Pulse Oximeter Blood Oxygen Monitoring Device

Pulse oximeter blood oxygen monitoring device is used in Non-Invasive optical measurement of arterial oxygen saturation in the blood, often it is used for continuous monitoring during surgery and as indicator of oxygen delivery to the peripheral tissues (such as the finger, earlobe, or nose). It is electrically safe and painless.

Note: The saturation of oxy-haemoglobin (haemoglobin is the protein which is bound to the red blood cells and is the carrier of oxygen) is the measure of the average amount of oxygen bound to each haemoglobin molecule for transport into bloodstream.

The oximeter sounds a warning alarm as soon as it detects any abnormal change in the level of oxygen of patients under anaesthesia or any other disease condition.

What is the need for pulse oximeter ?

• Body cells need oxygen to perform aerobic respiration. Respiration is one of the key ways a cell gains useful energy.Oxygen transportation is performed through the circulatory system.

• Blood oxygen saturation is among one of the five vital physiological sign other four being temperature, respiratory rate, heart rate and blood pressure.

• This information about oxygen saturation helps the clinician or doctor assess the functioning of body’s respiratory and circulatory system or amount of oxygen being carried in the blood during surgery or other procedures involving sedation (for example, bronchoscopy), adjustment of supplemental oxygen as needed, effectiveness of lung medications, and patient tolerance to increased activity levels.

• Before oximeters, clinicians would only notice signs of hypoxia (oxygen starvation) when patient’s skin literally began turning blue, with oxygen saturation at about 85%.

• Other reasons for using pulse oximeter could also be the following:
  • Mechanical ventilation: To access the functioning of a ventilator to support breathing.
  • Sleep apnea: To detect the apnea during sleep i.e Periods of cessation of breathing during sleep.
  • Medical conditions: such as heart attack, congestive heart failure, chronic obstructive pulmonary disease (COPD), anemia, lung cancer, asthma, or pneumonia

What is the principle of pulse oximetry ?

It is based on the following two principles:

1. Deoxygenated and oxygenated hemoglobin absorb different wavelengths. Deoxygenated hemoglobin (Hb) has a higher absorption at 660 nm and oxygenated hemoglobin (HbO2) has a higher absorption at 940 nm.
2. Pulsatile nature of blood flow in the arteries.

Working of pulse oximetry ?

• Oxygenated haemoglobin absorbs a greater amount of infrared light while allowing more red light to pass through.  Deoxygenated haemoglobin does the opposite.
• And with each pulsation or heartbeat there is a slight increase in the volume of blood flowing through the arteries.
• By calculating the absorption by haemoglobin in the patient’s blood stream at these two different wavelengths, the oximeter can calculate the level of oxygenation present.

Design of pulse oximeter device ?

• To measure oxygen saturation it is composed of a clip-like device called probe that is clipped to body spot with good blood flow typically finger but also earlobe or nose.
• Inside the probe is a light emitter that sends out two streams of light: one red and one infrared.  Opposite the emitter is a photodetector which receives and measures these streams of light as they pass through.
• This signal is then passed to microprocessor which calculates the differences and converts the information to a digital display unit or multiparameter health monitor.
• A healthy body in general should never fall below 95% oxygen saturation. 

Limitation of pulse oximeter ?

Use of pulse oximetry in cases of smoke or carbon monoxide inhalation is not helpful because oximetry cannot distinguish between normal oxygen saturation in the hemoglobin and the carboxyhemoglobin saturation of hemoglobin that occurs with inhalation of smoke or carbon dioxide.

In general there are certain factors or conditions may interfere with the results of the test :

• Decreased blood flow to the peripheral vessels as without minimum amount of blood, saturation measurement can’t be taken.
• Light shining directly on the oximetry probe as it would affect optical measurement.
• Movement of the area to which the probe is attached.
• Severe anemia (decreased red blood cells and hence low haemoglobin).
• Extreme warmth or coolness of the area to which the probe is attached
• Excessive sweating of the area to which the probe is attached
• Recent injection of contrast dye as it would affect the light absorption.
• Smoking tobacco.
• If a finger probe is to be used, patient may be asked to remove fingernail polish as it would affect the light absorption.

References and Further reading:

1. http://www.robots.ox.ac.uk/~neil/teaching/lectures/med_elec/notes6.pdf
2. http://www.robots.ox.ac.uk/~neil/teaching/lectures/med_elec/notes6.pdfNovel wearable and wireless ring-type pulse oximeter with multi-detectors by Huang CY, Chan MC, Chen CY, Lin BS
3. Pulse oximetry by Amal jubran