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Biomedical Instrumentation

Biomedical Instrumentation
Biomedical Instrumentation A. Intro ECG B18/BME2 Dr Gari Clifford (Based on slides from Prof. Lionel Tarassenko) Biomedical Instrumentation B18/BME2 Who am I  UL in Biomed Eng  Dir CDT in Healthcare Innovation IBME  Signal Processing Machine Learning for Clinical Diagnostics  mHealth for Developing Countries  Low Cost Electronics  EWH / OxCAHT Biomedical Instrumentation B18/BME2 Vital signs monitoring Clinical need  Every day, people die unnecessarily in hospitals  20,000 unscheduled admissions to Intensive Care p.a.  23,000 avoidable inhospital cardiac arrests per annum  Between 5 and 24 of patients with an unexpected cardiac arrest survive to discharge  Vital sign abnormalities observed up to 8 hours beforehand in 50 of cases Biomedical Instrumentation B18/BME2 Identifying atrisk patients  Acutely ill patients in hospital (e.g. in the Emergency Dept) have their vital signs (heart rate, breathing rate, oxygen levels, temperature, blood pressure) continuously monitored but…  Patient monitors generate very high numbers of false alerts (e.g. 8695 of alarms MIT studies in ‘97 ‘06)  Nursing staff mostly ignore alarms from monitors (“alarm noise”), apart from the apnoea alarm, and tend to focus instead on checking the vital signs at the time of the 4hourly observations Biomedical Instrumentation B18/BME2 Continuous bedside monitoring in Emergency Department Biomedical Instrumentation B18/BME2 Course Overview 1. The Electrocardiogram (ECG) 2. The Electroencephalogram (EEG) 3. Respiration measurement using Electrical Impedance Plethysmography/Pneumography 4. Oxygen Saturation using Pulse Oximetry 5. Noninvasive Blood Pressure Biomedical Instrumentation B18/BME2 Course text books  Biomedical Engineering Handbook, Volume I, nd 2 Edition, by Joseph D. Bronzino (Editor), December 1999, ISBN: 084930461X  Medical Instrumentation: Application and Design, 3rd Edition, by John G. Webster (Editor), December 1997, ISBN: 0471153680 Biomedical Instrumentation B18/BME2 Relevant lecture notes  OPAMP CIRCUITS – Year 1, pages 1 to 42  FILTER CIRCUITS – Year 1, pages 1 to 15  INSTRUMENTATION – Year 2, pages 1 to 4, 1718, 22 to 28 and 38 to 52.  Please email val.mitchelleng.ox.ac.uk if you would like copies of the above.  Course website: http://www.robots.ox.ac.uk/gari/teaching/b18/ Biomedical Instrumentation B18/BME2 Quick Vote  Do you want all these lectures printed out each day (You can use laptops etc to take notes, just don’t check your email.) Biomedical Instrumentation B18/BME2 ToDo (for you)  Sign up on weblearn for revision sessions (15 max per session)  B18 (Undergrad)  Question sheet 1: three sessions, 9 a.m. noon on Friday of Week 7, in LR4  Question sheet 2: three sessions 9 a.m. noon on Friday of Week 8, in LR4  MSc:  Question sheet 1: a single session for all students, 3 5 p.m. on Friday of Week 7, in LR3  Question sheet 2: a single session for all students, 3 5 p.m. on Friday of Week 8, in LR3 Hand in sheets before hand Biomedical Instrumentation B18/BME2 Biomedical Instrumentation 1. The Electrocardiogram (ECG) Biomedical Instrumentation B18/BME2 The Electrocardiogram  If two surface electrodes are attached to the upper body (thorax), the following electrical signal will be observed:  This is the electrocardiogram or ECG Biomedical Instrumentation B18/BME2 The origin of the ECG  Atrial and ventricular contractions are the result of carefully timed depolarisations of the cardiac muscle cells • The timing of the heart cycle depends on:  Stimulus from the pacemaker cells  Propagation between muscle cells  Nonexcitable cells  Specialised conducting cells (AtrioVentricular Node) Biomedical Instrumentation B18/BME2 Important specific structures  Sinoatrial node = pacemaker (usually)  Atria  After electrical excitation: contraction  Atrioventricular node (a tactical pause)  Ventricular conducting fibers (freeways)  Ventricular myocardium (surface roads)  After electrical excitation: contraction Biomedical Instrumentation B18/BME2 Excitation of the Heart Biomedical Instrumentation B18/BME2 Excitation of the Heart Biomedical Instrumentation B18/BME2 Cardiac Electrical Activity  Putting it al together: Biomedical Instrumentation B18/BME2 Approximate model of ECG  To a first approximation, the heart can be considered to be an electrical generator.  