Transcript Future uses of the Photoplethysmograph in our research

The Photoplethysmograph as an
instrument for physiological
measurement
Tomás Ward
Department of Electronic Engineering,
NUI Maynooth
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Overview
•What is the PPG?
•Common Uses of the PPG
•How we use the PPG
•How we intend to use the PPG
•Conclusion
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What is the
Photoplethysmograph?
• The PPG is an optical means of conducting
a plethysmography.
• So what is a plethysmography and how do
we do it optically?
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What is Plethysmography?
• PG is a term for a set of noninvasive
techniques for measuring volume changes
in parts of the body (even the whole body)
• Commonly measured volume changes are:
– those caused by breathing (lung and chest expansion)
– those caused by blood being forced into vessels
(such as arteries,veins and capillaries)
– those caused in the heart as it pumps
Currently of interest to us
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How are PGs commonly acquired ?
•
•
•
•
The 2 main techniques are
Volume Displacement Plethysmography
Electrical Impedance Plethysmography
The above two methods are flexible
• It is also possible to acquire a PG using
• Ultrasonic or X -Ray imaging
• A photoplethsymograph refers to a technique whereby
localised volume changes due to an optically
absorbant/scattering substance (e.g .blood ) are
measured.
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The PPG
• Usually the tissue under investigation is bathed
with light of a suitable wavelength (usally NIR)
and the resultant scattered light is measured with a
silicon photodiode
• Two modes
– Transmissive mode - fingers / toes / earlobe
– Reflective - forehead / cheek
• Received signal is assumed to be a measure of
volume changes due to localised blood flow
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Common uses of the PPG
The Finger PPG
I0
I
Vout
I
Time - s
This signal is very similar to the peripheral blood pressure waveform
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How does the PPG work?
• 15% of blood by weight is hemoglobin inside the
Red blood cells (RBC or Erythrocytes)
• The total Hb (THb) can have one of the
following forms
–
–
–
–
reduced or non-oxygenated Hb (HbR)
Oxyhemoglobin (HbO2)
Carboxyhemoglobin (HbCO)
Methemoglobin (metHb)
99% of
THb
• How do these various forms interact with light?
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Optical measures from Radiative
Tranfer theory - I
• Transmittance of light through an
absorbing medium is defined by
I
T
I0
• where I is the transmitted intensity and I0 is
the incident intensity.
• Absorbance is given by
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A   log10 T
Optical measures from Radiative
tranfer theory - II
It can be shown that the absorbance can be further expressed as
where  is the molar absorptivity (in cm-1 M-1), and l is the path
length (usually in cm), and c is the molar concentration.
This is known as Beer’s Law
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Optical Absorbance of HbR and
HbO2 and H2O in 1cm cuvette vs 
Spectral
Window
In cuvette: obeys Beers Law ie we can relate I/I0 to c,  and l
In real blood: Hb in erthrocytes (RBCs)
resulting in much scattering and reflection by the RBC
membranes and otherDepartment
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Diffuse Transmission no RX
Scattering Followed by absorption
Diffuse Reflection
Diffuse Transmission and RX
Specular Reflection
IR - LED
I0
Skin
Fat
Capilliary Bed
Venule
Arteriole
I
Transmissive PPG
Photodiode
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Diffuse Transmission
IR - LED
Scattering Followed by absorption
Diffuse Reflection and RX
Diffuse Transmission
0
Specular Reflection
I
Photodiode
I
Skin
Fat
Capilliary Bed
Venule
Arteriole
Reflective PPG
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So what really is the PPG a measure of?
• Hard to say! Literature unsatisfactory on the subject
• The name conventionally suggests that this device should measure
volume by optical methods. Really it detects changes in blood
perfusion in limbs and tissues.
• As arterial pulsations fill the capillary bed the changes in volume of
the blood vessels modify the absorption, reflection and scattering of
the light. Also the amount of HbO changes resulting in additional
modulation. So the picture is more complicated than Beer Lambert
Law
• As a raw signal it is best used to show the timing of events such as
heart beats,
• With additional processing it can provide a “fairly” accurate
measure of relative peripheral volume change and relative blood
pressure change
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• Principle involved most useful for oximetry
The use of a PPG for determining
oxygen saturation levels Oximetry and Pulse Oximetry
• Oximetry is the determination of the oxygen
content of tissue blood
• The measure used is oxygen saturation
SpO2 which is HbO/THb (as a percentage)
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Principle of Pulse Oximetry
• By using light at 2 different wavelengths
one at the isobestic wavelength we can
determine the ratio of HbO2 to HbR and
hence local oxygen levels (ideally!!)
