FACULTY OF ENGINEERING DEPARTMENT OF BIOMEDICAL ENGINEERING BME

FACULTY OF ENGINEERING DEPARTMENT OF BIOMEDICAL ENGINEERING BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın Lecture Note 4: ECG Systems BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 1

ECG Systems BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 2

What is an ECG? The electrocardiogram (ECG) is a representation of the electrical events of the cardiac cycle. Each event has a distinctive waveform, the study of which can lead to greater insight into a patient’s cardiac pathophysiology. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 3

The Normal Conduction System BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 4

The Normal Conduction System BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 5

ECG is a surface measurement of the electrical potential generated by electrical activity in cardiac tissue. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 6

ECG The human heart can be considered as a large muscle whose beating is simply muscular contraction. Therefore contractions of the heart cause a potential to be developed. The measurement of the potential produced by cardiac muscle is called electrocardiology. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 7

What types of pathology can we identify and study from ECGs? Arrhythmias Myocardial ischemia and infarction Pericarditis Chamber hypertrophy Electrolyte disturbances (i. e. hyperkalemia, hypokalemia) • Drug toxicity (i. e. digoxin and drugs which prolong the QT interval) • • • BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 8

Waveforms and Intervals BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 9

2. Ventricular depolarization 3. Ventricular repolarization 1. Atrial depolarization BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 10

Different Segments of ECG P wave: the sequential activation (depolarization) of the right and left atria QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously) ST-T wave: ventricular repolarization U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex) QRS duration: duration of ventricular muscle depolarization QT interval: duration of ventricular depolarization and repolarization RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate) PP interval: duration of atrial cycle (an indicator or atrial rate BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 11

ECG basics • • Amplitude: Bandwidth: 1 -5 m. V 0. 05 -100 Hz • Largest measurement error sources: – Motion artifacts – 50/60 Hz powerline interference • Typical applications: – Diagnosis of ischemia – Arrhythmia – Conduction defects BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 12

ECG Leads are electrode setups which measure the difference in electrical potential between either: 1. Two different points on the body (bipolar leads) 2. One point on the body and a virtual reference point with zero electrical potential, located in the center of the heart (unipolar leads) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 13

Cardiac Axis by Different Leads BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 14

Einthoven Triangle: Note potential difference for each lead of triangle BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 15

Each lead gives a slightly different representation of electrical activity of heart BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 16

ECG Leads The standard ECG has 12 leads: 3 Standard Limb Leads 3 Augmented Limb Leads 6 Precordial Leads The axis of a particular lead represents the viewpoint from which it looks at the heart. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 17

Standard Limb Leads BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 18

Precordial Leads Adapted from: www. numed. co. uk/electrodepl. html BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 19

12 -Lead ECG measurement • • Most widely used ECG measurement setup in clinical environment Signal is measured non-invasively with 9 electrodes + 1 reference electrode (right leg) Einthoven leads: I, II & III Goldberger augmented leads: VR, VL & VF BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 Precordial leads: V 1 -V 6 20

12 Lead ECG System BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 21

12 Lead ECG System BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 22

Block Diagram of a Basic ECG System Lead Selector Isolated Power Source BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 23

ECG Pre-Amplifier ECG pre-amplifier is a bioelectric differential amplifier. It includes; • High impedance input of bioelectric amplifier • Lead selector switch • 1 m. V calibration pulse source • Means of protecting amplifier from high voltage discharge such as a defibrillator discharge used on a patient • Pre-Amplifier contains an instrumentation amplifier as well as an isolation amplifier for patient safety BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 24

Isolation Amplifier • Needed for safety! Isolates the patient from high voltages and currents to prevent electric shock by introducing a specific barrier between passage of current from the power line to the patient. • Can be done by using light (photo emitter and photo detector) or a transformer (set of inductors that are used in a step up / step down configuration) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 25

Common Mode Rejection BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 26

ECG Amplifier Circuit With Right Leg Driver BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 27

Common Mode Voltage (CMV) • If 2 inputs are hooked together into a differential amplifier driven by a common source with respect to ground, then the common mode voltage should be the same and the ideal output should be zero. However practically you will see a voltage. • CMV is composed of 2 parts: – DC electrode offset potential – 50 Hz AC induced interference caused by magnetic and electric fields from power lines and transformers • Capacitively coupled into circuit BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 28