This generator drives (ionic) currents into the upper body (the thorax) which can be considered to be a passive, resistive medium  Different potentials will be measured at different points on the surface of the body Biomedical Instrumentation B18/BME2 Recording the ECG P1 R T1 P1 RA LA P2 R P R T2 P2 RL LL Points P1 and P2 are arbitrary observation points on the torso; R is the resistance between them, and R , R are lumped P T1 T2 thoracic medium resistances. . Biomedical Instrumentation B18/BME2 Typical ECG signal Biomedical Instrumentation B18/BME2 Components of the ECG waveform • Pwave: a small lowvoltage deflection caused by the depolarisation of the atria prior to atrial contraction. • QRS complex: the largestamplitude portion of the ECG, caused by currents generated when the ventricles depolarise prior to their contraction. Biomedical Instrumentation B18/BME2 Components of the ECG waveform • Twave: ventricular repolarisation. • PQ interval: the time interval between the beginning of the P wave and the beginning of the QRS complex. • QT interval: characterises ventricular repolarisation. Biomedical Instrumentation B18/BME2 Recording the ECG  To record the ECG we need a transducer capable of converting the ionic potentials generated within the body into electronic potentials  Such a transducer is a pair of electrodes and are:  Polarisable (which behave as capacitors)  Nonpolarisable (which behave as resistors)  Both; common electrodes lie between these two extremes  The electrode most commonly used for ECG signals, the silversilver chloride electrode, is closer to a non polarisable electrode. Biomedical Instrumentation B18/BME2 Silversilver chloride electrode  Electrodes are usually metal discs and a salt of that metal.  A paste is applied between the electrode and the skin.  This results in a local solution of the metal in the paste at the electrodeskin interface. Some of the silver dissolves + into solution producing Ag ions: +  Ag → Ag + e  Ionic equilibrium takes place when the electrical field is balanced by the concentration gradient and a layer of + Ag ions is adjacent to a layer of Cl ions. Biomedical Instrumentation B18/BME2 Electrodeelectrolyte interface Electrode e e e Ag Ag Ag Ag Current I + Ag Cl + Ag + Ag Cl Cl + Ag Cl Gel Illustrative diagram of electrodeelectrolyte interface in case of AgAgCl electrode Biomedical Instrumentation B18/BME2 Silversilver chloride electrode  Electrodes are usually metal discs and a salt of that metal.  A paste is applied between the electrode and the skin.  This results in a local solution of the metal in the paste at the electrodeskin interface.  Ionic equilibrium takes place when the electrical field is balanced by the concentration gradient and a layer of Ag+ ions is adjacent to a layer of Cl ions.  This gives a potential drop E called the halfcell potential (normally 0.8 V for an AgAgCl electrode) Biomedical Instrumentation B18/BME2 Silversilver chloride electrode  The double layer of charges also has a capacitive effect. Electrode  Since the AgAgCl electrode is + + + + + + + Ag Ag Ag Ag Ag Ag Ag Cl Cl Cl Cl Cl Cl Cl primarily nonpolarisable, there is a Gel large resistive effect.  This gives a simple model for the electrode. Skin  However, the impedance is not infinite at d.c. and so a resistor must be added in parallel with the capacitor. + Ag → Ag + e Biomedical Instrumentation B18/BME2 Silversilver chloride electrode  The double layer of charges also has a capacitive effect.  Since the AgAgCl electrode is primarily nonpolarisable, there is a large resistive effect.  This gives a simple model for the electrode.  However, the impedance is not infinite at d.c. and so a resistor must be added in parallel with the capacitor. Biomedical Instrumentation B18/BME2 The Overall Model  The resistors and capacitors may not be exactly equal.  Half cell potentials E and E' should be very similar.  