Ecg Cardiac Cycle

Typically, the ECG is measured from the surface of the skin, which can be done by placing two electrodes directly on the skin and reading the potential difference between them. This is possible because these signals are transmitted throughout the body. Again, as stated above, the detected waveform features depend not only on the amount of cardiac tissue involved, but also on the orientation of the electrodes with respect to the dipole in the heart. In other words, the ECG waveform will look slightly different when measured from different electrode positions, and typically an ECG is obtained using a number of different electrode locations (e.g., limb leads or precordial) or configurations (unipolar, bipolar, modified bipolar). which, fortunately, have been standardized by universal application of certain conventions.

3.1. Bipolar Limb Leads

The three most commonly employed lead positions used today are referred to as leads I, II, and III. For the purposes of explaining the position of these leads, imagine the torso of the body as an equilateral triangle as illustrated in Fig. 4. This forms what is known as Einthoven's triangle (named for the Dutch scientist who first described it). Electrodes are placed at each of the vertices of the triangle, and a single ECG trace (lead I, II, or III) is measured along the corresponding side of the triangle using the electrodes at each end (because each lead uses one electrode on either side of the heart, leads I, II, and III are also referred to as the bipolar leads). The plus and minus signs shown in Fig. 4 indicate the polarity of each lead measurement and are notably considered as the universal convention. (For reasons that become clear in this chapter, the vertices of the triangle can be considered to be at the wrists and left ankle for electrode placement, as well as the shoulders and lower torso.)

Electrocardiograma
Fig. 3. A typical lead II electrocardiogram (ECG) waveform is compared to the timing of atrioventricular and semilunar valve activity, along with which segments of the cardiac cycle the ventricles are in systole/diastole.
Pasco Ekg Sensor

Fig. 4. The limb leads are attached to the corners of Einthoven's triangle on the body. Each lead uses two of these locations for a positive and a negative lead. The plus and minus signs indicate the orientation of the polarity conventions. Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed,, 2003. Reproduced with permission of the McGraw-Hill Companies.

Fig. 4. The limb leads are attached to the corners of Einthoven's triangle on the body. Each lead uses two of these locations for a positive and a negative lead. The plus and minus signs indicate the orientation of the polarity conventions. Modified from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed,, 2003. Reproduced with permission of the McGraw-Hill Companies.

As an example, if the lead II ECG trace shows an upward deflection, it would mean that the voltage measured at the left leg (or bottom apex of the triangle) is more positive than the voltage measured at the right arm (or upper right apex of the triangle). One notable time-point at which this happens is during the P wave. Imagine the orientation of the heart as shown in Fig. 5, with the action potential propagation across the atria creating a dipole pointed downward and to the left side of the body. This can be represented as an arrow (shown in Fig. 5) showing the magnitude and direction of the dipole in the heart. This dipole would, overall, create a more positive voltage reading at the left ankle electrode than at the right wrist electrode, thus eliciting the positive deflection of the P wave on the lead II ECG.

Fig. 5. The net dipole occurring in the heart at any one point in time is detected by each lead (I, II, and Ill) in a different way because of the different orientations of each lead set relative to the dipole in the heart. In this example, the projection of the dipole on all three leads is positive (the arrow is pointing toward the positive end of the lead), which gives a positive deflection on the electrocardiogram during the P wave. Furthermore, lead II detects a larger amplitude signal than lead Ill does from the same net dipole (i.e., the net dipole projects a larger arrow on the lead II side of the triangle than on the lead Ill side). LA, left arm; LL, left leg; RA, right arm; SA, sinoatrial. Figure from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003.

Fig. 5. The net dipole occurring in the heart at any one point in time is detected by each lead (I, II, and Ill) in a different way because of the different orientations of each lead set relative to the dipole in the heart. In this example, the projection of the dipole on all three leads is positive (the arrow is pointing toward the positive end of the lead), which gives a positive deflection on the electrocardiogram during the P wave. Furthermore, lead II detects a larger amplitude signal than lead Ill does from the same net dipole (i.e., the net dipole projects a larger arrow on the lead II side of the triangle than on the lead Ill side). LA, left arm; LL, left leg; RA, right arm; SA, sinoatrial. Figure from D.E. Mohrman and L.J. Heller (eds.), Cardiovascular Physiology, 5th Ed., 2003.