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Probe - Transmissive
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Probe - Reflective
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Measures of the
received signals are
processed to yield
SpO2 values
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Practical Pulse Oximetry
• Due to scattering effects the actual output of a pulse
oximeter is not the linear function1 of average SpO2 that
theory predicts
1
Beer-Lambert
Law
Vout
Empirical
Calibration
normalised
0
0
%
100 - SpO2
• Actual SpO2 is found via a lookup table
• For absolute measurement calibration with blood sample
required
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• Does show relative changes - still clinically useful
Section Summary
• PPG produces a measure of blood perfusion
changes in a local area of tissue
• Pulse Oximetry signal is produced using a
PPG calculated at two or more wavelengths
and provides a measure of SpO2 and hence
relative local oxygen consumption by tissue
as a function of time
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Our current use of the PPG
• Measurement of Pulse Transit Time (PTT)
– MEng work Michael Maguire
– Collaboration: Diarmuid O’Shea, Leo Kevin, Charles
Markham
• Assessment of vascular function
– New research
– Collaborators: Michael Maguire, Douglas Leith,
Patricia Fitzgerald
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Measurement of Pulse Transit
Time using the PPG
• What is PTT?
• Pulse transit time is the time an arterial
pressure wave takes to travel between two
points along the same artery.
• Why measure PTT?
– Because it allows a noninvasive measurement
of arterial blood pressure
– may also allow measurement of certain other
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cardiovascular
parameters noninvasively
Direct Invasive Measurement of PTT
• Elastic Theory2 linear relationship Pulse
Wave Velocity and Diastolic BP
PWV
PTT
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• PTT decreases
with increasing BP
Conventional Noninvasive
Measurement of Pulse Transit Time
using the ECG and finger PPG
R
Finger
PPG
P
T
ECG
PTT
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Typical Results (Geddes et al., 1981)
High scatter, averaging required for even moderate accuracy
Can we improve on this?
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Our Method of Measurement of
Pulse Transit Time
• Direct measurement of PTT
• Brachial Reflective PPG replaces ECG
• Experiment: Actual continuous BP taken
with Portapress system along with ECG
– Allows PTT as measured using both methods to
be correlated with BP
• Collaboration: St Vincent’s Hospital
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Pulse Transit Time
Site 1
Site 2
0.35
ECG
PPGf
PPGb
0.3
0.25
mV
0.2
PPGb
0.15
0.1
PPGf
0.05
0
-0.05
1.95
2
2.05
Samples
2.1
2.15
• Improved result over ECG method
• Discrepancy between measures could be an
indicator of isovolumetric contraction
variablilty (C. Markham)
• Integration of additional
data
(HR) may yield
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improved relationship (Barschdorf et al.)
x 10
5
Potential of Additional
parameterization of Peripheral
Vascular system
• Currently looking at conducting stepresponse measurements for assessing state
of peripheral vascular system
–
–
–
–
Occlude artery under investigation
Rapidly allow blood back into arterial system
Monitor PPG
May yield information on compliance / arterial
narrowing
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• Collaboration:
Beaumont Hospital
Future uses of the Photoplethysmograph
in our research - a Brain Computer
Interface (BCI)
• Collaborators: Charles Markham, Gary McDarby
• A BCI in the context we discuss here is a
wearable device that will allow a human
user to control their environment via
thought processes alone.
• Next slides
– Why a BCI?
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Why bother?
• There exist people with such profound
disabilities that they have NO means of
communicating with the outside world.
– People with amyotrophic lateral sclerosis and
brainstem stroke for example
• The immediate goal is to provide these users,
who may be completely paralyzed or "locked
in," with basic communication capabilities so
that they can express their wishes to caregivers,
operate simple word processing programs, or
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even control a neuroprosthesis.
How should we proceed?
• Many severely disabled people can
communicate through the use of switches
Yes
No
Scanning Communication S/W
Output = “YES”
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Can we make a Mind Switch?
• Yes
– IF we can come up with a physiological
measurement modality that will allow different
neurological or thought processes to be
distinguished
– Then if a user can voluntarily reproduce a
thought process we can measure noninvasively
then we have our “Mind switch”
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Current BCIs ( EEG-based )
• Are all based on electrical potentials
(electroencephalograph/EEG) recorded
from the scalp (25bits/min, long training)
• Visual Evoked Potentials, P300, Slow
Cortical potentials, Sensorimotor Cortex
Rhythms
– Problems
• Long training times - non-intuitive
• Messy electrodes
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• Signal averaging
required
EEG problems
• The EEG is a representation of groups of waves
produced by the electrical activity of the cortex
averaged at a given point.
• EEG is a crude modality, akin to trying to discern
what is going on at a football game through
listening to the reactions of the crowd!!
• We require an imaging modality allows us to see
the brain function related to its anatomy.
• One such modality is Functional magnetic
resonance imaging
(fMRI)
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fMRI - basic principle
• Application of a large external magnetic field causes magnetically
active atomic nuclei to become oriented parallel to the applied
field.
• This resting orientation may be disturbed with an external RF
(radio frequency) pulse.
• After the RF pulse, the nuclei fall back in line with the external
magnetic field and, in so doing reemit the radio-frequency energy
as a signal that can be detected by a receiver coil. The frequency
of this signal reflects number of elements in the nucleus, the
strength of the external magnetic field, and the effect of
surrounding material.
