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)


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.


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.


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)


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


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.


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.


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.


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


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


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


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!


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


Measurement of

electrical impedance

The simplest method is to use a two-electrode

system: same electrodes for current injection

and voltage measurement.

i v


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


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


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.



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.


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


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


Recall OSA example

SNA: Sympathetic Nerve Activity (recorded from peroneal nerve)


What happens when trachea is impeded?


Therapy?

Continuous Positive Airway Pressure


Compliance?

Comfort?

Convenience?

Cost?

Social Acceptability?


ECG-derived respiration


EDR – two basic types

Amplitude Modulation

QRS height (or area)

Due to observational axis

changes

Frequency Modulation

RSA

Due to parasmypathetic

modulation


ECG-derived respiration

Concentrate on

deriving it from

peaks of ECG

Form time series

Need to resample –

why?


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?


Sample and hold


Linear interpolation


Polynomial interpolation


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?


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


EDR and HRV

ECG lead (top), HR (middle) and respiration (lower)

How can we measure respiratory rate from this?

What does phase tell us?


Appendix – IP circuitry


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.




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.

lecture c. respiration - university of oxfordgari/teaching/b18/lecture_slides/b...b12/bme2...

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