lead" created by hooking up the other two limb locations to the negative lead of the ECG amplifier. In other words, each lead has its positive end at the corresponding limb lead and runs toward the heart, the location of its "negative" end, directly between the other two limb leads. These are referred to as the augmented unipolar limb leads. The voltage recorded between the left arm limb lead and the neutral reference lead is called lead aVL; similarly, the right arm limb lead is aVR, and the left leg lead is aVF (Fig. 8).

The remaining 6 of the 12 lead recordings are the 6 chest leads. These leads are also unipolar; however, they measure electrical activity in the traverse plane instead of the frontal plane. Similar to the unipolar limb leads, a neutral reference lead is "created," this time using all 3 limb leads connected to the negative ECG lead, which basically puts it in the center of the chest. The 6 positive, or "exploring," electrodes are placed as shown in Fig. 9 (around the chest) and are labeled V1 through V6 (the V meaning voltage). These chest leads are also known as the precordial leads. Figure 9A shows a simple cross-section (looking superior to inferior) of the chest, depicting the relative position of each electrode in the traverse plane. Figure 9A also shows a typical waveform obtained from each of these leads. The 3 bipolar limb leads, 3 unipolar limb leads, and 6 precordial leads make up the 12-lead ECG.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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