1Chapter 6 Biopotential Amplifiers2Introduction to Biopotential Amplifiers Common properties of biopotential amplifiersElectrocardiographyCardiac vector modelECG leads and lead configurationsEinthovens TriangleMeasuring ECG36.1 Basic Requirements4Biopotential AmplifiersHigh I/P impedanceElectrically isolated input circuitLow output impedanceAppropriate frequency rangeHigh gain!Easy calibration56.2 The ECG6心臟的傳導系統7心臟電訊之傳導8心電訊號與生臟動作關係910Dipole ModelThe 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 vector11electric dipole: 由兩個大小相等符號相反的點荷所組成。設有距離d 兩個大小相等符號相反的點荷 qP點 , 距中心點為 z電偶極所建立之電場E隨1/r3化, r為電偶極中心到p點之距離電場E 之方向和 dipole moment vector 電偶極所建立之電場 E隨距離快速衰qdp =v12Standard Locations?Potentials generated by the heart appear throughout the bodyMeasurements made at different locations will generate different recordings.Need standard positions for electrode placement so that clinical evaluations of ECG can be made13ECG Lead ConfigurationsA 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.a1a2a1M+14Frontal Plane Bipolar LeadsEinthoven Triangle15The Einthoven Triangle16Lead ILead 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 beseen 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.17Lead IILead 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. 18Lead IIILead 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.19Cardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and IIICardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and IIICardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and IIICardiac Vector Through the Cardiac Cycle As Seen By Leads I, II and III2425Unipolar Measurements:Wilson Central TerminalThree additional leads are used for frontal plane measurements. These are the measurements at the specific electrode, with respect to a reference electrodeOne commonly used reference electrode is the Wilson Central Terminal, obtained through a resistive network, combining limb electrodesThe new set of leads obtained by combining the standard limb electrodes to the Wilson terminal form the augmented leadsThese 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.26Wilsons Central Terminal27Augmented Leads:aVRAugmented Lead (RIGHT) avRThe negative electrode is the central terminal.The positive electrode is connected to the RIGHT ARM.The axis is -150 degrees.28Augmented LeadsaVLAugmented Lead (LEFT) avL The negative electrode is the central terminal.The positive electrode is connected to the LEFT ARM.The axis is -30 degrees.29Augmented LeadsaVFAugmented Lead (FEET) avF The negative electrode is the central termininal.The positive electrode is connected to the LEFT LEG.The axis is +90 degrees.30All Frontal Leads31Frontal Leads3233Transversal LeadsAdditional 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.343536SummaryOf 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 hearts 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 com