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Principles of Biomedical Systems & Devices PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Lecture 7: The Electrocardiogram This Week in PBS&D PBS&D – Fall 2004 – Polikar       Cardiac conduction system Electrocardiography Cardiac vector model ECG leads and lead configurations Einthoven’s Triangle Measuring ECG http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Cardiac Conduction System PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 The Electrocardiogram PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 The ECG PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 The ECG PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 The Modern ECG PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Electrocardiograph PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Holter Monitor PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Dipole Model PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  The electrical activity of the heart can be modeled with a vector quantity: an electric dipole, M whose magnitude and direction changes in time. Also called the cardiac vector Standard Locations? PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  Potentials generated by the heart appear throughout the body  Measurements made at different locations will generate different recordings.  Need standard positions for electrode placement so that clinical evaluations of ECG can be made ECG Lead Configurations PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  A pair of electrodes, or a combination of several electrodes through a resistive network that gives an equivalent pair is called a lead.  A vector that connects a lead electrode pair is the lead vector.  If the cardiac vector M is known, the voltage generated at any lead can be easily computed. vai = M ⋅ a i a2 M a1 θ υa1 + Frontal Plane Bipolar Leads Einthoven Triangle PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 I − II + III = 0 The Einthoven Triangle PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Lead I PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Lead I is the first of three standard limb leads (I, II, III). Limb leads measure cardiac depolarization in the frontal (coronal) plane. The negative electrode is connected to the RIGHT ARM. The positive electrode is connected to the LEFT ARM. The axis is 0 degrees. When an action potential starts on the right and proceeds toward the left side of the heart, a positive inflection will be seen in lead one. This holds true for all leads. Whenever a current proceeds toward a positive electrode, an upright inflection is seen on the EKG tracing. Lead II PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Lead II is used alone quite frequently. Normal rhythms present with a prominant P wave and a tall QRS. The negative electrode is connected to the RIGHT ARM. The positive electrode is connected to the LEFT LEG. The axis is +60 degrees. In all the limb leads, the electrodes may be positioned close to the torso. For convenience, they are often placed at the shoulders and hips. Lead III PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Lead III is the last of the three standard limb leads. The negative electrode is connected to the LEFT ARM. The positive electrode is connected to the LEFT LEG. The axis is 120 degrees. Cardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and III Cardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and III Cardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and III Cardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and III Unipolar Measurements: Wilson Central Terminal PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  Three additional leads are used for frontal plane measurements.  These are the measurements at the specific electrode, with respect to a reference electrode  One commonly used reference electrode is the Wilson Central Terminal, obtained through a resistive network, combining limb electrodes  The new set of leads obtained by combining the standard limb electrodes to the Wilson terminal form the augmented leads  These leads provide additional vector views of cardiac depolarization in the frontal plane. Unlike leads I, II, III, the augmented leads utilize WCT, a central negative terminal. This virtual "electrode" is calculated by the EKG computer to measure vectors originating roughly at the center of the heart.  Note that the voltage at Wilson’s terminal is zero (why?) Wilson’s Central Terminal PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Augmented Leads: aVR PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Augmented Lead (RIGHT) avR The negative electrode is the central terminal. The positive electrode is connected to the RIGHT ARM. The axis is -150 degrees. Augmented Leads aVL PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Augmented Lead (LEFT) avL The negative electrode is the central terminal. The positive electrode is connected to the LEFT ARM. The axis is -30 degrees. Augmented Leads aVF PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Augmented Lead (FEET) avF The negative electrode is the central termininal. The positive electrode is connected to the LEFT LEG. The axis is +90 degrees. All Frontal Leads PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Frontal Leads PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 Transversal Leads PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  Additional set of six leads, placed on the chest, also known as the precordial leads. These too are unipolar, that is they measure the potential with respect to WCT. Summary PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  Of the 12 leads, the first six are derived from the same three measurement points. Therefore, any 2 of these 6 leads include the same information as the other four.  Over 90% of the heart's electric activity can be explained with the dipole source model (Geselowitz, 1964). To evaluate this dipole, it is sufficient to measure its three independent components. In principle, two of the limb leads (I, II, III) could reflect the frontal plane components, whereas one precordial lead could be chosen for the anterior-posterior component. The combination should be sufficient to describe completely the electric heart vector.  To the extent that the cardiac source can be described as a dipole, the 12-lead ECG system could be thought to have three independent leads and nine redundant leads.  However, in fact, the precordial leads detect also nondipolar components, which have diagnostic significance because they are located close to the frontal part of the heart. Therefore, the 12-lead ECG system has eight truly independent and four redundant leads.  The main reason for recording all 12 leads is that it enhances pattern recognition. This combination of leads gives the clinician an opportunity to compare the projections of the resultant vectors in two orthogonal planes and at different angles. PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404 References PBS&D – Fall 2004 – Polikar http://engineering.rowan.edu/~polikar/CLASSES/ECE404  Bioelectromagnetism, Principles and Applications of Bioelectric and Biomagnetic Fields, J. Malmivuo and R. Plonsey, http://butler.cc.tut.fi/~malmivuo/bem/bembook/  Univ. of Maryland, Emergence Medical Interest Group, http://davidge2.umaryland.edu/~emig/ekgtu03.html  The Alan E. Lindsay ECG Learning Center, http://medstat.med.utah.edu/kw/ecg/index.html
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