Now, imagine how that same action potential propagation would appear on the other lead placements, leads I and Ill, if placed at the center of Einthoven's triangle (Fig. 5, right side). Each of these lead placements can be thought of as viewing the electrical dipole from three different directions: lead I from the top, lead II from the lower right side of the body, and lead Ill from the lower left side, all looking at the heart in the frontal plane. In this example, the atrial depolarization creates a dipole that gives a positive deflection for all three leads because the arrow's projection onto each lead (or in other words, measuring the cardiac dipole from each lead) results in the positive end of the dipole pointed more toward the positive end of the lead than the negative end. This is why atrial depolarization (P wave) appears as a positive deflection for each lead (although wave magnitude is different in each). Ventricular depolarization, however, is a bit more complicated, as is discussed in detail in Section 3.2.; briefly, it results in various directions of the Q- and S-wave potentials depending on which lead trace is utilized for recording.

3.2. Electrical Axis of the Heart

The direction and magnitude of the overall dipole of the heart at any instant (represented by the arrow in Fig. 5, for example) is also known as the heart's "electrical axis," which is a vector originating in the center of Einthoven's triangle such that the direction of the dipole is typically assessed in degrees. The convention for this is to use a line horizontal across the top of Einthoven's triangle as 0° and move clockwise downward (pivoting on the negative end of lead I) as the positive direction. It should be noted that the electrical axis is actually changing direction throughout the cardiac cycle as different parts of the heart depolarize/repolarize in different directions. Fig. 6 shows the dipole spreading across the heart during a typical cardiac cycle, beginning with atrial depolarization. Each panel is accompanied by a diagram of the corresponding deflections on each ECG lead (I, II, and Ill). Keep in mind that, at certain points, the electrical axis of the heart may give opposite deflections on the various ECG leads.

As can be observed in Fig. 6, depolarization begins at the sinoatrial node in the right atrium, forming the P wave. The atria depolarize downward and to the left, toward the ventricles, followed by a slight delay at the atrioventricular node before the ventricles depolarize. The initial depolarization in the ventricles normally occurs on the left side of the septum, creating a dipole pointed slightly down and to the right. This gives a negative deflection of the Q wave for leads I and II; however, it is positive in lead Ill. Depolarization then continues to spread down the ventricles toward the apex, which is when the most tissue mass is depolarizing at the same time, with the same orientation. This gives the large positive deflection of the R wave for all three leads. Ventricular depolarization then continues to spread through the cardiac wall and finally finishes in the left ventricular lateral wall. This results in a positive deflection for leads I and II; however, lead Ill shows a lower R-wave amplitude along with a negative S-wave deflection.

After a sustained depolarized period (the S-T segment), the ventricles then repolarize. This occurs anatomically in the opposite direction of depolarization. However, one must keep in mind that the arrow in Fig. 6 represents the electrical axis of the heart (or the dipole) and does not necessarily show the direction that the repolarization wave is moving. Thus, even though the wave is moving from epicardium to endocardium (the direction of repolarization), the dipole (and therefore the electrical axis) remains in the same orientation as during depolarization. This explains why the T wave is also a positive deflection on leads I and II and negative (or nonexistent) on Lead Ill. The ventricles are then repolarized, returning the signal to its baseline potential (value).

During the cardiac cycle, the electrical axis of the heart (viewed from the plane of the limb leads) is always changing in both magnitude and direction. The average of all instantaneous electrical axis vectors gives rise to the "mean electrical axis" of the heart. Most commonly, it is taken as the average dipole (or electrical axis) direction during the QRS complex since this is the highest and most synchronized signal on the waveform. Typically, to find the mean electrical axis, area calculations

Fig. 6. The net dipole of the heart (indicated by the arrow) as it progresses through one cardiac cycle, beginning with firing of the sinoatrial (SA) node and finishing with the complete repolarization of the ventricular walls. Each heart shows the charge separation inside the myocardium, along with a corresponding Einthoven's triangle diagram below it, which displays how the net dipole is defected by each of the bipolar limb leads. Notice the change in direction and magnitude of the dipole during one complete cardiac cycle. AV, atrioventricular. Modified from L. R. Johnson (ed.), Essential Medical Physiology, 3rd Ed., 2003. With permission of Elsevier.

Fig. 6. The net dipole of the heart (indicated by the arrow) as it progresses through one cardiac cycle, beginning with firing of the sinoatrial (SA) node and finishing with the complete repolarization of the ventricular walls. Each heart shows the charge separation inside the myocardium, along with a corresponding Einthoven's triangle diagram below it, which displays how the net dipole is defected by each of the bipolar limb leads. Notice the change in direction and magnitude of the dipole during one complete cardiac cycle. AV, atrioventricular. Modified from L. R. Johnson (ed.), Essential Medical Physiology, 3rd Ed., 2003. With permission of Elsevier.

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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