Chapter 11: THE RESTING ELECTROCARDIOGRAM ELECTROCARDIOGRAPHIC LEADS
To record an ECG, an electric circuit between the heart and the electrocardiograph must be completed.11 For this purpose, electrodes are placed on different parts of the body surface and are connected to the instrument by means of cables.il Thus the whole system consists of an instrument, electrodes, cables, and leads.
An ECG lead can be defined as a pair of terminals with designated polarity, each of which is connected either directly or via a passive-active network to recording electrodes. In 1913, Einthoven et al.3 developed a method of studying the electrical activity of the heart by representing it graphically in a two-dimensional geometric figure, namely, an equilateral triangle. There are several simplifying assumptions on which Einthoven's hypothesis is founded3-13: (1) The body is a homogeneous volume conductor. Although the conductivity of the various tissues is not the same, the differences are not great enough to invalidate that the body can be considered as a homogeneous volume conductor. (2) The sum of all the electric forces, or the mean of all the forces generated during the cardiac cycle, can be considered as originating in a dipole located in the electrical center of the heart. (3) Electrodes placed on the right arm (RA), left arm (LA), and left leg (LL) are used to pick up the potential variations on these extremities. Standard (bipolar) leads (I, II, and III) are obtained by recording, respectively, the potential differences between LA and RA, LL and RA, and LL and LA. These leads record potential variations in a single frontal plane only. (4) Attachment between these limb electrodes, on the forearms and limbs, corresponds to a position in the root of the corresponding limb. For example, an electrode in the right forearm records the electrical activity that reaches the right shoulder. It should be pointed out that when the electrodes are placed proximally to the roots of the extremities, they lose their relatively "far" distance from the heart. Hence Einthoven's equilateral theory does not hold. The latter is of importance to understand why leads placed proximally to the roots of the extremities, such as those used for exercise testing and coronary care unit and Holter monitoring, by being only "equivalent" to the corresponding bipolar leads, are in some cases markedly different from the "true" standard bipolar leads.
The sum of the potentials from the right arm (RA), left arm (LA), and left leg (LL) is equal to zero throughout the cardiac cycle with respect to any point at the body surface.3,5,6,13 Lead wires attached to electrodes on each limb are connected together, through 5000-W resistors, at a point. When this common point- Wilson's central terminal-is attached to the negative pole of the ECG machine and an "exploring" electrode is connected to the positive pole, the potential variations recorded will be those of the latter only. A lead taken by this method is called a unipolar lead. Actually, the central terminal is not zero because the RA, LA, and LL are not equidistant from each other and from the heart, the body tissues vary in resistance, and the heart and extremities do not lie in exactly the same plane in the body. The potential of the central terminal has been said to average around 0.3 mV.9
At present, unipolar extremity leads are obtained by disconnecting the input to the central terminal of Wilson from the extremity being explored. This results in a one-and-a-half increase in their voltage. These augmented (a) extremity leads are the ones usually used for clinical electrocardiography and are labeled aVR, aVL, and aVp.5,9,13
The unipolar precordial ECG is obtained by placing the exploring electrode (connected to the positive pole of the ECG machine) on the classic six locations of the anterior and left portions of the chest.5,6,13 The central terminal is used as the indifferent electrode. Precordial (V) leads yield a positive deflection when facing positive charges and negative deflections when facing negative charges.5,6,12,13,15-17 They do this according to what Wilson called the solid-angle concept.513 A solid angle is merely an imaginary cone extending from the site in the chest throughout the heart. The precordial electrode is at its apex, and its base is at the opposite epicardial surface.13 This concept is most important to understand precordial lead morphologies. According to Wilson's scalar concept of electrocardiography, this occurs because the solid angle subtended by the corresponding lead records the electrical activity from the regions of the heart over which the lead is placed as well as from distant regions.513 Thus, if V2 is placed over (thereby facing) the right ventricle, part of the initial positive ventricular deflection reflects right ventricular activation, with the corresponding electrical forces moving toward the electrode.13 Most portions of the terminal S wave represent activation of muscle other than the right ventricle (septum and free left ventricular wall), reflecting electrical forces moving away from the electrode.13 Acceptance that the amount of muscle activity recorded by various unipolar leads is not the same implies different "real" duration of depolarization and repolarization, irrespective of that supposedly resulting from the projections of a vector on an idealized horizontal lead axis (see sections on QT dispersion and vectorcardiography). For practical purposes, the peak of the r (or R) wave in precordial leads gives a rough estimate of the moment of arrival of excitation (intrinsicoid deflection) at the muscle underneath the electrode.13 This encompasses a considerable number of muscle fibers (given by the solid-angle concept), however-in fact, a greater number than if the electrode is placed directly on the epicardial surface.13 In the latter case, the moment of arrival of excitation at the electrode affects a lesser number of fibers and is thus given by the intrinsic deflection.13
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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.