Oxygen Saturation Measurements using Pulse Oximetry
Zahoor Ali a, Mushtaq Ahmed b, Ishaq Khan a,,Nadeem Nisara
a Optics Laboratories P.O. Box 1021, Islamabad, Pakistan
b National Institute of Lasers and Optronics, Islamabad, Pakistan
Abstract
The oxygen saturation, i.e. the fraction of oxidized hemoglobin, in the blood has been measured by using the technique of pulse oximetry. It involves irradiating the human tissue by the light sources and analyzing the transmitted signal using a high sensitivity photodiode and an oscilloscope.
Our measured oxygen saturation of the examined male tissues, for an age group of 25-35 years and at an altitude of 600 m, ranges from 90-97%. Moreover, for a single healthy individual, i.e. the one with normal hemoglobin, it was measured to be 95 ±0.5% which is in agreement with the clinically accepted value.
Key words
Oximetry, oxygen saturation measurement, transmission oximetry, oximetry at lower altitude, photodiode
1. Introduction
Pulse oximetry estimates arterial hemoglobin oxygen saturation using analysis of light wave forms through a capillary tissue bed. The measurement of oxygen saturation in hemoglobin has become a standard in operating rooms, critical care unit, and emergency health care [1]. Here we will concentrate on the technique of pulse oximetry for the non- invasive measurement of arterial oxygen saturation in the blood. For the patients at the risk of respiratory failure, it is important to monitor the efficiency of gas exchange in the lungs; i.e. how well the arterial blood is oxygenated (as opposed to whether or not air is going in and out of the lungs). Preferably, such information should be available to clinicians on continuous basis (rather than every few hours). Both of these requirements can be fulfilled non-invasively with the technology of Pulse Oximetry. The clinical applications of pulse oximetry in medicine are as important as oxygen to life [2]. It is a part of any clinical investigation where there is remotest risk of hypoxemia [2]. Oxygen saturation values may vary with amount of oxygen utilization by the tissues. In some cases its value will be different for the same person during rest and activity such as ambulation or positioning [3]. In this paper oxygen saturation of healthy individual of age between 25-35 years has been recorded at an altitude of 600m by using a photodiode in photoconductive mode in the state of rest.
It was discovered in 1860`s that the coloured substance in blood, hemoglobin, was carrier of oxygen [4]. At the same time, it was noticed that the absorption of visible light by a hemoglobin solution varied with oxygenation. This is because the two common forms of molecules, oxidized hemoglobin (HbO2) and reduced hemoglobin (Hb) have different optical spectra in the wavelength range from 500 to 1000 nm as shown in Fig (1).
Once inside the lungs, the oxygen rapidly crosses the membrane and is absorbed by red blood cells to transport oxygen to body tissues [5]. Hemoglobin has tremendous affinity for oxygen that it is able to hold oxygen even when its partial pressure falls up to a point [5]. Oxygen saturation, which is often referred to as SpO2 is defined as the ratio of oxidized hemoglobin (HbO2) to total hemoglobin present in the blood (HbO2 + Hb).
(1)
Arterial SpO2 is a parameter measured with oximetry and is normally expressed as a percentage. Under normal physiological conditions arterial blood is 97% saturated, whilst venous blood is 75% saturated [4]. It is possible to use the difference in absorption spectra of HbO2 and Hb for measurement of arterial oxygen saturation because wavelength range between 600-1000 nm is also the range for which there is least attenuation of light by body (e.g. tissue and pigmentation absorb blue, green and yellow light but water absorbs longer infrared wavelength).
The intensities I and I are shown in Fig (2). By measuring transmitted light through the fingertip (or earlobe) at two different wavelengths, one red (650nm) and the other infrared (880nm), the oxygen saturation of the arterial blood in the finger (or earlobe) can be determined. If transmitted light through arterial blood is influenced by the relative concentration of HbO2 and Hb then light intensity will decrease logarithmically with path length according to well-known Beer-Lambert Law.
It is clear that general oximetry equations is valid for Pulse Oximetry where R is given by
(2)
Where is wavelength of visible light (650nm) and is that of IR (880nm). I and I are the AC and DC components of light as shown in Fig (5). So by knowing value of R a direct conversion of SpO2 is possible due to empirical calibration curve shown in Fig (3). The reported average values of oxygen saturation of healthy individuals were 92.0±.015% with a range of 84%-99% at an altitude of 3,900m-4,000m [6]. At sea level, normal oxygen saturation is 97-99% [5]. At 5,000 feet it might drop to 95% and at 10,000 feet to 90%. These figures vary significantly between individuals and may even change in the same person over time [5].