Hence V should represent the actual difference of ionic potential between the two points on the body where the electrodes have been placed. Biomedical Instrumentation B18/BME2 Electrode placement V = (potential at LA) – (potential at RA) I V = (potential at LL) – (potential at RA) II V = (potential at LL) – (potential at LA) III The right leg is usually grounded (but see later) Biomedical Instrumentation B18/BME2 ECG Amplification  Problems in ECG amplification  The signal is small (typical ECG peak value 1mV) so amplification is needed  Interference is usually larger amplitude than the signal itself Biomedical Instrumentation B18/BME2 st 1 Problem: Electric Field Interference  Capacitance between power lines and Electrical power system system couples current into the patient 50 pF  This capacitance varies but it is of the order of 50pF (this corresponds to 64MΩ at 50Hz ... recall Xc=1/C )  If the right leg is connected to the common RA LA ground of the amplifier with a contact impedance of 5kΩ, the mains potential will appear as a 20mV noise input. RL LL the 50 Hz interference is common to 5kΩ both measuring electrodes (common mode signals) Biomedical Instrumentation B18/BME2 The solution  The ECG is measured as a differential signal.  The 50Hz noise, however, is common to all the electrodes.  It appears equally at the Right Arm and Left Arm terminals.  Rejection therefore depends on the use of a differential amplifier in the input stage of the ECG machine.  The amount of rejection depends on the ability of the amplifier to reject commonmode voltages. Biomedical Instrumentation B18/BME2 Common Mode Rejection Ratio (CMRR) v = v + v v = A v + A v A A in cm d out cm cm d d d cm CMRR = A / A d cm (ratio of differential gain to common mode gain) Biomedical Instrumentation B18/BME2 Three OpAmp Differential Amplifier Biomedical Instrumentation B18/BME2 Three OpAmp Differential Amplifier ' ' vv vv vv Ad1 = 1 1 1 2 2 2 i R R R 2 1 2 R R ' 2 2 v (1 )v v 1 1 2 R R 1 1 . R R ' 2 2 v (1 )v v 2 2 1 R R 1 1 2R ' ' 2 vv (vv )(1 ) 2 1 2 1 R 1 2R 2 1 A = d1 R 1 Biomedical Instrumentation B18/BME2 Three OpAmp Differential Amplifier ' ' vv vv vv Ad1 = 1 1 1 2 2 2 i R R R 2 1 2 R R ' 2 2 v (1 )v v 1 1 2 R R 1 1 R R ' 2 2 v (1 )v v 2 2 1 R R 1 1 2R ' ' 2 vv (vv )(1 ) 2 1 2 1 R 1 When v = v = v , A = 1 1 2 cm cm Biomedical Instrumentation B18/BME2 Three OpAmp Differential Amplifier ' ' vv vv vv Ad1 = 1 1 1 2 2 2 i R R R 2 1 2 R R ' 2 2 v (1 )v v 1 1 2 R R 1 1 R R ' 2 2 v (1 )v v 2 2 1 R R 1 1 2R ' ' 2 vv (vv )(1 ) 2 1 2 1 R 1 A . A d1 d 2 CMRR = CMRR is product of CMRR A . A cm1 cm2 for each input amplifier Biomedical Instrumentation B18/BME2 nd 2 problem: Magnetic Induction  Current in magnetic fields induces voltage in the loop formed by patient leads RA LA  The solution is to minimise the coil area (e.g. by twisting RL LL the lead wires together) Biomedical Instrumentation B18/BME2 rd 3 problem: Source impedance unbalance  If the contact impedances are not balanced (i.e. the same), then the body’s commonmode voltage will be higher at one input to the amplifier than the other. Biomedical Instrumentation B18/BME2 rd 3 problem: Source impedance unbalance  If the contact impedances are not balanced (i.e. the same), then the body’s commonmode voltage will be higher at one input to the amplifier than the other.  Hence, a fraction of the commonmode voltage will be seen as a differential signal.  see problem on example sheet Biomedical Instrumentation B18/BME2 Summary  Output from the differential amplifier consists of three components:  The desired output (ECG)  Unwanted commonmode signal because the commonmode rejection is not infinite  Unwanted component of commonmode signal (appearing as pseudodifferential signal at the input) due to contact impedance imbalance Biomedical Instrumentation B18/BME2 Driven rightleg circuitry  The commonmode voltage can be controlled using a Driven rightleg circuit.  A small current (1µA) is injected into the patient to equal the displacement currents flowing in the body. Biomedical Instrumentation B18/BME2 Driven rightleg circuitry LA + A1 R2 Ra RA LA A4 R1 + Ra R2 RA A2 RL LL + RL R0 Biomedical Instrumentation B18/BME2 Driven rightleg circuitry Biomedical Instrumentation B18/BME2 Driven rightleg circuitry  The commonmode voltage can be controlled using a Driven rightleg circuit.  A small current (1µA) is injected into the patient to equal the displacement currents flowing in the body.  