1. The Photoplethysmograph as an instrument for physiological measurement
Tomás Ward
Department of Electronic Engineering,
NUI Maynooth

2. Overview What is the PPG? Common Uses of the PPG How we use the PPG
How we intend to use the PPG
Conclusion

3. 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?

4. 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

5. 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.

6. 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

7. Common uses of the PPG The Finger PPG
Time - s
Vout I
This signal is very similar to the peripheral blood pressure waveform

8. 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)
How do these various forms interact with light?
99% of THb

9. Optical measures from Radiative Tranfer theory - I
Transmittance of light through an absorbing medium is defined by
where I is the transmitted intensity and I0 is the incident intensity.
Absorbance is given by

10. 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

11. 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 other tissues

12. I0 I IR - LED Capilliary Bed Venule Arteriole Transmissive PPG
Diffuse Transmission no RX
Scattering Followed by absorption
IR - LED
Diffuse Reflection
Diffuse Transmission and RX
Specular Reflection I0
Skin
Fat
Capilliary Bed
Venule
Arteriole I
Transmissive PPG
Photodiode

13. I0 I Capilliary Bed Venule Arteriole Reflective PPG
Diffuse Transmission
IR - LED
Photodiode
Scattering Followed by absorption
Diffuse Reflection and RX
Diffuse Transmission
Specular Reflection I0 I
Skin
Fat
Capilliary Bed
Venule
Arteriole
Reflective PPG

14. 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
Principle involved most useful for oximetry

15. 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)

16. 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!!)

17. Probe - Transmissive

18. Probe - Reflective

19. Measures of the received signals are processed to yield SpO2 values

20. 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
Actual SpO2 is found via a lookup table
For absolute measurement calibration with blood sample required
Does show relative changes - still clinically useful 1
Beer-Lambert Law
Vout
normalised
Empirical Calibration
0 % SpO2

21. 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

22. 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

23. 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 cardiovascular parameters noninvasively

24. Direct Invasive Measurement of PTT
Elastic Theory2 linear relationship Pulse Wave Velocity and Diastolic BP
PTT decreases with increasing BP
PWV
PTT

25. Conventional Noninvasive Measurement of Pulse Transit Time using the ECG and finger PPG
PTT

26. Typical Results (Geddes et al., 1981)
High scatter, averaging required for even moderate accuracy
Can we improve on this?

27. 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

28. Pulse Transit Time 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 improved relationship (Barschdorf et al.)

29. Potential of Additional parameterization of Peripheral Vascular system
Currently looking at conducting step-response 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
Collaboration: Beaumont Hospital

30. 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?
And How.

31. 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 even control a neuroprosthesis.

32. How should we proceed?
Many severely disabled people can communicate through the use of switches
Click!
Output = “YES”
Yes No
Scanning Communication S/W

33. 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”

34. 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
Signal averaging required

35. 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)

36. 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 which results in an increase in local oxygen levels

37. 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 increases.

38. 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 is that this increase is small (usually less than 2%) and easily obscured by noise and different artifacts.

39. Anatomy fMRI Moving fingers on right hand - the anatomy
Moving fingers on right hand - the fMRI image

40. Typical patterns as you read this text!

41. 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

42. 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

43. Typical DOT system

44. Right finger movement experiment as “seen” by DOT system

45. How DOT compares with other brain imaging modalities

46. Can we do this ? Wait and see
Project is funded and will commence start of April 2002

47. 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

48. 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

49. Partial Pressure
John Dalton ( ) - (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

50. 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

51. 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)

52. 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
% SpO2
Also we can say

53. Pulse Oximetry Theory - II
… Beer’s Law I0 I
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
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
independent of SpO2

54. 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