Biomedical Instrumentation Revision Session

Transcription

1 Biomedical Instrumentation eision Session B8/BME Dr Gari Clifford Biomedical Instrumentation B8/BME

2 The Electrocardiogram If two surface electrodes are attached to the upper body (thorax), the following electrical signal will be obsered: This is the electrocardiogram or ECG Biomedical Instrumentation B8/BME

3 st Problem: Electric Field Interference Capacitance between power lines and system couples current into the patient Electrical power system This capacitance aries but it is of the order of 50pF (this corresponds to 64MΩ at 50Hz... recall Xc=/C ) 50 pf If the right leg is connected to the common ground of the amplifier with a contact impedance of 5kΩ, the mains potential will appear as a ~0mV noise input. A LA the 50 Hz interference is common to both measuring electrodes! (common mode signals) L LL 5kΩ Biomedical Instrumentation B8/BME

4 The solution The ECG is measured as a differential signal. The 50Hz noise, howeer, is common to all the electrodes. It appears equally at the ight Arm and Left Arm terminals. ejection therefore depends on the use of a differential amplifier in the input stage of the ECG machine. The amount of rejection depends on the ability of the amplifier to reject common-mode oltages. Biomedical Instrumentation B8/BME

5 Common Mode ejection atio (CM) in = cm + d A d & A cm out = A cm cm + A d d CM = A d / A cm (ratio of differential gain to common mode gain) Often expressed in power: CMM = log0(ad/acm) Biomedical Instrumentation B8/BME

6 Three Op-Amp Differential Amplifier Biomedical Instrumentation B8/BME

7 Biomedical Instrumentation B8/BME Three Op-Amp Differential Amplifier Ad = ) )( ( ) ( ) ( i. A d =

8 Biomedical Instrumentation B8/BME Ad = ) )( ( ) ( ) ( i When = = cm, A cm = Three Op-Amp Differential Amplifier

9 Biomedical Instrumentation B8/BME Ad = ) )( ( ) ( ) ( i cm cm d d A. A A. A CM = Three Op-Amp Differential Amplifier CM is product of CM for each input amplifier

10 nd problem: Magnetic Induction Current in magnetic fields induces oltage in the loop formed by patient leads The solution is to minimise the coil area (e.g. by twisting the lead wires together) A LA The noise is now common to both inputs and is cancelled at differential amplifier L LL Twisted pair cabling minimises cross talk in communication lines Biomedical Instrumentation B8/BME

11 3 rd problem: Source impedance unbalance If the contact impedances are not balanced (i.e. the same), then the body s common-mode oltage will be higher at one input to the amplifier than the other. Biomedical Instrumentation B8/BME

12 3 rd problem: Source impedance unbalance If the contact impedances are not balanced (i.e. the same), then the body s common-mode oltage will be higher at one input to the amplifier than the other. Hence, a fraction of the common-mode oltage will be seen as a differential signal. Biomedical Instrumentation B8/BME

13 Summary Output from the differential amplifier consists of three components: The desired output (ECG) Unwanted common-mode signal because the common-mode rejection is not infinite Unwanted component of common-mode signal (appearing as pseudo-differential signal at the input) due to contact impedance imbalance Biomedical Instrumentation B8/BME

14 ECG - BP PPG - ESP Biomedical Instrumentation B8/BME

15 Blood pressure measurements Blood pressure is generally recorded using two measurements (in mmhg): Systolic Pressure Diastolic Pressure Blood pressure is usually reported as "Systolic oer Diastolic ; e.g. 0/70 is a systolic pressure of 0 mmhg and a diastolic pressure of 70 mmhg. Biomedical Instrumentation B8/BME

16 Systolic blood pressure Systolic blood pressure (SBP) is the arterial pressure when the heart is beating (during systole). It is, broadly speaking, the highest pressure present in the arterial (and ascular) system. It is a reflection of how hard the heart is pumping. Biomedical Instrumentation B8/BME

