Pulse oximetry is a test that is used to measure the oxygen saturation level or the oxygen level in the blood. In other words, it tells whether the heart and the lungs are supplying enough oxygen to meet the body’s requirements.
It is a quick, non-invasive, and painless procedure.
They are called pulse oximeters since they respond to pulsations, such as those in pulsating capillaries present in the part of the body where the test is performed.
The device may be incorporated into a multiparameter patient monitor that assesses multiple parameters. Most of the pulse oximeters also display the pulse rate.
They may be used in hospital settings for both indoor and outdoor patients. They are routinely used in operation theatres, intensive care units, and physician’s or pulmonologist’s office. In some cases, the doctor may advise a patient to have a pulse oximeter for home use. Such home use oximeters are usually portable and battery-operated types.
Principle of Pulse Oximetry
The oximeter probe consists of two parts, a pair of light-emitting diodes (LEDs) and a light detector (called a photo-detector). The beams of light are shone through the tissues of body parts (of fingertip/earlobe) from one side of the probe to the other. The blood and tissues absorb some of the light emitted by the probe.
It is based on the principle that oxygenated hemoglobin and deoxygenated hemoglobin differ in their ability to absorb red and near-infrared (IR) light.
Oxygenated hemoglobin absorbs greater amounts of IR light and lower amounts of red light than does deoxygenated hemoglobin (well-oxygenated blood has a higher concentration of oxygenated hemoglobin and appears bright red to the eye because it scatters more red light than does deoxygenated hemoglobin).
Deoxygenated hemoglobin, on the other hand, absorbs more red light and appears less red.
Based on this difference, pulse oximeters emit two wavelengths of light, red at 660 nm and near-IR at 940 nm from a pair of small light-emitting diodes located in one arm of the finger probe. The light that is transmitted through the fingertip is then detected by the photodiode present on the opposite arm of the probe.
The relative amount of red and IR light absorbed is used by the pulse oximeter to determine the amount of hemoglobin bound to oxygen. This is displayed on the electronic screen as a percentage of oxygen saturation in the blood ( SpO2).
Indications and Purpose
A pulse oximeter is used to monitor the health of a person with any type of condition that can affect the oxygen levels of the blood, whether in the hospital or at home. Its various indications include:
- To diagnose the cause of symptoms like shortness of breath
- To monitor heart, lung or other diseases that affect the blood oxygen levels
- Heart failure
- Heart attack (cardiovascular diseases)
- Congenital heart defects
- Lung cancer
- Acute respiratory distress syndrome
- Pulmonary embolism
- To monitor the blood oxygen level during or after surgery
- To assess the effectiveness of a new lung medicine
- To evaluate how helpful a ventilator is
- To assess the ability to tolerate increased physical activity as during exercise
- To evaluate whether the breathing stops momentarily while sleeping – a condition called sleep apnea
- To determine the effectiveness of supplemental oxygen therapy
Sites Where Pulse Oximeters Can be Applied
A pulse oximeter is a small clip-like photoelectric device that can be placed on the finger, earlobe, nose, forehead, or toe. The most common site where the test is performed is the fingertip.
Types of Pulse Oximeters Probes
Reusable clip probes (finger, nose, earlobe)
These can be used easily and rapidly. They are also cost-effective in outpatient settings since a single probe can be used for multiple patients because only a single reading of SpO2 is required.
Single-patient adhesive probes (finger, forehead)
A single-use tape probe is fixed to the body part using a sticky adhesive. Its advantages include lesser chances of hospital-acquired infections and secure placement of the probe in case of excessive patient movement (when continuous monitoring is required).
- The pulse oximeter is placed on the finger, earlobe, or toe. The most common site where the test is performed is the fingertip. You will be asked to remove your nail polish if it is being attached to a finger.
- It is a completely painless procedure. However, you may feel a small amount of pressure at the site.
- The probe should be placed correctly to ensure accurate reading. It should not be too tight (as it would constrict the circulation) or too loose (as it may fall off or let external light pass through it).
- To ensure that the oximeter is giving a proper reading, count your pulse for one minute, and compare the value with the pulse rate displayed on the oximeter. If they are the same, it means that the reading is proper.
- The probe is kept on for as long as the pulse and oxygen saturation need to be monitored.
- In case a single reading is required, the values appear within a few seconds and the device may be removed quickly.
- During surgery, the probe is attached before the start of surgery and removed only after you no longer require supervision.
- If used to monitor the stamina for physical activity, the device is kept in place during the time of the exercise as well as during the recovery period.
- After the test is over, the clip or probe is removed.
