Waleed Abdel Aziz Salem, PhD.Electrical DepartmentBenha Faculty of Engineering, Benha University

E472 E472 Hospital Instrumentation

Topic

• 1-Electrocardiography (ECG)• 2- Physiological pressure measurements• 3- Defibrillator • 4- Pacemakers• 5- Intensive & Coronary Care Units• 6- Electrosurgery Generators• 7- Medical Ultrasound• 8- The Human Respiratory Measurement• 9- Computers in Biomedical Equipment

• 10- Experimental Work (Biomedical Measurement System)

Introduction to Biomedical Equipment TechnologyBy Joseph Carr and John Brown

Electrocardiography (ECG)

Schematic Representation of

Electro-Conduction System

• SA Node• AV Node• Bundle of

His• Bundle

Branches• Purkinjie

Fibers

• SA: serves as a pacemaker for the heart ,fires electrical impulses but under control of the central nervous system

• AV: Operate like a delay line to retard the advance of action potential along the internal electroconduction system toward the ventricles

• Purkinje Fibers: Excite the muscle cells of the ventricles

• The contaraction of many muscles cells at one time creates electrical signal that can detected by electrodes

• Sequence: Depolarization occurs in the sinoatrial (SA) node; current travels through internodal tracts of the atria to the atrioventricular (AV) node; then through Bundle of His, which divides into right and left bundle branches; left bundle branch divides into left anterior and posterior fascicles.

ECG Review

Electrocardiograph (ECG)

• Components:– P wave = Atrial Contraction– QRS Complex = Ventricular Systole– T Wave = Refractory Period

• Typical measurement from right arm to left arm• Also see 1 mV Calibration pulse

Different Segments of ECGDepolarization:Electrical activation of the myocardium. Repolarization: Restoration of the electrical potential of the

myocardial cell. P wave: the sequential activation (depolarization) of the

right and left atriaQRS complex: right and left ventricular depolarization

(normally the ventricles are activated simultaneously) 0.06 to 0.10 seconds

ST-T wave: ventricular repolarizationPR interval: time interval from onset of atrial depolarization

(P wave) to onset of ventricular depolarization (QRS complex) P-R interval is 0.12 to 0.20 seconds

Different Segments of ECG (Cont.)

QRS duration: duration of ventricular muscle depolarization

QT interval: duration of ventricular depolarization and repolarization (0.34 and 0.42 seconds)

RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate)

PP interval: duration of atrial cycle (an indicator or atrial rate

Typical LeadsRA = right armLA = Left armLL = left legRL = right legC = Chest

Different leads result in different waveform shapes and amplitudes due to different view and are called leads

Cardiac Axis by different Leads

• ECG Electrodes: Two arrangements, bipolar and unipolar leads.

• Bipolar Lead: One in which the electrical activity at one electrode is compared with that of another.

• Unipolar Lead: One in which the electrical potential at an exploring electrode is compared to a reference point that averages electrical activity,

Cardiac Axis by different Leads

Standard Limb Leads: I, II, III; bipolar, form a set of axes 60° apart Lead I: Composed of negative electrode on the right arm and positive electrode on the left arm. Lead II: Composed of negative electrode on the right arm and positive electrode on the left leg. Lead III: Composed of negative electrode on the left arm and positive electrode on the left leg. Augmented Voltage Leads: aVR, aVL aVF; unipolar ; form a set of axes 60° apart but are rotated 30° from the axes of the standard limb leads. aVR: Exploring electrode located at the right shoulder. aVL: Exploring electrode located at the left shoulder. aVF: Exploring electrode located at the left foot.

