Biological Electrical Potentials: Their display and recording

Question 1

Compare the amplitude of the signal in the conducting system of the heart to that of the electrocardiogram. Explain this.

Answer 1

Amplitude of the cardiac action potential is 90mV

Amplitude of QRS complex: 1 - 2 mV

This biological signal is attenuated by the impedance of the tissues that exist between the conducting system of the heart and the skin electrode.

Question 2

Why does the QRS complex have a larger amplitude versus the p wave

Answer 2

Atrial walls thinner, less muscle mass = smaller potential

Question 3

How do EMG potentials differ from ECG potentials and why

Answer 3

EMG –> much shorter duration - 5 -10 ms (versus ECG QRS 110 ms)
–>Skeletal muscle depolarises more rapidly than cardiac

EMG –> spikes versus complex pattern of ECG
–> Skeletal muscle cells do not depolarise in synchrony with adjoining cells unlike the the cardiac functional syncytium.

Question 4

What determines the amplitude of the skeletal muscle EMG spike

Answer 4

Depends on the number of muscle fibres simultaneously stimulated. This, in turn, depends on the number of fibres stimulated by the motor neurone

(e.g. eye muscle: ± one motor neurone to 8 muscle fibres versus limb muscle: one motor neurone to 200 muscle fibres

Question 5

Differentiate the 1.Frequency distribution and 2. range of potentials of the EMG, ECG and EEG

Answer 5

EMG: 1 Hz - 1000 Hz and 10 uV - 50mV

ECG: 0.05 Hz - 100Hz and 100 uV - 5mV

EEG 1 Hz - 50 Hz and 20 uV - 200uV

Question 6

Describe the names, frequencies and LOC for the various types of EEG waves

Answer 6

Good BAT Dad (mnemonic)

Gamma: 35 Hz - Concentration / focus

Beta: 14 - 30 Hz - Awake with active ext. attention

Alpha: 8 - 14 Hz - Awake and resting

Theta: 4 - 8 Hz - Sleep

Delta: < 4 Hz - Deep Sleep

Question 7

Define the ‘black box’ concept

Answer 7

Black box 1: Input transducer
Black box 2: Amplifier
Black box 3: Recorder / Display device

These boxes simplify the collection of electrical components for the measurement and display of biological signals.

As long as voltage, current and frequency range or compatible between the boxes the system will work.

Question 8

Define an electric cell

Answer 8

This is a device that converts stored chemical energy into electricity

Question 9

What is an electrode in contact with the skin

Answer 9

Metal - gel - skin (+electrolytes within) form an electric cell which generates a potential

Question 10

What are the problems that arise at skin surface electrodes

Answer 10

1. Change in impedance if electrode moves
2. Loss of contact
3. Polarization (by biological signal current flowing through) –> fixed using a good electrode (silver chloride based)

Question 11

Why is it useful that biological signals from the heart, brain and skeletal muscle have disparate cell types and hence difference frequency and voltages?

Answer 11

Various amplifiers and filters can be used so that specific biological signals from specific organs can be transduced, isolated and displayed with the minimization of interference from the biological signals emitted from other cell types.

Question 12

How does the biological signal from the brain differ from that of the heart

Answer 12

ECG (0.1 - 100 Hz and 1mV)
Heart: myocardial cells contract in synchrony (via the functional syncytium formed by cardiac myocytes. therefore a biological signal of relatively higher amplitude is formed ( ±1mV)

EEG (1 - 50 Hz and 0.02 - 0.2 mV)
Brain:
1. Produces smaller and more complex signals
2. Combined potential of many post-synaptic potentials from sheets of large symmetrically arranged pyramidal cells in the cortical layers 3 and 4.
3. Delta: 0 - 4 Hz. Theta 4 - 8 Hz. Alpha 8 - 13 Hz. Beta > 13Hz

EMG (1 - 1000 Hz and 0.01 mV - 50 mV)
Skeletal muscle
1. Spikes (no organized complexes as cels not interconnected like myocardium)

Question 13

What is the function of an EMG

Answer 13

It is used to investigate spontaneous or evoked electrical potential from muscle
Can differentiate between
1. Primary muscular disorders
2. Neuromuscular junction abnormalities
3. Nerve disease or lesions

Question 14

What is an amplifier and why are they necessary when measuring biological signals

Answer 14

Devices that convert a small weak electrical signal to a more powerful output. with as little signal distortion as possible.

Essential for interpreting biological signals as the amplitude of the signal at the surface of the skin is too small with too much noise and interference.

Increase amplitude and filters in the amplifier

Filter components within the amplifier device filter out allow the amplifier to concentrate on the range most relevant to the biological signal of interest

Question 15

What is the bandwidth of an amplifier

Answer 15

The range of frequencies which an amplifier works over with minimum loss of signal amplification (or gain) is called the bandwidth

Question 16

What is a differential amplifier

Answer 16

A differential amplifier has two inputs.

