Heart Pulse Oximeter


A Heart Pulse Wave is the change in the volume of a blood vessel that occurs when the heart pumps blood. Pulse sensor monitors and senses the volume change using the photoelectric pulse wave methods that classified into two types: reflection and transmission. During the refection method, infrared, red, or green light are emitted towards the body and measure the amount of light reflected using a photodiode or phototransistor. However, during the transmission method, red or infrared light are emitted from the body surface and detecting the change in blood flow during heart beats as a change in the amount of light transmitted through the body. One of the applications that use transmission is Pulse oximeter.

Pulse Oximeter is a device that is used to monitor a person's oxygen saturation quickly and easily. It can measure the level of oxygen within the blood, especially in the arterial blood without a use of invasive method. It measures the oxygen saturation of pulsating blood which is known as SpO2. Heart rate is automatically calculated alongside the percentage of oxygen.

Oxygen saturation:

Oxygen in blood is specifically carried by hemoglobin molecules. The hemoglobin without oxygen is called as deoxygenated hemoglobin. On the other hand, the hemoglobin with oxygen is called as oxygenated hemoglobin. Oxygen saturation refers to the percentage of the available of oxygenated hemoglobin. For example, when half of the hemoglobin molecules are oxygenated, that means the oxygen saturation is fifty percent. Hence, the relationship between the amount of oxygenated hemoglobin and oxygen saturation is linear.

How pulse oximeter work?

Pulse oximeter mainly consists of two major components, light source, and light detector. The finger should be placed between the light source and light detector. The light will pass from the light source through the finger to the detector. Part of the light will be absorbed by the finger, and the other part will reach the light detector. Pulse oximeter uses the property that oxygenated hemoglobin and deoxygenated hemoglobin absorb different amount of light at different wavelengths.


Pulse oximeter emits two types of lights, red light as a wavelength of 650nm and infrared light as a wavelength of 950nm. The pulse oximeter measures oxygen saturation by comparing how much red light and infrared light is absorbed by the blood. Depending on the amounts of oxyhemoglobin and deoxyhemoglobin present, the ratio of the amount of red light absorbed compared to the amount of infrared light absorbed changes. Using this ratio, the pulse oximeter can measure the oxygen saturation. The internal circuits and the processing components of the pulse oximeter do some calculations as well. The device must compensate for the ambient light, finger size, and the absorbance of the rest of the tissues. There is a reference curve saved in the memory of the device to calibrate the incoming readings and to normalize them. This increases the precision of the results that it provides.

Factors that affect the accuracy of a pulse oximeter readings:

  1. Inaccurately positioning of the pulse oximeter

The finger should fit well, and the probe should not be too tight because it can constrict the blood circulation. It should not be too loose as the pulse oximeter may fall off or let the light come in.

  1. Nail polish or pigment on fingers

The nail polish color can absorb light emitted by the oximeter and interfere with the detection of oxygenated hemoglobin. Hence, it is always best to remove nail polish from one finger. However, in case of an emergency the probe may be turned sideways. In case of hennas pigmentation, the henna can block the signal. The oximeter may detect a pulse but is unable to measure SpO2 due to the pigment blocking the signal. As a result, you can select a toe or ear lobe instead of finger.

  1. Bright light on the probe

Bright light such as sunlight or the operating light on the probe may interfere with the light detector and cause inaccuracy.

  1. Movement of the patient

The patient should hold the hand steady and not shake. The movement will affect the resulted wave form and the detected amount of SpO2.

  1. Poor perfusion

Oximeters need a flow of blood through the finger to function. Some oximeters give an indication of the blood flow detected.

  1. Carbon monoxide poisoning

Patients involved in fires or who have inhaled smoke, may have significant amount of hemoglobin combined with carbon monoxide. Most oximeters cannot detect this and tend to exaggerate the SpO2 value.

  1. Skin thickness

  2. Living in high altitude

Which finger is best for pulse oximeter?

We assume that’s right middle finger and the right thumb have the most accurate value and reflects the arterial oxygen saturation. Index finger dominantly is fed from deep palmer archers created from radial artery, but the middle finger receives both ulna and radial arterial blood supply. Consequently, there is a higher perfusion in the middle finger. This seems reasonable to expect the highest and most accurate SPO2 value. However, this is insignificant, you can use any finger just make sure it fades properly.

References:

1. https://www.rohm.com/electronics-basics/sensor/pulse-sensor

2. https://www.sciencedirect.com/topics/engineering/heart-beat

https://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication

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