Fundamentals and Design of Pulse Oximeter

Background

The human body is composed of trillions of cells, each of which requires the continuous supply of oxygen (O_2) and the excretion of carbon dioxide (CO_2) for metabolism and survival. Therefore, measuring the amount of oxygen present in blood or tissues could help assess a person’s health and overall well-being [1].

Blood oxygenation is the measure of oxygen present in arterial or venous blood, while the measure of oxygen present in the vascular tissue or microvascular bed is referred to as tissue oxygenation. Blood oxygenation assesses how well oxygen binds to hemoglobin, indicating the adequacy of pulmonary gas exchange. Tissue oxygenation indicates the adequacy of tissue perfusion and is primarily determined by the local delivery of oxygenated blood [2]

Oxygen enters the lungs and then passed into the blood. The blood carries oxygen to the various organs in our body. The main way oxygen is carried in our blood is through hemoglobin. The hemoglobin without oxygen is called deoxygenated hemoglobin (deoxy-Hb) and the hemoglobin with oxygen is called oxygenated hemoglobin (oxy-Hb) [3].

Oxygen levels in the blood or tissue can be measured using chemical, optical, or colorimetric techniques. Optical methods have several advantages over chemical or calorimetric techniques as light is virtually harmless, and can easily be generated and detected by optoelectronic components (e.g., photodiodes). Also, with the availability of lasers and optical fibers, the measurement of oxygenation from within the body cavities is easily possible with great accuracy and precision. “Oximetry” is the general term used to refer to optical methods of measuring oxygen levels, and is widely used in clinical environments. In oximetry, the level of oxygen in the arterial and venous blood is indirectly determined by measuring the arterial oxygen saturation (SaO_2) and the venous oxygen saturation (SvO_2) [3].

SvO_2 is a measurement of the oxygen remaining in the blood after passing through the capillary bed, which indicates moment-to-moment variation between oxygen supply and demand. It can be monitored using fiber-optic catheters. SaO_2 is a measurement of oxygen saturation in the arteries. An estimation of SaO_2 at the peripheral capillary is called SpO_2, which is the primary focus. Unlike traditional SaO_2 measurement, which is normally conducted invasively via a blood test with a blood gas analyzer, SpO_2 can be measured by non-invasive methods. Monitoring SpO_2 provides a quick and convenient assessment of the user’s oxygenation status. The most widely used device for SpO_2 monitoring is a pulse oximeter, which is often attached to the finger for measurement purposes [1].

There has been a constant effort to develop optical techniques for measuring blood and tissue oxygenation. Currently, there are a vast number of optical techniques utilizing light to measure the oxygen content in either tissues or blood. Spectrophotometry forms the basis of all oximetry techniques, which include spectrophotometry, Co-oximetry, pulse oximetry, optical fiber venous oximetry, Near-infrared spectroscopy (NIRS), Resonance Raman Spectroscopy, etc. [3].

The relationship between hemoglobin and oxygen

Hemoglobin (Hb) exhibits positive cooperativity.

• When one O2 molecule binds to one of hemoglobin's four binding sites, the affinity to the oxygen of the three remaining available binding sites increases; i.e. oxygen is more likely to bind to hemoglobin bound to one oxygen than to unbound hemoglobin [4].
• This property results in a sigmoidal oxygen dissociation curve allowing for more rapid loading of oxygen molecules in oxygen-rich environments (i.e. alveolar capillaries of the lungs) and easier offloading in oxygen-deficient environments (i.e. metabolically active tissues) [4].

 

               Fig 1: oxygenated and deoxygenated configuration of the Hb molecule [4].

Hemoglobin is composed of four subunits (2 alpha, 2 beta in adults) and exists in two forms:

• Taut (T): deoxygenated form with low affinity for O2, therefore, it promotes release/unloading of O2.
• Relaxed (R): oxygenated form with high affinity for O2, therefore oxygen loading is favored.
• T and R configurations lead to different electromagnetic absorption and therefore different emission of light.

The main element(s) of the Pulse Oximeter

The LED's energy is incident at two different wavelengths into vascular tissue and detects the changes in transmitted energy. A photodetector in the sensor perceives the non-absorbed light from the LEDs that is converted into a voltage signal called photoplethysmogram (PPG). The maximal and minimal values of the PPG pulse reflect light irradiance transmitted when artery's blood volume is minimal or maximal, respectively. The PPG signal consists of a pulsatile alternating part (AC) component and a slowly varying DC component with a magnitude that is determined by the nature of the matter through which the light passes.

The DC component represents the light absorption of the tissue, venous blood, and non-pulsatile arterial blood. The AC component represents the pulsatile arterial blood. The amplitude of the pulsatile component varies cyclically and in synchrony with the pumping action of the heart. As the volume of blood in arteries and arterioles starts to increase during systole, the proportion of incident light absorbed also increases. This is possible because the arterial walls are elastic so their diameter changes in response to variations in transmural pressure. Conversely, during diastole, the light absorption decreases with a decrease in blood volume, creating a rapidly alternating signal [3].

