The Photoplethysmograph as an instrument for physiological measurement

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. Currently of interest to us 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

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. I0 I Vout I Time - s Common uses of the PPGThe Finger PPG This signal is very similar to the peripheral blood pressure waveform

8. 99% of THb 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?

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. Spectral Window Optical Absorbance of HbR and HbO2 and H2O in 1cm cuvette vs  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. Diffuse Transmission no RX Scattering Followed by absorption DiffuseReflection Diffuse Transmission and RX Specular Reflection IR - LED I0 Skin Fat Capilliary Bed Venule Arteriole I Transmissive PPG Photodiode

13. Diffuse Transmission Scattering Followed by absorption DiffuseReflection and RX Diffuse Transmission Specular Reflection IR - LED Photodiode 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 1 • Due to scattering effects the actual output of a pulse oximeter is not the linear function1 of average SpO2that 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 Beer-Lambert Law Vout normalised Empirical Calibration 0 0 % 100 - 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. R Finger PPG P T ECG PTT Conventional Noninvasive Measurement of Pulse Transit Time using the ECG and finger PPG

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. Click! Output = “YES” How should we proceed? • Many severely disabled people can communicate through the use of switches 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 (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

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