17 Diastolic pressure Diastolic blood pressure (DBP) is the arterial pressure when the heart is not beating (during diastole). It is, broadly speaking, the lowest pressure present in the arterial (but not ascular) system. It is a reflection of the resistance against which the heart is pumping. Biomedical Instrumentation B8/BME

18 Oscillometry Marey (885) noted that the pressure in a essel containing an arm fluctuated with the beating of the heart. The magnitude of these pressure fluctuations aried with the applied pressure Modern ersion inoled an inflatable cuff around the arm Biomedical Instrumentation B8/BME

19 Pressure fluctuations First assumption: The start of the pressure fluctuations occurs at systolic pressure. The end of the pressure fluctuations occurs at diastolic pressure. It turns out, howeer, that this is not the case. Biomedical Instrumentation B8/BME

20 Pressure fluctuations Empirical studies hae shown that: The onset of the oscillations occurs well aboe systolic pressure. The oscillations do not disappear until well below diastolic pressure. Biomedical Instrumentation B8/BME

21 Pressure fluctuations The pressure at which the oscillations hae their maximum amplitude is the Mean Arterial Pressure (MAP). Biomedical Instrumentation B8/BME

22 Empirical determination of systolic and diastolic pressures Systolic pressure = cuff pressure when the oscillation amplitude is 55% of the maximum amplitude Diastolic pressure = cuff pressure when the oscillation amplitude is 85% of the maximum amplitude Maximum oscillation Oscillation amplitude time time Systolic pressure diastolic pressure Biomedical Instrumentation B8/BME

23 Oscillometry system oeriew Biomedical Instrumentation B8/BME

24 NIBP Deice specs Dynamic range = 0-50 mmhg Maximum output from pressure sensor = 50 mv Differential amplifier typical gain = 0... Therefore max output of differential amp = V Quantization: 0. mv per mmhg LPF with cut-off of 0.05Hz DC supply 5-9V Max output for amplifier & LPF ~8V. DC gain = Biomedical Instrumentation B8/BME

25 Oscillometry system oeriew The signal from the pressure transducer has two components: the underlying pressure of the cuff the cardiac-synchronous oscillations Biomedical Instrumentation B8/BME

26 Pressure measurement system The pressure measurement system consists of the following: A pressure transducer to sense the cuff pressure (including the cardiac-synchronous oscillations) Amplification and filtering Analogue-to-digital conersion Biomedical Instrumentation B8/BME

27 Amplification and filtering Instrumentation amplifier (as with ECG or EEG measurement) is used to amplify the differential pressure transducer output. Amplifier output is low-pass filtered to derie the cuff pressure signal and high-pass filtered to extract the cardiacsynchronous fluctuations. Biomedical Instrumentation B8/BME

28 Analogue-to-digital conersion Sampling frequency: Accurate peak detection requires 0 samples per cycle of cardiac-synchronous fluctuations choose a sampling frequency of 50 Hz or aboe for that channel. Amplitude resolution: 8-bit accuracy should be sufficient for A-D conersion: Low-pass filtered cuff pressure signal needs to be resoled to or mmhg in a range of 0 to 300 mmhg High-pass filtered cardiac-synchronous fluctuations will be digitised with 0.5% accuracy. Biomedical Instrumentation B8/BME

29 Signal processing Software will perform the following functions: Initiating the measurement cycle and driing the cuff controller (or telling the user to pump ) eading in the digitised data ecording the amplitude of the cardiac-synchronous fluctuations at the different cuff pressures Computing mean, systolic and diastolic pressures Computing pulse rate, if required (It is of course possible for the low-pass and high-pass filtering of the oerall cuff pressure signal also to be performed in software after A-D conersion of the amplifier output.) Biomedical Instrumentation B8/BME

30 eision / Exam Tips ead past papers practice answering them in the allotted time ecap the connection between signals ECG, BP, PPG, resp eise simple electronics Ohm s law & Kirchoff s first law!!! Just trace around the circuit and remember the current is consered at any junction Biomedical Instrumentation B8/BME

31 ECG - BP PPG - ESP Biomedical Instrumentation B8/BME

32 Biomedical Instrumentation B8/BME