                                        Fig 2: The photoplethysmography (PPG) pulses [12]

The transmitted light through the tissue decreases during systole, when blood is ejected from the left ventricle into the vascular system, thereby increasing the peripheral arterial blood volume and increases during diastole. AC is the difference between the maximal (I_d) and minimal (I_s) light transmission through the tissue; DC is the mean light transmission through the pulse [12].

                                    Fig 3: Light absorption profile in the pulse oximetry [3]

The light intensities I_s and I_d, corresponding, respectively, to the systolic and diastolic phases of the cardiac cycle, are measured at two different wavelengths (red and infrared) to estimate the arterial oxygen saturation SpO_2.

The absorbance of light energy by the pulsatile arterial blood is determined using the Beer-Lambert law.

A_λ = ε_λ.L. C                                                                                      Eq. 1

Where A_λthe light absorbance at a given wavelength is, ε_λ is the molar absorber at wavelength 𝝀, C is the concentration of the absorber, and L is the optical path length traveled by the light.

A_λ = ε_λ.L. C = log_10(T) = log_10(I_i/I_t )                                     Eq.2

A = absorbance, i_i= intensity of original light passing from LEDs, i_t = intensity of light transmitted after it has passed through the patient’s body part

Here, i_t is an intensity of light transmitted after it has passed through the patient’s body part. So the light falls of an image sensor. Rewriting Eq. (1) to include the baseline transmittance (I_d) during diastole and the pulsatile transmittance (I_s) during systole (Fig 1), the light absorbance of the arterial blood is given as:

A_λ =ε_hb.L.C_hb=log_10 (I_t/I_s ) = log_10 (I_d/I_s ) = -log_10 (I_s/I_d )      Eq. 3

ε_hb= molar extinction coefficient of hemoglobin

C_hb = concentration of hemoglobin

L = length travel between systolic and diastole phase

The pulsatile component (AC) of optical absorption originates from the pulsatile arterial blood, and the non-pulsatile component (DC) contains contributions from non-pulsatile arterial blood, venous blood, and other tissues. This pulsatile component (AC) allows for differentiation between the absorbance due to non-pulsatile components (skin, venous blood, and other tissues) from the absorbance due to the pulsatile arterial blood.

So, by subtracting the minimum transmitted light from the peak transmitted light in each wavelength, the effects of other tissues are corrected for allowing for measurement of only the arterial blood. There is a need to isolate the contribution of hemoglobin in arterial blood to the total absorption because the idea is to sense only arterial blood volume change.

Since the AC component of the transmitted signal represents the difference between I_s & I_d at the wavelength 𝝀 (AC_λ= I_d-I_s), Eq. 3 can be written as:

A_λ= -log_10 ((I_d- AC_λ)/I_d ) = -log_10 (1-AC_λ/I_d ) = AC_λ/I_d        Eq.4 (using log expansion and approximation )

As mentioned earlier, the AC component accounts only for 1% to 2% of total absorption. The consequence of the AC signal being so small is that slowly varying DC components can be considered roughly equal to I_d.

A_λ= (AC_λ/DC_λ ) = (AC/DC)_λ                                                              Eq. 5

The AC component of the PPG signals represents the light absorbed by the pulsatile arterial blood. This AC component is superimposed on a DC signal that captures the effects of light absorbed by other blood and tissue components (e.g., venous and capillary blood, bone, water, etc.). The ratio of the AC signal to the DC level is called the perfusion index (PI).

Critical Analysis of Perfusion index and selection of wavelength

The measurement of SpO_2 is based on the differential absorption of light by HbO_2 and Hb at two wavelengths. Depending upon the optical absorption spectrum of HbO_2 and Hb as shown in Fig. 4, it is possible to select two wavelengths λ1 and λ2 such that absorbance by HbO2 is more at λ2 than at λ1, while the absorbance by Hb is more at λ1 than at λ2.

                                                Fig 4: Oxy and Deoxy Hemoglobin Absorption [4]

The oxygenated hemoglobin (HbO_2) absorbs more infrared light and allows more red light to pass through whereas the deoxygenated hemoglobin (Hb) allows more infrared light to pass through and absorbs more red light. This allows deriving an oximetry index, which is a normalized difference of absorption at two different wavelengths. The idea is to select a wavelength that is more sensitive to change in oxygen in the arterial blood vessel and has less absorption so that maximum transmitted light is detected. The red region wavelength (660-670) nm has minimum absorption and sensitivity.

