Oxygen plays an important role in the survival and functioning of the human body. A presence of a normal level of oxygen in the blood is important for the proper functioning of different parts of the body. Oxygen level may slightly dip down temporarily at different times of the day which is considered normal. Persistent low levels of oxygen may indicate the presence of a serious infection or a disease. In order to check the oxygen levels in the blood, pulse oximetry test is recommended.Importance of the testPulse oximetry is a way to measure the amount of oxygen being carried in the blood. The blood oxygen level is checked using a small, non-invasive device called a pulse oximeter. The blood oxygen level measured with an oximeter is called oxygen saturation level (O2sat or SaO2).A pulse oximeter is a small hand-held device with a wire probe and it is attached to a person's finger, toe or earlobe. It is relatively inexpensive and is more practical for home use. The device releases beams of light that pass through the blood in the finger, earlobe or toe to measure the oxygen level. These light beams are 'read' to calculate the percentage of oxygen carried in the blood. It also provides a reading of the heart rate.Pulse oximetry is widely performed in all patients in critical care, during the intraoperative and postoperative period and during transport of patients to monitor oxygen saturation. Many people who require oxygen supplementation at home need to monitor the requirement of oxygen depending on oxygen saturation level. Such patients may benefit by using pulse oximetry at home.Pulse oximetry explainedThe principle of pulse oximetry is based on the amount of red and infrared light absorption by the oxygenated and deoxygenated blood. Oxygenated blood absorbs more infrared light and allows the red light to pass through. Deoxygenated blood absorbs red light and allows the infrared light to pass through. The level of oxygen in the blood is measured through these changes in the absorption of the red and infrared lights. Oxygen level in the blood may drop due to many reasons.Pulse oximetry monitoring is required in cases of:SuffocationChokingInfections such as pneumoniaDrowningDiseases such as emphysema, lung cancer and lung infectionsInhaling of poisonous chemicalsHeart failure or a history of heart attackAllergic reactionsAdministration of general anaesthesiaSleep apnoeaAssessment of viability of limbs after plastic and orthopaedic surgeryPerforming the testA small clip-like device is placed on the finger, toe or earlobe. A small amount of pressure can be felt at the tip of the toe or finger, although it is a painless procedure. The patient will be asked to remove nail enamel before starting the test as nail enamel may affect the accuracy of the test. The test will be performed according to the required time of monitoring of the oxygen saturation..Interpreting the resultsThe results are produced in the graphical representation and in the form of waves denoting the oxygen levels. The oxygen level in the blood recorded on the pulse oximeter is considered normal between 95-100 per cent. A value below 92 per cent is considered as low oxygen level. This indicates that oxygen is not getting supplied properly to the cells and tissues in the body.AdvantagesPulse oximetry is a continuous and non-invasive method of monitoring oxygen levels in the blood of a person. It is a reliable method and has a rapid response time. The pulse oximeter device is self- calibrated and possesses no risk of burns. It is a simple, portable 'all-in-one' monitor of oxygenation, pulse rate and rhythm and is suitable for use in all settings. Monitoring with pulse oximetry continues to be a critical component of the standard of care in critically ill patients.DisadvantagesPulse oximetry may be less effective in a critically ill or injured patient due to hypotension or shock. Bright overhead lights in operation theatre may cause the oximeter to be inaccurate and the signal may be interrupted by surgical diathermy (providing heat to a body part through electric current). Pulse oximetry cannot distinguish between different forms of haemoglobin. Other conditions like motion and shivering may also affect pulse oximeter recordings.Dr Gourish Karande is an Intensivist and consultant Physician.