Pulse Oximeter

Oxygen saturation Red blood cells have hemoglobin. A molecule of hemoglobin can transport up to four molecules of oxygen, more than that amount is said to be saturated with oxygen. If all the places of union with the hemoglobin are transporting oxygen, it is said that the hemoglobin has saturation of 100%. Most blood hemoglobin combines with oxygen during its passage through the lungs. A healthy individual, breathing the air at sea level, will have an arterial blood saturation of 95-100%. Extreme altitudes will affect these figures. Venous blood collected from tissues contains less oxygen and normally contains a saturation of about 75%.[1]

Heart rate in beats per minute, average every 5 to 20 seconds. Some have a pulse curve that reflects the strength of the detected pulse. This curve indicates how tissues are perfused. The strength of the signal drops if the circulation begins to be inadequate.[1]

Pulse oximeter sensor

The pulse oximeter sensor consists of two parts, the light emitting diodes and a detector called the photo detector. The beams of light shine through the tissues from one side of the sensor to another. Blood and tissues absorb some of the light emitted. The light absorbed by the blood varies with the saturation of hemoglobin. The detector photo detects the light emitted as the blood pulses through the tissues and the microprocessor calculates the value for the oxygen saturation.[1]

Physical basic

The pulse oximetry consists mainly of two principles, spectrophotometry and plethysmography. The first one defines that oxyhemoglobin and reduced hemoglobin have a level of absorption of red and infrared light differently. The second principle is that the volume of arterial blood and the absorption of light by said volume, vary with the arterial pulse. The method is based on the alternate emission of two different wavelengths (red and infrared) which are transmitted by an emitter to a photodetector through oxygenated blood and Reduced, nails, skin and bone. The pulse oximetry consists mainly of two principles, spectrophotometry and plethysmography. The first one defines that oxyhemoglobin and reduced hemoglobin have a level of absorption of red and infrared light differently. The second principle is that the volume of arterial blood and the absorption of light by said volume, vary with the arterial pulse. The method is based on the alternate emission of two different wavelengths (red and infrared) which are transmitted by an emitter to a photodetector through oxygenated blood and Reduced, nails, skin and bone. [2]

Law of Beer-Lambert

Pulse oximetry is the technique used to calculate the level of oxygen saturation in blood using the absorption of light. When the finger is inserted between the emitter and the photoreceptor, the emitted light must pass through the different tissues, it is then when a percentage of light will be absorbed by the finger and the rest by the photoreceptor. The amount of light that the finger absorbs depends mainly on three parameters:

1. The concentration of blood that absorbs light; the higher the concentration absorbance.

2. The length of the path that the light travels when crossing the blood that absorbs it.

3. Absorbent composition, reduced hemoglobin and oxyhemoglobin absorb

the light in different form.[2]

DEVELOPMENT

Taking account the theory of Lambert, we design a basic circuit to determinate the oximetry starting with the use of infrared and red diode.

Materials:

*RESISTANCES OF 100, 470, 1K, 10K, 47K, 100K Ohm. *electrolytic capacitor of 1 uF Operational amplifier LF353 or LM358 *Infrared led detector, Infrared led and red led. *Potenciometer of 100Kohms *ARDUINO CARD *LCD DISPLAY 2X16. *WIRE.

Now, as you can see, we did a amplifier and voltage comparator to detect the pulse.

then use with the arduino card the lecture of ac and dc voltaje of diodes to can apply the f lambert formula.

After assembling the circuit, the following code allows us to detect the pulses and the oximetry according to the reading state of the switch from digital input 7 in arduino card.

* first of all, We start by initializing variables and determine ports to use.

*then we put what kind is the ports like inputs or outputs of circuit. Also, we call the clock interrupts to determinate the pulse per minute.

* in the void loop we execute the complete code. when entrada is high. we read 4 analog inputs to determinate the AC and DC voltage of each led to obtain the oximetry.

*void ISR_Blink() its a function to reset the clock each 15 seg, in this way, we can

* void pulso() read the digital input

Results and analysis of results

The results show that the stage of acquisition of the signal is working properly because it was possible to visualize the pulse train, which allows us to determine the beats per minute, in addition to this there is a slight interference in the area of comparison, because the diodes, emitter and receiver are affected by light intensity, since there is no suitable environment, generating alterations in the results obtained for pulse; However, the variation of the resistance is made that allows the comparison stage and the data is taken, making the circuit work in the best possible way for these measurements. To obtain the values of oxygen saturation was used a red LED, along with the infrared, which intermittently lit by instructions in Arduino. The infrared LED was implemented to capture the reduced hemoglobin and the red led for the oxyhemoglobin and the signals generated with these LEDs towards the emitter were obtained the percentage of oxygen saturation performing a digital analogue conversion through Code, to be able to visualize on a LCD the SPO2 of the evaluated person.

Oxygen Saturation graph

then with the state of pulse we detect each pulse with a rising flank

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GLOSARY

Spectrophotometry: is a scientific method used to measure how much light a chemical absorbs, by measuring the intensity of light when a light beam passes through the sample solution, based on Beer-Lambert's Law.

This measurement can also be used to measure the amount of a known chemical in a substance.

Plethysmography: is a method based on the measurement of pressure and volume changes that is used to measure parameters oriented to the diagnosis of pulmonary or cardiovascular diseases.

The wavelength: is the real distance a disturbance goes through in a certain time interval. That time interval is the interval between two consecutive maximums of some physical property of the wave.

The absorption: of electromagnetic radiation is the process by which said radiation is captured by matter. When absorption occurs within the range of visible light, it is called optical absorption. This radiation, when absorbed, can either be re-emitted or transformed into another type of energy, such as heat or electrical energy.

Hemoglobin: It is a protein in red blood cells that carries oxygen. The hemoglobin test measures the amount of hemoglobin in your blood.

Reduced hemoglobin: When hemoglobin loses oxygen, it is called reduced hemoglobin, and it presents the dark red color of venous blood.

Oxyhemoglobin: When hemoglobin is attached to oxygen, it is called oxyhemoglobin or oxygenated hemoglobin, giving the intense red appearance characteristic of arterial blood.

BIBLIOGRAPHICAL REFERENCES

[1]Lifebox.org, 2018. [Online]. Available: http://www.lifebox.org/wp-content/uploads/WHO-Pulse-Oximetry-Training-Manual-Final-Spanish.pdf.

[2]Riunet.upv.es, 2018. [Online]. Available: https://riunet.upv.es/bitstream/handle/10251/91753/L%C3%93PEZ%20-%20Dise%C3%B1o%20e%20implementaci%C3%B3n%20de%20un%20pulsiox%C3%ADmetro.pdf?sequence=1