• fMRI is tuned to the magnetic properties of hemoglobin and can
distinguish HbO2 from
HbR and so can image neural activity
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which results in an increase in local oxygen levels
fMRI - principle more detail - I
• When neurons fire, they consume oxygen and this
causes the local oxygen levels to briefly decrease
and then actually increase above the resting level as
nearby capillaries dilate to allow more oxygenated
blood into the active area. fMRI works by imaging
blood oxygenation, a technique called BOLD (Blood
Oxygen Level Dependence). The BOLD paradigm
relies on brain mechanisms which overcompensate
for oxygen usage (activation causes an influx of
oxygenated blood in excess of that used and
therefore the local
oxyhemoglobin
concentration
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increases.
fMRI - principle more detail - II
• Oxygen is carried to the brain in the hemoglobin molecules
of red blood cells. Luckily for fMRI, the magnetic
properties of hemoglobin differ when it is saturated with
oxygen compared to when it has given up oxygen.
Technically, deoxygenated haemoglobin is "paramagnetic"
and thefore has a short T2 relaxation time. As the ratio of
oxygenated to deoxygenated haemoglobin increases, so to
does the signal recorded by the MRI. Deoxyhemoglobin
increases the rate of depolarization of hydrogen nuclei
creating the NMR signal thus decreases the intensity of the
T2 image. The bottom line is that the intensity of images
increases with the increase of brain activation. The problem
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is that this increaseDepartment
is small
(usually
less than 2%) and
easily obscured by noise and different artifacts.
• Anatomy
fMRI
Moving fingers on right
hand - the anatomy
Moving fingers on right
hand - the fMRI image
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Typical patterns as you read this
text!
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So why not just use fMRI?
• fMRI is highly sensitive to movement of the head - the
head must be clamped in place.
• Subjects responses must not involve speaking and at most
only small movements etc.
• Useful imaging still requires task repetition and image
averaging.
• The MRI machine is very noisy and somewhat
claustrophobia-provoking.
• The technique can induce heating of the brain.
• Very expensive (several million dollars)
• Large magnetic fields required (up to 4 Tesla)
• Not portable
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Alternative to fMRI Monitoring
Cerebral Surface activity with
Near-Infrared imaging
• Use of oximetry approach (double PPG) at
suitable NIR wavelengths
• Use an array of
POX sensors
• Build up map of
cortical neural activity
• Called Diffuse Optical Tomography
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Typical DOT system
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Right finger movement experiment as
“seen” by DOT system
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How DOT compares with other brain
imaging modalities
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Can we do this ?
• Wait and see
• Project is funded and will commence start
of April 2002
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Conclusion
• The PPG is a deceptively useful
physiological instrument
• Last 12 months has spawned a number of
interesting experiments
• Expect more useful applications in the
future
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References
1 p397 in Noninvasive Instrumentation and Measurement in
Medical Diagnosis, Robert B Northrop, CRC Press 2002, NUI
Maynooth Library
2 Geddes, Hughes and Babbs, 1969 (reference incomplete from
Geddes Psychophysiology paper)
3 p241 in Noninvasive Instrumentation and Measurement in
Medical Diagnosis, Robert B Northrop, CRC Press 2002, NUI
Maynooth Library
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Partial Pressure
John Dalton (1766-1844) - (gave us Dalton's atomic theory)
The total pressure of a mixture of gases equals the sum of
the pressures that each would exert if it were present alone
The partial pressure of a gas:
The pressure exerted by a particular component
of a mixture of gases
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Volume Plethysmography
• Simplest Example: Measurement of limb
volume using pneumatic sphyganometer
cuff
• Inflated to P0 << BPdiastolic
• If the limb expands against bladder by V
it will cause P=P0(V/V0) allowing V to be
calculated3
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Impedance Plethysmography
• Usually ac current source (30-75kHz, high freq has
less physiological effect such as electroshock <1mA pk), impedance
changes with blood flow
• Major applications
– occulsive impedance plethysmography used to
detect clots in deep leg veins
– Measurement of depth of respiration and rate
in ICU (air intake varies impedance)
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Pulse Oximetry Theory - I
At 650nm (NIR) with concentration C and path length L held
constant the absorbitivity or extinction coefficient varies with
Saturation percent of Hb as
Sp02=Hb/THb
 
 mSpO
r
r max
2
 r (SpO2 )
 r max
Also we can say
  650 nm
r max   Hb
 r min
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0
100% SpO2
 r min   HbO
Pulse Oximetry Theory - II
( Br  r CL )
I  I 010
… Beer’s Law
I0
Br is the non-Hb absorption of
tissues at 650nm this light is
converted to a proportional (Ka)
voltage before being fed through a
log10(x) nonlinearity (KL) to yield
I
VLr=KLlog10(KaIor)=KLlog10(KaI0)-KL(Br+rCL)
VLr=KLlog10(KaI0)-KL(Br+CL(rmax-mSp02))
Now the light from the 805 nm isobestic LED yields
 Bi
I  I 010
independent of SpO2
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Pulse Oximetry Theory - III
VLi=KLlog10(KaI0)-KL(Bi)
We subtract VLi from VLr to get Vo
Vo approx (SpO2)(KLCLm)+KL(Bi-Br)
ie output is a linear fn of SpO2 but only if Beer’s Law were to
hold
in fact output is a monotonically inc. fn of SpO2
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