Analysis to reduce noise in ECG • Right leg driver circuit is used in a feedback configuration to reduce 50 Hz noise and to drive the noise on the patient to a lower level. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 29

“ 6” ways to reduce Noise in ECG • • • Common Mode Rejection (differential Amplifier) Right Leg Drive (feedback loop to decrease noise) Shielding of wires Isolation amplifier Notch filter to reduce 50 Hz noise Bandpass filter to reduce noise below and above ECG bandwidth (0. 05 -100 Hz) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 30

High Pass Active Filters Attenuates frequencies lower than the cutoff frequency is 1/(2 ) =1/ 2 Ri. Ci Rf Ci Ri - A Vinput Ci + Ri Ii 0 Voutput Rf Voutput IRf BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 31

Low Pass Active Filters = Integrator Cf Attenuates frequencies higher than the cutoff frequency Rf Ri - A Vinput + Voutput Cf Vinput Ri 0 Ii BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 Rf ICf Voutput IRf 32

Defibrillator Protection Circuit • Defibrillator = a high voltage electrical heart stimulator used to resuscitate heart attack victims • When the physician uses defibrillator, the high voltages and currents discharged onto patient can cause damage to medical equipment(specificly preamplifier circuitry), BUT physician still needs to view ECG of the patient while defibrillating. • How do you protect your medical equipment from excessive voltages and currents? BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 33

Protection Devices in ECGs: Neon Glow Lamps • Neon Glow Lamps are pair of electrodes mounted in a glass envelope filled with low pressured neon gas or a mix of other inert gases. • Normally impedance across the electrodes is very high (so lamps are not conductive) but when the potential across the electrodes reaches to the ionization point of the gas, impedance suddenly drops making the lamps conductive (they light up) creating a short circuit to ground, transferring excess energy safely to the ground without damaging the amplifier. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 34

Protection Devices in ECGs: Zener Diodes • Zener Diode: is a diode which allows current to flow in the forward direction in the same manner as an ideal diode, but also permits it to flow in the reverse direction when the voltage is above a certain value known as the breakdown voltage. • In this configuration when a larger potential than the breakdown voltage is applied to the system (i. e. Defibrillator discharge) the zener diode allow the current to flow in reverse direction and shunts it to the ground thus preventing any damage to the amplifier. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 35

Protection Devices in ECGs: Current-Limiting Diodes • Current Limiting Diodes are electronic devices that limit current to a maximum specified value for the device. • These diodes consists of a n-channel JFET-transistor • They act as a resistor as long as the current level remains below the limiting value. If current tends to rise above the limit (as in the case of a Defibrillator discharge), it will be clamped and limited preventing any excess current reaching the amplifier thus protecting it. BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 36

Electro-Surgery Unit (ESU) Filtering • During surgery simultaneous ECG recording is also required. • ESU can introduce high frequency signals with frequencies ranging from 100 KHz to 100 MHz and with magnitudes up to few k. Volts into the ECG signal. This interferance can heavily distort the ECG signal. • ESU introduces: – DC offsets – and obscures the ECG signal BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 37

• Even though the ECG frequency range is between 0. 05 -100 Hz and all the higher frequency signals are attenuated by using low-pass filters at filtering stage (slide 32), ESU signal still manages to disort ECG. • Because; Ø Internal junction points of the ECG amplifier can rectify high frequency signals like ESU signal and the parasitic capacitance between the leg connections of the amplifier further filters this signal to create a dc ofset potential. (Whenever the ESU device is triggered during the surgery ECG baseline oscilates up and down. ) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 38

Ø And the ESU signal can leak through amplifier and low pass filter layers obscuring the ECG signal. • ECG needs to be of diagnostic quality so the ESU noise should be eliminated • Common technique is to use a pi-type 3 -layered RC filter BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 39

RC Filters Vs Frequency FH • Low Pass Filters will pass frequencies lower than cutoff frequency of FH =1/2 RC Vs FL • High Pass Filters will pass frequencies greater than cutoff frequency of FL =1/2 RC BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 40

3 -Layered RC ESU Filter RC Filter RA Defibrillator Protection Circuit ECG Differential Amplifier LA LC filter design can also be used BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 41

Some Sample ECG Circuits High-precision analog front end of a portable ECG application. 3 patient electrodes (only 3 bipolar leads) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 42

• ECG system with 5 -patient electrodes (7 -leads) BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 43

Standalone ECG with Compact Design BME 311 LECTURE NOTE 4 - ALİ IŞIN, 2014 44