Types of Leads

Bipolar Limb Leads: are those designated by Lead I, II, III which form Einthoven Triangle:– Lead I = LA connected to noninverting input and RA connected to

inverting Input

– Lead II = LL connected to noninverting input and RA connected to inverting input and LA shorted to RL

– Lead III = LL connected to noninverting input and LA connected to inverting input and RA shorted to RL

LL LL LL

Einthoven Triangle:Note potential difference for each lead of triangle

Carr and Brown Figure 8-3

Each lead gives a slightly different representation of electrical activity of heart

Unipolar Limb Leads: augmented limb leads: leads that look at composite potential from

3 limbs simultaneously where signal from 2 limbs are summed in a resistor network and then applied to an inverting amplifier input and the remaining limb electrode is applied to the non-inverting input

Lead aVR = RA connected to non-inverting input while LA and LL are summed at inverting input

augmented (amplified) Voltage for Right arm (aVR)

Lead aVL = LA connected to non-inverting input while RA and LL are summed at inverting input

augmented (amplified) Voltage for Left arm (aVL)

Lead aVF = LL connected to non-inverting input while RA and LA are summed at inverting input

augmented (amplified) Voltage for Foot (aVF)

Types of Leads

Unipolar Limb Leads:

Types of Leads

LL LL LL

Types of Leads

Unipolar Chest Leads: measured with signals from certain specified locations on the chest applied to amplifiers non-inverting input while RA LA, and LL are summed in a resistor Wilson network at amplifier inverting inputs

Types of Leads

Unipolar Chest Leads

Wilson’s Central Terminal

• Configuration used with Unipolar Chest Leads where RA LA and LL are summed in resistor network and this is sent to the inverting input of an amplifier

Electrocardiograph Traces from different leads

Normal ECG with RA, LA, LL connected

Artrial Tachycardia with RA, LA, LL connected

Ventricular Tachycardia with RA, LA, LL connected

Block Diagram of ECG

ECG Pre-Amplifier

• High Impedance input of bioelectric amplifier• Lead selector switch• 1mV calibration source• Means of protecting amplifier from high voltage

discharge such as a defibrillator used on a patient• Amplifier will have instrumentation amplifier as

well as isolation amplifier

Isolation Amplifier

• Needed for safety! Want to isolate patient from high voltages and currents to prevent electric shock where there is specifically a barrier between passage of current from the power line to the patient.

• Can be done using light (photo emitter and photo detector) or a transformer (set of inductors that are used in a step up / step down configuration)

Isolation of Signal of Patient from Power needed for safety

Typical Representation of an Isolation Amplifier

Common Mode Rejection

• Until now we assumed Amplifiers were ideal such that the signal into each terminal would completely cancel lead to complete common mode rejection

• However with practical Op Amp there is not perfect cancellation thus you are interested in what common mode rejection is.

Simplistic Example of ECG Circuit

Would like to analyze what type of common mode voltage (CMV) can be derived

Common Mode Voltage (CMV)

• If 2 inputs are hooked together into a differential amplifier driven by a common source with respect to ground 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– 60Hz AC induced interference caused by magnetic and

electric fields from power lines and transformers• This noise is a current from in signal, common and ground wires• Capacitively coupled into circuit• (Other markets that work at 220-240 V will experience 50Hz noise)

Analysis to reduce noise in ECG

• Common Mode Rejection: – Instrumentation amplifier

(EX. INA128) using a differential amplifier which will cancel much of the 60 Hz and common DC offset currents to each input

– If each signal is carrying similar noise then the some of the noise will subtract out with a differential amplifier

Analysis to reduce noise in ECG

• Right leg driver circuit is used in a feedback configuration to reduce 60 Hz noise and drive noise on patient to a lower level.

Analysis to reduce noise in ECG

• Isolation Amplifier also will attenuate noise

• Shielding of cables further reduce noise

Review of Five 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 60 Hz noise

How to overcome offset voltage

Instrumentation Amplifier Gain (A1,A2,A3) = Non-Inverting Amplifier A4

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• If you had 300 mV of DC offset sent through two gains of 10 and then 50 you would have an offset of (300mV)(10)(50) = 150V thus you would saturate your amplifiers and not see any of your signal

• 3V offset after first set of noninverting amplifiers goes through differential amplifier A3 which reduces the offset voltage.

