Lectures 4 & 5 - Biopotentials & ECGs

What does the Hodgkin & Huxley model show?

Shows electrical characteristics of excitable cells

What is the source of biopotentials?

- Electrical signals that originate within biological tissue as a result of electrochemical activity of excitable cells
- This excitation causes a change in electric field properties propagating through the body
- Action potentials can be measured and are the source of biopotentials

Define biopotential

- Electric potential that is measured between points in living cells, tissue and organisms which accompanies all biochemical processes
- Transfer of information between and within cells

Name three examples of biopotentials

- ECG
- EEG
- EMG

What does an ECG show?

- Recording of the electrical activity of the heart and is an important diagnostic tool in cardiology
- Signal originates from action potentials of cardiac muscle cells
- Relates mechanical and electrical activities of the heart

What does an EEG show?

Recording of neuronal electrical activity in the brain

What does an EMG show?

Recording of the electrical potential generated by the contractions of skeletal muscle cells

Name three characteristics of biopotentials?

1. Almost all biopotentials are in the low frequency range

Name the four chambers of the heart

1. Right atrium
2. Right ventricle
3. Left atrium
4. Left ventricle

What is the difference between diastole and systole?

Diastole - the resting phase where the heart muscle relaxes and the chambers fill with blood
Systole - The pumping phase where the heart muscle contracts and pushes blood out into the blood vessels

What are the five main components in the conduction system?

1. Sinoatrial SA node
2. Atrioventricular AV node
3. Bundle of His
4. Bundle branches
5. Purkinje fibres

Describe the three phases of heart conduction

Phase 1. Spontaneous depolarisation of SA node
Spread of depolarisation signal to atria then to AV node
Phase 2. SA repolarisation
Depolarisation of Bundle of His and then Purkinje fibres = ventricles contract and atria relax
Phase 3. Repolarisation of AV node, bundle of His and Purkinje fibres
Ventricles relax

On an ECG grid, what does each square represent?

x - axis
Small square - 1mm in length = 0.04 seconds
Larger square - 5mm in length = 0.2 seconds

y axis
Small square - 1 mm in length = 0.1mV
Larger square - 5mm in length = 0.5mV

Describe how ECG recordings are picked up

- Physiological potentials that arise from ionic movement create a measurable potential difference (voltages) on the skin surface.
- These voltages can be measured with a transducer.

What does a transducer do?

Converts ionic potentials to electrical potentials

What does an electrode do?

Measures potential difference between two points on the body

What are the two main types of electrodes?
Describe how they behave

- Polarisable
Behave as capacitors
- Non-polarisable
Behave as resistors

In electrocardiology, what does a vector represent?

Both the magnitude and direction of the action potential generated by an individual cardiac muscle
Sum of all the individual vectors generated by depolarisation waves make up the electrical axis

When the depolarisation wave travels towards a recording lead, which way do the results deflect?

Positive/Upward

When the depolarisation wave travels away a recording lead, which way do the results deflect?

Negative/Downwards

How is the overall magnitude of the heart's electrical potential measured?

From leads at different angles and is recorded over a period of time

Name three considerations during ECG measurement

1. Lead positioning
2. Skin-electrode contact
3. Shielding from interference

Horizontal axis
Rhythm
Vertical axis
Base line
Calibration
Several lines
Pattern
Patients condition

Time
Frequency of the beat
Voltage
No peaks
Rectangles at the edges
12 lead ECG
Small or larger peaks
Normal or abnormal

No electrical activity

Depolarisation travelling towards sensor

Depolarisation travelling away from the sensor

Repolarisation travelling towards the sensor

Repolarisation travelling away from the sensor

Einthoven's triangle

What does the P wave reflect?

Atrial depolarisation

What does the PR interval represent?

- Distance between the onset of the the P wave to the onset of the QRS complex
- Determines whether impulse conduction from the atria to the ventricles in normal

What does the PR segment reflect?

- Slow impulse conduction through the AV node
- Serves as a baseline

What does the QRS complex represent?

Depolarisation of the ventricle

Define the QRS duration

Time interval from the onset to the end of the QRS complex

What does the T wave represent?

Rapid repolairsation

What does the QT interval represent?

Ventricular activation and recovery

Define left ventricular hypertrophy

Enlargement and thickening of the walls of your heart's main pumping chamber

Why can left ventricular hypertrophy develop?

In response to a factor such as high BP or a heart condition that causes the left ventricle to work harder

State the stages of ECG signal processing and analysis

1. Signal amplification
2. Analogue to Digital conversion
3. Noise reduction
4. Data compression
5. Feature selection
6. Signal classification
7. Interpretation

How is ECG amplification achieved?

Instrumentation amplifier

What are the requirements for the instrumentation amplifier?

- High amplification
- High input impedance
- Ability to reject electrical interference

What is used for ADC?
State the specs

ΣΔ ADCs of high resolution and a low noise level

What are the stages of ADC?

1. Sampling
2. Quantising
3. Coding

What is meant by sampling in ADC?

- Analogue signal is sampled whilst the intermediate values of the signal are discarded
- Number of values within a time period constitutes the sampling rate

Define the Nyquist thereom

- When samples are taken below a certain rate, the samples may not adequately capture the characteristics of the signal.
- The “characteristics” are related with the frequencies of the signal and the rate should be double of the maximum frequency

State the Shannon-Nyquist Thereom

- In order to reconstitute a digitised sine wave signal it is necessary to take at least 2 samples per cycle of the signal
- This means that the minimum sampling frequency must be greater than twice the highest frequency component in the signal

What is the formula for praction sample frequencies, fs?

5 x fmax to 20 x fmax

What is meant by quantisation in ADC?

- The Voltage Amplitude of the analogue signal is quantised or sliced up into discrete digital levels
- Each voltage must be represented by one of these quantised levels after conversion
- The number of digital levels available depends on the number of bits of the converter
- A 3-bit converter would slice the voltage range into 8 discrete levels numbered 0 to 7

Define resolution in ADC

- Indicates the number of discrete digital values that an ADC can produce at its output
- Expressed as the number of bits of the converter

Define voltage resolution in ADC

Full input voltage range of the ADC divided by the number of discrete intervals on digitisation

Describe coding in ADC

Each level is allocated a binary digital number

Where does high frequency noise arise from?

- Overlap of ECG and EMG signal frequencies (20-80Hz)
- Power supply interference (50 or 60Hz)
- Neighbouring high-powered devices

Where does low frequency noise arise from?

- Results from patient movement
- Changes in body electrode contact

Define data transformation

Process of data compression, where information is reduced to a limited number of discriminatory features

Define feature extraction

Process where important features are extracted from the ECG signal

Define ECG signal classification

Process involves discrimination of PQRST complexes by means of analysing its shape

ECG atrial fibrillation detection

What are the four classifications for the analysis of the heart rhythm disturbances?

1. Irregular rhythms
2. Substitute rhythms
3. Premature stimulations
4. Tachyarrhythmias

What is the objective of ECG interpretation?

Understand the results of ECG testing and offer an interpretation in order to provide diagnosis and consider further testing