BME 311: BIOMEDICAL INSTRUMENTATION ILecturer: Ali Işın

BME 311: BIOMEDICAL INSTRUMENTATION I Lecturer: Ali Işın

Lecture Note 4: ECG Systems

FACULTY OF ENGINEERING

DEPARTMENT OF BIOMEDICAL ENGINEERING

ECG Systems

ECG Systems

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.

The Normal Conduction System

The Normal Conduction System

The Normal Conduction System

ECG

ECG is a surface measurement of the

electrical potential generated by

electrical activity in cardiac tissue.

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.

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)

Waveforms and Intervals

2. Ventricular depolarization

3. Ventricular repolarization

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

ECG basics

• Amplitude: 1-5 mV

• Bandwidth: 0.05-100 Hz

• Largest measurement error sources:

– Motion artifacts

– 50/60 Hz powerline interference

• Typical applications:

– Diagnosis of ischemia – Arrhythmia

– Conduction defects

ECG Leads

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)

Cardiac Axis by Different Leads

Einthoven Triangle:

Einthoven Triangle:

Note potential difference Note potential difference

for each lead of triangle

for each lead of triangle

Each lead gives a slightly different

representation of electrical activity of heart

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.

Standard Limb Leads

Precordial Leads

Adapted from: www.numed.co.uk/electrodepl.html

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: V_R, V_L & V_F Precordial leads: V₁-V₆

12 Lead ECG System

12 Lead ECG System

Block Diagram of a Basic ECG System

Isolated Power Source Lead Selector

ECG Pre-Amplifier

ECG pre-amplifier is a bioelectric differential amplifier.

It includes;

•High impedance input of bioelectric amplifier

•Lead selector switch

•1mV calibration pulse source

•Means of protecting amplifier from high voltage

discharge such as a defibrillator discharge used on a

patient

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)

Common Mode Rejection

ECG Amplifier Circuit With Right Leg

Driver

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

– 50Hz AC induced interference caused by magnetic and electric fields from power lines and transformers

• Capacitively coupled into circuit

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.

“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)

High Pass Active Filters

Attenuates frequencies lower than the cutoff frequency.

cutoff frequency is 1/(2) =1/ 2RiCi

-

+

Vinput

Ri A

Rf Ci

Ri Rf Voutput

0

I_Rf Ii

Vinput Ci

Low Pass Active Filters = Integrator

Attenuates frequencies higher than the cutoff frequency

Voutput

Cf

-

Vinput

+

Ri A

Rf

Ri Rf

0 Vinput

Cf

I_Cf

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 pre- amplifier 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?

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

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.

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

Electro-Surgery Unit (ESU) Filtering

• During surgery simultaneous ECG recording is also required.

• ESU can introduce high frequency signals with

frequencies ranging from 100KHz to 100 MHz and

with magnitudes up to few kVolts into the ECG signal.

This interferance can heavily distort the ECG signal.

• ESU introduces:

– DC offsets

– and obscures the ECG signal

• 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;

 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

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

RC Filters

• Low Pass Filters will pass frequencies lower than cutoff frequency of FH

=1/2RC Vs

Frequency F_H

Vs

3-Layered RC ESU Filter

Defibrillator Protection Circuit

ECG

Differential Amplifier RA

LA

RC Filter

LC filter design can also be used

Some Sample ECG Circuits

• ECG system with 5-patient electrodes (7-leads)

Standalone ECG with Compact Design