A bit of history
At the core of SATHeart is a wonderful engine that extract many health parameters from the blood pulse wave as measured using sensor called photoplethysmogram (PPG). The principle of the PPG is known since the early 1930s but it is only recently that due to the miniaturisation of the light source and photodetectors, as well as the quality of these electronic components, that we have seen a revival of these technologies [1,2]. The principle of PPG is explained on this Post in Figure 4.
The nature of the photoplethysmogram
The exact origin of the PPG is still not fully explained as there are many factors that can influence the interaction of light with biological tissues. Up to our knowledge, the PPG has its source in three main factors:
- the reflection of the light by the vessels wall movements due to the blood pressure wave propagating from the heart to the periphery tissues and organs.
- the absorption of the light by the hard and soft tissues, and liquids such as water and blood
- the absorption of light by the blood cells as they flow through our arteries and veins
Light is energy in the form of a particle called photon as well as a wave that propagate at very high speed. Both of these aspects of light are important when it comes to understand the interaction between her and the objects or fields that she encounter on her path. The light energy is transformed in other forms of energy such as linear motion, rotation and eventually heat which is the last stage of her transformation. These different transformations depend on two factors, her intensity (number of photons per second) and her wavelength (the colour of the photon). There are many advantages to use multi-wavelength PPG sensors due to the diversity of the phenomena that we can capture  and sensors are currently being developed as shown in the illustration below.
The PPG signal is very sensitive to motion artefacts . It is thus highly advisable to record the PPG at total rest and if possible at the same time of the day.
The different waves in the photoplethysmogram
Illustration of the multiple wavelets composing a single heart beat pulse waveThe PPG wave is composed of different waves of various shapes, amplitudes, and timing. In the simplest model, we can think of the PPG waveform as a superposition of many different wavelets as we can also observe when we are watching the ocean [5,6,7]. These blood pressure waves are currently thought of coming from the multiple reflection back and forth from heart to peripheral branching a bit like a water wave reflecting on the border of the container, the source being the heart pump. The many reflected waves are attenuated due to loss of energy when bouncing against the walls, as well as when interfering with each other and with different organs.
The figure above (top panel) shows an example of such wavelets in the different phases of the heart activity. The green waves compose the systolic phase (ejection of the blood) and the red ones to the diastolic phase (filling of the heart). The bottom panel shows the composition of these waves in the systole and diastole.
The information contained in the photoplethysmogram
The blood pulse wave as captured by the PPG signal contains a lot of information about the heart pump, the regulation of the heart beat rhythm by the autonomic nervous system and conscious breathing and many features related to the organs visited by the blood pressure wave and the state of the arteries. This plethora of valuable information can be used by both the clinicians, medical scientists and people at home. Among other parameters, SATHeart can compute the following with high fidelity and accuracy :
- Heart rate
- Interbeat intervals variability indices
- Breathing rate and depth
- Systolic and Diastolic heart functions
- Reflected wave timing and index
- Arterial stiffness
Most commercial use of the PPG signal does not go beyond the two first parameters and still with limited scope when concerned with the second one. SATHeart is thus offering to his customers a wonderful tool to assess the effect of medicine, exercises and sleep on health.
Higher level of photoplethysmogram interpretation
The PPG parameters explored in the previous section can be incorporated into a higher level of interpretation such as for detecting and predicting abnormal states of health. SATHeart has been involved into Eastern medicine pulse reading since more than 10 years and is also proposing to his customers such higher level of interpretation of the pulse wave.
Probably the most difficult health situation to assess is mental stress or any related mental dysfunction such as depression and anxiety. SATHeart and other teams have been working for a long time on this topic and has performed scientific research that proves that mental stress is indeed encoded into the pulse wave shape and not only the heart rhythm, thus giving deeper insights into the impact of mental stress on the entire body [8,9,10].
No doubts that we are living a new era of the use of light sensors for the diagnosis of health state, and SATHeart is at the forefront of this digital light renaissance of the ancient knowledge of pulse wave reading.
- Challoner, A. V. J., and C. A. Ramsay. “A photoelectric plethysmograph for the measurement of cutaneous blood flow.” Physics in Medicine & Biology 19.3 (1974): 317.
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- Liu, Jing, et al. “Multi-wavelength photoplethysmography method for skin arterial pulse extraction.” Biomedical optics express 7.10 (2016): 4313-4326.
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- Millasseau, Sandrine C., et al. “Contour analysis of the photoplethysmographic pulse measured at the finger.” Journal of hypertension 24.8 (2006): 1449-1456.
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- Couceiro, Ricardo, et al. “Assessment of cardiovascular function from multi-Gaussian fitting of a finger photoplethysmogram.” Physiological measurement 36.9 (2015): 1801.
- Banerjee, Swati, et al. “A Two Step Gaussian Modelling to Assess PPG Morphological Variability Induced by Psychological Stress.” Computing 44 (2017): 1.
- Charlton, Peter H., et al. “Assessing mental stress from the photoplethysmogram: a numerical study.” Physiological measurement 39.5 (2018): 054001.
- Celka, P. et al, “Influence of mental stress on the pulse wave features of photoplethysmograms”, to be published, 2018