The first pulse oximeters, which were manufactured in early 1980`s used to compute the value of arterial SpO. However Beer- Lambert law on which Oximetry is based, does not take into account the multiple scattering of light by the red blood cells. Although oximetry is a different technique however the effect of scattering is partially compensated because scattering is wavelength dependent. Equation (1) is therefore an over simplification. Fig (3) shows graph between ratio R and oxygen saturation %S one using Beer-Lambert law and other based on empirical data. Instruments based on the Beer-Lambert law tend to give erroneous estimates of the true value of oxygen saturation (especially for SpO2 values below 85%). There have been attempts to modify the theory in order to take scattering into account. So most pulse oximeters now used look up table derived from calibration studies on large number of healthy volunteers whose oxygen saturation is also measured invasively.
2. Experimental Details
In our work no pulse oximeter of any registered firm has been used. The pulse oximetry was done only by a simple photodiode connected in photoconductive mode. The signal was detected by an oscilloscope and the readings were taken and compared with the calibration curve [4]. The circuit diagram of sensitive photo detector is shown in Fig (4). The detector measured the oxygen saturation with ± 0.5% error. This error can be minimized to 0.2%-0.3% by neglecting hand motion with respect to photodiode. Experiment was performed with two light sources one visible 650nm wavelength and other IR 880nm wavelength. This sensitive photo detector was able to measure the intensity even coming out of a finger or earlobe. Block diagram of the experimental setup is given in Fig (6). To perform this experiment one should place finger near the window of the photodiode to measure DC level of light, which has some value as IDC. The important task of experiment is to detect AC level of light passing through finger. This AC level arises from the pulse of a human and its width is equal to heart beat time of each person. The intensity of transmitted light through arterial blood will vary according to the heart beat. The detector output signal will therefore consist of an AC signal superimposed over a constant DC level. We recorded this AC signal on the scope. The pulse shape of AC level obtained is shown in Fig (5).
Results and Discussion
In our experiment, the number of pulses obtained for a normal person were almost equal to the number of heart beats per minute i.e. 72 pulses/ min. The AC signal measured is always almost 1-3 % of the DC signal. The value of R can be determined by putting the values of AC and DC signal in equation (2).By using Empirical Calibration curve in Fig (3) we can get an oxygen saturation value corresponding to each value of R. The experimental results are given as follow:
At wavelength (650 nm)
= 188 mv
= 185 mv
While at wavelength (880 nm)
= 8.2 mv
= 8.0 mv
By putting these values in equation (2) we get R=0.65
This according to the calibration curve in Fig (3) gives oxygen saturation ≈ 95 or 96%.
The experiment was repeated 5 times for a single person and got same result with ±0.5% error. This error can be minimized to 0.2%-0.3% by neglecting the hand motion with respect to photodiode. Similarly the experiment was performed on a number of healthy individuals and the value of R obtained having range from 0.6-0.7 which correspond oxygen saturation from 90%-97% at the attitude of 600m.
4. Conclusion
The oxygen saturation in the artery blood of the examined healthy individuals was measured to be 95-97%, for an age group of 25-35 years at an altitude of 600 m.
Normal oxygen saturation values are 97% to 99% in the healthy individual [3]. An oxygen saturation value of 95% is clinically accepted in a patient with a normal hemoglobin level, which in accordance to our experimental value
5. References
1. Byungsool Moon, Analysis of pulse oximetry signals through statistical signal processing techniques, “ECE 538 statistical signal processing 2004,”
2. H.-J.Priebe, Books review, pulse oximetry 2nd edn, “ British journal of anesthesia 89 (5); 802-805 (2002),”
3. Sandra L. Schutz, Oxygen saturation monitoring by pulse oximeter, “ AACN procedure manual for critical care, fourth edition 2001,”
4. N. Townsend, Michaelmas, “medical electronics, P 32-54, 2001,”
5. Hypoxia, oxygen and pulse oximetry, “ An article by Dr. Fred Furgang, MD,”
6. Cynthia M. Beal et.al, Percent of Oxygen Saturation of Arterial Hemoglobin Among Bolivian Aymara at 3,900–4,000 m, “American journal of physical anthropology 108, 41-51(1999),”
6. Figures Caption
(1) Figure 1: Absorption spectra of Hb an HbO2 (the isobestic point is the wavelength at which the absorption by the two forms of the molecule is the same).
(2) Figure 2: Intensity in and out through the artery
(3) Figure (3): Relationships between the ratio R and oxygen saturation of the patient.
(4) Figure (4): Circuit diagram of the photo detector used in the experiment.
(5) Figure (5): Wave shape of the heart beat at wavelength 650 nm
(6) Figure (6): Block diagram of experimental setup
7. Figures
Fig (1). Absorption spectra of Hb an HbO2 (the isobestic point is the wavelength at which the absorption by the two forms of the molecule is the same).
Fig. (2). Intensity in and out through the artery
Fig (3). Relationships between the ratio R and oxygen saturation of the patient.
Fig (4). Circuit diagram of the photo detector used in the experiment.
Fig (5). Wave shape of the heart beat at wavelength 650 nm
Fig. (6). Block diagram of Experimental setup.
11