The body acts as a summing junction in a feedback loop and the commonmode voltage is driven to a low value.  This also improves patient safety (R0 is v. large – see notes). Biomedical Instrumentation B18/BME2 Other patient protection  (Defib Protection)  Isolation  Filtering  Amplification  Antialias filtering  Digitization Biomedical Instrumentation B18/BME2 Static defibrillation protection  For use in medical situations, the ECG must be able to recover from a 5kV, 100A impulse (defibrillation)  Use large inductors and diodes Biomedical Instrumentation B18/BME2 Patient Isolation  Optoisolators  DCDC Converters Biomedical Instrumentation B18/BME2 RF Shielding Emissions  Electromagnetic compatibility (EMC)  the ability of a device to function (a) properly in its intended electromagnetic environment, and (b) without introducing excessive EM energy that may interfere with other devices  Electromagnetic disturbance (EMD)  any EM phenomenon that may degrade the performance of equipment, such as medical devices or any electronic equipment. Examples include power line voltage dips and interruptions, electrical fast transients (EFTs), electromagnetic fields (radiated emissions), electrostatic discharges, and conducted emissions  Electromagnetic interference (EMI)  degradation of the performance of a piece of equipment, transmission channel, or system (such as medical devices) caused by an electromagnetic disturbance  Electrostatic discharge (ESD)  the rapid transfer of electrostatic charge between bodies of different electrostatic potential, either in proximity in air (air discharge) or through direct contact (contact discharge)  Emissions  electromagnetic energy emanating from a device generally falling into two categories: conducted and radiated. Both categories of emission may occur simultaneously, depending on the configuration of the device Biomedical Instrumentation B18/BME2 Testing Biomedical Instrumentation B18/BME2 Electrical safety (from Lecture B) Physiological effects of electricity:  Electrolysis  Neural stimulation  Tissue heating Biomedical Instrumentation B18/BME2 Electrolysis  Electrolysis takes place when direct current passes through tissue.  Ulcers can be developed, for example if a d.c. current of 0.1 mA is applied to the skin for a few minutes.  IEC601 limits the direct current ( 0.1 Hz) that is allowed to flow between a pair of electrodes to 10 μA. Biomedical Instrumentation B18/BME2 Neural stimulation  An action potential occurs if the normal potential difference across a nerve membrane is reversed for a certain period of time.  This results in a sensation of pain (if sensory nerve has been stimulated) or muscle contraction (if motor nerve has been simulated). Biomedical Instrumentation B18/BME2 Hazards of neural stimulation • The effects of neural stimulation depend on the amplitude and frequency of the current, as well as the location of the current injection.  If the current is injected through the skin, 75 mA – 400 mA at 50 Hz can cause ventricular fibrillation. Beware: under normal (dry) conditions, the impedance of the skin at 50 Hz is usually between 10 kΩ and 100 kΩ; if the skin is wet, the impedance can be 1 kΩ or less.  If the current is directly applied to the heart wall (e.g. failure of circuitry in a cardiac catheter), 100μA can cause ventricular fibrillation. Biomedical Instrumentation B18/BME2 Tissue heating  The major effect of highfrequency ( 10 kHz) electrical currents is heating.  The local effect depends on the current amplitude and frequency as well as the length of exposure.  Think about your mobile phone usage… Biomedical Instrumentation B18/BME2 Electricity can also be good for you…  Electrical shock is also applied to patients in clinical practice for therapeutic purposes.  These applications make use of the neural stimulation effect:  Pacemakers (to stimulate the heart)  Defibrillators (to stop ventricular fibrillation)  Implantable Stimulators for Neuromuscular Control (to help paralysed patients regain some neuromuscular control). Biomedical Instrumentation B18/BME2 Electricity can also be good for you… Biomedical Instrumentation B18/BME2
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