Lecture C. Respiration - University of Oxfordgari/teaching/b18/lecture_slides/B...B12/BME2...
Transcript of Lecture C. Respiration - University of Oxfordgari/teaching/b18/lecture_slides/B...B12/BME2...
B12/BME2 Biomedical Instrumentation
Biomedical
Instrumentation Lecture C. Respiration
B18/BME2
Dr Gari Clifford
(Based on slides from
Prof. Lionel Tarassenko)
B12/BME2 Biomedical Instrumentation
The respiratory system
Respiration is the act of inhaling
and exhaling air in order to
exchange oxygen for carbon
dioxide. It is synonymous with
breathing and ventilation.
The lungs are where the gas
exchange takes place. The lungs
consist of a series of tubes which
repeatedly fork into smaller
tubes.
Eventually these tubes terminate
in tiny sacks called the alveoli.
There are 300 million alveoli in
the adult lung.
B12/BME2 Biomedical Instrumentation
The lungs
The alveoli have very thin walls.
Small blood vessels (capillaries)
are very close to the air in the
alveoli, allowing gas exchange to
take place.
A surface area of approximately
75m2 is provided for the
exchange of gas.
The total volume of air in the
lungs varies with inhalation and
exhalation.
B12/BME2 Biomedical Instrumentation
Measuring respiration
Two parameters of interest:
Breathing rate
Depth (or amplitude) of breathing
Two types of methods:
Air flow measurement (pressure sensor /
thermistor)
Measurement of change in volume of the lung
(Plethysmography)
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Measurement of airflow
Basic principle:
A heated element is inserted in a mouth tube.
The amount of electrical current needed to
maintain the heated element at a constant
temperature is proportional to the airflow.
The flow signal can be integrated to obtain
volume information
Alternatively, use thermistor
Air from lungs different temp to ambient T
Needs a fast thermal time const for probe
Think about the Nyquist freq of resp ...
Disadvantage: obstructive / obtrusive
flow
Mouth tube
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Inductance plethysmography
Basic principle:
This uses a pair of wires,
each attached in a zig-zag
pattern to a highly compliant
belt (one belt is placed around
the ribcage, the other around
the abdomen).
Each wire forms a single loop,
which is excited by a low-level
radio-frequency signal.
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Inductance plethysmography
Basic principle:
Changes in cross-sectional
area result in changes in self-
inductance, which can be
measured (after demodulation).
The output is proportional to
the local cross-sectional area
encircled by the loop.
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Electrical Impedance
Plethysmography
Basic principle:
The change in resistivity of the lungs with inspiration and
expiration can be measured by injecting a small high-
frequency current across the chest and measuring the
resulting voltage.
The change in voltage during the breathing cycle, ΔV, is
proportional to the change in electrical impedance of the
chest, ΔZ, since ΔZ = ΔV / I, where I is the high-frequency
(of constant amplitude) current.
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Methods for
measuring respiration
The most frequently
used method in ICU is
electrical impedance
plethysmography
In sleep clinics, flow
meters on masks or
inductance
plethysmography
B12/BME2 Biomedical Instrumentation
Modelling the electrical
impedance of the thorax
If we model the thorax
as a cylinder, then the
electrical impedance Z
is given by:
Length L Resistivity ρ
Area A
Z = ρL/A
B12/BME2 Biomedical Instrumentation
Resistivity values
for relevant tissues
Typical resistivity values for human tissues
Blood 1.6 Ωm
Muscle transverse > 4 Ωm
longitudinal 1.5 - 2.5 Ωm
Lung (+ air) 10 - 20 Ωm
Bone > 100 Ωm
Fat 16 Ωm
B12/BME2 Biomedical Instrumentation
Modelling the electrical
impedance of the thorax
The electrical impedance
Z will increase during
inspiration, as the lungs
have a higher resistivity
when filled with air.
Length L Resistivity ρ
Area A
However, we need to bear in mind that L and A also change when we breathe!
B12/BME2 Biomedical Instrumentation
Choice of frequency for Electrical
Impedance Plethysmography
Adequate SNR requires a current of about 1mA
At low frequencies, a 1mA current causes an unpleasant shock
At low frequencies, electrode contact impedance can be high
At frequencies above 100 kHz, stray capacitance makes the design of
the circuitry difficult
A frequency between 20 kHz and 100 kHz is usually chosen
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Measurement of
electrical impedance
The simplest method is to use a two-electrode
system: same electrodes for current injection
and voltage measurement.
i v
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Two-electrode system
The movement of the thorax with breathing will cause changes in
electrode contact impedance.