Also, the IR region wavelength (910 – 930) nm has almost constant absorption for both HbO_2 and Hb. The central wavelength of both the red and IR region is chosen.

Problems or disadvantages of existing commercial pulse oximeter

Many researchers have attempted different oxygen saturation (SpO_2) measurement approaches using noncontact methods. Humphreys et al. [5], [6] used a CMOS camera with LED arrays that emit two different wavelengths as the light source for noncontact pulse oximetry. Due to the low frame rate and sensitivity to ambient light, the noise in the measured PPG signals was too large to obtain accurate SpO_2 values. Wieringa et al. [7] also used a CMOS camera, but with three different wavelengths to investigate the feasibility of an “SpO_2 camera.” However, no SpO_2results were presented due to poor SNR of the PPG signals. Kong et al. [8] used two CCD cameras, each mounted with a narrow bandpass filter to capture PPG signals at two different wavelengths (520 and 660 nm) in ambient lighting conditions. Tarassenko et al. [9] and Bal et al. [10] Used a camera to calculate SpO_2 based on the PPG information obtained from the RGB channels under ambient lighting conditions.

Commercial pulse oximeters are subjected to several sources of inaccuracies. The fact is that there are discrepancies in accuracy between pulse oximeters, in part, as a result of the differences in their calibration processes and algorithms employed for their signal processing. One of them is observed when oxygen saturation drops below 70%, the scattering difference between these two wavelengths affects the SpO2 estimation [11].

Certain limitation and problems with the existing pulse oximeter are mentioned below:

• Varied pressure applied from finger to the contact sensor cause inaccurate SpO2 readings.
• Skin irritation occurs in some individuals, especially infants, during extended monitoring periods.
• The currently used pulse oximeter has a single photodiode that sense the transmitted light intensity at a single point. Thus, it provides very less information and leads to inaccuracy.
• The problem of variable ambient light
• The problem of patient moving hand/finger cause inaccurate readings
• The problem of EM interference due to extra circuitry
• The problem of calibration
• The problem of optical shunting: the misalignment of LEDs, finger, and Photodetector

References

1. Shao, D., Liu, C., Tsow, F., Yang, Y., Du, Z., Iriya, R., ... & Tao, N. (2015). Noncontact monitoring of blood oxygen saturation using camera and dual-wavelength imaging system. IEEE Transactions on Biomedical Engineering, 63(6), 1091-1098.
2. Kyriacou, P., Budidha, K., & Abay, T. Y. (2019). Optical techniques for blood and tissue oxygenation. Encyclopedia of Biomedical Engineering, ed R. Narayan (Oxford: Elsevier), 461-472.
3. Pulse oximeter [Available online] https://www.howequipmentworks.com/pulse_oximeter/, Accessed on 10th may, 2020.
4. Pulse oximeter basic principle [available online] https://medicine.uiowa.edu/iowaprotocols/pulse-oximetry-basic-principles-and-interpretation, accessed on 12th may, 2020.
5. Humphreys, K., Ward, T., & Markham, C. (2006, January). A CMOS camera-based pulse oximetry imaging system. In 2005 IEEE Engineering in Medicine and Biology 27th Annual Conference (pp. 3494-3497). IEEE.
6. Humphreys, K., Ward, T., & Markham, C. (2007). Noncontact simultaneous dual wavelength photoplethysmography: a further step toward noncontact pulse oximetry. Review of scientific instruments, 78(4), 044304.
7. . Wieringa, F. P., Mastik, F., & van der Steen, A. F. (2005). Contactless multiple wavelength photoplethysmographic imaging: A first step toward “SpO 2 camera” technology. Annals of biomedical engineering, 33(8), 1034-1041.
8. Kong, L., Zhao, Y., Dong, L., Jian, Y., Jin, X., Li, B., ... & Wu, H. (2013). Non-contact detection of oxygen saturation based on visible light imaging device using ambient light. Optics express, 21(15), 17464-17471.
9. Tarassenko, L., Villarroel, M., Guazzi, A., Jorge, J., Clifton, D. A., & Pugh, C. (2014). Non-contact video-based vital sign monitoring using ambient light and auto-regressive models. Physiological measurement, 35(5), 807.
10. Bal, U. (2015). Non-contact estimation of heart rate and oxygen saturation using ambient light. Biomedical optics express, 6(1), 86-97.
11. Von Chong, A., Terosiet, M., Histace, A., & Romain, O. (2019). Towards a novel single-LED pulse oximeter based on a multispectral sensor for IoT applications. Microelectronics Journal, 88, 128-136.
12. Yossef Hay, O., Cohen, M., Nitzan, I., Kasirer, Y., Shahroor-karni, S., Yitzhaky, Y., ... & Nitzan, M. (2018). Pulse oximetry with two infrared wavelengths without calibration in extracted arterial blood. Sensors, 18(10), 3457.