THE PPG: theory and applications

1. THE PPG: theory and applications Dr Christina Orphanidou christina.orphanidou@eng.ox.ac.uk

2. Photoplethysmography Plethysmograph: Greek, plethynein, to increase, graphein, to record It is an instrument used mainly to determine and register the variations in blood volume or blood flow in the body. These transient changes occur with each heartbeat. It can be measured using air, impedance, photoelectric and strain gauge methods. Photoelectric plethysmography, also known as photoplethysmography (PPG) is a non-invasive method to detect cardio-vascular pulse wave that propagates through the body using a light source and a detector. Compared to other types of plethysmography it is easy to set up, low in cost and does not require direct skin contact.

3. Some optics I placed my prism at this (light) entrance, that it might be refracted to the opposite wall. It was at first a pleasing divertissement to view the vivid and intense colours produced thereby. (Sir Isaac Newton, Philosophical Transactions 1672) Light was, therefore, recognised to contain all colours of the visible spectrum of electro -magnetic energy. Newton’s theory lead to the development of techniques for spectral analysis, that is the ability to detect elemental composition by defining the unique ‘light’ absorption “finger- prints” Beer-Lambert law: the concentration of absorbant in solution can be determined as a Mathematical function of the amount of light transmitted through the solution, provided that the intensity of incident light...[]...is known.

4. How the is PPG obtained As blood is pumped through the cardiovascular system, the pressure pulse causes a change in the volume of blood at any given point. This change in volume caused by the pressure pulse is detected by illuminating the skin with the light from a Light Emitting Diode (LED) and then measuring the amount of light either transmitted or reflected to a photodiode. Each cardiac cycle appears as a peak. From wikipedia

5. Components of the PPG The PPG waveform comprises a pulsative (‘AC’) physiological waveform attributed to Cardiac synchronous changes in blood volume with each heart beat, which is superimposed on a slowly varying (‘DC’) baseline with various lower frequency components attributed to respiration and other activities of the cardiovascular and nervous system . The AC component usually has its fundamental frequency around 1 Hz, depending on heart rate. With suitable filtering and amplification, both the AC and DC can be extracted for subsequent pulse wave analysis.

6. Optical considerations • The wavelength of optical radiation is important in light-tissue interactions for 3 reasons: • The main constituent of tissue is water that absorbs light very strongly in the ultraviolet • and the longer infrared wavelengths. The shorter wavelengths of light are also strongly • absorbed by melanin. There is, however, a window in the absorption spectra of water • that allows visible (red) and near infrared light to pass more easily, thereby facilitating the • measurement of blood flow of volume at these wavelengths. • 2. The depth to which light penetrates the tissue for a given intensity of optical radiation • depends on the operating wavelength. • 3. Significant differences exist in absorption between oxyhaemoglobin (HbO2) and reduced • haemoglobin (Hb) except at the isobestic wavelength which would show no differences. • Red and near infrared wavelengths are commonly used. • The latter is important since being able to measure the amount of each of these two gives • the oxygen saturation, an important measure of someone’s health:

7. Other considerations Placement of probe, interface pressure, movement artefacts, subject posture etc affect The PPG measurement. There is no international protocol but usually the following sites Are chosen because of the absoption characteristics of the tissue at these sites.

9. PPG outputs depending on location

10. Measurement artefacts (from finger probe) • Movement artefact • Hand of finger tremor • Coughing • Big breath

11. Blood oxygen saturation Pulse oximetry represents one of the most significant technological advances in clinical Patient monitoring in the past few decades. It utilizes PPG measurements to obtain Information about the arterial blood oxygen saturation (Sp02) as well as heart rate. Sp02 can be determined by shining red and then infrared light through vascular tissue, with rapid switching between the wavelengths. The amplitudes of red and near infrared AC signals are sensitive to changes in Sp02 because of the differences in the light Absorption of oxyhaemoglobin (HbO2) and reduced haemoglobin (Hb). The technique is limited by the fact that it relies upon a peripheral pulse to be present, and the accuracy falls at low saturation levels.

12. Heart rate The AC component of the PPG pulse is synchronous with the beating heart and therefore can be a source of heart rate information.

13. Respiration Physiological monitoring of breathing interval is important in many clinical settings, Including critical neonatal care, sleep study assessment and anaesthetics. Respiration causes variation in the peripheral circulation, making it possible to monitor breathing using a PPG sensor attached to the skin. The low frequency respiration-induced intensity variations (RIIV) are well documented. --By band-pass filtering the signal in the relevant range. --By looking at heart rate variability and the effect of respiration on that.