Problems of offset voltageand how to correct

Other Corrections for Offset

• Feedback circuit where output of A4 goes through HPF of A5 so only responses larger than cutoff frequency pass through thus the DC offset is attenuated

R and C should be switched because this is really a LPF

Affect of High Pass Filter of A5• Feedback through HPF has a

time constant of RC• 3 Modes:

– Diagnostic Mode (most time) where

RC = 1x10-6F*3.2x106Ώ = 3.2 sec

Cutoff Freq = 1/(2πRC) = 0.05Hz

– Monitor Mode (medium time) where

RC = 1x10-6F*318x103Ώ = 0.318 sec

Cutoff Freq = 1/(2πRC) = 0.5Hz

– Quick Restore (least time) where

RC = 1x10-6F*80x103Ώ = 0.08 sec

Cutoff Freq = 1/(2πRC) = 2Hz

With Feedback the DC offset is eliminated and thus can have a gain of 50 on the 2nd Non-inverting Amplifier Stage without Saturating the Circuit

Drawn IncorrectlyR and C should be switched

Defribillator

• A Defribillator = a high voltage electrical heart stimulator used to resuscitate heart attack victims

• When a physician applies this high voltage the high voltages and currents can cause damage to medical equipment BUT physician still needs to view ECG of the patient

• How do you protect your medical equipment from excessively voltages and currents?

Protection Devices in ECGs: Glow

Lamps

• Glow Lamps are pair of electrodes mounted in a glass envelope in a atmosphere of lower pressure neon gain or a mix of inert gases

• Typically impedance across electrodes is high but if voltage across electrodes exceeds ionization potential of gas then impedance drops so you create a short to ground so vast majority of current goes safely to ground and avoids your amplifiers

Protection Devices in ECGs: Zener

Diodes

• Diode: device that conducts electricity in one direction only

• Zener Diode: “Turns-On” when a minimum voltage is reached so in this configuration if a large voltage is applied (ie defibrillator) the zener diode will allow current to flow and shunts it to grounds thus current goes to ground and not to the amplifiers

Protection Devices in ECGs: Current-

Limiting Diodes

• Diode: device that conducts electricity in one direction only

• Diode acts as a resistor as long as current level remains below limiting point. It current rises above the limit, the resistance will change and the current will become clamped

• Can also use a varistor (variable resistor) which functions like a surge protector that clips spikes in voltages

Types of Defibrillator Damage

• Defibrillator is 6X greater than normal working voltage so damage will eventually occur

• Two forms of Damage:– Both Amplifier inputs are blown thus readout is a flat line– One amplifier input is blown so the ECG appears distorted

• Cause is from zener diodes becoming open or from glow lamps becoming defective from an air leak, or recombination or absorption of gases

• Recommended that lamps are changed every 1-2 years or sooner if ECG is in Emergency Room

Effect of Voltage Transient on ECG

• Sometime a high voltage transient is applied to the patient (defibrillator) which cause magnitudes much greater than biopotential signal (ECG) which saturates the amplifier

• Once the voltage transient signal is removed the ECG signal takes time to recover

Example of bandwidth and magnitude of various biopotentials

ECG is approximately 1 mV and spans from DC to 500 HzBook assumes Diagnostic mode is 0.05 Hz to 100 Hz

Electromyography (EMG)Electroencephalography (EEG)Electrooculography (EOG)

Electro-Surgery Unit (ESU) Filtering

Electro-Surgery Unit (ESU) Filtering

• While a surgeon is conducting surgery he/she will want to see their patient’s ECG

• ESU can introduce frequencies into the ECG of 100KHz to 100 MHz and with magnitudes up to kVolts which can distort the ECG

• ESU introduces:– DC offsets– Obscures the signal

• ESU needs to be of diagnostic quality thus you must eliminate higher frequencies which are noise

Correct for high frequency noise using LPF so ECG can function with ESU

RC Filters

• Low Pass Filters will pass frequencies lower than cutoff frequency of FH =1/2RC

Vs

Frequency

• High Pass Filters will pass frequencies greater than cutoff frequency of FL =1/2RC

FH

Vs

FL

Circuit Schematic of an example of ECG

•Lead I (LA – RA) means LA is going to the noninverting input and RA is going to inverting input•Precordial are the chest leads

Block diagram of Entire ECG Circuit

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QUESTIONS?

THANK YOU