1. Inverts one of the input signals –> changing its sign from positive to negative or vice versa
2. Sums the two inputs together

This means that signals of equal magnitude common to both inputs such as mains signals and other noise will sum to zero. Therefore any signal which is the same to both sources e.g. 50 Hz mains interference will be eliminated and the remaining signal will be amplified, hopefully the signal you are interested in. This is known as common mode rejection

Question 17

What is common mode rejection

Answer 17

Common mode rejection is carried out by differential amplifiers.

Differential amplifiers have two inputs: One of the input signals is inverted, changing its sign from negative to positive or vice versa. The inputs are then summed together. This means that signals of equal amplitude common to both inputs such as mains signals and other noise sum to 0. Therefore any signal which is the same to both sources e.g. 50Hz mains interference will be eliminated and the remaining signal will be amplified, hopefully the signal you are interested in.

Question 18

What is the common mode rejection ratio CMRR

Answer 18

The ratio of the unwanted signal: wanted signal

E.g. CMRR of 10 000 : 1 would require an interference signal at both electrodes 10 000 times greater than the desired signal at one electrode to achieve the same output as the biological signal on the display

Question 19

What are filters and why are they needed if common mode rejection works to remove interference

Answer 19

Despite common mode rejection, some interference with signal frequencies outside the bandwidth of interest may still be present in the signal to be amplified. Filters are essential to enhance the quality of the signal to be analysed and reject all other currents.

Question 20

What are high pass filters

What are low pass filters

Answer 20

High pass
- Eliminate all currents with a low frequency allowing currents with high frequency to pass through to be analysed

Low pass
- Eliminate all currents with a high frequency
allowing currents with low frequency to pass through to be analysed

Question 21

What is a band pass filter

Answer 21

A filter allows through a specific bandwidth. For example between 10 - 100 HZ eliminating frequencies above and below

Question 22

What is a notch filter

Answer 22

A filter that rejects a specific band of frequency

E.g. ECG machines –> eliminate frequencies between 48 to 52 Hz to remove interference from mains electricity

Question 23

What are the most common sources of interference

Answer 23

Electrical interference is the most common

1. Mains 50 Hz (ECG or EMG interefernce)
2. Radio
3. Mobile phones

Question 24

What is the frequency range of surgical diathermy and what filter can be used to remove interference from this device

Answer 24

0.5 - 1 MHz

Easy to eliminate with a low pass filter

Question 25

Apart from filters, what should be used to ensure minimal interference from surgical diathermy

Answer 25

Good co-axial shielding of cables

Question 26

What are the problems with patient monitoring in MRI scanners?

Answer 26

1. Powerful magnet
   - -> no magnetic metals within close range of the scanner as they interfere with image the scanner produces OR make objects move dangerously outside or within the body
2. Radio-transmitter and receiver used to communicate control room –> may interfere with patient monitoring

Question 27

How can problems related to patient monitoring within an MRI scanner be overcome

Answer 27

Pulse oximetry –> use long leads with fibre-optic cables

Another small radiotransmitter sometimes used at a non-interfering frequency to transmit ECG info outside the MRI room

Question 28

Why is there a Faraday cage around the MRI room

Answer 28

To prevent radio waves getting in or out

Question 29

What type of electrodes are used in the motor nerve stimulator (peripheral nerve stimulator

Answer 29

Two small pre-gelled silver/silver chloride pad electrodes.

Question 30

Where are the negative and positive electrodes positioned when trying to stimulate the ulnar nerve during peripheral noerve stimulation and neuromuscular monitoring

Answer 30

Negative electrode placed over the ulnar nerve at the wrist.

Positive electrode is positioned a few cm proximal

Question 31

What is the ‘gate theory’ of pain and what electrical stimulators used in pain treatment are employed in line with this theory

Answer 31

Rubbing an injured area or the application of heat or ice reduces the severity of pain. 1965 gate theory was postulated as an explanation for this. Gate theory purports that nociceptive impulses are transmitted in C fibres and touch pressure and vibration sense are carried by A delta fibres alone. Both C and A delta fibres converge on the same second order interneurons and hence it was believed that the nociceptive information would be reduced by transmission of the non-nociceptive information by these second order interneurons. IT has subsequently been discovered that there are A delta fibres which transmit nociceptive signal alone and hence gate theory has been disproved.

Electrical pain stimulators are used in the management of chronic pain = TENS: Transcutaneous Electrical Nerve Stimulation 60 mA and 60 - 380 us. Tingling and warm sensation for patient –> varied response and titrate frequency and current.

Question 32

What is the ordered category scale for pain assessment

Answer 32

1 = no pain
2 = mild pain
3 = moderate pain
4 = severe pain

Question 33

What is the visual analogue scale for pain assessment

Answer 33

100 mm line with 0 mm no pain and 100 mm worst pain ever

Patient marks their pain severity on the line