B18/BME1 laboratory in Medical Instrumentation

LAB RECORD

NAME / College
Attendance / Preparation / Mark (short)

Before coming into the lab, complete the preparation (Section P) and read through the handout.

Safety. Any equipment attached to the body is battery operated or electrically isolated. Standard risks for using computers apply

Please ask the demonstrators if you need help

Preparation

P1: ECG Physiology

(i)Explain briefly how the components of the ECG trace (P wave, QRS complex and T waves) are correlated with the electrical activity of the atrial and ventricular muscle

(ii)Referring to the position of the electrodes, explain why you might expect lead II to give the largest amplitude of the QRS complex

(iii)In which lead(s) might you expect to find changes from the norm in (i) the left ventricle and (ii) the right ventricle and why?

P2: Einthoven’s triangle calculation

Using the figure as a guideuse the cosine rule to write down  in terms of the lead voltage: VI, VII, VIII

cos θ =

P3: Instrumentation

(i)Why is it important for the differential amplifier to have a high common mode rejection ration (CMRR)?

(ii)Describe briefly whichcomponents ensure the subject is isolated from mains electricity

(iii)Why are filters likely to be necessary?

P4: Fourier analysis
  1. State the first three harmonics present in a periodic signal of frequency 1Hz. What components might you expect to find in the Fourier series of an ECG signal if the heart beat is 1.1Hz?
  1. The sampling rate to acquire an ECG signal is normally above 100Hz. Look at Fig 1.and say why it needs to be much higher than the heart rate.

SECTION A

A1: Parameter settings

Sampling rate:

Time scale factor(5:1, 10:1 etc) / Vertical scale (channel 1) / Vertical scale (channel 2) / Pulse rate

Exercise A2: Filters

Exercise A2.1 the signal

(i)Write a few notes on the difference (if any) between the signals acquired in the time domain (Chart mode) before and after applying the band pass filter. (Note that there is a d.c. filter built into the hardware.)

Exercise A2.2: spectrum

(i)Note the magnitude of the 50Hz component with and without the filter.

No filter / Filter
Amplitude

In dB, how much does the filter reduce this component?

(ii)Note down the frequency of the first few significant maxima in the spectrum. Identify any associated with the pulse rate, breathing or other identifiable causes.

(iii)Increasing the number of samples increases the length of the window used in the FFT and therefore improves the resolution of the FFT. However because it effectively averages over a longer window it reduces the rate at which you can identify changes.

Note down the frequency of the fundamental heart beat when you use 32k samples. Note below how this changes as you reduce the number of samples (estimate the frequency from the spectrum in each case)..

No of samples / Frequency of fundamental heart cycle
1k
2k
4k
8k
16k
32k

Estimate the error in centre frequency if you take 8k samples. How any independent spectra could you observe per minute?

(Note: time to acquire the data = Number of samples x sampling interval)

Exercise A3: The ECG Trace
  1. On your printout, identify the PQRST points on each channel.
  2. Which R peak is the largest? Why do you think this is so?
  3. Fill in the table below

From each R-R cycle over a breathe in-out / Cycle 1 / Cycle 2 / Cycle 3 / Cycle 4 / Cycle 5 / Average
Cycle time (R-R)
Heart rate=1/cycle time

You may observe some variation in the heart rate over the breathing cycle. What is the cause of this?

Exercise A4: Movement Artefacts

In each case belowdescribe and explain briefly the changes in the signal and its spectrum caused by the artefact.

(i)Hand tremor

(ii)Deep and regular breathing

Exercises A5: Effect of exercise

You need to take measurements whilst the subject’s heart rate is still elevated.

R-R interval / QRS / S-T / T-P / P-R
Before exercise
After exercise
  1. What happened to the R-R interval and the heart rate after exercise?
  1. Note that the R-R interval consists of the sum of QRS, S-T, T-P and P-R. Which of these become shorter when the heart rate increased?
Errors:Describe the main sources of error in your experiments

Session B

B1 Bioradio FiLTers

Note down any Bioradio filters used.

The filter in this software simulates an analogue filter

B2 Einthoven’s triangle – subject sitting

For Einthoven’s triangle it is important to measure the peak value of each signal. Using your data, fill in the following table. Calculate  for at least one cycle, all three if you have time

R(Lead I) /  R(Lead II) /  R(Lead III) / 
Peak 1
Peak 2
Peak 3

Mean and standard deviation of 

B3 Einthoven’s triangle – subject Lying down
R(Lead I) /  R(Lead II) /  R(Lead III) / 
Peak 1
Peak 2
Peak 3

Mean and standard deviation of 

Comment on your results, and any errors in the experiment

Section C: EEG signals

Exercise C1.1
Any significant peaks in the spectrum / Amplitude of 50Hz component
Eyesopen
Eyesclosed

Comment on these results.

Exercise C1.2: Eyesclosed data
  1. Print out the graphs requested in the handout. Label each with the order of the filter.
  2. Note the four highest components remaining after the 6th order filte. Can yousuggest a cause for each

Frequency / Cause
Spectral component 1
Spectral component 2
Spectral component 3
Spectral component 4

Exercise C1.3:Eyesopen data

Sketch the spectrum of the filtered signal noting any bands of activity you can identify

Exercise C2: Parameters

Note down the amplifier settings, time scale and any filter settings used

Exercise C3: Artefacts

Fill in the table below:

Peak amplitude / O1-A2 / FP1-A2
Effect of blinking - amplitude of peaks observed
Effect of eye movement – amplitude of movements observed
Additional terms in spectrum from deep breathing
Clenching teeth

Would these signals be a problem inobservations of alpha, beta and theta waves?

Exercise C4.1; Alpha waves

On the spectra of the two signals you have just acquired, note any peak which might result from alpha waveactivity (sketch them below or print them out)

O1-A2 / C3-A2

Referring to Figures C1 and C2 explain on which channel you might expect to see alpha wave activity most strongly? Did this occur in your experiment?

Exercise C4.2 (optional): beta waves

Sketch any difference between the spectra in the beta range you induced htrough a task. Note the task involved.

O1-A2 / C3-A2

Referring to Figures C1 and C2 explain on which channel you might expect to see beta wave activity most strongly? Did this occur in your experiment?

Errors: Describe the main sources of error in your experiments