Interpretation of Pulse Oximeter Readings
An oxygen saturation level of 95 % or more is considered normal for a healthy individual. A value below 95% is considered low and indicates hypoxemia (meaning reduced oxygen reaching the body tissues).
However, the ‘normal’ desirable value for people having specific diseases can vary. For example, in severe COPD, the patients are asked to maintain their pulse oximeter levels (SpO2) between 88% to 92%.
In case of significantly low value, the health care provider may carry out other investigations or put the patient on specific treatment depending upon their condition.
For example, if the test was carried out to determine the effectiveness of supplemental oxygen therapy, a lower value indicates the need for more oxygen.
Difference between Pulse Oxymetry and Arterial Blood Gas (ABG) Analysis
A pulse oximeter is a medical device that indirectly monitors the oxygen saturation level of the blood. This is different from measuring the oxygen saturation level directly by examining a blood sample drawn from a blood vessel.
The reading of peripheral oxygen saturation (SpO2) obtained by pulse oximetry is not always the same as the arterial oxygen saturation (SaO2) obtained from arterial blood gas analysis (which is ideally more accurate and desirable). However, the two values show a significant correlation, and hence pulse oximetry (because of its safety, convenience, and being needle-free) is a valuable procedure for measuring oxygen saturation in routine use.
However, pulse oximetry only measures hemoglobin oxygen saturation. It does not measure respiratory or circulatory oxygen sufficiency and is not a substitute for blood gases measured in a laboratory. It does not provide information about base deficit, carbon dioxide levels, blood pH, or bicarbonate (HCO3−) concentration.
Accuracy of Pulse Oximeter
The oxygen saturation level obtained from a pulse oximeter is quite accurate. The reading provided by most oximeters gives a reading 2-3 % above or below the actual saturation value (as provided by an arterial blood gas analysis). However, this test may not provide accurate results in certain conditions and has a few limitations as discussed below.
Limitations of Pulse Oximetry
Presence of substances other than oxygen in the blood
As pulse oximetry measures only the amount of bound hemoglobin, a false reading will occur if hemoglobin binds to substances other than oxygen
- Carbon Monoxide: Hemoglobin has a higher affinity for carbon monoxide as compared to oxygen. In the presence of carbon monoxide, these molecules will attach to the patient’s hemoglobin replacing oxygen molecules. This can result in falsely high SpO2 reading despite the patient being actually hypoxemic. So pulse oximeter readings should not be used in case of carbon monoxide poisoning, smoke inhalation, and heavy cigarette smoking.
- Methemoglobin: A high level of methemoglobin in the blood results in a SpO2 reading of around 85% irrespective of the actual oxygen saturation level. A high level of methemoglobin can occur due to exposure to certain chemicals and medications.
Blood Volume Deficiency
Conditions, such as hypothermia (causing cold hands), hypovolemia or hypotension, may cause a reduction in blood flow in the body even though the oxygen saturation is normal. Hence the pulse oximeter will give a low reading.
Hemoglobin Deficiency (Anemia)
In the presence of severe anemia, when there is insufficient hemoglobin in the blood, the body tissues can suffer hypoxia in spite of having a high blood oxygen saturation level. Thus the value obtained by pulse oximeter in such a condition carries no significance.
Irregular signals can give incorrect results on a pulse oximeter. This can occur due to patient’s movements such as in shivering. To prevent this, hold the hand steady or wait till the patient has stopped shaking.
Presence of strong external light (sunlight or operating lights) while performing the test may cause inaccurate readings. This can be avoided by shielding the sensors from bright lights.
Strong electromagnetic fields in the vicinity of the pulse oximeter may also affect the values.
Fingernail Polish or other pigments
Nail polish may interfere with the readings. Hence it is important to tell the patient to remove the nail polish before taking the reading. Similarly, henna applied on the fingers may provide inaccurate results.
If a patient is having artificial nails that are too long, the reading may be inaccurate. For this, the nail should be clipped shorter or the probe can be put sideways. Alternatively, the toe or another site can be used.
Dark-colored skin can give falsely high readings. In such a case, choose a site where the skin is lighter in color.
Intravenous dyes (eg. methylene blue, indigo carmine, etc) can cause false readings.
Role of Pulse Oximeters in COVID-19 Era
Pulse oximeters can help in the early detection and management of COVID-19 infections and their complications, as this infection may cause low blood oxygen saturation and hypoxia. There is still no consensus on whether people on a large scale should be recommended home monitoring with a pulse oximeter for Covid-19. However many doctors are advising people to keep the device at home for self-monitoring.