This artefactual change in Z will be superimposed on the desired
quantity (change in Z due to change in lung resistivity).
Tissue Impedance
v i
B12/BME2 Biomedical Instrumentation
i v
The current flows through
two outer electrodes and
voltage is sensed between
two inner electrodes.
If the voltage-sensing
amplifier has an infinite input
impedance, the errors
caused by the changes in
electrode contact impedance
are eliminated.
Four-electrode system
B12/BME2 Biomedical Instrumentation
The current flows through
two outer electrodes and
voltage is sensed between
two inner electrodes.
If the voltage-sensing
amplifier has an infinite input
impedance, the errors
caused by the changes in
electrode contact impedance
are eliminated.
Four-electrode system
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Electrical Impedance Plethysmography
Conclusions
Advantage:
Simple to use, non-invasive
Disadvantages:
Not sufficiently accurate to quantify volume
change
Often used simply to extract breathing rate
(Impedance Pneumography), in which case
two-electrode method is adequate.
B12/BME2 Biomedical Instrumentation
Electrical Impedance Plethysmography
Conclusions
Often used simply to extract breathing rate
(Impedance Pneumography), in which case
two-electrode method is adequate.
One breathing cycle
Impedance change due to blood movement during the cardiac cycle
B12/BME2 Biomedical Instrumentation
Diagnostic use
of respiration information
Monitoring the very young and the elderly for
cessation of breathing (apnoeas)
Diagnosis of Obstructive Sleep Apnoea (OSA) or
Cheyne-Stokes breathing (an abnormal type of
breathing characterized by alternating periods of
shallow and deep breathing)
Look for alternating chest and abdominal signals
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Recall OSA example
SNA: Sympathetic Nerve Activity (recorded from peroneal nerve)
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What happens when trachea is impeded?
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Therapy?
Continuous Positive Airway Pressure
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Compliance?
Comfort?
Convenience?
Cost?
Social Acceptability?
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ECG-derived respiration
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EDR – two basic types
Amplitude Modulation
QRS height (or area)
Due to observational axis
changes
Frequency Modulation
RSA
Due to parasmypathetic
modulation
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ECG-derived respiration
Concentrate on
deriving it from
peaks of ECG
Form time series
Need to resample –
why?
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Pre-processing
Derive features (t,x) – what are they?
Select resample rate – what is a good rate?
Remove noisy beats – why? how?
Select resampling interpolation method – which one?
Estimate respiration rate? How?
Time domain - how?
Frequency domain – how?
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Sample and hold
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Linear interpolation
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Polynomial interpolation
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Warnings
If there is ‘too much’ missing data, the more complex
interpolation methods (e..g cubic spline) become
unstable – constraints become too loose
Interpolation/resampling frequency has to be chosen
wisely
Not too high to avoid instability
Too low and you will cut corners off the data
Also - look out for noise in the time series – remove
anomalous intervals!
Extrapolation outside of boundaries?
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Resampling introduces noise
-- 0.25Hz Sine wave
(6Hz sampling)
* Downsampled (~1Hz)
-∆- Linear interp
-+- Cubic spline
Sample and hold introduces flat regions (low freq) and corners (high freq)
Linear interpolation does the same, although not as pronounced
Cubic spline interpolation smoothes the edges, with only small amounts
of low frequency and high frequency noise
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EDR and HRV
ECG lead (top), HR (middle) and respiration (lower)
How can we measure respiratory rate from this?
What does phase tell us?
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Appendix – IP circuitry
B12/BME2 Biomedical Instrumentation
Current source i causes a constant current to flow through the
thorax. ΔZ is the change in electrical impedance due to breathing.
Ideally, the input impedance of the voltage amplifier is much greater
than Z2 or Z3.
Four-electrode system
B12/BME2 Biomedical Instrumentation
Four-electrode system
The output of the amplifier is a large a.c. signal (at a frequency between
20 kHz and 100 kHz), the amplitude of which is modulated a small
amount by iΔZ (at the breathing frequency).
Usually ΔZ, which contains the desired information, is only 1/1000th of Z.
iΔZ may be demodulated by any AM detector, for example by a diode
followed by a low-pass filter or (better) by a phase-sensitive detector.